DISSERTATION
DISSERTATION
ON
ON
“
“
ARCHITECTURAL ACOUSTICS AND ITS
ARCHITECTURAL ACOUSTICS AND ITS
TREATMENT
TREATMENT
”
”
Submitted by:
Submitted by:
SAIF SIDDIQUI
SAIF SIDDIQUI
091110025
091110025
Under the guidance of
Under the guidance of
Dr. ANUPAMA SHARMA
Dr. ANUPAMA SHARMA
DEPARTMENT OF ARCHITECTURE AND PLANNING
DEPARTMENT OF ARCHITECTURE AND PLANNING
Maulana Azad National Institute of Technology, Bhopal
Maulana Azad National Institute of Technology, Bhopal
APRIL 2013 APRIL 2013
MAULANA AZAD NATIONAL INSTITUTE OF TECHNOLOGY,
MAULANA AZAD NATIONAL INSTITUTE OF TECHNOLOGY,
BHOPAL DEPARTMENT OF ARCHITECTURE AND PLANNING
BHOPAL DEPARTMENT OF ARCHITECTURE AND PLANNING
DECLARATION
DECLARATION
This Dissertation in subject AR 494, entitled
This Dissertation in subject AR 494, entitled ““ARCHITECTURAL ACOUSTICSARCHITECTURAL ACOUSTICS
AND ITS TREATMENT
AND ITS TREATMENT”” is being submitted as part of requirement for eighth is being submitted as part of requirement for eighth
semester of Bachelor of Architecture by the undersigned for evaluation. semester of Bachelor of Architecture by the undersigned for evaluation.
The matter embodied in this dissertation is either my own work or compilation of The matter embodied in this dissertation is either my own work or compilation of
others’ work, acknowledged properly. If, in future, it is found that the above statement others’ work, acknowledged properly. If, in future, it is found that the above statement
is false, then I have no objection in withdrawal of my Dissertation and any other is false, then I have no objection in withdrawal of my Dissertation and any other action taken by the Institute.
action taken by the Institute.
Date: Date: SAIF SIDDIQUI SAIF SIDDIQUI 091110025 091110025
APRIL 2013 APRIL 2013
MAULANA AZAD NATIONAL INSTITUTE OF TECHNOLOGY,
MAULANA AZAD NATIONAL INSTITUTE OF TECHNOLOGY,
BHOPAL DEPARTMENT OF ARCHITECTURE AND PLANNING
BHOPAL DEPARTMENT OF ARCHITECTURE AND PLANNING
DECLARATION
DECLARATION
This Dissertation in subject AR 494, entitled
This Dissertation in subject AR 494, entitled ““ARCHITECTURAL ACOUSTICSARCHITECTURAL ACOUSTICS
AND ITS TREATMENT
AND ITS TREATMENT”” is being submitted as part of requirement for eighth is being submitted as part of requirement for eighth
semester of Bachelor of Architecture by the undersigned for evaluation. semester of Bachelor of Architecture by the undersigned for evaluation.
The matter embodied in this dissertation is either my own work or compilation of The matter embodied in this dissertation is either my own work or compilation of
others’ work, acknowledged properly. If, in future, it is found that the above statement others’ work, acknowledged properly. If, in future, it is found that the above statement
is false, then I have no objection in withdrawal of my Dissertation and any other is false, then I have no objection in withdrawal of my Dissertation and any other action taken by the Institute.
action taken by the Institute.
Date: Date: SAIF SIDDIQUI SAIF SIDDIQUI 091110025 091110025
MAULANA AZAD NATIONAL INSTITUTE OF TECHNOLOGY,
MAULANA AZAD NATIONAL INSTITUTE OF TECHNOLOGY,
BHOPAL
BHOPAL
DEPARTMENT OF ARCHITECTURE AND PLANNING
DEPARTMENT OF ARCHITECTURE AND PLANNING
CERTIFICATE CERTIFICATE
This is to certify that the Dissertation entitle
This is to certify that the Dissertation entitle ““ARCHITECTURALARCHITECTURAL ACOUSTICS AND ITS TREATMENT
ACOUSTICS AND ITS TREATMENT”” is a piece of research work done byis a piece of research work done by
Saif Siddiqui
Saif Siddiqui under my guidance and supervision and to the best of my under my guidance and supervision and to the best of my knowledge and belief that this dissertation is:
knowledge and belief that this dissertation is:
(i)
(i)
Embodies the work of the candidate himself;Embodies the work of the candidate himself;(ii)
(ii)
has duly been completed;has duly been completed;(iii)
(iii)
Up to the standard both in respect of contents and language forUp to the standard both in respect of contents and language for beinbeing refg referred erred to thto the exae examinerminer..
Recommended Recommended Dr. Anupama Sharma Dr. Anupama Sharma Associate Professor, Associate Professor,
Department of Architecture and Planning Department of Architecture and Planning MANIT, Bhopal.
MANIT, Bhopal. Date
ACKNOWLEDGEMENT
I deemed it my privilege to extend my profound gratitude and appreciation to all those who have directly or indirectly involved themselves in helping me to proceed with the Dissertation work.
My sincere appreciation and thanks to Supervisor/guideDr. Anupama Sharma for their diligent attention towards the dissertation throughout all stages of work. Their comments and criticism have been invaluable.
I am thankful to all faculty members for their inspiration, without which it was impossible to finish the task.
The writing of this dissertation has been one of the most significant academic challenges I have ever taken. Though the following dissertation is an individual work, I could never have reached the heights or explored the depths without the help of books published by various authors, the e-books available on the internet and websites providing information related to my dissertation topic.
SAIF SIDDIQUI 091110025
Table of Content
Declaration ...Error! Bookmark not defined. Acknowledgement ...Error! Bookmark not defined.
Table of Content ... 4
Chapter-1. Synopsis ... 7
1.1 Title ... 7
1.2 Needs and Concerns ... 8
1.3 Aim ... 8
1.4 Objectives... 8
1.5 Scope ... 8
Chapter-2. Introduction ... 9
2.1 Acoustics ... Error! Bookmark not defined. 2.2 History of Acoustics ... 10
2.3 Sound and its Mechanism ... 16
2.4 Noise ... Error! Bookmark not defined. Chapter-3. Acoustical Treatment Of Various Spaces ... 35
3.1 Classrooms ... 35
3.2 Concert Hall ... 36
3.3 Office ... 38
3.4 Studio ... 39
3.4 Theatre ... 40
Chapter-4. Acoustic Materials ... 41
Chapter-5. Acoustical Treatments ... 42
5.2 Specialtiy Construction Materials ... 44 5.3 Floors ... 46 5.4 Stringers ... 4 9 5.2 Ceilings ... 50 5.3 Walls ... 52 5.4 Doors ... 54 5.5 Windows ... 5 7 Chapter-6. Conclusion ... 60 Chapter-7. References ... 60
Chapter-1. Synopsis
1.1 Title
Architectural Acoustics And Its Treatment
1.2 Introduction
Architectural acoustics refers to the control of sound and vibrations within buildings. Although architectural acoustics was first applied to opera houses and concert halls, this branch of acoustical engineering applies to any enclosed area, whether concert halls, office spaces, or ventilation ducts.
The acoustics of rooms are often considered to ensure speech intelligibility and privacy. One thing that can affect speech intelligibility is standing waves. A standing wave results from a sound wave reflected 180 degrees out of phase with its incident wave, which often occurs for at least one specific frequency when two walls are placed parallel to each other. To avoid this, many rooms are designed with angled walls. A second potential cause of poor speech intelligibility is reverberation. This effect can be reduced through porous absorbing materials. Examples of these include glass or mineral fibers, textiles, and polyurethane cell foams. Since the absorption of each material is different for different frequencies of sound, the materials used often vary based on the intended purpose of the room, though compound partitions, or layered combinations of different materials, make more effective absorbers. A third common technique for room acoustics is the use of masking. Masking is the canceling or drowning out of other sounds. Although this raises the overall sound pressure, masking can make irritating noises less distracting and add speech privacy As these examples highlight, room acoustics are a regular part of architectural design.
Reducing ventilation noise serves as another example of applied architectural acoustics. Many heating, ventilation, and air conditioning systems have silencers. Silencers can actively cancel noise by electronic feed forward and feedback techniques, or muffle the sound by either having sudden changes in cross section or walls with absorbent linings.Architectural acoustics involves the control of sound for ventilation, rooms, and anything else indoors.
1.3 Needs and Concerns
In today’s architectural environment, good acoustical design isn’t a luxury – it’s a
necessity. Acoustics impacts everything from employee productivity in office settings to performance quality in auditoriums to the market value of apartments, condominiums and single-family homes. While the science behind sound is well understood, using that science to create desired acoustical performance within a
specific building or room is complex. There’s no single acoustical “solution” that can
be universally applied to building design. Each built environment offers its own unique set of acoustical parameters. The acoustical design for a business conference room, for instance, differs greatly from the design needed for a kindergarten classroom. Understanding these differences and knowing how to utilize building materials, system design and technologies are key factors behind successful acoustical design. This research will provide basic background on the science and measurement of sound, as well as insights into some of the principles of architectural acoustical design.
1.4 Aim
To study the architectural acoustical designing of spaces.
1.5 Objectives
To study the sound and its mechanism Study acoustics of an enclosure..
To study treatment of moise..
1.6 Scope
Since this is an architectural report, the literature study will cover study of acoustics in an architectural space. This research will provide basic background on
Introduction to sound, as well as insights into acoustical designing of spaces principles and noise reduction techniques.
1.7 Methodology
Literature survey
1. Basics of acoustics
2. Various acoustical treatments 3. Relevant case studies.
study of the acoustical materials for treatments acoustical measures for respective enclosures.
Chapter-2. Introduction
2.1 Acoustics
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics technology may be called an acoustical or audio engineer. The application of acoustics can be seen in almost all aspects of modern society with the most obvious being the audio and noise control industries.
The word "acoustic" is derived from the Greek wordακουστικός (akoustikos), meaning "of or for hearing, ready to hear" and that fromἀκουστός (akoustos), "heard, audible", which in turn derives from the verbἀκούω (akouo), "I hear".
The Latin synonym is "sonic", after which the termsonics used to be a synonym for acoustics and later a branch of acoustics. Frequencies above and below the audible range are called "ultrasonic" and "infrasonic", respectively.
The acoustical environment of a workspace is typically given little or no attention during project planning and design. The functionality and aesthetics of the workspace are usually the primary focus of the designer. Too often overlooked, are the factors contributing to the productivity of employees occupying the workspace. Providing a comfortable environment for employees contributes significantly to their optimum performance and reduced absenteeism. Workspace comfort is really a combination of factors that includes day lighting and electric lighting, indoor environmental quality, temperature, and acoustics. The assault on ears in the workplace can come from traffic noise outside, mechanical equipment in adjacent spaces, and copiers, phones, and voices within the workspace.
2.2 History of Acoustics
The historical development of architectural acoustics is similar to other fields of building design, in comprising two parallel strands of ideas – the science and mathematics of the subject on the one hand, leading to improved understanding of the phenomena, and the methods used by designers when faced with the challenge of a new building on the other, especially when the task differs markedly from precedent. The two nineteenth-century classic works on the physics of acoustics (Helmholz 1863; Strutt 1877-78) hardly mentioned the acoustics of theatres or other rooms, and the science they contained only began to be used by architectural acousticians in the mid-twentieth century. While these two branches of knowledge are closely related, it was not a case of theory leading to practice, or vice versa: the two were symbiotic.
2.2.1 Acoustics in The Ancient World
Vitruvius on acoustics and theatre design
The Roman engineer Vitruvius devoted several chapters of his book on building design and construction to the location and design of theatres (Vitruvius, Book V). He advised that they should be located away from winds and from “marshy districts and other unwholesome quarters” and also on their orientation with respect to the sun and the surrounding terrain. He addressed key geometric issues such as the plan and section, sight lines, numbers and locations of entrances and exits, and finally considered the subject of acoustics. This highly theoretical section was not his own; he was repeating what he found in various Greek treatises on acoustics from two or
three centuries earlier which, in turn, probably had their origins in Pythagoras who first developed the subject around 530 BC. Vitruvius dealt with acoustics from several points of view. First he introduced harmonics – “an obscure and difficult branch of
musical science, especially for those who do not know Greek”. This science explained
the pitch of notes and the intervals between them in the Greek musical scale, as well as why some combinations of notes are concordant and others discordant. Next Vitruvius discussed sound in the auditorium – in particular the need for sound of all pitches to travel from the stage to the ears of every member of the audience by a
direct route, in the manner of waves created by a pebble thrown into water. This led logically to both raked seating and the semi-circular plan. He advised against vertical reflective surfaces that would prevent sound reaching the upper tiers of seats since this particularly impairs the intelligibility of word endings which, in Greek and Latin, are vital to comprehension. Such reflected waves, he wrote, can also interfere with the direct waves and distort sounds for the listener. These explanations differ remarkably little from how we would put it today. Thirdly, Vitruvius explained that the site of a theatre itself must be carefully selected taking account of acoustics: it must not have an echo, nor give reflections that can lead to direct (incident) and reflected sounds interfering.
Vitruvius also discusses the use of sounding vessels – nowadays called Helmholz resonators, after the nineteenth-century German physicist who explained how they function – which, he says, reinforce certain frequencies of the human voice and can increase intelligibility. These open-ended vessels were made of bronze and tuned to six notes of the chromatic scale. Two sets of six were arranged beneath a tier of seats symmetrically either side of the centre line of the theatre. If the theatre were particularly large, two additional sets of vessels should be installed in higher rows, each a few semi tones lower in pitch – a total of thirty six different notes. Vitruvius admits he knows of no theatres that had actually been built in Rome with sounding
vessels. The reason, he explains, is that “the many theatres that are constructed in
Rome every year contain a good deal of wood which does not lead to the same
problems with reflections as stone”. Also, he says, the timber panels themselves can resonate in a manner similar to the air in a sounding vessel and so improve intelligibility. As to the effectiveness of sounding vessels, they are known today not to
the Roman theatres were as good as the Greek ones, we do not know, but there is no doubt that both were designed with great understanding of acoustics and expertise in using this understanding to achieve demonstrably better results.
One final recommendation from Vitruvius on acoustics was for a senate house. The height of a senate house should be half the width of the building, he says, and coronae, or cornices, made of woodwork or stucco, should be fixed half way up the inside faces of the walls around the entire room. Without these, he says, the voices of men engaged in discourse are lost in the high roof. With coronae, the sound of the
voices is ‘detained before rising’ and so is more intelligible to the ear. Acoustics In The Mediaeval And Renaissance Eras
No significant writings on the acoustics of buildings survive from mediaeval or Renaissance times (Hunt 1978). Vitruvius was published in the late 15th century and would have been known by most designers of large buildings. However, it is not possible to identify precise ways in which his guidance was followed, either in cathedrals or, from the late Renaissance, in theatres. The development of music from the 12th century provides evidence of a good understanding of the acoustics of cathedrals; however, their legendary acoustic qualities are more indebted to the skill of composers and musicians than to the buildings themselves or their designers. They have long reverberation times because sound waves are reflected many times with little loss of intensity which means that musical rhythms have to be slow to be intelligible, and percussive instruments must not be used to avoid the inevitable machinegun effect of any echo. The acoustic of the space favours those instruments with a gradual attack to each note, and which sustain their notes – for example the organ, flute, violin and the human voice. For speech, however, the long reverberation time is a disaster. As the distance between speaker and listener increases, so the sound reaching the ear directly is increasingly swamped by the reflected sounds arriving by indirect, longer sound paths. Speech is thus generally unintelligible at any distance greater than a few meters, which phenomenon has an interesting architectural effect. Since it is the longer wavelengths of lower notes that are more effectively reflected, people talking in cathedrals are naturally and unconsciously inclined to whisper, irrespective of any reverence for the religious nature of the buildings they may feel, because whispering removes the lower frequencies of the human voice.
Early Modern Design Guidance – Eighteenth Century
As in the ancient world of building described by Vitruvius, it was the intelligibility of speech that drew the attention of 17th and 18th century building designers to the acoustic performance of building interiors, especially in two types of building –
theatres and the debating chambers used by politicians. During the eighteenth century the importance of room acoustics was further heightened with the invention of a number of musical instruments such as the harpsichord and fortepiano, and the growing popularity, in elite circles at least, of chamber music. The new instruments used ingenious mechanisms and large sounding boards to produce plucked and
percussive notes with unprecedented speed and at much greater volumes than earlier instruments such as the lute, harp and clavichord. When played in a room with a very live acoustic, the individual notes became indistinguishable and the objectives of the instrument makers and musicians were ruined.
Throughout Europe the second half of the eighteenth century saw a boom in theatre building in the major cities, and designers generally learned from the acoustic disasters of the early century. By the late eighteenth century it was common practice
to use the ceiling or soffit above the front of the stage as a ‘sounding board’ (actually a reflector) and the ceiling over the orchestra pit to ‘throw the voice forward’ from the
stage to the back of the stalls and to the galleries. The first design guides for theatres discussed acoustics alongside the equally important issue of line-of-sight (Patte 1782,
Saunders 1790, Rhode 1800, Langhans 1810). These and others followed Patte’s
example in showing ray diagrams to visualise sound paths.
Wallace Clement Sabine – “Father Of Architectural Acoustics”
The man who has no rival in being called “the father of architectural acoustics” was
Wallace Clement Sabine (1868-1919) (Sabine 1922; Beyer 1999, pp.186-191; Thompson 1992, 2002). Sabine was a lecturer in physics in the department of natural philosophy at Harvard University and was approached in 1895 to advise on how to improve the poor acoustics of a new lecture theatre in the University’s Fogg Art
Museum. This lecture theatre had been designed to emulate a classical Greek theatre and followed the same principles of acoustic design that Vitruvius had written down. These addressed the need of intelligibility by focusing mainly on maintaining the
volume of the direct sound that reached the listener’s ear. The speaker was placed
above the level of the front row of the audience; the seating was raked upwards towards back of the auditorium; and a wall was placed behind the speaker to reflect sound into the auditorium. However, such principles were intended for open-air theatres and took no account of sound reflected from walls or the roof. In an enclosed room these reflected sounds also reach the listener’s ear and, since there will be many
sounds, arriving at different times, the result is confusion with direct sound from a speaker competing with reflections of earlier sounds. Sabine realised this was how intelligibility was lost, like many before him. Being a physicist, however, his approach was to conduct experiments to measure how the loudness of the reflections was influenced by the reflecting surfaces in the lecture theatre. His aim was to discover the relationship between the dimensions of the room and the rate at which a sound became quieter and eventually became inaudible. He called this rate of decay the reverberation time and defined it as the time, in seconds, for a sound to decay to one millionth of its original loudness (a fall of 60dB). Sabine had to work at night to ensure all extraneous sounds were avoided. He used a single organ pipe with a frequency of 512 Herz (an octave above middle C). In 1895 there were no microphones or audio-electronics Proceedings of the Third International Congress on Construction History, May 2009 and the judgement as to when the sound was inaudible was made by the experimenter himself. An electric chronograph recorded the times to one-hundredth of a second. By covering more and more of the
auditorium’s wooden seats with soft cushions, he showed that the reverberation time was inversely proportional to the number of seats covered with cushions. He repeated the experiments in eleven other rooms in the university, with volumes ranging from a
lecture theatre of 9300 cubic metres down to an office of just 35 cubic metres. From the results he derived the equation for which his name is well-known giving the relationship between the reverberation time (RT), of a room, in seconds, its volume (V), in cubic metres, and the area (A), in square metres, of sound-absorbing surfaces in the room. (1) Sabine used this equation to give an objective means of comparing different auditoria and, in particular, to compare the proposed design for the new Boston Music Hall with the Leipzig Gewandhaus, on which its overall shape was based, and the old Music Hall in Boston. He was able to specify, for the first time, the precise degree of sound absorption in the interior of the new Boston hall needed to achieve the same reverberation time as the Leipzig Gewandhaus whose seating
capacity it exceeded by 70%, and volume by 40%. Sabine’s predictions were accurate
and the acoustic of the new hall was widely praised. He had fulfilled his goal of
overcoming the “unwarranted mysticism” that then surrounded the subject of
architectural acoustics and, most importantly, achieved “the calculation of reverberation in advance of construction”. Sabine was soon being approached by the
owners of various types of room to advise on how to rectify their acoustic problems. Often this followed the failed attempts by others to deal with the problems. Sabine noted the persistent use of a traditional but wholly ineffective remedy which involved stretching a grid of steel wires in the top of a church, theatre or court room which suffered too much reverberation on the mistaken believe that the wires would resonate and absorb sound. In New York and Boston he had seen theatres and churches with just four or five wires stretched across the room while in other auditoria several miles of wire had been used, all without the slightest effect. As part of his diagnosis of acoustic problems he would sometimes plot a contour map showing the distribution of the sound intensity. This helped him identify the source of the worst sound reflections from the walls and ceiling and hence reduce them by using sound-absorbing panels or adding decorations that would break up strong reflections from large plane surfaces. Sabine also turned his attention to the design of new theatres and how best to create a near-uniform acoustic experience for every member of the audience. To help him in these studies he used the newly-perfected
schlieren method of photography to show sound waves passing through air in two-dimensional models of auditoria (Fig.2). He was thus able to show in plan and
section, how sound waves were reflected and broken up as they emanated from the stage into the auditorium. Outside the field of building structures this was probably the first use of a scale model to investigate the engineering behaviour of a building.
Fig 3.3 Photographs showing the progress of sound waves through a model of a theatre.
The development of design methods for the acoustics of auditoria has followed the same pattern observed in other branches of building engineering design. Initially designers used their own experience to observe and improve their art and collected their experience in the form of simple design rules which could be passed on to other designers. In acoustics this approach was known in ancient times and has continued even into the twentieth century. The technical difficulty of measuring acoustic phenomena delayed a truly scientific approach to understanding acoustics until the
late eighteenth century (over a century later than for structural engineering). The first scientific concept in acoustics, defined in quantitative terms by Sabine in the 1890s, was the reverberation time whose relationship to the dimensions of a room was expressed as an empirical quantity known as the absorptivity of the surfaces of the room. This approach remains the most important in acoustic design today. The testing of scale models together with the use of non-dimensional constants was developed in acoustics simultaneously with their use in the design of building structures, first in the 1930s and more widely in the 1960s. Their use consolidated the understanding of acoustic phenomena and laid the foundation for creating mathematical models using computers.
2.3
Sound and its Mechanism
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing. Sound also travels through plasma
2.3.1 Propagation of sound
Sound is a sequence of waves of pressure that propagates through compressible media such as air or water. (Sound can propagate through solids as well, but there are additional modes of propagation). Sound that is perceptible by humans has frequencies from about 20 Hz to 20,000 Hz. In air at standard temperature and pressure, the corresponding wavelengths of sound waves range from 17 m to 17 mm.
During propagation, waves can be reflected, refracted, or attenuated by the medium.
Fig. 3.2 Travelling of sound waves
The behaviour of sound propagation is generally affected by three things:
A relationship between density and pressure. This relationship, affected by
temperature, determines the speed of sound within the medium.
The propagation is also affected by the motion of the medium itself. For
example, sound moving through wind. Independent of the motion of sound through the medium, if the medium is moving, the sound is further transported.
The viscosity of the medium also affects the motion of sound waves. It
determines the rate at which sound is attenuated. For many media, such as air or water, attenuation due to viscosity is negligible.
When sound is moving through a medium that does not have constant physical properties, it may be refracted (either dispersed or focused).
2.3.2 Perception of Sound
The perception of sound in any organism is limited to a certain range of frequencies. For humans, hearing is normally limited to frequencies between about 20 Hzand 20,000 Hz (20 kHz),[3] although these limits are not definite. The upper limit generally
dogs can perceive vibrations higher than 20 kHz, but are deaf to anything below 40 Hz. As a signal perceived by one of the major senses, sound is used by many species for detecting danger, navigation, predation, and communication. Earth's atmosphere, water, and virtually any physical phenomenon, such as fire, rain, wind, surf, or earthquake, produces (and is characterized by) its unique sounds. Many species, such as frogs, birds, marine and terrestrial mammals, have also developed special organs to produce sound. In some species, these produce song and speech. Furthermore, humans have developed culture and technology (such as music, telephone and radio) that allows them to generate, record, transmit, and broadcast sound. The scientific study of human sound perception is known as psychoacoustics.
2.3.3 Physics of Sound
The mechanical vibrations that can be interpreted as sound are able to travel through all forms of matter: gases, liquids, solids, and plasmas. The matter that supports the sound is called the medium. Sound cannot travel through a vacuum.
Longitudinal and transverse waves
Sound is transmitted through gases, plasma, and liquids as longitudinal waves, also called compression waves. Through solids, however, it can be transmitted as both longitudinal waves and transverse waves. Longitudinal sound waves are waves of alternating pressure deviations from the equilibrium pressure, causing local regions of compression and rarefaction, while transverse waves (in solids) are waves of alternating shear stress at right angle to the direction of propagation.
Matter in the medium is periodically displaced by a sound wave, and thus oscillates. The energy carried by the sound wave converts back and forth between the potential energy of the extra compression (in case of longitudinal waves) or lateral displacement strain (in case of transverse waves) of the matter and the kinetic energy of the oscillations of the medium.
Figure 3.3 Sinusoidal waves of various frequencies; the bottom waves have higher frequencies than those above. The horizontal axis represents time.
Sound waves are often simplified to a description in terms of sinusoidal plane waves, which are characterized by these generic properties:
Frequency, or its inverse, the period
Wavelength Wave number Amplitude Sound pressure Sound intensity Speed of sound Direction
Sometimes speed and direction is combined as a velocity vector; wave number and direction are combined as a wave vector.
Transverse waves, also known as shear waves, have the additional property, polarization, and are not a characteristic of sound waves.
2.3.4 Speed of Sound
The speed of sound depends on the medium the waves pass through, and is a fundamental property of the material. In general, the speed of sound is proportional to the square root of the ratio of the elastic modulus (stiffness) of the medium to its density. Those physical properties and the speed of sound change with ambient conditions. For example, the speed of sound in gases depends on temperature. In 20 °C (68 °F) air at sea level, the speed of sound is approximately 343 m/s (1,230 km/h; 767 mph) using the formula "v = (331 + 0.6 T) m/s". In fresh water, also at 20 °C, the speed of sound is approximately 1,482 m/s (5,335 km/h; 3,315 mph). In steel, the speed of sound is about 5,960 m/s (21,460 km/h; 13,330 mph).[6] The speed of sound is also slightly sensitive (a second-order anharmonic effect) to the sound amplitude, which means that there are nonlinear propagation effects, such as
2.3.5 Measurement of Sound
Sound is measured in dB (decibels). The decibel (dB) is a logarithmic unit that indicates the ratio of a physical quantity (usually power or intensity) relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities.[1] A decibel is one tenth of a bel, a
seldom-used unit named in honor of Alexander Graham Bell.
Fig. 3.4 Various sounds and their dB units.
2.3.6 Acoustic Terms
Reverberation
enclosed space, when a sound source stops emitting
room caused by continued multiple reflections is called reverberation. Re time plays a crucial role in the quality of music and the ability to understand s given space. When room surfaces are highly reflective, sound continues to reverberate. The effect of this condition is described as a live space with a long rev time. A high reverberation time will cause a build-up of the noise level in a space. of reverberation time on a given space are crucial to musical conditions and und speech. It is difficult to choose an optimum reverberation time in a multi-functio different uses require different reverberation times. A reverberation time that is op a music program could be disastrous to the intelligibility of the spoken word. Co reverberation time that is excellent for speech can cause music to sound dry and fla
Reflections
Reflected sound strikes a surface or several surfaces before reac receiver. These reflections can have unwanted or even
consequences. Although reverberation is due to continued
reflections, controlling the Reverberation Time in a space does not
space will be free from problems from r
Reflective corners or peaked ceilings can create a “megaphone” effect potentiall
annoying reflections and loud spaces. Reflective parallel surfaces lend themselves t acoustical problem called standing waves, creating a “fluttering” of sound betwee
surfaces.
Reflections can be attributed to the shape of the space as well as the material on th Domes and concave surfaces cause reflections to be focused rather than dispersed cause annoying sound reflections. Absorptive surface treatments can help to elim reverberation and reflection problems.
Noise Reduction Coefficient
The Noise Reduction Coefficient (NRC) is a single-number index for rating how ab particular material is. Although the standard is often abused, it is simply the average o frequency sound absorption coefficients (250, 500, 1000 and 2000 Hertz rounded to t 5%). The NRC gives no information as to how absorptive a material is in the lo frequencies, nor does it have anything to do with the material’s barrier effect.
Sound Transmission Class (STC):
The Sound Transmission Class (STC) is a single-number rating of a
material’s or assembly’s barrier effect. Higher STC values are more
sound transmission performance at a range of frequencies from 125 Hertz to 4000 Hertz. This range is consistent with the frequency range of speech. The STC rating does not assess the low frequency sound transfer. Special consideration must be given to spaces where the noise transfer concern is other than speech, such as mechanical
equipment or music.
Even with a high STC rating, any penetration, air-gap, or “flanking” path can seriously
degrade the isolation quality of a wall. Flanking paths are the means for sound to transfer from one space to another other than through the wall. Sound can flank over, under, or around a wall. Sound can also travel through common ductwork, plumbing or corridors.
2.4
Noise
Fig 3.5 graph showing noise levels
In relation to sound, noise is not necessarily random. Sounds, particularly loud ones, that disturb people or make it difficult to hear wanted sounds, are noise. For example, conversations of other people may be called noise by people not involved in any of them; any unwanted sound such as domesticated dogs barking, neighbours playing loud music, portable mechanical saws, road traffic sounds, or a distant aircraft in quiet countryside, is
called noise.
Acoustic noise can be anything from quiet but annoying to loud and harmful. At one extreme users of public transport sometimes complain about the faint and tinny sounds emanating from the headphones or earbuds of somebody listening to a portable audio player; at the other the sound of very loud music, a jet engine at close quarters, etc. can
cause permanent irreversible hearing damage.
Sound intensity follows an inverse square law with distance from the source; doubling the distance from a noise source reduces its intensity by a factor of four, or 6 dB.
2.4.1 Reverberation time
With this theory described, the reverberation time can be defined. It is the time for the
level of energy to decrease of 60 dB. It depends on the volume of the room V and the equivalent
absorption area a :T60 =0.16V a Sabine formula
This reverberation time is the fundamental parameter in room acoustics and depends trough the equivalent absorption area and the absorption coefficients on the frequency. It is used for several measurement :
• Measurement of an absorption coefficient of a material • Measurement of the power of a source
• Measurement of the transmission of a wall
2.4.2 Controlling Noise
Controlling Noise Between Spaces
Controlling noise between spaces is frequently an issue in residential projects and office spaces. Noise will travel between spaces at the weakest
points, such as through a door or outlet. There is no reason to spend money or effort to
improve the walls until all the weak points are controlled.
General rules of thumb for controlling noise between spaces:
A wall must extend to the structural deck in order to achieve optimal isolation.
Walls extending only to a dropped ceiling will result in inadequate isolation.
Sound will travel through the weakest structural elements, which, many times, are
the doors or electrical outlets.
When the mass of a barrier is doubled, the isolation quality (or STC rating)
increases by five, which is clearly noticeable.
Installing insulation within a wall or floor/ceiling cavity will improve the STC
rating by about 4-6 dB, which is clearly noticeable.
Often times, specialty insulations do not perform any better than standard batt
insulation.
Metal studs perform better than wood studs. Staggering the studs or using dual
studs can provide a substantial increase in isolation.
Location: Newspaper office building
Area of concern: Space between CEO office and boardroom
Additional information: Noise usually travels through spaces at several different points. Controlling only one point is like trying to save a sinking boat by patching only one hole when 10 holes exist. You must be thorough to ensure effective results. Questions to ask client:
Please describe the problem.
Does the wall go all the way up to the deck and is it sealed airtight? Does it just go
up to the dropped ceiling? Are there any penetrations through the wall?
Are there any penetrations through the wall?
Could the noise be going around the wall? Are there any air gaps? Under the door?
At the perimeter of the wall? At the window mullion? Etc?
What materials are used in the space(s)? What are your confidentiality needs?
Client feedback:
The CEO is distracted by noise from the boardroom when there are meetings in
progress. There are also confidentiality issues.
The wall does not go up to the deck, it ends at the dropped ceiling. There are no penetrations other than the door.
The noise could be going around the wall by means of the door.
The materials used in this space are carpet, painted drywall and acoustic tile on the
ceiling. There are two return air ducts about two feet apart, separated only by the wall.
Confidentiality is an issue to some degree, but not a security problem.
Evaluation: In this particular project, there was a door and a window between the two spaces and the ceiling did not go up to the deck. To improve the acoustics, an upgraded sealer was added to the doors and a flexible, vinyl barrier was placed on top of the ceiling above the two spaces (since the wall could not be extended to the deck). Creating a completely confidential space is very difficult and extremely expensive. Since confidentiality was an issue, but not a security matter, this improvement proved successful.
If further improvements were needed, the next step would be to install a sound
masking system.
essential, a very expensive door was installed. This door had an STC rating of 65, but the surrounding walls had an STC rating of 50. In this case, the walls served as the
weakest point, rather than the door. It’s important to note that the isolation quality of
an assembly is dictated by the weakest element of the assembly.
Controlling Noise from the Outside
When noise from the outside is a distraction, the windows are often to blame. Exterior walls will typically block at least
between 45 to 50 dB of sound, but even a very high quality window might not even block 40 dB. When possible, controlling noise at the source is usually the best solution. Sometimes a barrier can be built around the noise source. Other times, the noise source can be relocated.
General rules of thumb for controlling noise from the outside:
Typically, the noise transfer will go through the weakest structural element, such
as the door, window or ventilation duct.
When applicable, it is best to control exterior noise at the source.
The isolation provided by a door is only as good as the extent to which it is sealed.
If air can get around or under the door, so can sound.
The majority of exterior noise enters through the windows. Dual-pane windows
with increased air space can improve isolation.
If the noise cannot be reduced to a satisfactory level, consider trying to mask the
annoying noise with a more pleasant noise such as a water feature.
Case Study
Location: Private residence
Area of concern: A neighbor’s pool motor created an annoying hum that could be heard in
Additional information: In this case, the first thing to do is to check the weakest points, such as windows and doors. Windows can be replaced with upgraded varieties, or acoustical inserts can be added for further control. Originally, acoustic absorption was mistakenly added to the inside of the room. This actually made the problem worse. Although the noise level within the room decreased, the absorption did nothing to reduce
the exterior noise.
Questions to ask client:
Describe the problem. What is the noise source?
Where does the noise seem to be coming from? Under the door? Through the
window? Through the ceiling? Etc.?
What changes have already been made?
Ideally, what improvements would you like to see?
Client feedback:
An annoying hum is heard in the master bedroom. It interrupts sleep and interferes
with other activities such as watching television and reading.
The noise is coming from the motor from the neighbor’s pool pump.
The windows are upgraded and an acoustic sealant has been applied to the doors. Ideally, the noise would be inaudible, or at least not distracting.
Evaluation: In this situation, encapsulating the noise source was the best solution. Vibration dampening was also used to control the noise. This solution completely met the client's needs. Additional comments: There are certain noises that are difficult to control at the source, such as traffic noise. In such cases, look to control the noise at the path by erecting a barrier, such as a wall. Vegetation provides little, if any, noise reduction. If air can pass through, so can sound.
Controlling Noise Within a Space
When controlling noise within a space, there are usually two main problems to remedy: a noisy space due to reverberation or a noisy space due to equipment noise.
General rules of thumb for controlling noise within a space:
You have to at least double the absorption in a space before there is a noticeable
difference. Every time you double the absorption, the reverberant noise field is
reduced by 3 dB, which is classified as “just perceptible.”
Adding absorption to a space can provide a clearly noticeable improvement if the
space is fairly reverberant to begin with. The practical limit for noise reduction from absorption is 10 dB, which sounds half as loud.
The improvement will not be as noticeable as you get closer to the noise source. Carpet is not a cure-all. In fact, it is typically only 15-20% absorptive. It would
take four times as much carpet to have the same impact as a typical acoustic material, which is about 80% absorptive.
Case Study 1
Location: Retirement Village
Area of concern: Multi-purpose clubhouse
Additional information: The original thought was that the sound system needed to be upgraded or fixed because it wasn’t “working” properly. Further review showed that it
was the lack of absorption in the room, not the sound system that was causing the problems.
Questions asked of client:
Please describe the problem.
What are the dimensions of the space? What activities take place in this room?
Is there a noise issue? A sound system issue? A reverberation ("echo") problem? When is it the loudest?
Please describe the ceiling. Is it domed? Peaked? Flat?
What materials are used in this room? Drywall? Wood? Carpet? Tile?
Client feedback:
The room is too loud whenever there is a group in it, especially during dinners. It’s difficult to hear presenters and understand announcements. Small group
conversations are hindered by excessive surrounding noise.
The space is 65'L x 54'W x 18'H.
The room is used for large dinners, performances, presentations, and other group
activities.
The or iginal assumption was that the problem was the sound system, but we don’t
have problems hearing announcements when the room is quiet. It must be a noise issue within the room itself.
It’s the loudest during dinner when everyone is talking at once. It is not difficult to hear a presenter when there is no other noise. Presenters on stage do complain about reflections.
The ceiling is flat drywall.
Drywall and carpet are used throughout the room. Draperies and curtains are used
on the stage.
Evaluation: After speaking with the client and visiting the site, it was obvious that a lack of absorption was causing the excessive noise in the room. Frequently, in a situation such as this, a reflective ceiling, which is a large area that will project noise back down to the
floor, causes a majority of problems.
Addressing the ceiling alone would improve the noise level, but would not protect performers from the problematic reflections called slap-back*. There are a variety of products available for such applications. The products you choose are dependent upon the look and feel of the room and your budget. In this case, acoustics improved as a result of adding material to the ceiling (to control the overall noise) and acoustic wall paneling to the back wall (to control slap-back and the overall reverberation time).
*Slap- back = A reflective back wall will reflect, or “slap,” the noise back to the source
Case Study 2
Location: Headquarters for a large credit card company
Area of concern: Credit card processing center
Additional information: The first step in solving a problem related to equipment noise is to call the manufacturer. Sometimes there is a problem in the installation or in the equipment operation. Certain pieces of equipment have a retrofit noise reduction kit that can be
purchased to reduce problems.
Questions to ask client:
Please describe the problem.
What are the dimensions of the space? What activities take place in this room?
Is there a noise issue? A sound system issue? A reverberation ("echo") problem? When is it the loudest?
Is it difficult to hear someone speaking when there is no loud noise? Please describe the ceiling. Is it domed? Peaked? Flat?
What materials are used in this room? Drywall? Wood? Carpet? Tile?
Client feedback:
The processing center houses equipment that generates noise at 85-90 dB.
Workers are annoyed by this noise and the company is on the borderline of an
OSHA violation.
The space in question is 260'L x 90'W x 20'H.
This room facilitates automated printing and folding of statements and stuffing
envelopes.
Equipment noise is the primary problem.
It is the loudest when all of the equipment is operating, which is during business
hours.
the equipment itself. The level of improvement is related to the reverberance of the space. The more reverberant a space is, the more dramatic the possible improvement. For this project, the space was not too reverberant, so the improvement would not be remarkable, but it would be noticeable. Hanging vinyl-covered acoustic baffles from the ceiling, particularly the areas directly above the equipment, controlled the noise from emanating within the space, but did not reduce the noise level for the equipment operator (though it did help the other operators).
If adding absorption does not provide enough noise control, it might be necessary to isolate the noisy areas from the quieter areas. Doing so would result in the implementation of a hearing protection program for those employees working in the unavoidably louder areas. In this case, enclosing the equipment with an acoustic shield (of plexi-glass) reduced the noise level for the operator by about 10 dB. The combination of the absorptive material and the acoustic shield reduced the overall noise by about 4 dB for all employees
in the area, which met the client’s needs and br ought them into OSHA compliance.
Controlling Outside Noise
In certain situations, an outside space must be protected from the
surrounding outside noise. Encapsulation, barriers, increased distance or masking
source are some possible
General rules of thumb for controlling outside noise:
By doubling the distance from a noise source, the level is reduced by 6 dB
noticeable amount. The reduction will not be experienced to this extent with a li such as a railroad or freeway (the reduction is around 4-1/2 dB).
A barrier must block the line-of-sight between the source and the receiver in
effective.
You will typically not need a barrier with a surface weight/density greater
pounds/square foot, as long as there are no openings in the wall.
It is difficult to reduce the noise by more than 10 dB with a barrier wall. Noise barriers can be solid walls, berms or a combination of the two.
The noise wall must be continuous with no openings to be effective. If ai
through the wall, so will sound.
Vegetation, such as trees and bushes, provides very little, if any, noise reduction Case Study
Location:
Area of concern: A column burial area with a meanderi
Additional information: This space needed to facilitate a solemn and contemplative set minimizing distractions from a nearby street. Originally, a concrete block wall was us
results were not
Questions to ask client:
Describe the problem.
Describe the ambient noise conditions. Are there any existing barriers?
What is the desired result?
Client feedback:
The cemetery is next to a relatively busy road. The traffic noise is distracting
who expect a quiet, intimate setting.
Aside from the traffic noise, there are no other major noise sources in the area. A concrete block wall was used, but the results were not sufficient.
The desired result is a relaxed, meditative atmosphere that is aesthetically cons
the rest of the space.
Evaluation: Since it was not feasible to increase the barrier wall height, a sound masking system (that is typically used in an office environment) was implemented in this case. To blend in with the atmosphere, rock speakers that generated pink noise were placed along the meandering path. Water features served as additional
These fountains also eliminated hot and cold zones and created a consistent noise through the entire space. Water features alone would only work when a visitor was
standing directly next to the water.
Additional comments: In many cases, the best outdoor solution is a barrier wall. Other solutions include encapsulating a noise source (such as an emergency generator) and adding distance between the receiver and the noise source.
2.4.3 Noise Standrards
Noise Isolation Class (NIC)
Test: NIC is a method for rating a partition's ability to block airborne noise transfer.
RelatedCode: UBC/IBC and STC
General Information: Similar to a field STC test, NIC is often specified on certain projects (such as spaces with operable walls, hotels, education facilities). For a
field STC test, the individual transmission loss measurements are modified based on the reverberation time, the size of the room and the size of the test partition. The NIC does not include these modifications and simply measures he Transmission
Loss between125and4,000Hz.
Strength: Tests the isolation performance of the assembly in the field. It is good include an NIC performance requirement within your spec for operable and demounta walls.
Weakness: The NIC rating is highly dependent on the field conditions of the tested spa Because of this, the tested rating might not be achieved in other spaces or projects.
Noise Criteria (NC)
Code: This industry standard (also an ANSI standard) usually pertains to HVAC or
Enforcement: This standard is often required for certain certifications (such as government medical facilities) or included in client specifications/standards (for example, some companies have NC standards that their buildings must meet).
General Information: An NC level is a standard that describes the relative loudness of a space, examining a range of frequencies (rather than simply recording the decibel level). This level illustrates the extent to which noise interferes with speech intelligibility. NC should be considered for any project where excessive noise would be irritating to the users, especially where speech intelligibility is important. There are a few spaces where speech intelligibility is absolutely crucial, including:
Recording studios Lecture halls Performance halls Courtrooms Libraries Worship centers Educational facilities
For some areas, such as machine shops or kitchens, it is not essential to maintain a particularly low NC level.
NC Level Strength: It is important for design professionals to specify NC ratings to protect their designs (within reason – specifying an acceptable NC level does not have to be a burden on the budget). Doing so speaks to your reputation as a responsible
architect or designer and limits your liability.
NC Level Weakness: NC does not account for sound at very low frequencies. In spite of numerous efforts to establish a widely accepted, useful, single-number rating method for evaluating noise in a structure, a variety of techniques exist today. The vast majority of acoustic professionals use the NC standard, but it is still important to be aware of the other acceptable methods that do account for low frequency levels,
Room Criteria (RC) measures background sound in a building over the
frequency range 16 Hz to 4000 Hz. This rating system requires two steps: determining the mid-frequency average level and determining the perceived balance between high and low frequency sound. To view the recommended
ANSI levels for room criteria for various activity areas, click here.
Balanced Noise Criteria (NCB) is based on the ANSI threshold of audibility
pure-tones and is defined as the range of audibility for continuous sound i specified field from 16 Hz to 8000 Hz.
Sound Transmission Clas (STC)Code:
STC rates a partition's or material's ability to block airborne sound.
Enforcement: Appendix Chapter 35 of the ’88 and ’91 UBC, Appendix Chapter 12, Division II of the ’94 and ’97 UBC will be contained in the forthcoming IBC. Although not all municipalities have adopted this appendix chapter, it is still
recognized as an industry standard.
General Information: The Uniform Building Code (UBC) contains requirements for sound isolation for dwelling units in Group-R occupancies (including hotels, motels,
apartments, condominiums, monasteries and convents).
UBC requirements for walls: STC rating of 50 (if tested in a laboratory) or 45 (if tested
in the field*).
UBC requirements for floor/ceiling assemblies: STC ratings of 50 (if tested in a laboratory) or 45 (if tested in the field*).
* The field test evaluates the dwelling’s actual construction and includes all sound aths.
Definitions:
Sound Transmission Class rates a partition’s r esistance to airborne sound
transfer at the speech frequencies (125-4000 Hz). The higher the number, the better the isolation.
STC Strength: Classifies an assembly’s resistance to airborne sound
transmission in a single number.
frequencies and provides no evaluation of the barrier’s ability to block low
frequency noise, such as the bass in music or the noise of some mechanical equipment.
Recommended Isolation Level
An assembly rated at STC 50 will satisfy the building code requirement,
however, residents could still be subject to awareness, if not understanding, of loud speech. It is typically argued that luxury accommodations require a more stringent design goal (as much as 10dB better – STC 60). Regardless of what STC is selected, all air-gaps and penetrations must be carefully controlled and sealed. Even a small air-gap can degrade the isolation integrity of an assembly.
Chapter-3. Acoustical Treatments of Various Spaces
3.1
Classrooms
Tips/Considerations Recommended reverberation time is 0.4-1.0 seconds (depending on the size o
the space).
Numerous studies demonstrate how chronic noise exposure (i.e., noise found in
the community, as well as noise to which we are voluntarily exposed) negatively impacts education. For more information, readProgressing the Learning Curve.
Noise from air-conditioning/heating units or other equipment on the premises
can impact the educational environment. In addition to an NC specification for inside the classroom, specify a maximum dB level for all equipment in and around the school.
Consider the impact of noise from nearby freeways, busy roads, train tracks and
other transportation- or industrial-related sources. Identify noise sources in the vicinity and assess the possible impact. Based on this assessment, take the
o Noise from adjacent classrooms can be easily transmitted into other
classrooms, particularly in an open-classroom setting. It is vital to control the noise transfer between spaces. Keep in mind that STC ratings only address noise isolation from 125 Hz to 4000 Hz. Low frequency sounds (below 125 Hz) are not accounted for in an STC rating. Even if you specify a high STC rating for the wall, it will not allow for privacy if the wall only extends to the ceiling, or just above the ceiling. To ensure isolation, the wall must extend to, and seal to, the deck.
Even if everything else is controlled perfectly, the space might not be usable i
the background noise (e.g. HVAC system) is too loud. To help protect your design, the NC level should not exceed 25 to 35. When specifying NC, specify an actual rating, such as NC 25, rather than a range, such as NC 25-30. Although specifying a lower number will ensure minimal background noise, it might be cost prohibitive to achieve. Be realistic about the amount of acceptable noise and the project's budget when specifying an NC level.
3.2
Concert Hall
Goal:To create an optimal acoustic environment suitable for performance enhancement and audibility while protecting the hearing health of the individuals using that space.
Tips/Considerations
o The reverberation time will depend on what type of concert is performed.
For classical or orchestral music, a higher reverberation time would be appropriate (approximately 2 sec), for a rock concert, a lower reverberation time would be appropriate (approximately 1 sec). Find a happy medium, perhaps 1.5 sec. This only applies to indoor venues.
o It is vital to control the reflections from the back wall. If you don't
control them, the presentation could reflect off the back wall and "slap back" to the presenter(s). This won't necessarily impact the audience,
but could be disastrous and distracting for the people on stage. Because of this, it's usually necessary to splay or tilt the back wall to avoid slap back. A concave back wall could compound this problem. If you can't avoid a concave back wall, it's imperative that it be treated with absorptive material.
o Control the reverberation time on the stage. Ideally, the reverberation
time in the stage area should be the same as in the house. Since the stage area might have a higher ceiling than the rest of the auditorium, more absorptive materials might be required in this area. Frequently, the back wall of the stage, and possibly one or two of the side walls, is treated with an acoustically absorptive material, typically black in color.
o Beware of potential noise impact to your space from exterior sources
and/or excessive HVAC noise. To help protect your design, the NC level should not exceed 25 to 35. When specifying NC, specify an actual rating, such as NC 30, rather than a range, such as NC 30-35. Although specifying a lower number will ensure minimal background noise, it might be cost prohibitive to achieve. Be realistic about the amount of acceptable noise and the project's budget when specifying an NC level.
o Some concert attendees have sued (and won) over experiencing hearing
loss at a concert. Beware of potentially dangerous, excessive noise levels. Some venue operators regulate the noise levels to help alleviate the potential noise impact on surrounding areas and on the audience.
o For outdoor venues, be sure to check on local noise ordinances. Even i
they don't exist, you should still take steps to control excessive noise impact to the surrounding community.
Especially outdoors, be concerned about exterior noise impact on the venue. Often this will decide the location of the site. For instance, be aware of surrounding airports (flight paths), freeways, railroads and industrial sites.
Tips/Considerations
o Typical reverberation time is between 0.4 and 1 second.
o Absorptive materials will most likely be necessary for the ceiling.
o Even if the reverberation time is optimally controlled, reflections from
the walls can be problematic. Parallel reflective surfaces can cause an annoying condition called flutter echo or standing wave. Ideally, at least two non-parallel walls should be treated with acoustically absorptive material. It might not be necessary to completely treat the wall as long as the critical zone (normally from 3'-7') is treated with a material that has an NRC of at least 0.50, ideally at least 0.80.
o Draperies typically provide very little, if any, absorption.
o Beware of potential noise impact to your space from exterior sources
and/or excessive HVAC noise. To help protect your design, the NC level should not exceed 25 to 35. When specifying NC, specify an actual rating, such as NC 30, rather than a range, such as NC 25-30. Although specifying a lower number will ensure minimal background noise, it might be cost prohibitive to achieve. Be realistic about the amount of acceptable noise and the project's budget when specifying an NC level.
o Awareness of activity in adjacent spaces is typical in most offices.
However, if the transmitted speech is intelligible, it becomes far more distracting. Additionally, confidentiality and speech privacy can become a serious concern. Noise transfer is due to the isolation quality of a wall assembly, as well as any potential flanking paths. The isolation quality of an assembly is largely determined by the weakest point of the assembly. Any air-gap can substantially degrade the isolation quality of the assembly. Even if the assembly has a high STC rating, a variety of flanking paths can allow noise transmission and speech to be understood between spaces. Some of the sound paths that can contribute to potential noise transfer are:
Wall Assembly Door Assembly
Penetrations (outlets)
Air-Gap between wall and window mullion Flanking over the wall/through the ceiling
