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.
Increasing air space in a wall or window assembly will improve 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.
Further comments: In another office space, where complete confidentiality was
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
the master bedroom.
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?
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 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 causing a delay.
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.
There are no communication issues when the equipment is not running.
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
mechanical noise impact.
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
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