Sound Absorption.
Sound may also be absorbed by the solid. The acoustic energy is converted to heat energy as a result of frictional forces within the solid. Large amounts of sound may be absorbed with little effect on the temperature of the absorbing material.
Most hard solid surfaces are highly sound reflective. Open cell or porous materials are the most effective sound absorbers. The long, narrow, twisting air paths give rise to considerable friction between vibrating air particles and the fibres or cell walls. The friction converts much of the sound energy into heat and the process is referred to as ‘sound absorption’.
Increasing the thickness or density of a porous material will increase its sound absorption. Increasing the thickness is the most effective method of increasing the sound absorption of a material, particularly at the lower frequencies.
A material’s ability to absorb sound is expressed by its sound absorption coefficient, which is sometimes denoted by αand defined as:
1 –
(
)
The sound absorption coefficient is reported as a decimal, e.g. α = 0.75 would mean that 75% of the incident sound energy was absorbed while 25% was reflected.
A more convenient method of describing sound absorption is to use the single number NRC (Noise Reduction Coefficient). NRC is the arithmetic average of the sound absorption coefficients at the four frequency of 250Hz, 500Hz, 1000Hz and 2000Hz. NRC is usually rounded to the nearest 0.05 as per Australian Standard AS1045 : 1988 ‘Acoustics - Measurement of Sound Absorption in a Reverberation Room’.
Sound energy reflected from surface Sound energy incident on surface
α =
FIG A6.
COMMON FLANKING TRANSMISSIONS PATH.
1. Ceiling plenums, floors, walls. 2. Poor seals between structural elements and around service penetrations.
3. External air-borne paths.
4. Heating and ventilation ducting. 5. Rigid plumbing connections and
penetrations.
6. Back-to-back cabinets and switches/power outlets.
For many porous absorbers such as rockwool and glasswool, sound absorption coefficients or NRCs are commonly greater than 1.00. For example:
• 75mm thick Bradford Glasswool Supertel™ (32kg/m3) NRC = 1.09 • 50mm thick Bradford Fibertex™350 Rockwool
(60kg/m3) NRC = 1.05
Although it is theoretically impossible to have sound absorption coefficients greater than 1, as this would mean that more sound is absorbed by the material than is incident on it, NRCs greater than 1 do occur in laboratory testing as a result of the measuring techniques and the sound field within the testing facility.
Sound absorption coefficients are measured on a linear scale and so do not relate directly to decibels. The effect of sound absorption on sound pressure level is discussed under ‘Reverberation Control’.
Sound absorption materials do not absorb equal amounts of sound in all frequencies. Thus it is necessary to determine the sound absorption coefficient for each
octave band, or more preferably for each one third octave band. The sound absorption coefficients of some typical building materials are listed in Table A3.
Sound absorption coefficients may be determined in an acoustic laboratory by two different methods. The simplest of these uses a device called an ‘impedance tube’ and its use is covered by AS/NZS1935 ‘Acoustics – Determination of Sound Absorption Coefficient and Impedance in Impedance Tubes’. A more involved method uses a specifically constructed room known as a reverberation room. This method is set down in AS1045 : 1988 ‘Acoustics – Measurement Of Sound Absorption Coefficients In A Reverberation Room’.
The impedance tube method being simpler, and therefore cheaper, has been f avoured by some manufacturers of acoustic products. It has a major limitation however in that it only allows for normal incidence of sound as shown in Figure A8(a). In practice, sound will impinge on the sound absorbent material from all directions.
TABLE A3. TYPICAL VALUES OF SOUND ABSORPTION COEFFICIENTS.
Typical Building Materials Frequency (Hz)
125 250 500 1000 2000 4000 NRC
Reflective Sound Absorption Coefficients (α)
Terrazzo Flooring on concrete 0.01 0.01 0.01 0.01 0.01 0.01 0.01
Concrete 100mm 0.01 0.01 0.02 0.02 0.03 0.04 0.02 Exposed Brick 0.05 0.03 0.03 0.04 0.05 0.05 0.05 Fibrous Cement 0.04 0.05 0.06 0.08 0.04 0.06 0.05 Timber Floor 0.15 0.12 0.11 0.07 0.07 0.08 0.10 Plasterboard 0.30 0.20 0.15 0.05 0.05 0.05 0.10 Glass window 4mm 0.30 0.25 0.18 0.12 0.07 0.05 0.15 Hardboard 0.10 0.10 0.15 0.15 0.10 0.10 0.15
Suspended Plasterboard Ceiling 0.20 0.20 0.15 0.10 0.05 0.05 0.15 Aerated lightweight concrete 0.01 0.15 0.25 0.20 0.20 0.20 0.20
Absorptive
Thick Pile Carpet 0.15 0.25 0.50 0.60 0.70 0.70 0.50
Open Cell Polyurethane Foam 25mm 0.10 0.25 0.55 0.70 0.75 0.85 0.55
Polyester 25mm 0.10 0.25 0.55 0.60 0.75 0.75 0.55
Perforated Metal Pan Ceiling with Glasswool backing 0.30 0.65 0.55 0.65 0.70 0.60 0.65 Bradford Flexitel™Glasswool 25mm 0.10 0.33 0.66 0.90 1.03 0.79 0.75 Bradford Supertel™Glasswool 50mm 0.25 0.66 1.01 1.04 1.10 1.13 0.95 Bradford 50mm Fibertex™350 Rockwool 0.21 0.69 1.13 1.15 1.16 1.18 1.05 Refer to Appendix C for ‘Sound Absorption Coefficients’ of Bradford Insulation products.
The reverberation room allows for this random incidence as shown in Figure A8(b). For some applications such as ceilings and air conditioning ducts or glazing, glancing incidence as shown in Figure A8(c) predominates. As can easily be seen, data obtained by using normal sound incidence will be totally inappropriate for evaluating performance in glancing incidence situations.
It is important therefore to check by which method, published sound absorption coefficients have been determined. All leading Australian manufacturers publish data measured in accordance with AS1045-1988 ‘Acoustics – Measurements of Sound Absorption in a Reverberation Room’. Some imported products may claim performance on the basis of overseas standards. Such performance data is not necessarily in accordance with the Australian standard.
Sound absorption coefficients may also be calculated empirically from the flow resistivity of porous or fibrous absorbers. The flow resistivity is usually measured by an American Standard test method, ASTM C522-73, as there is no Australian Standard for this test.
The use of flow resistivity data enables prediction of the sound absorption coefficients for composite materials and thus minimises the number of laboratory tests required. As with all empirical calculations, predictions should be compared to actual test data to ensure the validity of the calculations.
Fibrous materials such as Bradford Rockwool and Glasswool are extremely efficient absorbers of sound at
mid to high frequencies. Low frequency absorption is influenced by the thickness of the material. The sound absorption coefficients of Bradford Rockwool and Glasswool products are shown in Appendix C of this guide.
Further improvement in low frequency sound absorption may be achieved by using Bradford Rockwool or Glasswool thicknesses greater than 50mm or by using an air space behind. For optimum acoustic absorption particularly at low frequencies, the air space should be at least as thick as the rockwool or glasswool insulation.
The sound absorption for a surface is the product of the sound absorption coefficient and the area of the surface. The unit is the Sabin, where 1 Sabin is the amount of absorption provided by 1 square metre of surface with an absorption coefficient of 1. There is a trend to replace the Sabin with ‘equivalent absorption area’. The calculation is still the same, however units of square metres are used.
Reverberation.
When sound is produced within an enclosed space such as a room, the first sound which a listener hears is that which arrives directly from the source. The next sound to be heard will be that which has been reflected from one wall of the enclosure. After this, sound which has been reflected from two, three, or more surfaces will successively arrive.
These multiple reflected or reverberant sounds combine with each other and the direct sound to form the resulting sound field as shown in Figure A9. Not only does the reverberant sound increase the level of sound, it also increases its duration. This causes distortion of the sound with particularly detrimental effects on speech and music. When long delays occur between the arrival of direct and reflected sound, distinct echoes can be heard.
Sound can take 2 paths in a room: the direct sound and the reflected sound. The total sound level is the sum of the direct and reflected sounds. The reflected sound will lose energy when striking the boundaries of the room. Some of this reflected sound will be transmitted and some absorbed, so that the amount of sound reflected will be less than that striking the boundary.
For a continuous noise source, a steady-state situation will develop where the rate of sound energy entering the room from the noise source will be balanced by the rate of sound energy leaving the room by transmission and absorption. Sound Source Direct Sound Reflected Sound FIG A7.
DIRECT AND REFLECTED SOUND.
FIG A8.
TYPES OF SOUND INCIDENCE.
(a) (b) (c)
Normal
REVERBERATION TIME.
Reverberation Time (RT) is the time it takes a sound to travel from its source to and from reflecting surfaces and gradually become inaudible. More technically speaking, RT60is the time taken for the reverberant sound pressure
level to decrease by 60dB after the direct sound has ceased.
The reverberation time of any room depends primarily upon the degree of sound reflection from the room boundaries and objects within the room. The more reflective surfaces in the room, the longer will be the reverberation time. Room dimensions also have an effect.
As sound levels fall due to absorption and transmission at solid boundaries, it follows that where sound has to travel further between reflections (ie larger rooms), it will take longer for the sound pressure level to fall, resulting in longer reverberation times.
Rooms used for different purposes need different reverberation times. Churches, concert halls and music studios may require reverberation times of up to 2 or 3 seconds, while for broadcasting studios and open plan offices appropriate reverberation times may be below 0.5 seconds.
Reverberation time affects both the room acoustics and the noise level. Short reverberation times result in lower noise levels and what is commonly called ‘dead’ acoustics, while long reverberation times result in higher noise level, or ‘live’ acoustics. For everyday purposes, reverberation time criteria can be classified as shown in Table A4. The optimum reverberation time depends upon the intended use of the room.
TABLE A4.
OPTIMUM REVERBERATION TIMES.
Room Reverberation Typical Example Acoustics Time (sec)
Dead 0.6 Hotel and airport lounges, Surgeries and consulting Rooms, Kindergarten. Medium Dead 0.6 - 0.9 Classrooms, Restaurant,
Large open-plan offices. Medium 0.9 - 1.1 Lecture rooms, General
Offices, Hospital Wards. Medium Live 1.1 - 1.4 Board Rooms,
Conference Rooms, Assembly Halls.
Live 1.4 Music Rooms,
Concert Halls.
Figure A10 from Australian Standard AS2107 : 1987 shows optimum reverberation times for various rooms. Reverberation times are usually quoted for frequency of 500Hz or 1kHz. Ideally, the reverberation time at higher frequencies should be the same as that at 500Hz, but in practice some reduction in reverberation time at frequencies above 2000Hz is almost inevitable. For good music listening condition the reverberations time at frequencies below 500Hz should increase while for speech there should be little deviation from the value at 500Hz.
REVERBERATION CONTROL.
Increasing the amount of sound absorption within a room reduces both the reverberant sound pressure level and the reverberation time.
The effect on reverberant sound pressure level is a 3dB reduction for each doubling of absorption. Thus, in a highly reflective room the addition of small amounts of sound absorbing materials will have a marked effect on the sound pressure level, while in a highly absorptive room the addition of large amounts of sound absorbing materials may have little effect.
Reverberation control as a means of noise control is limited by two factors. Firstly, it is not possible to reduce the total sound pressure level below that due to direct air borne sound transmission from source to receiver. Secondly, very large amounts of sound absorption may make the room unacceptably ‘dead’ by reducing the reverberation time too much.
The reverberation time depends on the room volume and the total sound absorption present in the room. It may be calculated by:
Direct Sound = Reflected Sound = Sound
Source
FIG A9.
Midfrequency Re v erber ation Time (sec) Room Volume (m3) 3.0 2.0 1.0 0.7 0 50 100 500 1000 10000 100000 Churches
Music Studios and Concer t Halls Oper a Houses Speech A uditor iums Variety Enter tainment Theatres Speech Studios Film and TV Studios
FIG A10. MEAN REVERBERATION TIMES (FROM AS2107 : 1987).
Equation Nº5
Where:
T = reverberation time (sec) V = room volume (m3)
A = Sαtotal absorption (Sabins) Where:
S = room surface area (m2)
α= average sound absorption coefficient for room surfaces
0.162 V A T =
Note: Equation Nº5 shows that doubling the amount of absorption in the room halves the reverberation time.
For highly sound absorbent rooms such as recording studios, the reverberation time is more correctly calculated by:
Equation Nº6
The use of CSR Bradford Rockwool or Glasswool insulation is the most effective means of absorbing sound and reducing overall sound levels in enclosed areas.
0.162 V – S ln (1 – α) T =