Room Noise Reduction, Transondent Facings, Resonant Panels, etc.

EXAMPLE PROBLEM (REVERBERATION TIME)

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A classroom 60 ft long by 35 ft wide by 15 ft high has sound absorption coefficients α of 0.30 for walls, 0.04 for ceiling, and 0.10 for floor. All α are at 500 Hz.

Find the reverberation time T at 500 Hz in this space with no occupants and no sound-absorbing treatment.

1. Compute the room volume V.

V=60X35X15=31,500ft

2. Compute the surface areas S.

Ceiling S =60X35=2100ft

Walls S = 2 X 35 X 15 = 1050 ft

S = 2X60X 15= 1800ft

Floor S = 60 X 35 = 2100 ft

3. Compute the total room absorption a using a = Σ S α .

Total a = 1149 sabins

Note: Include air absorption in total for large rooms at frequencies greater than 1000 Hz.

4. Compute the reverberation time T using T = 0.05 (V/a)

T = 0.05 (V÷a) = 0.05 X 31,500÷1149 = 1575÷1149 = 1.37s at 500 Hz

Find the reverberation time T if 50 percent of the ceiling surface (along the perimeter of the room) is treated with acoustical panels at α of 0.85. The central area remains sound-reflecting to help distribute sound energy from lectern end toward rear of the room.

1. Compute the total room absorption a using a = Σ S α.

2. Compute new reverberation time T.

T = 0.05 (V÷a) = 0.05x31,500÷1999 = 1575÷1999 = 0.79s at 500Hz

The reverberation time is reduced to below 1 s with 50 percent ceiling treatment for unoccupied conditions. This represents a reduction of (1.37 - 079)/1.37 x 100 = 42 percent, which is a “clearly noticeable change. Absorption provided by teachers and students will further reduce reverberation depending on the number of occupants, their distribution throughout the room, and the clothing worn.

HOW TO COMPUTE SURFACE AREAS

To find total absorption in a room, first compute the surface areas of ceiling, walls, and floor and then multiply by their respective sound absorption coefficients. Next, add absorption from occupants and furnishings. A wide variety of surface shapes, along with corresponding formulas to find area, are shown below. Areas of irregular shapes can be found by subdividing the surface into smaller areas of equal widths. The more divisions by parallel lines, the greater the accuracy. For alternate methods to compute areas of irregular shapes, see p. 667 in J. N. Boaz (ed.), Architectural Graphic Standards, Wiley, New York, 1970.

Note: For a review of trigonometry, see pp. 144-145 in M. D. Egan, Concepts in Architectural Lighting, McGraw-Hill, 1983. A comprehensive self-study review of mathematics for architecture is presented by M. Salvadori, Mathematics in Architecture, Prentice-Hall, Englewood Cliffs, N.J., 1968.

The buildup of sound levels in a room is due to the repeated reflections of sound from its enclosing surfaces. This buildup is affected by the size of the room and the amount of absorption within the room. The difference in decibels in reverberant noise levels, or noise reduction, under two conditions of room absorption can be found as follows:

NR = 10 log (a2÷a1)

ROOM NOISE REDUCTION

The buildup of sound levels in a room is due to the repeated reflections of sound from its enclosing surfaces. This buildup is affected by the size of the room and the amount of absorption within the room. The difference in decibels in reverberant noise levels, or noise reduction, under two conditions of room absorption can be found as follows:

NR = 10 log = (a2÷a1)

where NR = room noise reduction (dB)

a1 = total room absorption after treatment (sabins)

a2 = total room absorption before treatment (sabins)

The chart below also can be used to determine the reduction of reverberant noise level within a room due to changing the total room absorption. For example, if the total amount of absorption in a space can be increased from 700 to 2100 sabins, the reduction in reverberant noise level NA will be about 5 dB. (See dot on chart scale at absorption ratio of a = 2100/700 = 3.) Since absorption efficiencies vary with frequency, the NR should be calculated at all frequencies for which sound absorption coefficients are known.

Practical upper limit of improvement for most situations.

The NR is the reduction in reverberant noise level. This does not affect the noise level very near the source of sound in a room. Also, as indicated on the chart, a reduction in reverberant noise level of 10 dB (an increase in absorption of greater than 10 times the initial value before treatment) is the practical upper limit for most remedial situations.

EXAMPLE PROBLEM (ROOM NOISE REDUCTION)

A small room 10 ft by 10 ft by 10 ft has all walls and floor finished in ex posed concrete. The ceiling is completely covered with sound-absorbing spray- on material. Sound absorption coefficients α are 0.02 for concrete and 0.70 for spray-on material, both at 500 Hz.

Find the noise reduction NR in this room if sound-absorbing panels are added to two adjacent walls. The sound absorption coefficient a is 0.85 for panels at 500 Hz.

1. Compute the surface areas S.

S = 5 x 10 x 10=500 ft^2 of concrete

S = 10 X 10 = 100 ft^2 of spray-on material

2. Compute the total room absorption a with spray-on material on the ceiling.

a₁ = Σ S α = (500 X 0.02) + .(100 X 0.70) = 10 + 700 sabin’

3. Compute the total room absorption a2 with sound-absorbing panels covering two walls and spray-on material on ceiling.

a₂ = (300X 0.02) + (200X 0.85) + (100 X 0.70) = 6 + 170 + 70 = 246 sabin

4. Compute the noise reduction NR.

NR= 10log (a₂÷a₁) = 10log (246/80) = 10log (3.075 X 10^0)

= 10(0.4878) = 5dB

This would be a ‘noticeable improvement. With no treatment, the total absorption in the room would only be 600 X 0.02 = 12 sabins. Therefore, treating the ceiling alone provides

NR = 10 log (80/12)= 10 log 6.67 = 10(0.8241) = 8 dB

which is a “significant” reduction. However, initial conditions of all hard surfaces in unfurnished rooms rarely occur.

Find the noise reduction NR if all four wall surfaces are treated with fabric- covered panels and the floor is carpeted. The sound absorption coefficient α of the carpet is 0.50 at 500 Hz.

1. Compute the total room absorption a3 with sound-absorbing panels on all walls, spray-on material on ceiling, and carpet on floor.

a₃ = Σ S α = (400X0.85) + (100 X0.70) + (100X0.50)

= 340 + 70 + 50 = 460 sabins

2. Compute the noise reduction NR for these improvements compared to room conditions of spray-on ceiling treatment alone.

NR = 10log (a3/a1) = 10log(460/80) = 10 log (5.75 x 10^0)

= 10(0.7597) = 8 dB

The results from both parts of the problem are summarized below.

Note: The NRs given in the above table would not be as great at low frequencies be cause sound absorption coefficients usually are smaller at low frequencies than at mid- or high frequencies.

NOISE REDUCTION FOR HIGH-NOISE ENVIRONMENTS

Low Ceiling, Machines Widely Spaced

In the example shown below, machines are widely spaced so that in stalling efficient sound-absorbing treatment on the ceiling and upper walls can reduce reverberant noise levels throughout the room. However, the sound- absorbing treatment will be of little benefit to the individual equipment operators in the free field because the direct sound energy will reach the operator before it reaches the sound-absorbing materials.

High Ceiling, Machines Closely Spaced

In the example of closely spaced machines in a room with a high ceiling, room surface treatment can be effective if reverberant noise levels are higher than the free-field noise of some machines. A reduction in reverberation will help make machine noise more directional (by reducing the reflected sound), allowing workers to be more responsive to their own machines. However, operators of closely spaced machines may be in the free field of several machines, which would be unaffected by ceiling and upper-wall treatment.

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Enclosure to Contain Machine Noise

The sound-isolating enclosure shown below can be designed to provide noise reduction near the source so individual operators can be close to their machines without experiencing high noise levels. Enclosures can be designed with operable viewing panels to allow rapid access when needed (see section 4 for sound-isolation principles, materials, and constructions).

Note: Where noisy machines are located close to walls, sound-absorbing wall treatment may provide useful noise reduction.

References

P. D. Emerson et al., Manual of Textile Industry Noise Control, Center for Acoustical Studies, North Carolina State University, 1978 (contains over 20 case studies).

P. Jensen et al., Industrial Noise Control Manual, U.S. Department of Health, Education, and Welfare, December 1978 (contains over 60 case studies on a wide variety of industries).

R. B. Newman and W. J. Cavanaugh, “Design for Hearing,” Progressive Architecture, May 1959.

W. G. Orr, Handbook for Industrial Noise Control, National Aeronautics and Space Administration, NASA SP-5 108, 1981.

TRANSONDENT FACINGS

Sound-transparent facings (called transondent) may range from 5 to 50 percent or more open area, depending on absorption requirements. Facings tend to reduce the effectiveness of sound-absorbing materials by reflecting high-frequency sound waves. In general, the lower the percentage of open area in the facing, the less absorption of high-frequency sound energy. Sizes of holes, number of holes per unit area, and dimensions of solid area between openings also affect the reduction in absorption. Transondent facings such as perforated sheet metal, expanded metal, or punched and pressed metal can be used alone in front of sound-absorbing materials, or in combination with wood slats or other large-scale protective elements.

Examples of open metal materials and a table of perforation sizes and spacings for facing materials are shown below.

Note: When painting open facings, use rollers, not sprayers, to reduce the likelihood that the openings will become blocked. Be careful also to avoid using facings with very tiny holes which may easily become clogged with paint.

* Do not exceed this spacing for hardboard material (e.g., pegboard).

** Most suitable for wall materials. Holes are small enough to discourage jabbing with sharp objects and large enough so facing can be carefully painted without becoming clogged.

Reference

W. R. Farrell, “Sound Absorption for Walls,” Architectural and Engineering News, October 1965.

PERFORATED FACINGS

Perforated facings can be used to protect and conceal porous sound- absorbing materials or, if highly transparent to sound waves, to conceal sound- reflecting or diffusing surfaces. When used over a solid backup surface without fuzz (fibrous materials) in the cavity, perforated facings can act as multiple volume resonators to selectively absorb sound with the individual holes sharing a common volume. Partitioned (or subdivided) cavities can provide wider absorption near the resonant frequency.

As shown by the graph below, the thinner the facing, the more efficient the absorption of sound energy at mid- and high frequencies. The higher the percentage of open area (from numerous, closely spaced perforations to re duce size of solid areas), the more efficient the absorption of sound energy at high frequencies. Sound transparency increases as the size of the holes and number of holes per unit area increases, and as the distance between holes decreases.

The critical frequency for circular perforations, above which sound absorption efficiency drops off rapidly, can be found as follows:

fc = 40P/D

where fc = critical frequency (Hz)

P = open area (%)

D = hole diameter (in)

For example, 25 percent open perforated facing with 1/4-in-diameter holes will have a critical frequency of

fc = 40 x 25 / 0.25 = 4KHz

Precise analysis should also take into account the thickness of the facing and depth of the airspace behind the facing (cf., P. V. Brüel, Sound Insulation and Room Acoustics, Chapman and Hall, London, 1951, pp. 114-123).

Reference

T. J. Schultz, Acoustical Uses for Perforated Metals, Industrial Perforators Association, Milwaukee, Wis., 1986, pp. 14-20.

PROTECTIVE FACINGS FOR WALL ABSORPTION

When absorption of high-frequency sound energy is not critical, the open area of protective facings need only be greater than about 10 percent to control reverberation or noise buildup within rooms. As a consequence, a wide variety of textures and forms can be used to satisfy this requirement. When absorption is used to control echoes, however, protective facings should have a higher percentage of open area from numerous, closely spaced openings. To conceal the sound-absorbing material behind most facings, tint the material black by spraying with non-bridging water-base paint or use a dark sound- transparent protective cover (e.g., burlap or open-weave fabric).

Reference

R. B. Newman and W. J. Cavanaugh, “Acoustics” in J. H. Callender (ed.), Time-Saver Standards for Architectural Design Data, McGraw-Hill, New York, 1966, p. 622.

RESONANT PANELS

Resonant panels are sound-absorbing panels which are designed to pro vide low-frequency absorption ( 250 Hz). Example applications for resonant panels are music practice rooms, radio/TV studios, and the like. Resonant panels absorb energy from sound waves by vibrating at a frequency deter mined by the geometry and damping characteristics of the panel.

To decrease the resonant frequency, use wide spacings between supports (> 2 ft), thin panel materials (e.g., plywood, hardboard), and “deep” air space behind panels. To increase the resonant frequency, use close spacings between supports, thick panel materials (or perforated, thin panel materials with sound-absorbing material located close behind the panel), and shallow or narrow airspace behind panels.

It is prudent to test unique resonant panel designs in reverberation rooms to evaluate their performance. The resonant frequency fr can be estimated by:

fr = 170 / sqr-rt(wd)

where fr. = resonant frequency (Hz)

w = surface weight of panel (lb/ft^2)

d = depth of airspace behind panel (in)

Reference

V. O. Knudsen and C. M. Harris, Acoustical Designing in Architecture, Wiley, New York, 1950, p. 120 (paperback reprint is available from the Acoustical Society of America, 500 Sunnyside Blvd., Woodbury, NY 11797).

SUGGESTED SOUND-ABSORBING TREATMENT FOR ROOMS

Although the NRC rating method has the limitations presented earlier in this section, it can be an adequate index to evaluate sound-absorbing materials for use in treating the noncritical spaces listed below. The last two groups in the table represent many of the spaces where the NRC by itself does not pro vide sufficient information. Therefore, special study may be required to deter mine the specific absorption needs. For example, absorption for ceilings in open-plan offices, where sound can reflect over partial-height barriers, destroying speech privacy, should be evaluated only by noise isolation c/ass prime NIC’ ratings (see section 6), although a minimum NRC is given.

CHECKLIST FOR EFFECTIVE ABSORPTION OF SOUND

1. Apply sound-absorbing materials on surfaces that may contribute to excessive reverberation, produce annoying echoes, or focus sound energy. In auditoriums and similar facilities, use sound-absorbing materials to control echoes and reverberation. Excessive reverberation can seriously interfere with listening conditions, especially for hearing-impaired and older persons. A doubling of the existing absorption in a room will reduce the reverberation by one-half.

2. Do not use sound-absorbing materials on surfaces which should provide useful sound reflections (e.g., above lecterns in auditoriums). Sound-reflecting surfaces must have sound absorption coefficients well below 0.20 and be properly shaped and oriented (see section 3).

3. Use sound-absorbing ceilings to control the buildup of noise within rooms, un less the floor is carpeted and the room is filled with heavy draperies and other sound-absorbing furnishings. Sound-absorbing materials are commercially avail able that have a factory-applied surface finish which is reasonably durable for ceiling applications as well as satisfying appearance, light reflectance, and other architectural and fire safety requirements.

4. Place absorption on the walls of very high rooms, small rooms, or long and narrow rooms, where flutter echo may occur. In very large rooms with low ceilings, wall absorption is rarely beneficial unless needed to prevent flanking of sound energy around partial-height barriers in open plans. Sound-absorbing wall panels that have a fabric finish and hardened edges to maintain their shape are commercially available.

5. Be sure the mounting method used is best suited for the amount of absorption desired. The actual method of mounting is important because it will affect absorption efficiency. For example, sound-absorbing materials directly attached with mechanical fasteners (mounting A) are poor absorbers of low-frequency sound. However, when attached to furring supports (mounting D), they will provide more absorption at low frequencies; and when used in suspended ceiling systems (mounting E), they can provide considerable low-frequency absorption. To achieve maximum absorption from special sound-absorbing materials and units, such as suspended baffles and spaced absorbers, install them at the spacings recommended by manufacturers.

6. Do not overestimate the noise control benefits from sound absorption. Re member, it takes a doubling of the existing absorption to achieve only 3 dB of noise reduction! It requires an enormous increase in existing absorption to achieve 6 dB of noise reduction. Consequently, in most situations, 3 to 6 dB is the practical limit of noise reduction benefits from adding sound absorption to rooms.

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