Education: Acoustics 101
Four acoustic terms you need to be familiar with:
Reverberation
Reflections
Absorption - Noise Reduction Coefficient (NRC)
Isolation – Sound Transmission Class (STC)
Reverberation:
In an enclosed space, when a sound source stops emitting energy, it takes some time for the sound to become inaudible. This prolongation of the sound in the room caused by continued multiple reflections is called reverberation.
Reverberation time plays a crucial role in the quality of music and the ability to understand speech in a given space. When room surfaces are highly reflective, sound continues to reflect or reverberate. The effect of this condition is described as a live space with a long reverberation time. A high reverberation time will cause a build-up of the noise level in a space. The effects of reverberation time on a given space are crucial to musical conditions and understanding speech. It is difficult to choose an optimum reverberation time in a multi-function space, as different uses require different reverberation times. A reverberation time that is optimum for a music program could be disastrous to the intelligibility of the spoken word. Conversely, a reverberation time that is excellent for speech can cause music to sound dry and flat.
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Reflections:
Reflected sound strikes a surface or several surfaces before reaching the receiver. These reflections can have unwanted or even disastrous consequences. Although reverberation is due to continued multiple reflections, controlling the Reverberation Time in a space does not ensure the space will be free from problems from reflections.
Reflective corners or peaked ceilings can create a “megaphone” effect potentially causing annoying reflections and loud spaces. Reflective parallel surfaces lend themselves to a unique acoustical problem called standing waves, creating a “fluttering” of sound between the two surfaces.
Reflections can be attributed to the shape of the space as well as the material on the surfaces. Domes and concave surfaces cause reflections to be focused rather than dispersed which can cause annoying sound reflections. Absorptive surface treatments can help to eliminate both reverberation and reflection problems.
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Noise Reduction Coefficient (NRC):
The Noise Reduction Coefficient (NRC) is a single-number index for rating how absorptive a particular material is. Although the standard is often abused, it is simply the average of the mid-frequency sound absorption coefficients (250, 500, 1000 and 2000 Hertz rounded to the nearest 5%). The NRC gives no information as to how absorptive a material is in the low and high frequencies, nor does it have anything to do with the material’s barrier effect (STC).
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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 efficient for reducing sound transmission. For example, loud speech can be understood fairly well through an STC 30 wall but should not be audible through an STC 60 wall. The rating assesses the airborne 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.
For more information on Sound Transmission Class, visit STCratings.com.
Project Remedies :: 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.
http://www.acoustics.com/control_02.asp
Project Remedies :: 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.
http://www.acoustics.com/control_01.asp
Noise Reduction Coefficients (NRC) for Common Building Materials:
Brick, painted / .00 - .02
Brick, unpainted / .00 - .05
Carpet, indoor-outdoor / .15 - .20
Carpet, heavy on concrete / .20 - .30
Carpet, heavy on foam rubber / .30 - .55
Concrete (smooth), painted / .00 - .05
Concrete (smooth), unpainted / .00 - .20
Concrete (block), painted / .05
Concrete (block), unpainted / .05 - .35
Cork, floor tiles (3/4" thick) / .10 - .15
Cork, wall tiles (1" thick) / .30 - .70
Drapery, light weight (10oz.) / .05 - .15
Drapery, medium weight (14oz.), velour draped to half / .55
Drapery, heavy weight (18oz.), velour draped to half / .60
Fabric on Gypsum / .05
Fiberglass, 3-1/2" batt / .90 - .95
Fiberglass, 1" Semi-rigid / .50 - .75
Glass / .05 - .10
Gypsum / .05
Linoleum on Concrete / .00 - .05
Marble / .00
Plaster / .05
Plywood / .10 - .15
Polyurethane Foam (1" thick, open cell, reticulated) / .30
Rubber on Concrete / .05
Seating (occupied) / .80 - .85
Seating (unoccupied), metal / .30
Seating (unoccupied), wood / .30
Seating (unoccupied), fabric upholstered / .60
Seating (unoccupied), leather upholstered / .50
"Soundboard" (1/2" thick) / .20
Sprayed Cellulose Fibers (1" thick on concrete) / .50 - .75
Steel / .00 - .10
Terrazzo / .00
Wood / .05 - .15
Adapted from Architectural Acoustics, M. David Egan and Sound Analysis and Noise Control, John E.K. Foreman
STC ratings
http://www.stcratings.com/assemblies.html
http://www.stcratings.com/masonry.html
Typical Partition Types and Sound Transmission Examples
33 / Single layer of 1/2” drywall on each side, wood studs, no insulation (typical interior wall).
45 / Double layer of 1/2” drywall on each side, wood studs, batt insulation in wall (typical exterior wall).
46 / Single layer of 1/2” drywall, glued to 6” lightweight concrete block wall, painted both sides.
54 / Single layer of 1/2” drywall, glued to 8” dense concrete block wall, painted both sides.
55 / Double layer of 1/2” drywall on each side, on staggered wood stud wall, batt insulation in wall.
59 / Double layer of 1/2” drywall on each side, on wood stud wall, resilient channels on one side, batt insulation.
63 / Double layer of 1/2” drywall on each side, on double wood/metal stud walls (spaced 1” apart), double batt insulation.
72 / 8” concrete block wall, painted, with 1/2” drywall on independent steel stud walls, each side, insulation in cavities.
STC Sound Transmission Examples
25 / Normal speech can be easily understood.
30 / Loud speech can be understood fairly well, normal speech can be heard by not understood.
35 / Loud speech can be heard but not understood.
42 / Loud speech audible as a murmur.
45 / Loud speech not audible.
50 / Very loud sounds such as musical instruments (higher frequencies) can be heard but lack clarity.
60 / Very loud sounds such as musical instruments (higher frequencies) can be faintly heard.
75 / Most airborne noises blocked.
STC partition ratings taken from: “Noise Control in Buildings: A Practical Guide for Architects and Engineers”; Cyril M. Harris, 1994.