Noise, like light, bounces from one hard surface to another. In noise control work, this bouncing is called reverberation. If a soft, spongy material is placed on the walls, floors, and ceiling, the reflected sound is diffused and soaked up (absorbed). Sound-absorbing materials are rated either by their Sabin absorption coefficients (aSAB) at 125, 500, 1000, 2000, and 4000 Hz or by a single number rating called the noise reduction coefficient (NRC).
If a unit area of open window is assumed to transmit all and reflect none of the acoustical energy that reaches it, it is assumed to be 100% absorbent. This unit area of totally absorbent surface is called a sabin. The absorptive properties of acoustical materials are then compared with this standard. The performance is expressed as a fraction or percentage of the sabin (aSAB). The NRC is the average of the aSABs at 250, 500, 1000, and 2000 Hz rounded to the nearest multiple of 0.05. The NRC has no physical meaning. It is a useful means of comparing similar materials.
Inner Metal Duct
Glass Fiber Board Liner
Glass Fiber Board Liner mmrmsF*
SINGLE & DOUBLE WALL LINED DUCTS
Outer Metal Duct lass Fiber Blanket
Silencer" Sinusoid Glass Fiber er Honey-Comb Glass Fiber er Honey-Comb Glass Fiber
FIG. 6.7.1 Various types of acoustical duct lining, baffles, and silencers.
Sound-absorbing materials such as acoustical tile, carpets, and drapes placed on ceiling, floor, or wall surfaces can reduce the noise level in most rooms by about 5 to 10 dB for high-frequency sounds, but only by 2 or 3 dB for low-frequency sounds. Unfortunately, such treatment does not protect an operator of a noisy machine who is in the direct noise field. For greatest effectiveness, sound-absorbing materials should be installed as close to the noise source as possible.
If only a small or limited amount of sound-absorbing material is available, putting it in the upper trihedral corners of the room, formed by the ceiling and two walls, is the most effective use of it in a noisy room. Due to the process of reflection, the concentration of sound is greatest in the trihedral corners of a room. Additionally, the upper corner locations also protect lightweight fragile materials from damage.
Because of their light weight and porous nature, acoustical materials are ineffectual in preventing the transmission of either airborne or structure-borne sound from one room to another. In other words, if you can hear people walking or talking in the room or apartment above, installing acoustical tile on your ceiling will not reduce the noise transmission.
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