Acoustical absorption is used in a wide array of settings and applications. It is used to control echo, reduce the noise level in a space, make it easier to understand speach, and control focusing.
There are many different types of spaces where acoustical absorption is used, including:
- Shooting ranges
- Community centers
- Call centers
- Practice rooms
- Conference rooms
- Court rooms
Types of Absorption
All surfaced has some amount of absorption. A common way to evaluate absorption is with the Noise Reduction Coefficient (NRC). NRC is a scalar representation of the amount of sound energy absorbed upon striking a particular surface. An NRC of 0 indicates perfect reflection; an NRC of 1 indicates perfect absorption. The NRC is calculated based on 16 third-octave band frequencies from 125 to 4,000 hertz (Hz). NRC is being replaced by the Sound Absorption Average (SAA), which is described in the current ASTM C423 09a. The SAA is a single number rating of sound absorption properties of a material similar to NRC, except that the sound absorption values employed in the averaging are taken at the twelve one-third octave bands from 200 Hz to 2500 Hz, inclusive, and rounding is to the nearest multiple of 0.01.
There are many types of acoustical absorption. The type that is best for any application depends on the durability that is needed, aesthetics, cost and location.
The four common base acoustically absorptive materials are: fiberglass, foam, mineral wool and cotton. There are spray-on cellulose materials that can be spayed onto a surface.
Acoustic ceiling tiles are another common acoustically absorptive treatment. The most absorptive are made of fiberglass, but there are other types.
There are many types of coverings that can be used to increase durability and visual appeal. Some common coverings include:
- Perforated metal
Depends on use of space, size and other surfaces in the space. Commonly applied to ceilings and to walls at the height of the noise source.
Effectiveness of acoustic absorption can be determined by measuring Reverberation time.
Reverberation time is a measure of how long sound stays present within a space after it is made. More specifically, reverberation time is defined as the time required for the level of sound in a room to drop 60 dB after the signal is turned off. The reverberation time within a space can be controlled by the ratio of sound-absorptive surface area to sound-reflective surface area.
The change in reverberation time is used to calculate the reverberant noise reduction or increase. The following table shows an approximation of human sensitivity to changes in reverberation time and the corresponding reverberant sound level. Noise is measured in decibels (dBA). Because people respond differently to sound at different frequencies, a weighted scale (dBA) is used to approximate the sensitivity of the human ear. Note that a 6 dBA change is required for the sound level change to be clearly noticeable.
Table 1 – Human Sensitivity to Reverberation Time and the Corresponding Sound Level Reduction
|% Reduction in Reverberation Time||Reduction in Sound Level (dBA)||Change in Apparent Loudness|
|50||3||Just barely perceptible|
|75||6||Clearly noticeable reduction|
|90||10||About half as loud|
|99||20||About quarter as loud|
The reverberation time within a space can be controlled by the ratio of sound-absorptive surface area to sound-reflective surface area.
Some treatment materials can cost as little as $1 per sq ft. Costs vary depending on application, location and desired aesthetic.
Noise Engineers provides information and resources to help people address acoustical issues. In these episodes my goal is to provide resources, inexpensive tools, rules of thumb when dealing with acoustical issues. I would like to explain basic acoustic principles and answer any questions. I will describe actual projects to make this as practical as possible.