26 Acoustical Absorption

acoustical absorption

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.  We now offer a Free Room Acoustics Analysis (noiseexpert.com/free-room-analysis/).  Just enter your project informaton and we will get back to you with an evaluation of the room dimentions, room finishes, reverberation time, direct and reflected noise paths, and the location of acoustical treatment.

Noise Expert can help provide and install acoustically absorptive material – Acoustic Panels and Absorption.

Applications

There are many different types of spaces where acoustical absorption is used, including: theaters, churches, pools, gyms, practice rooms, conference rooms, open office plan spaces, shooting ranges, …

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 single number representing 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 tests at 16 third–octave band frequencies from 125 to 4,000 hertz (Hz).

There are many types of aboustical 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 celulose 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 used depending on durability and visual appeal. Some common coverings include: fabric, perforated metal, PVC, vinyl, wood, …

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

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
20 1 Imperceptible
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.

Cost

Treatment materials can cost between $1 to $15 per sq ft. Costs vary depending on application, location and desired aesthetic.

Thank you for visiting Noise Expert Acoustical Consulting – Please let us know if you have any questions or if you would like our assistance with acoustical evaluations. 480-332-9325, info [at] noiseexpert.com, www.noiseexpert.com. We now have offices in Phoenix, San Diego, Las Vegas, and Salt Lake City.

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Sound Solutions is is an acoustical consulting firm that conducts noise measurements, analysis, reporting, materials and installation for environmental and architectural noise issues.

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