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Basic Knowledge About Room Acoustics - Part I

Sound Waves

Sound waves are basically pressure variations travelling through the air.

Frequency and wavelength

The frequency of a sound wave indicates the rate of pressure variation or cycles. One full cycle is a change from high pressure, to low pressure, and back to high pressure. The number of these cycles, completed in one second is called the Hertz (Hz). A tone of 1000Hz frequency has 1,000 cycles per second.

The wavelength of a sound is the physical distance from the start of one cycle to the start of the next cycle. Wavelength is related to frequency by the speed of sound.

Sound velocity in air depends on atmospheric pressure and temperature, with the latter being the more significant factor. The velocity at 0??C is 332 meters per second, rising by 0.6 meters per second for each ??C increase in temperature.


The fluctuation of air pressure created by sound waves is a change above and below normal atmospheric pressure. This is what the human ear responds to. The greater the pressure change, the louder the sound.

One microbar is equal to one millionth of atmospheric pressure. The threshold of pain is around 200 microbars. This wide amplitude range of sound is often referred to in decibels. Sound Pressure Level (dB SPL), relative to 0.0002 microbar (0dB SPL). 0dB SPL is the threshold of hearing and 120dB SPL is the threshold of pain. A 10dB SPL increase is perceived to be twice as loud.


A sound wave can be reflected by a surface or object if that surface is physically as large, or larger, than the wavelength of the sound wave. Because low-frequency sounds have long wavelengths they can only be reflected by large surfaces or objects. Reflection is the source of echo, reverb, standing waves and diffusion.


This occurs when an indirect sound is delayed long enough to be heard by the listener as a distinct repetition of the direct sound.


This consists of many reflections of a sound, maintaining the overall sound in a room for a time even after the direct sound has stopped.

Standing Waves

These occur in a room at certain frequencies related to the distance between parallel walls. The original sound and the reflected sound will begin to reinforce each other when the wavelength is equal to the distance between the two walls. Typically, this happens at low frequencies due to their longer wavelengths and the difficulty in absorbing them.


This is the bending of a sound wave as it passes through some change in the density of the transmission medium. This change may be due to physical objects or it may be due to atmospheric effects such as wind or temperature gradients.


A sound wave will bend around obstacles in its path which are smaller than its wavelength. Because a low frequency wave is much longer than a high frequency wave, the low frequencies will bend around objects that the high frequencies cannot.

Passive Absorption

When sound passes through an acoustically absorptive material like acoustic fiberglass, the sound waves are forced to change directions many times and travel great distances before the sound passes completely through the absorptive material.

Each time the sound waves change direction a percentage of the energy is absorbed by conversion to heat. When there is a reflective surface behind the absorber, (such as a wall) the sound which passes through the absorber will be reflected back and through the absorber once again. Absorbers work best when there is some sort of a reflective surface behind them.

Absorbers behave differently as they are moved away from the wall surface. In recording studio situations it is crucial to have an air gap behind the absorber to increase the mid and low frequency absorption.

Long wavelength, low frequency sound waves are much more difficult to attenuate with porous absorptive materials. This is because the thickness of the absorptive material must be at least 1/4 of the wavelength of the lowest frequency to be absorbed. For 200Hz the wavelength is 1.7 meters so a theoretical thickness of 400mm is required to absorb or attenuate this wavelength.

The Sound Absorption Coefficient of a material can be expressed as a value between 0 and 1 where 0 represents no absorption (perfect reflection) and 1 represents total absorption. It can also be represented as a percentage. 

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