Basic Room Acoustics
Sound waves are basically pressure variations travelling through the air. When the sound wave travels, it compresses air molecules together at one point. This is known as the high pressure zone. After the compression, an expansion of molecules occurs. This is known as the low pressure zone. This process continues along the path of the sound wave until the energy becomes too weak to hear.
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 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 varying amount of air molecule pressure compressing and expanding is related to the apparent loudness arriving at the ear. The greater the pressure change, the louder the sound. The ear is capable of detecting a pressure change as small as 0.0002 microbar. 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. 1dB is about the smallest change in SPL that can be heard. A 3dB change is generally noticeable and a 6dB change is very noticeable. 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 (by a distant reflective surface) 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.
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.
When sound passes through an acoustically absorptive material like mineral wool insulation or acoustic foam, 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.
Direct vs Ambient
Direct sound becomes weaker as it travels away from the sound source at a rate controlled by the inverse square law. When the distance from a sound source doubles, the sound level decreases by 6dB.
The phase of a single frequency sound wave is always described relative to the starting point of the wave or 0°. The pressure change is zero at this point. The peak of the high pressure zone is at 90°, and the pressure change falls to zero again at 180°. The peak of the low pressure zone is at 270° and the pressure change rises to zero at 360° for the start of the next cycle.