The 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. For 200Hz the wavelength is 1.7 meters so a theoretical thickness of 400mm is required to absorb or attenuate this wavelength. 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. Long wavelength, low frequency soundwaves are much more difficult to attenuate with porous absorptive materials. In recording studio situations it is crucial to have an air gap behind the absorber to increase the mid and low frequency absorption. Absorbers work best when there is some sort of a reflective surface behind them.Ībsorbers behave differently as they are moved away from the wall surface. 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. Each time the sound waves change direction a percentage of the energy is absorbed by conversion to heat. When sound passes through an acoustically absorptive material like acoustic 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. 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. This change may be due to physical objects or it may be due to atmospheric effects such as wind or temperature gradients.Ī sound wave will bend around obstacles in its path which are smaller than its wavelength. This is the bending of a sound wave as it passes through some change in the density of the transmission medium. Typically, this happens at low frequencies due to their longer wavelengths and the difficulty in absorbing them. 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. These occur in a room at certain frequencies related to the distance between parallel walls. 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. 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. Reflection is the source of echo, reverb, standing waves and diffusion. Because low-frequency sounds have long wavelengths they can only be reflected by large surfaces or objects. ReflectionsĪ 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. A 10dB SPL increase is perceived to be twice as loud. A 3dB change is generally noticeable and a 6dB change is very noticeable. 1dB is about the smallest change in SPL that can be heard. 0dB SPL is the threshold of hearing and 120dB SPL is the threshold of pain. Sound Pressure Level (dB SPL), relative to 0.0002 microbar (0dB SPL). This wide amplitude range of sound is often referred to in decibels. The threshold of pain is around 200 microbars. One microbar is equal to one millionth of atmospheric pressure. The ear is capable of detecting a pressure change as small as 0.0002 microbar. The greater the pressure change, the louder the sound. The varying amount of air molecule pressure compressing and expanding is related to the apparent loudness arriving at the ear. The fluctuation of air pressure created by sound waves is a change above and below normal atmospheric pressure. The velocity at 0☌ is 332 metres per second, rising by 0.6 metres per second for each ☌ increase in temperature. Sound velocity in air depends on atmospheric pressure and temperature, with the latter being the more significant factor. Wavelength is related to frequency by the speed of sound. The wavelength of a sound is the physical distance from the start of one cycle to the start of the next cycle. A tone of 1000Hz frequency has 1,000 cycles per second. The number of these cycles, completed in one second is called the Hertz (Hz). One full cycle is a change from high pressure, to low pressure, and back to high pressure. The frequency of a sound wave indicates the rate of pressure variation or cycles. This process continues along the path of the sound wave until the energy becomes too weak to hear. After the compression, an expansion of molecules occurs. When the sound wave travels, it compresses air molecules together at one point. Sound waves are basically pressure variations travelling through the air.
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