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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.
Loudness
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.
Reflections
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.
Echo
This occurs when an indirect sound is delayed long
enough to be heard by the listener as a distinct
repetition of the direct sound.
Reverberation
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.
Refraction
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.
Diffraction
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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