Audio, Image and Video Processing : September 2012

Anechoic Chambers

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Anechoic chambers are rooms designed to stop the reflection of sound.
They are designed using wedge shaped panels made of an absorbent material so that any sound produced in the room will not echo or reverberate.
They are a useful tool for bands and orchestras to record music in as it produces a completely different quality of sound.

Echoes and Reverberations

ECHO
An echo is the perceived reflection of sound from a surface.
The fraction of sound level that is reflected is known as the reflections coefficient.
0 <= α <= 1
The time difference between the echo and the direct sound depends on the distances travelled and the speed of sound.
The smallest distance for sound to travel to be described as an echo is 33m. However because it is an echo the sound has to travel to eg. a wall and bounce back therefore travelling 17m each way.
The difference must be greater than about 100ms to be perceived as an echo.

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REVERBERATION
The persistence of a sound after its source has stopped, caused by multiple reflection of the sound within a closed space.
The reverberant character of a room is due to the accumulated reflections from all the surfaces (walls, floors,ceiling and other objects) adding to the direct sound.
Multiple level reflections occur as well as single ones.

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Inverse Square Law

The intensity of the sound received varies inversely as the square of the distance R from the source
ie. as 1/R².
In open air, sound will be roughly nine times less intense at a distance of 3m from its origin, as at a distance of 1m.
Objects in our surroundings produce reflection (echo), absorption and scattering of the sound waves.
The inverse square law is not always applicable in direct measurements of the intensity of sound.
However is it a rough indication when you are away from reflecting surfaces eg. open air away from walls.

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Decibels

The sound intensity of a normal conversation speech is around 10⁵ (ie. 100,000) times that of whispered speech.
To easily graph and discuss these huge range of values;
sound intensity (power) level is defined using a logarithm and is measured in decibels (dB)
eg. Sound Intensity Level = 10log₁₀(Isound/Istandard) dB
which means that a 10 times multiplication in sound intensity corresponds to an additional 10dB of sound level, and an increase from sound intensity P₁ to 2*P₁ (a two times increase in power) corresponds to 10log₁₀(2*P₁/P₁)=3dB

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Harmonics

The vibration as a whole produces the fundamental (basic) tone, and the other vibrations produce the various harmonic tones.
A harmonic is an integer multiple of the fundamental frequency.
Eg. 2 x fundamental, 3 x fundamental etc.
So a sound consisting of components at frequencies of 1000Hz and 3000Hz would contain a 3rd harmonic of the 1kHz fundamental.

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Wavelength

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For a transverse wave, the wavelength is the distance between two successive crests or troughs.
For a longitudinal wave, it is the shortest distance between two peak compressions.

Amplitude

The amplitude of a sound wave is the degree of motion of air molecules within the wave.
This corresponds to the extent of rarefaction and compression that accompanies the wave.
The greater the amplitude of the wave, the harder the molecules strike the ear drum and the louder the sound is transduced.
The amplitude of a sound wave can be expressed in absolute units by measuring the actual distance moved by the air molecules, or the pressure difference in the compression and rarefaction, or the energy involved.
Eg. Ordinary speech produces sound energy at a power level of about one hundred-thousandths of a watt (10⁻⁵W = 10µW)

Velocity

The velocity of the wave, which is the speed at which it advances, is equal to the wavelength times the frequency.
The frequency of the wave is the number of vibrations (complete cycles) per second. The unit is the Hertz (Hz).
Velocity (m/s) = wavelength (m) x frequency (Hz)
v = ƛ x f m/s
So if 1kHz tone in air has a wavelength:
ƛ = v/f m
= 333/1000 m
= 0.333 m
which is slightly longer than a 12" ruler.

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COMPRESSION:
INCREASE IN DENSITY
RAREFACTION:
DECREASE IN DENSITY

A sound wave is a series of alternate compression and rarefaction events in the medium eg. air.

http://lossenderosstudio.com/rarefaction.jpg

Sound is transmitted as a wave motion through a medium such as air, water or metal.
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If the vibration is at right angles to the direction of motion the wave is known as transverse wave.
An example of a transverse wave is the ripple on the surface of water, they move up and down out from the disturbance.

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Key Words:

Thursday 27 September 2012