1. Sound Principles of Sound
Sound is a variation in the pressure of the air of a type which has an effect on our ears and brain. These pressure variations transfer energy from a source of vibration that can be naturally occurring, such as by the wind, or produced artificially such as by speech. A vibrating object compresses adjacent particles of air as it moves in one direction and leaves particles of air “spread out” as it moves in the other direction. The displaced particles pass on their additional energy, and a pattern of compressions and rarefactions travels out from the source, while the individual particles return to their original positions.
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2. Sound
The mechanical vibrations of sound move forward using a wave motion. This means that, although the individual particles of material such as air molecules return to their original position, the sound energy travels forward. The front of the wave spreads out equally in all directions unless it is affected by an object or other material in its path. The waves are longitudinal in type because the particles travel in the same direction as the wave. Sound waves can travel in solids, liquids or gases, but cannot travel in a vacuum.
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3. Frequency (f) is the number of cycles per second. Unit: hertz (Hz).
Sound
Wavelength (l) is the distance between any two repeating points on a wave.   Unit: metre (m).
Frequency (f) is the number of cycles per second. Unit: hertz (Hz).
Velocity (v) is the distance moved per second in a fixed direction. Unit: metres per second (m/s).
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4. Where V = velocity in m/s f = frequency in Hz l = wavelength in m
Sound
For every vibration of the sound source, the wave moves forward by one wavelength. Therefore the velocity of a wave is given by the frequency times the wavelength.
V = f x l
Where V = velocity in m/s
f = frequency in Hz
l = wavelength in m
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5. Sound
Worked Example
A particular sound wave has a frequency of 440Hz and a velocity of 340m/s. Calculate the wavelength of this sound.
V = f x l
l = V / f
l = 340 / 440 = m
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6. Material Velocity (m/s)
Sound
The velocity of sound varies with the density and elasticity of the medium through which it is passing.
Material Velocity (m/s)
Air (20oC) Water (25oC) Softwood Steel Granite
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7. Sound Intensity Level (SIL) = 10 log10 ( I / Io) dB
Decibels are calculated using the sound pressure or sound intensity, and comparing it with a reference value which is the value at the threshold of hearing.
Sound Intensity Level (SIL) = 10 log10 ( I / Io) dB
Sound Pressure Level (SPL) = 20 log10 (P / Po) dB
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8. Sound
Example
Calculate the SIL produced by a sound with an intensity of 2.6 x 10-4 W/m2.
I = 2.6 x Io = 1 x 10-12
SIL = 10 x log ( 2.6 x / 1 x )
= 10 x log 2.6 x 108
= 10 x
= dB
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9. = 87.16 dB The increase is therefore 3.01dB
Sound
Calculate the increase in SIL produced when a sound intensity of 2.6 x 10-4 W/m2 doubles to 5.2 x 10-4 W/m2
There are two ways of doing this. One way is to simply repeat the previous calculation with the new intensity and find the difference.
I = 5.2 x Io = 1 x 10-12
SIL = 10 x log ( 5.2 x / 1 x )
= 10 x log 5.2 x = 10 x
= dB The increase is therefore 3.01dB
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10. Sound
Alternatively, the calculation could be carried out by using the original sound intensity as the reference.
I = 5.2 x Io = 2.6 x 10-4
SIL = 10 x log ( 5.2 x / x 10-4 )
= 10 x log = 10 x = dB
Therefore it can be seen that when sound intensity is doubled, the SIL increases by 3dB.
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11. Sound
Noise
Any unwanted sound is referred to as noise. The effect of noise will depend upon a variety of factors including the use of the space where the noise is heard, the duration of the noise and the frequency components of the noise.
When noise levels are measured using an electronic sound meter it is usual to weight the response of the meter so that it more closely resembles that of the human ear. There are various standard weighting scales, the one most commonly used is known as the A scale and sound levels thus weighted are given in dB(A).
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12. Do Do fill and seal all masonry joints with mortar.
Do keep the cavity leaves separate below ground floor level.
Do ensure that any external cavity wall is stopped with a flexible closer at the junction with the separating wall, unless the cavity is fully filled with mineral wool or expanded polystyrene beads (seek manufacturer’s advice for other suitable materials).
Do control flanking transmission from walls and floors connected to the separating wall as described in the guidance on junctions.
Do stagger the position of sockets on opposite sides of the separating wall.
Do ensure that flue blocks will not adversely affect the sound insulation and that a suitable finish is used over the flue blocks (see BS :1986 and seek manufacturer’s advice).

13. Do not
Do not try and convert a cavity separating wall to a type 1 (solid masonry) separating wall by inserting mortar or concrete into the cavity between the two leaves.
Do not change to a solid wall construction in the roof space as a rigid connection between the leaves will reduce wall performance.
Do not build cavity walls off a continuous solid concrete slab floor.
Do not use deep sockets and chases in the separating wall, do not place them back to back.