Sound
Noise is a major comfort criterion and has considerable effect on the well being of human beings . Noise is generated by friction due to moving parts , compression , explosion etc ,. Mechanical forces create vibration of components which radiate noise in the frequencies of the mechanical source . Deep frequencies cause rumbling which is transported via the structural elements of a building and can be experienced by the sense of touch and body vibrations . Higher frequencies are transported by air . The vibration compresses and expands the air around the noise source and the varying pressure waves are transmitted in all directions .
Sound pressure Lp
Sound pressure is the noise emmitted from an object in a series of high frequency pressure waves which move through the air in a similar pattern to the water ripples caused when a stone is thrown into a pond . It radiates outwards from the sound source and is reflected from objects and surfaces in its path . The magnitude of a sound pressure wave is measured in pascals ( N / m 2 ) but in order to correspond with the human perception of sound . A logorithmic scale is used with decibel ( dB ) units . Most internationally accepted scales use zero decibels as a sound pressure wave of 0.00002 pascals in height which is approximately the threshold of human hearing . As this is a logarithmic scale each time the size of the pressure wave increases by a factor of ten the decibel scale increases by the number ten , ie ., 70 dB represents a pressure wave 1000 times greater than 40dB . This corresponds to the human perception of sound which would also rate the 1000 times increase in sound pressure as a 30 times increase in loudness . Sound pressure is given the symbol Lp . Lp = n dB re 2 x 10 -5 Pa The human ear can normally detect sound to as low as 2 x 10 -5 Pa . Sound pressure is projected at a specific distance from the source and is effected by the surroundings .
Sound power Lw
Energy is required to generate a sound pressure wave and the size of the wave is directly related to the amount of energy used . A continuous sound will only be produced if continuous power is available . Sound power can be measured in watts but it is more convenient to use a logarithmic scale and decibel units . As a basis for the sound power scale 1 picowatt is generally taken to be 0 dB . In order to find a measure for noise a ratio is taken between the sound power and a reference sound power of
P 0
= 10 -12 Watts Sound power is NOT distance dependant . Sound power is the property of the noise emmitting object and sound pressure is used to measure the pressure waves which carry the sound to the ear .
Sound power is given the symbol Lw . Lw = n dB re 10 -12 W Example : The human voice has an average sound power of 10 -6 W . Lw = 10 -6 / 10 -12 = 10 6 Translated into logarithmic figures : Lw = 10 x log 10 -6 = 10 x 6 = 60 dB If two sound sources of equal power ( 60 dB or 10 W each ) were active their added sound level would be as follows : ( 10 -6 + 10 -6 )/ 10 -12 = 2 x 10 -6 / 10 -12 = 2 x 10 -6 = 2 x 10 -6 Lw = 10 log ( 2 x 10 -6 ) = 63 dB Therefore : 60 dB + 60 dB = 63 dB .
Therefore if two sound sources have the same sound power add 3 dB .
For several sound sources of having the same sound level refer to the following graph .
Level difference ( dB )
Number of the same sound sources
When sound sources of differing sound levels are to be added refer to the following table .
Level increase ( dB )
Addition of Equal Sound Levels Sources
Addition of Different Sound Levels Sources
dB Difference
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