YORK ® AIR-CONDITIONING PRODUCTS
Sound
Sound power is given the symbol Lw.
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.
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
Sound pressure Lp
Lw = 10 log (2 x 10 -6 ) = 63 dB
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.
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.
The magnitude of a sound pressure wave is measured in pascals (N/m )
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.
2
ADDITION OF EQUAL SOUND LEVELS SOURCES
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.
Number of the same sound sources
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
When sound sources of differing sound levels are to be added refer to
the following table.
ADDITION OF DIFFERENT SOUND LEVELS SOURCES
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.
dB Difference
83