Chiller sound measurement
Chiller sound power
Sound Power is the property of the chiller only and can be used directly to compare the Sound Power of the chillers of one manufacturer against another . Most Sound Power data is quoted with reference to standard ISO 3744 which is entitled ‘ Sound Power Levels of noise Sources ’ and is sub-titled ‘ Engineering methods of determination of sound power levels for sources in free field conditions over a reflective plane ’. This standard refers only to Sound Power values . Sound power values are not distance-dependant The basic method described by the standard involves averaging a number of sound pressure measurements taken all over an imaginary surface around a chiller in free field conditions . Providing that background noises are within prescribed limits all of the sound measured at the surface must be coming from the chiller and the sound output ( power ) can be calculated as follows : Lw = Lp + 10 x Log 10
( S )
Where : Lw = Sound power in dB ( reference value : 10 -12 W ) Lp = Average value of the sound pressure measurements in dB
( Reference value : 2 × 10 -5 Pa ) S = Measuring area in square meters
Chiller sound pressure
Chiller manufacturers use the parallelepiped method ( rectangular box ) of constant distance D from the surface of the chiller to measure the Sound Pressure . The use of this shape does not change the calculated Sound Power for the chiller . For example to calculate the sound pressure at 10 metres for a chiller with an ‘ A ’ weighted overall Sound Power of 107dB ( A ) re 10 -12 W and a rectangular surface area at 10 metres distance of 1778 m 2 using the parallelepiped surface : Sound pressure at a distance of 10 meters = 107 - 10 x log 10 ( 1778 m 2 ) = 107 - 32.5 = 74.5 dB ( A )( reference value 2 x 10 -5 Pa ) Some manufacturers quote sound Pressure levels at a distance of one metre . When a chiller can be anything up to 10 metres in length this distance is illogical , as the sound level will change depending upon the position along the unit . That is near to the compressor will be noisier than near to the control panel etc , Even a distance of 5 metres is too close in the case of very large chillers .
Deduction from Sound Power Level to Obtain Sound Pressure Levels at Various Distances from a Chiller
Distance “ D ” in meters 5 10 25 50 Deduction from sound power -28 -32 -39 -45
D
D
LIQUID COOLER
D
D
YORK Air-Conditioning Products 113