Product Manual ALR2-3
ALR 110F – 150F
17
Sound Power Levels
Acoustical consultants may require sound power octave band data to perform a detailed acoustical
analysis. The previous tables present sound power levels per ARI Standard 370 “Sound Rating of
Large Outdoor Refrigerating and Air Conditioning Equipment”. These standards were developed to
establish uniform methods of determining the sound power radiated by large outdoor and indoor
equipment. The aforementioned methods are based on providing sound power levels by octave band
and the overall ‘A’ weighted value. Sound pressure measurements are taken over a prescribed area
around the unit and the data is mathematically calculated to give the sound power, dB.
Sound Reduction due to Distance from the Unit
The distance between a source of sound and the location of the sound measurement plays an
important role in minimizing sound problems. The equation below can be used to calculate the sound
pressure level at any distance if the sound power is known.
Lp=Lw-(20 log r) + (10 log Q) - .5
Lp = sound pressure
Lw = sound power
r = distance from unit in feet
Q = directionality factor
The directionality factor, "Q", is a dimensionless number that compensates for the type of sound
reflection from the source. For example, a unit sitting on a flat roof or ground with no other reflective
surfaces or attenuation due to grass, snow, etc., between source and receiver: Q=1.
Figure 6, "Q" Definition, Plan View, Unit Located in Center
Uniform Spherical Radiation
Q=1 no reflecting surface
Uniform Hemispherical Radiation
Q=2 single reflecting surface
Uniform Radiation over ¼ of sphere
Q=4 two reflecting surfaces
With Q=1, no reflecting walls, the equation simplifies to:
Lp = Lw - (20)(log r) –0.5
With Q=2, for a unit sitting on a flat roof or ground with one wall as a reflective surface the equation
simplifies to:
Lp = Lw - (20)(log r) + 2.5
The equations are reduced to table form in Table 9 for various distances and the three most usual
cases of "Q" type of location.
Table 9, dB Conversion of Sound Power to Pressure for Distance
dB Reduction from Sound Power at the Source
to Sound Pressure at Referenced Distance
Distance From Sound Source
ft (m)
Q=1
Q=2
Q=4
30 (9)
30.0
27.1
24.0
50 (15)
34.5
31.6
28.5
75 (23)
38.0
35.1
32.0
100 (30)
40.5
37.6
34.5
150 (46)
44.0
41.1
38.0
200 (61)
46.5
43.6
40.5
300 (91)
50.0
47.6
44.0
Содержание ALR 110F
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