Building Design and Construction Handbook - Merritt - Ventech!

11.150 SECTION ELEVEN
p
SPL � 20 log (11.13)
10 po
Sound power level refers to the power of a sound source relative to a reference
power of 10 �12 W. (Note: At one time, 10 �13 W was used; thus, it is imperative
that the reference level always be explicitly stated.)
The ear responds in a roughly logarithmic manner to changes in stimulus intensity,
but approximately as shown in Table 11.25.
Another comparison, which gives more meaning to various levels, is shown in
Table 11.26.
Measurement Scales. Most measurements or evaluations of sound intensity or
level are made with an electronic instrument that measures the sound pressure. The
instrument is calibrated to read pressure levels in decibels (rather than volts). It can
measure the overall sound pressure level throughout a frequency range of about 20
to 20,000 Hz, or within narrow frequency bands (such as an octave, third-octave,
TABLE 11.25 Subjective Effect of Changes
in Sound Characteristics
Change
in sound
level, dB Change in apparent loudness
3 Just perceptible
5 Clearly noticeable
10 Twice as loud (or 1 ⁄2)
20 Much louder (or quieter)
TABLE 11.26 Comparison of Intensity, Sound Pressure Level, and
Common Sounds
Relative intensity
SPL
dBA* Loudness
100,000,000,000,000 140 Jet aircraft and artillery fire
10,000,000,000,000 130 Threshold of pain
1,000,000,000,000 120 Threshold of feeling
100,000,000,000 110
10,000,000,000 100 Inside propeller plane
1,000,000,000 90 Full symphony or band
100,000,000 80 Inside automobile at high speed
10,000,000 70 Conversation, face-to-face
1,000,000 60
100,000 50 Inside general office
10,000 40 Inside private office
1,000 30 Inside bedroom
100 20 Inside empty theater
10 10
1 0 Threshold of hearing
* SPL as measured on A scale of standard sound level meter.