Building Design and Construction Handbook - Merritt - Ventech!

WALL, FLOOR, AND CEILING SYSTEMS 11.149
but there is a significant noise level above which even loud (shouting) speech is
scarcely intelligible. Such a level, if long continued, can prove damaging to the
hearing mechanism. At still higher levels, pain can be experienced, and immediate
physical damage (often irreversible) may occur.
Complete silence (if such a thing could occur) would be most unpleasant. Humans
would quickly become disoriented and distressed. Somewhere between silence
and the uncomfortably loud, however, is a wide range that humans find acceptable.
11.78 MEASUREMENT OF SOUND
Generally, absolute numbers obtained from measurements have little significance
in acoustics. Instead, measurements are almost always compared with some base
or reference, and they are usually quoted as levels above or below that reference
level. The levels are usually ratios of observed values to the reference level, such
as 2:1, 1:2, 1:10, because rarely are simple, linear relationships found between
stimuli and effects in humans.
Zero level of sound pressure is not a true physical zero; that is, the absence of
any sound pressure at all. Rather, zero level is something of an average threshold
of human hearing, of sound at about 1000 Hz. The physical pressure associated
with this threshold level is very small, 0.00002 Pa (N/m 2 ).
Changes in human response tend to occur according to a ratio of the intensity
of the stimuli producing the response. In acoustics, the ratio of 10:1 is called a bel,
and one-tenth of a bel, a decibel (dB). Thus, power and intensity levels, in dB, are
computed from
quantity measured
Level � 10 log (11.10)
10 reference quantity
Intensity level (IL), dB, for example, represents the ratio of the intensity being
measured to some reference level, and is given by
where I � intensity measured, W/cm 2
I o � reference intensity � 10 �16 W/cm 2
I
IL � 10 log (11.11)
10 Io
Since intensity varies as the square of the pressure, intensity level also is given
by
where p � pressure measured, Pa
p o � reference pressure � 0.00002 Pa
2 p p
IL � 10 log10 � 20 log 10
(11.12)
2 p p
o o
Sound pressure level (SPL), dB, to correspond with intensity level, is defined
by