Introduction to Acoustics

Typical outdoor setting
A-weighted sound level in dB re 20 µPa
120
110
100
90
80
70
60
50
40
30
20
10
0
Noisy urban (daytime)
Commercial retail area
Suburban (daytime)
Suburban (nighttime)
Grand Canyon (along river)
Hawaii volcanoes (crater overlook)
Grand Canyon (remote trail)
Haleakala (in crater, no wind)
Non-park setting
Park setting
Fig. 23.4 Comparison of outdoor A-weighted levels in different
environments. (After Miller [23.7], with permission)
The plus signs indicate that the two loudspeakers are
operating in phase. The distance between all loudspeakers
is d. In the second case (b) the two loudspeakers are
operated out of phase and form a dipole with a dipole
moment in the direction shown. In this case, the total
power radiated relative to monopole radiation can be
shown [23.6]tobe
Wd = 1
24 (kd)2 Wm , (23.14)
where k is the wave number (ω/c) andd is the distance
between the two sources. If two dipoles with opposite
23.1 Instruments for Noise Measurements
23.1.1 Introduction
The most common quantity for control and assessment
of noise is the frequency-weighted sound level, as measured
by a sound level meter. Exponential time weighting
Noise 23.1 Instruments for Noise Measurements 965
dipole moments are combined, the result is a quadrupole
– as illustrated in (c). In this case, the total power radiated
relative to monopole radiation can be shown [23.6] to
be:
Wq = 1
120 (kd)4 Wm . (23.15)
It can be seen that at small values of kd (low frequencies),
dipole and quadrupole sources of sound are much less
efficient in radiating sound than monopoles. The dipole
case is especially important because one way to reduce
the source noise level is to create a secondary source
of opposite phase near the primary sources. The sound
radiated by the secondary source partially cancels the
sound radiated by the primary source, and the radiation
efficiency is reduced.
Radiation Efficiency of Machines
The sound power radiated by a source is also a very small
fraction of the mechanical power driving the source.
Shaw [23.4] studied the radiation efficiency of a number
of practical sources, and found that only a small fraction
of the mechanical power was converted to sound.
Fig. 23.2 shows the relationship between A-weighted
sound power and mechanical power for a wide variety
of sources.
23.0.4 Sound Pressure Level
of Common Sounds
Many authors have shown the relationship between
sound pressure level and many common sounds. Data
adapted from Burgé [23.5] areshowninFig.23.3. Aweighted
sound pressure levels range from about 25
to 110 dB, which is a range in sound pressure of approximately
1:18 000. Similar data, but at lower sound
pressure levels, have been adapted from Miller and are
shown in Fig. 23.4. The vertical scales are labeled differently
to conform to the practice of each author. As can
be seen, there is a considerable difference for certain
common terms; environmental levels may vary greatly
in urban and suburban areas.
may also be employed for measurements of frequencyweighted
sound level. Sound pressure levels are usually
measured through constant-percentage-bandwidth filters.
Sound intensity is commonly used for localization
of noise sources and for direct determination of sound
Part G 23.1