TM 5-805-4/AFJMAN 32-1090
from octave band levels. This is done by subtract-
that many acoustical materials perform well when
their dimensions are comparable to or larger than
ing the decibel weighting from the octave band
levels and then summing the levels logarithamati-
the wavelength of sound. Thus, a l-inch thickness
tally using equation B-2. But it is not possible to
of acoustical ceiling tile applied directly to a wall
determine accurately the detailed frequency con-
is quite effective in absorbing high-frequency
tent of a noise from only the weighted sound
sound, but is of little value in absorbing low-
levels. In some instances it is considered advanta-
frequency sound. At room temperature, a lo-feet-
geous to measure or report A-weighted octave
long dissipative muffler is about 9 wavelengths
band levels. When this is done the octave band
long for sound at 1000 Hz and is therefore quite
levels should not be presented as "sound levels in
effective, but is only about 0.4 wavelength long at
50 Hz and is therefore not very effective. At an
dB(A)", but must be labeled as "octave band sound
elevated exhaust temperature of 1000 deg. F, the
levels with A-weighting", otherwise confusion will
wavelength of sound is about 2/3 greater than at
result.
room temperature, so the length of a correspond-
B-8. Temporal Variations.
ing muffler should be about 2/3 longer in order to
be as effective as one at room temperature. In the
Both the acoustical level and spectral content can
design of noise control treatments and the selec-
vary with respect to time. This can be accounted
tion of noise control materials, the acoustical
for in several ways. Sounds with short term
performance will frequently be found to relate to
variations can be measured using the meter aver-
the dimensions of the treatment compared to the
aging characteristics of the standard sound level
wavelengths of sound. This is the basic reason why
meter as defined by ANSI S1.4. Typically two
it is generally easier and less expensive to achieve
meter averaging characteristics are provided,
high-frequency noise control (short wavelengths)
these are termed "Slow" with a time constant of
and more difficult and expensive to achieve low-
approximately 1 second and "Fast" with a time
frequency noise control (long wavelengths).
constant of approximately 1/8 second. The slow
response is useful in estimating the average value
B-10. Loudness.
of most mechanical equipment noise. The fast
The ear has a wide dynamic range. At the low end
response if useful in evaluating the maximum
of the range, one can hear very faint sounds of
level of sounds which vary widely.
about 0 to 10 dB sound pressure level. At the
B-9. Speed of sound and Wavelength.
upper end of the range, one can hear with clarity
and discrimination loud sounds of 100-dB sound
The speed of sound in air is given by equation
pressure level, whose actual sound pressures are
B-15:
100,000 times greater than those of the faintest
where c is the spped of sound in air in ft./set, and
sounds. People may hear even louder sounds, but
tF is the temperature in degrees Fahrenheit.
in the interest of hearing conservation, exposure to
1/2
c = 49.03 x (460 + tF)
(eq B-15)
very loud sounds for significant periods of time
a. Temperature effect. For most normal condi-
should be avoided. It is largely because of this
tions, the speed of sound in air can be taken as
very wide dynamic range that the logarithmic
approximately 1120 ft./sec. For an elevated tem-
decibel system is useful; it permits compression of
perature of about 1000 deg. F, as in the hot
large spreads in sound power and pressure into a
exhaust of a gas turbine engine, the speed of
more practical and manageable numerical system.
sound will be approximately 1870 ft./sec. This
For example, a commercial jet airliner produced
higher speed becomes significant for engine muf-
100,000,000,000 ( = 1011) times the sound power of
fler designs, as will be noted in the following
a cricket. In the decibel system, the sound power
paragraph.
of the jet is 110 dB greater than that of the cricket
b. Wavelength. The wavelength of sound in air
(110 = 10 log 1011). Humans judge subjective
is given by equation B-16.
loudness on a still more compressed scale.
(eq B-16)
a. Loudness judgments. Under controlled listen-
ing tests, humans judge that a 10 dB change in
where {SYMBOL 108/f"Symbol"} is the wave-
length in ft., c is the speed of sound in air in
sound pressure level, on the average, represents
approximately a halving or a doubling of the
ft./sec, and f is the frequency of the sound in Hz.
loudness of a sound. Yet a 10-dB reduction in a
Over the frequency range of 50 Hz to 12,000 Hz,
sound source means that 90 percent of the radi-
the wavelength of sound in air at normal tempera-
ated sound energy has been eliminated. Table B-2
ture varies from 22 feet to 1.1 inches, a relatively
large spread. The significance of this spread is
shows the approximate relationship between sound
B-8
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