TM 5-805-4/AFJMAN 32-1090
a. Frequency unit, hertz, Hz. When a piano
the band width frequencies and the second column
string vibrates 400 times per second, its frequency
gives the geometric mean frequencies of the bands.
is 400 vibrations per second or 400 Hz. Before the
The latter values are the frequencies that are used
US joined the IS0 in standardization of many
to label the various octave bands. For example, the
technical terms (about 1963), this unit was known
1000-Hz octave band contains all the noise falling
as "cycles per second."
between 707 Hz (1000/square root of 2) and 1414
b. Discrete frequencies, tonal components. When
an electrical or mechanical device operates at a
characteristics of these filters have been standard-
constant speed and has some repetitive mechanism
ized by agreement (ANSI S1.11 and ANSI S1.6). In
that produces a strong sound, that sound may be
some instances reference is made to "low", "mid"
concentrated at the principal frequency of opera-
and "high" frequency sound. This distinction is
tion of the device. Examples are: the blade passage
somewhat arbitrary, but for the purposes of this
frequency of a fan or propeller, the gear-tooth
manual low frequency sound includes the 31
contact frequency of a gear or timing belt, the
through 125 Hz octave bands, mid frequency sound
whining frequencies of a motor, the firing rate of
includes the 250 through 1,000 Hz octave bands,
an internal combustion engine, the impeller blade
and high frequency sound includes the 2,000
frequency of a pump or compressor, and the hum
through 8,000 Hz octave band sound levels. For
of a transformer. These frequencies are designated
finer resolution of data, narrower bandwidth fil-
"discrete frequencies" or "pure tones" when the
ters are sometimes used; for example, finer con-
sounds are clearly tonal in character, and their
stant percentage bandwidth filters (e.g. half-
frequency is usually calculable. The principal fre-
octave, third-octave, and tenth-octave filters), and
quency is known as the "fundamental," and most
constant width filters (e.g. 1 Hz, 10 Hz, etc.). The
such sounds also contain many "harmonics" of the
spectral information presented in this manual in
fundamental. The harmonics are multiple of the
terms of full octave bands. This has been found to
fundamental frequency, i.e., 2, 3, 4, 5, etc. times
be a sufficient resolution for most engineering
the fundamental. For example, in a gear train,
considerations. Most laboratory test data is ob-
where gear tooth contacts occur at the rate of 200
tained and presented in terms of 1/3 octave bands.
per second, the fundamental frequency would be
A reasonably approximate conversion from 1/3 to
200 Hz, and it is very probable that the gear
full octave bands can be made (see d. below). In
would also generate sounds at 400, 600, 800, 1000,
certain cases the octave band is referred to as a
1200 Hz and so on for possible 10 to 15 harmonics.
"full octave" or "1/1 octave" to differentiate it
Considerable sound energy is often concentrated at
from partial octaves such as the 1/3 or 1/2 octave
these discrete frequencies, and these sounds are
bands. The term "overall" is used to designate the
more noticeable and sometimes more annoying
because of their presence. Discrete frequencies can
total noise without any filtering.
be located and identified within a general back-
d. Octave band levels (1/3). Each octave band
ground of broadband noise (noise that has all
can be further divided into three 1/3 octave bands.
frequencies present, such as the roar of a jet
Laboratory data for sound pressure, sound power
aircraft or the water noise in a cooling tower or
and sound intensity levels may be given in terms
waterfall) with the use of narrowband filters that
of 1/3 octave band levels. The corresponding octave
can be swept through the full frequency range of
band level can be determined by adding the levels
interest.
of the three 1/3 octave bands using equation B-2.
c. Octave frequency bands. Typically, a piece of
There is no method of determining the 1/3 octave
mechanical equipment, such as a diesel engine, a
band levels from octave band data. However as an
fan, or a cooling tower, generates and radiates
estimate one can assume that the 1/3 octave levels
some noise over the entire audible range of hear-
are approximately 4.8 dB less than the octave
ing. The amount and frequency distribution of the
band level. Laboratory data for sound transmission
total noise is determined by measuring it with an
loss is commonly given in terms of 1/3 octave band
octave band analyzer, which is a set of contiguous
transmission losses. To convert from 1/3 octave
filters covering essentially the full frequency
band transmission losses to octave band transmis-
range of human hearing. Each filter has a band-
sion losses use equation B-14.
width of one octave, and nine such filters cover the
range of interest for most noise problems. The
standard octave frequencies are given in table
B-l. An octave represents a frequency interval of
(eq B-14)
a factor of two. The first column of table B-l gives
B-5
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