TM 5-805-4/AFJMAN 32-1090
Table C-24. Frequency Adjustments (in dB) for TEFC Electric
frequency; or other sound frequencies for other
Motors.
line frequencies). Actually, the sound power level
of the transformer does not increase in this loca-
Octave
Value to
tion, but the sound analysis procedure is more
Frequency
be Subtracted
readily handled by presuming that the sound
From Overall SPL
power is increased. The C3 value is an approxima-
Band
tion of the noise of a transformer that has grown
(Hz)
(dB)
noisier (by about 10 dB) during its lifetime. This
happens occasionally when the laminations or
31
14
tie-bolts become loose, and the transformer begins
63
14
to buzz or rattle. In a highly critical location, it
would be wise to use this value. All of the table
125
11
C-30 values assume that the transformer initially
250
9
meets it quoted NEMA sound level rating. Field
measurements have shown that transformers may
500
6
actually have A-weighted sound levels that range
1000
6-
from a few decibels (2 or 3 dB) above to as much as
5 or 6 dB below the quoted NEMA value. Quieted
2000
7
transformers that contain various forms of noise
4000
12
control treatments can be purchased at as much as
15 to 20 dB below normal NEMA ratings. If a
8000
20
quieter transformer is purchased and used, the
lowered sound level rating should be used in place
A-weighted,
of the regular NEMA rating in equation C-17, and
1
dB(A)
the appropriate corrections from table C-30 se-
lected.
the manufacturer. On the basis of field studies of
C-21. Opening In A Wall.
many transformer installations, the PWL in octave
An opening, such as a door, window, or louvered
bands has been related to the NEMA rating and
vent, in an exterior wall of a noisy room will allow
the area of the four side walls of the unit. This
noise to escape from that room and perhaps be
relationship is expressed by equation C-17:
disturbing to neighbors. The PWL of the sound
Lw = NEMA rating + 10 log A + C,
that passes through the opening can be estimated
(eq C-17)
from equation C-18:
(eq C-18)
Lw = Lp + 10 log A - 10
where "NEMA rating" is the A-weighted sound
level of the transformer provided by the manufac-
where Lp is the SPL in the room at the location of
turer, obtained in accordance with current NEMA
the opening and A is the area, in ft.2, of the
Standards, A is the total surface area of the four
opening. (Note, the factor of - 10 is due to the use
of ft.2 for A, if m2 had been used then this factor
side walls of the transformer in ft.2, and C is an
would be 0) Once the PWL is estimated, the SPL
octave band correction that has different values for
at any neighbor distance can be estimated with
different uses, as shown in table C-30. If the exact
the use of chapter 5 material. For normal openings
dimensions of the transformer are not known, an
(windows or vents) without ducted connections to
approximation will suffice. If in doubt, the area
the noise source, it may be assumed that the
should be estimated on the high side. An error of
sound radiates freely in all directions in front of
25 percent in area will produce a change of 1 dB
the opening, but to the rear of the wall containing
in the PWL. The most nearly applicable C value
the opening, the barrier effect of the wall should
from table C-30 should be used. The Cl value
be taken into account. For ducted connections from
assumes normal radiation of sound. The C2 value
a sound source to an opening in the wall, the
should be used in regular-shaped confined spaces
sound is somewhat "beamed" out of the opening
where standing waves will very likely occur,
and may be assumed to have a directivity effect of
which typically may produce 6 dB higher sound
above one-half the amount given in table C-23 for
levels at the transformer harmonic frequencies of
air intake openings of large stacks.
120, 240, 360, 480, and 600 Hz (for 60-Hz line
C-21
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