TM 5-805-4/AFJMAN 32-1090
given a Class 1, 2 or 3 designation. Class 3 filters
(2) Prior to making any measurements, sur-
vey the room to determine how the levels vary
have the highest frequency discrimination and
over the space.
Class 1 have the lowest. It is recommended that
(3) Choose measurement locations that are
all octave band filter sets used for the evaluation
indicative of the critical use of the space.
of noise in buildings, with respect to compliance
(4) Verify and document the operation of the
with noise or vibration specifications, have a Class
mechanical equipment.
2 or higher designation. For cursory evaluation a
(5) Conduct the measurements using the slow
Class 1 will be sufficient.
meter response. Note, for sound level measure-
c. Microphones. Microphones are categorized by
ments, locations within 3 feet of reflecting surfaces
their frequency response, level sensitivity and
should be avoided if possible. For vibration mea-
directionality. Most commonly provided micro-
surements ensure that the accelerometer is prop-
phones will provide suitable frequency response
erly mounted and oriented in the desired direction.
(e.g. 10 to 10,000 Hz) and level sensitivity (30 to
(6) Upon completion of the measurements, re-
130 dB) for the evaluation of mechanical equip-
verify and document the operation of the equipment.
(7) If possible conduct measurements when
ment in buildings. The microphone directionality
the equipment is not in operation.
is important however. Measurement microphones
(8) As a final step check the operating order of
directionality is typically given as "free-field" or
"random incidence". Free field microphones are
the meter and then recalibrate.
b. Conducting measurements after the equipment
intended for use outdoors and the microphone
has been turned off is extremely helpful. A compari-
should be aimed at the sound source under investi-
son of the measurement with and without the
gation. Random incidence microphones are used
equipment in operation will indicate if the measure-
indoors where the reverberant sound is significant.
ments are indicative of the equipment or some other
There are adapters that can be applied to a free
extraneous source. If the level decreases after the
field microphone when used indoors.
equipment has been turned off, then the measure-
ments are indicative of the equipment under evalua-
bility and extended frequency response, accelerom-
tion. If the sound level does not decrease after the
eters are the most common vibration transducers.
equipment is turned off, then the measured level is
As a general rule the sensitivity of an accelerome-
not indicative of the equipment under evaluation. If
ter is directly proportional to the physical size (e.g.
the decrease is more than 2 dB but less than 10 dB,
larger accelerometers usually can measure lower
the measured levels after the equipment has been
vibration levels). And the frequency response is
shut down can be subtracted from the levels with
inversely proportional to the frequency response
the equipment (see appendix C). Usually it is best to
(e.g. accelerometers with an extended frequency
conduct these measurements at night or when the
response may be limited in measuring low vibra-
building is not in use. At these times it is easier to
tion levels.) Some accelerometers require a exter-
turn on and off equipment and extraneous sources
nal power supply in order to operate an pre-amp
are at a minimum.
that is incorporated into the accelerometer casing.
There exists a large variety of accelerometers and
10-4. Measurement Of Noise And Vibration
once the intended purpose is ascertained, the
Outdoors.
manufactures can provide guidance on the most
The consideration for measuring noise and vibra-
appropriate type and model.
tion outdoors is identical to that for indoor mea-
10-3. Measurement Of Noise And Vibration In
surements. The most significant factor is the envi-
Buildings.
ronmental influence on the transmission of the
sound. Environmental factors, such as wind, hu-
a. Noise measurements in buildings are usually
midity and temperature gradients can produce
made either to determine if RC or NC curves have
significant (e.g. 5, 10 dB or greater) variations in
been met or to search for the cause of their not
the measured sound level. Therefore it is impor-
having been met. In conducting sound or vibration
tant to document the environmental conditions at
measurements utilize the following procedure:
the time of the measurements. Ideally measure-
(1) Prior to making measurements ensure the
ments should only be made under neutral condi-
meter is in proper working order and calibrate the
tions (e.g. no wind, cloudy overcast day).
measurement system with the external calibrator.
10-2
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