774 NOISE
limit. An octave band is identified by its center frequency
defined as follows:fffcLu[]12/
(8.1)where f c the center frequency, f L the lower band limit,
and f u the upper band limit, all in Hz. The center frequen-
cies of the preferred octave bands in the audible range are
31.5, 63, 125, 250 and 500 Hz and 1, 2, 4, 8 and 16 kHz. The
center frequencies of the preferred one-third-octave bands are
those listed in the first column of Table 1 (Section 3).
Real-time analyzers and Fast-Fourier-Transform (FFT)
analyzers examine a signal in all of the selected frequency
bands simultaneously. The signal is then displayed as a bar-
graph, showing the sound level contribution of each selected
frequency band.
Sound intensity measurement. Vector sound intensity
is the net rate or flow of sound energy. Vector sound inten-
sity measurements are useful in determining noise source
power in the presence of background noise and for location
of noise sources. Sound intensity measurement systems uti-
lize a two-microphone probe to measure sound pressure at
two locations simultaneously.
The signals are processed to determine the particle veloc-
ity and its phase relationship to sound pressure.
Calibration. Acoustic calibrators produce a sound level
of known strength. Before a series of measurements, sound
measurement instrumentation should be adjusted to the cali-
brator level. Calibration should be checked at the end of each
measurement session. If a significant change has occurred,
the measured data should be discarded. Calibration data
should be recorded on a data sheet, along with instrumenta-
tion settings and all relevant information about the measure-
ment site and environmental conditions.
Background noise. When measuring the noise contribu-
tion of a given source, all other contributions to total noise
are identified as background noise. Let the sound level be
measured with the given source operating, and then let back-
ground noise alone be measured. The correction for back-
ground noise is given byCOR 10 lg 1 10
⎡ DIF 10⁄
⎣ ⎤⎦ (8.2)where DIF Total noise level – background noise level,
and the noise level contribution of the source in question is
given by:LSOURCETotal noise level COR.Background noise corrections are tabulated in Table 5 and
plotted in Figure 4. Whenever possible measurements should
be made under conditions where background noise is negli-
gible. When total noise level exceeds background noise by at
least 20 dB, then the correction is less than 1/20 dB. Such ideal
conditions are not always possible. Truck noise, for example,
must sometimes be measured on a highway with other moving
vehicles nearby. If the difference between total noise level andbackground noise is less than 5 dB, then the contribution of
the source in question cannot be accurately determined.HEARING DAMAGE RISKThe frequency range of human hearing extends from about
20 Hz to 20 kHz. Under ideal conditions, a sound pressure
level of 0 dB at 1 kHz can be detected. Human hearing is less
sensitive to low frequencies and very high frequencies.
Hearing threshold. A standard for human hearing has
been established on the basis of audiometric measurements
at a series of frequencies. An individual’s hearing threshold
represents the deviation from the standard or audiometric-
zero levels. A hearing threshold of 25 dB at 4 kHz, for example,
indicates that an individual has “lost” 25 dB in ability to hear
sounds at a frequency of 4 kHz (assuming the individual had
“normal” hearing at one time).
A temporary threshold shift (TTS) is a hearing thresh-
old change determined from audiometric evaluation before,
and immediately after exposure to loud noise. A measurable
permanent threshold shift (PTS) usually occurs as a result
of long-term noise exposure. The post-exposure audiometric
measurements to establish PTS are made after the subject has
been free of loud noise exposure for several hours. A com-
pound threshold shift (CTS) combines a PTS and TTS. There
is substantial evidence that repeated TTS’s translate into a
measurable PTS. Miller (1974) assembled data relating TTS,
CTS and PTS resulting from exposure to high noise levels.
Occupational Safety and Health Administration
(OSHA criteria. OSHA (1981, 1983) and the Noise Control
Act (1972) set standards for industrial noise exposure and
guidelines for hearing protection. OSHA criteria have resulted
in reduced noise levels in many industries and reduced the
incidence of hearing loss to workers. However, retrospective
studies have shown that some hearing loss will occur with
long-term exposure a OSHA-permitted sound levels.
The basic OSHA criterion level (CL) is a 90 dBA sound
exposure level for an 8 hour day. An exchange rate (ER)
of 5 dBA is specified, indicating that the permissible dailyTotal - background levelBackground noise correction.0–0.5–1–1.5–2–2.5
4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0FIGURE 4 Background noise correction.C014_003_r03.indd 774C014_003_r03.indd 774 11/18/2005 10:46:10 AM11/18/2005 10:46:10 AM