Basic Calculations for Occupational Safety and Environmental Health Professionals - Mathematics and Statistics for the Environment - page 424

Sound intensity (I) —Sound intensity at a specific location is the average rate at which sound
energy is transmitted through a unit area normal to the direction of sound propagation.
Sound level meter (SLM) —A device which measures sound and provides a readout of the result-
ing measurement. Some provide only A-weighted measurements, others provide A- and
C-weighted measurements, and some can provide weighted, linear, and octave (or narrower)
ban measurements. Some SLMs are also capable of providing time-integrated measurements.
Sound power —The total sound energy radiated by a source per unit time. Sound power can-
not be measured directly.
Sound pressure level (SPL) —A measure of the ratio of the pressure of a sound wave relative
to a reference sound pressure. Sound pressure level in decibels is typically referenced to
20 mPa. When used alone (e.g., 90 dB), a given decibel level implies an unweighted sound
pressure level.
Standard threshold shift (STS) —(1) OSHA uses the term to describe a change in hearing
threshold relative to the baseline audiogram of an average of 10 dB or more at 2000, 3000,
and 4000 Hz in either ear; used by OSHA to trigger additional audiometric testing and
related follow up. (2) NIOSH uses this term to describe a change of 15 dB or more at any
frequency, 5000 through 6000 Hz, from baseline levels that are present on an immediate
retest in the same ear and at the same frequency. NIOSH recommends a confirmation
audiogram within 30 days with the confirmation audiogram preceded by a quiet period of
at least 14 hours.
Threshold shift —Audiometric monitoring programs will encounter two types of changes in
hearing sensitivity: a permanent threshold shift (PTS) and a temporary threshold shift
(TTS). As the names imply, any change in hearing sensitivity which is persistent is consid-
ered a PTS. Persistence may be assumed if the change is observed on a 30-day follow-up
exam. Exposure to loud noise may cause a temporary worsening in hearing sensitivity
(i.e., a TTS) that may persist for 14 hours (or even longer in cases where the exposure
duration exceeded 12 to 16 hours). Hearing health professionals need to recognize that
not all threshold shifts represent decreased sensitivity, and not all temporary or perma-
nent threshold shifts are due to noise exposure. When a permanent threshold shift can be
attributable to noise exposure, it may be referred to as a noise-induced permanent threshold
shift (NIPTS).
Velocity —Speed at which the regions of sound producing pressure changes move away from
the sound source.
Wavelength —Distance required for one complete pressure cycle to be completed (1 wave-
length) and is measured in feet or meters.
Weighted measurements —Two weighting curves are commonly applied to measures of sound
levels to account for the way the ear perceives the “loudness” of sounds.
A-weighting —A measurement scale that approximates the “loudness” of tones relative to
a 40-db SPL 1000 Hz reference tone. A-weighting has the added advantage of being
correlated with annoyance measures and is most responsive to the mid frequencies,
500 to 4000 Hz.
C-weighting —A measurement scale that approximates the “loudness” of tones relative to a
90-dB SPL 1000 Hz reference tone. C-weighting has the added advantage of providing
a relatively flat measurement scale that includes very low frequencies.
15.7.2 o CCupational n oise e xposure
As mentioned earlier, noise is commonly defined as any unwanted sound. Noise literally surrounds
us every day, and is with us just about everywhere we go. However, the noise we are concerned with
here is that produced by industrial processes. Excessive amounts of noise in the work environment
(and outside it) cause many problems for workers, including increased stress levels, interference