Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No. 6945
BBN Systems and Technologies Corporation
One of the principal means of distinguishing the sounds of various
sources is by their distribution patterns of sound intensity with frequency.
When this distribution has its intensity concentrated at discrete frequencies,
the sound is tonal in character. When the distribution is spread over a broad
frequency range the sound is rough and noisy. Sound spectrum analyzers are
used to measure the distribution of sound intensity with frequency and thus
classify various sources by their spectra. These analyzers may provide either
a constant bandwidth analysis or a constant percentage bandwidth analysis.
The constant bandwidth analysis provides sound level data in a sequential
series of bands, each of constant bandwidth. The data are usually converted
to an equivalent l-Hz bandwidth to obtain standard comparison spectra. This
type of "spectrum level" analysis is generally used for engineering and
scientific purposes.
Constant percentage bandwidth analyzers have filter bandwidths which are
a given percentage of the band center frequency. The bandwidth is usually
specified as a fractional part of an octave. The 1/3 octave analyzer, which
has a bandwidth of 23% of the center frequency, is often used in analyzing
sounds of concern in human annoyance studies. This bandwidth has been found
to approximate the selectivity characteristic of human hearing in the middle
of the human hearing frequency range. It also approximates the hearing
selectivity of some of the marine mammal species which have been studied. As
a result, this type of spectrum characterization is used in this report to
describe the various sources of concern.
3.1.2 Temporal features
Most natural and man-made sound sources do not produce sound at a
constant output level. The temporal variation in level, and often in
frequency spectrum, is an important descriptive paranieter for sound from a
given source. Output level fluctuations are particularly of concern for this
study since the relationship between sound level and exposure duration in
producing behavioral effects in non-human species is not well known. Some
guidance can be obtained by review of studies of human annoyance reactions to
time-varying industrial noise exposure.
To aid this review, relevant procedures and terminology used in the study
of human response to fluctuating industrial noise sources are given below:
Exposure period - A
reference period of time for calculating a behavioral
response measure such as the equivalent sound level - one of the metrics
used to predict annoyance (this period is generally considered to be
eight hours for human response studies).
Source temporal characteristics -
Steady continuous source - A source with output level varying less than
k2.5 dB during an exposure period.
Fluctuating continuous source - A source with output level varying more
than 22.5 dB but not going below the ambient noise level during an
exposure period, .