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

Report No. 6945
BBN Systems and Technologies Corporation
Intermittent source - A source with more than one operating cycle during
an exposure period.
Intermittent impulsive source - A source with more than one operating
cycle during an exposure period where the output duration is less than
0.1 sec.
Equivalent sound level (L ) - The level of a continuous source that
provides the same acoustic eq energy as a fluctuating or intermittent source
. for the same exposure period. The value of L may be determined by a
continuous integration of the energy output ofqthe time-varying source
using the following relationship:
where
1
1 P pr(t)
L = 10 log- J (T) dt (dB)
e q T P 0 0
T is the time duration of the exposure period
P
pr(t) is the time-varying sound pressure in a specified bandwidth
Po is a referenced sound pressure (1 pPa).
It is often more convenient to do a statistical analysis using discrete
logarithmic step increments instead of a continuous integration of the
pressure signal. Steps with 5 dB intervals are recommended in Standard ISO/R
1996-1971 (Assessment of Noise With Respect to Community Response). The
procedure is based on the following equation:
where Ti is the time interval (expressed as a percentage of the exposure
period) for which the sound level is within the limits of class i
.(Li + 2.5 dB).
Li is the sound level in a selected band corresponding to the midpoint
of the class i.
Time Ratio or Duty-Cycle - The ratio of the total effective operating time in
an exposure period to the length of the exposure period for a specific source.
If an intermittent source produces identical output sequences during an exposure
period, Eq. 6 may be simplified as follows:
L = L +10Log(nT/T) (dB)
eq eqs s P
(7)
where L is the equivalent sound level of a single output sequence
eqs
n is the number of sequences in an exposure period