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

Report No. 6945
BBN Systems and Technologies Corporation
250 Hz to 100 kHz in various tests. He found that the threshold for tones
shorter than 0.1 to 0.2 s increased as the tone duration decreased. Tones
longer in duration than 0.1 to 0.2 s elicited similar threshold values
regardless of duration. . For high-frequency single clicks of 0.2 ms duration,
the threshold is about 20 dB higher than that for sounds longer than 0.1 to
0.2 s (Johnson 1968a). Likewise, Bullock and Ridgway (1972) found that evoked
potentials recorded in the cerebrum of Tursiops increased in amplitude as tone
duration increased. Also, evoked potentials recorded at the majority of locations
(but not all) in the auditory cortex of the harbor porpoise increased in
amplitude and decreased in threshold as tone duration increased (Popov et al.
1986).
Terhune (1988) recently performed a signal duration experiment on a
harbor seal. At most frequencies tested, thresholds to pulses of various
durations were similar as long as the duration was at least 50.111s. Thresholds
increased as duration decreased from 50 ms.
These results might suggest that single short-duration signals, such as
echolocation clicks or brief calls, will have higher thresholds than those
indicated on the audiograms. However, Bullock and Ridgway (1972) found
locations in the midbrain of Tursiops that appeared to be specialized for
processing very brief (30 kHz) clicks. These are all characteristics of Tursiops echolocation
signals. Given the importance of echolocation to toothed whales, it can be
assumed that neural processing is highly adapted for detection of echoes and
integration of successive echoes. Pinnipeds seem far less responsive to click
stimuli than are odontocetes (Bullock et al. 1971)
2.3.5 Auditory masking
Critical Ratios. The hearing threshold audiograms that have been
presented (Figs. 2.25 and 2.26) represent the lowest intensities of sound that
can be detected by an animal in the absence of noise. The sea is often a
noisy environment, even in the absence of man-made sounds, and background
ambient noise levels often mask the hearing thresholds of marine mammals. The
intensity by which a signal must exceed the spectrum level background noise in
order to be audible is termed the critical ratio (Hawkins and Stevens 1950;
Popper 1980). Critical ratios for marine mammals have been determined by
presenting a pure tone to a test animal while a background white noise* is
present. (Johnson 1968b; Terhune 1981 ; Fig. 2.28). A critical ratio of 20 dB
at a particular frequency means that a tone at that frequency would have to
have a level of at least 100 dB re 1 pPa to be heard over white noise with a
spectrum level of 80 dB re ( 1 p~a)2/~z.
*White noise is simply broadband noise in which all frequencies in the noise
spectrum are of equal intensity. In some masking experiments, the white
noise has been filtered and limited to some range of frequencies above and
below the test frequency. This should have little effect on the results as
long as the bandwidth of the noise exceeds masking bandwidth.