9. Echolocation in Dolphins
395
The predominant cue for the dolphin in the wall thickness experiment of
Titov (1972) was probably the difference in the arrival time of the echoes
from the front and back walls of the cylinder. The time difference in the
arrival of echoes from the front and back walls is
2th 2(OD - 2th)
't=-+----'----~
c,
Co
where c, is the longitudinal sound velocity of the cylinder material, Co is the
sound velocity in water, th is the wall thickness of the cylinder, and OD is
outer diameter. The difference in 't for one cylinder versus another cylinder
can be expressed as
where I1t ='t, - 't2 and 11th =th l - th 2 . Inserting the sound velocity for steel
and seawater along with a discrimination threshold of 0.2 mm into these
equations, we can estimate the dolphin's time difference discrimination
threshold as being I1t "' " 0.5I..ls.
Au and Pawloski (1992) also examined the wall thickness discrimination
capability of an echolocating T.truncatus. Our experimental geometry consisted of a dolphin stationing in a hoop with two targets 8 m from the hoop,
separated by a 22° azimuth. The standard target was a 3.81-cm outer diameter aluminum cylinder with a wall thickness of 6.35 mm. Comparison
targets with wall thickness both thinner and thicker than the standard were
used. The comparison targets had incremental differences in wall thickness
of ±O.2, ±O.3, ±OA, and ±O.8 mm from the standard target. The dolphin was
required to echolocate and to respond to the paddle that was on the same
side of the center line as the standard target. The dolphin's performance as
a function of wall thickness difference is shown in Figure 9.20. The 75%
correct response threshold corresponded to a wall thickness difference of
-0.23 mm for thinner targets and +0.27 mm for thicker targets. These results
compared well with the results of Titov (1972) who used a different paradigm and targets of different sizes and material composition. The echo
waveform, envelope, and frequency spectrum for the standard target and
the comparison targets having a wall thickness difference of -0.3 mm are
displayed in Figure 9.20. The dolphin was able to perform above threshold
for the targets associated with Figure 9.20. The envelope curves suggest that
if the dolphin used time domain cues, it may be able to perceive incremental
time differences of approximately 0.51..ls between highlight intervals, agreeing with I1t of approximately 0.51..ls obtained with Titov (1972) results.
Differences in the frequency spectra of the echoes from the standard and
comparison targets can be seen in Figure 9.20. The frequency spectra for
the thinner comparison targets resembled the spectrum of the standard
target, but were shifted slightly toward lower frequencies. The spectra for
395
The predominant cue for the dolphin in the wall thickness experiment of
Titov (1972) was probably the difference in the arrival time of the echoes
from the front and back walls of the cylinder. The time difference in the
arrival of echoes from the front and back walls is
2th 2(OD - 2th)
't=-+----'----~
c,
Co
where c, is the longitudinal sound velocity of the cylinder material, Co is the
sound velocity in water, th is the wall thickness of the cylinder, and OD is
outer diameter. The difference in 't for one cylinder versus another cylinder
can be expressed as
where I1t ='t, - 't2 and 11th =th l - th 2 . Inserting the sound velocity for steel
and seawater along with a discrimination threshold of 0.2 mm into these
equations, we can estimate the dolphin's time difference discrimination
threshold as being I1t "' " 0.5I..ls.
Au and Pawloski (1992) also examined the wall thickness discrimination
capability of an echolocating T.truncatus. Our experimental geometry consisted of a dolphin stationing in a hoop with two targets 8 m from the hoop,
separated by a 22° azimuth. The standard target was a 3.81-cm outer diameter aluminum cylinder with a wall thickness of 6.35 mm. Comparison
targets with wall thickness both thinner and thicker than the standard were
used. The comparison targets had incremental differences in wall thickness
of ±O.2, ±O.3, ±OA, and ±O.8 mm from the standard target. The dolphin was
required to echolocate and to respond to the paddle that was on the same
side of the center line as the standard target. The dolphin's performance as
a function of wall thickness difference is shown in Figure 9.20. The 75%
correct response threshold corresponded to a wall thickness difference of
-0.23 mm for thinner targets and +0.27 mm for thicker targets. These results
compared well with the results of Titov (1972) who used a different paradigm and targets of different sizes and material composition. The echo
waveform, envelope, and frequency spectrum for the standard target and
the comparison targets having a wall thickness difference of -0.3 mm are
displayed in Figure 9.20. The dolphin was able to perform above threshold
for the targets associated with Figure 9.20. The envelope curves suggest that
if the dolphin used time domain cues, it may be able to perceive incremental
time differences of approximately 0.51..ls between highlight intervals, agreeing with I1t of approximately 0.51..ls obtained with Titov (1972) results.
Differences in the frequency spectra of the echoes from the standard and
comparison targets can be seen in Figure 9.20. The frequency spectra for
the thinner comparison targets resembled the spectrum of the standard
target, but were shifted slightly toward lower frequencies. The spectra for