All recordings made off axis at ±90°, ±75°, ±60° and ±45° exhibited the presence of two distinct pulses for the corresponding singular sonar click observed on axis (). The interval between these double pulses was greatest ±90° off axis and progressively decreased with each 15° closer to the centre of the beam (). At ±30° off axis a single pulse was observed with multiple peaks, while at ±15° and 0° only one dominant peak was evident, which remained consistent throughout the experiment.
Figure 1 Sonar pulses produced by the beluga whale as recorded at 15° angular intervals in a 180° arc along the horizontal plane of the animal. The pulses shown are representative examples of the on-axis click and off-axis pulses measured separately (more ...)
Table 1 Variability between double pulses recorded off axis. This table reports the temporal separation between double pulses, the average calculated off axis peak-to-peak source level (SL) of the first pulse, the difference between the SLs of the first and second (more ...)
The inter-pulse intervals (IPI) between the double pulses () were not consistent with any reflective paths present in the experimental environment. The nearest reflective area, the water's surface, was 1
m above the animal's head, resulting in an expected pulse interval of approximately 850
μs. The nearest tank wall was 222
cm away, which would have produced a minimum pulse interval of approximately 1300
μs when the off-axis hydrophone was at the −90° position. Additionally, the IPIs did not change when the stationing hoop was placed deeper, shallower or moved horizontally, supporting the conclusion that external reflections could not be the cause of the second pulse.
IPIs were also not consistent with delays expected from reflections within the head, such as those from air sacs or cranial features within a few centimetres of the MLDB complex. Tissue has an acoustic impedance similar to water (Ludwig 1950
), so internal reflections would be expected tens of microseconds following the direct pulse, not hundreds as we observed (). In addition, the second received pulse was frequently greater in amplitude than the first (discussed further below), requiring the presence of a better than perfect reflective surface to be viable as a potential origin.
The lack of possible external or internal reflective surfaces as the origin of the second pulse leaves two separate sound sources as the only viable explanation. However, this will also be problematic if the sound transmission path between the two sources and the off-axis hydrophone is assumed to be exclusively through tissue. At ±90° off axis, assuming a sound speed constant of approximately 1500
, this places the two sources 38–46
cm apart, a distance too great to reconcile with the animal's head size (which measures 33
cm in diameter at the blowhole), unless a nonlinear transmission path is assumed.
On the other hand, if the two sources are presumed to be separated by air, with a constant of approximately 340
, the calculated separation is 8.7–10.4
cm, which is approximately consistent with the expected horizontal separation between the phonic lips (extrapolated from evidence presented by Cranford et al. (2000)
for a bottlenose dolphin, Tursiops truncatus
). A transmission path through air is plausible because the MLDB complex is bounded dorsally by the vestibular sac, a large air-filled structure involved in air recycling during phonation (Dormer 1979
). Both phonic lips terminate on the floor of this sac and can be observed vibrating when an endoscope is placed through the blowhole and into the vestibular sac (Cranford et al. 2000
). Furthermore, the amplitudes of the pulses measured off axis are consistent with the loss expected from sound crossing the air–tissue boundary. The off-axis pulses we measured have estimated source levels (SLs) between 33 and 41
dB lower than the clicks measured on axis (), which matches reasonably well with the 30
dB loss expected as a result of the air–water impedance difference (Caruthers 1977
). It should also be noted, however, that air–water impedance differences can be quite variable and difficult to predict in restricted spaces when the transmitted wavelength is equivalent or smaller to the space it is crossing due to reverberation effects (Weiss 1970
Based on these observations, we conclude that the most viable explanation for the presence of the off-axis double pulses is that the beluga used both phonic lips to produce simultaneous or near-simultaneous signals. The interval between pulses, we believe, represents the delay in transmission through the air space of the vestibular sac, although we presently cannot entirely exclude a nonlinear refractive path through tissue.
The two-source explanation is further supported by the observations made during several instances when the animal momentarily turned away from the target hydrophone to face the off-axis hydrophone. shows the click train from such a case as it was recorded by the hydrophone positioned 45° off-axis. A click-by-click analysis reveals that when the animal was directed towards the target hydrophone, the off-axis hydrophone received paired pulses. When the animal briefly turned to face the off-axis hydrophone, the two pulses converged into one and then separated again into two as the animal's attention was re-directed towards the target hydrophone. This pattern could only arise if both pulses originated within the animal's head.
Figure 2 An example of the changes in pulse structure observed on the off-axis hydrophone as the beluga momentarily turns to ensonify it. The double pulse structure is present while the animal is facing the target hydrophone, then gradually changes to a single (more ...)
Measurements of the on- and off-axis recorded signals reveal trends in how double pulses are produced. Examining the SLs and centre frequencies of the two pulses shows a right–left bias (). The relative difference in amplitude between pulses originating from the left and right phonic lips indicates that, on average, the left side produced higher amplitude pulses than the right (T-test: t=−8.60, p<0.001, d.f.=103). A similar comparison of the centre frequencies of the two pulses shows that, although the first pulse on both sides consistently has a higher centre frequency than the second, the difference between them is significantly greater on the left side (T-test: t=−7.30, p<0.001, d.f.=93). This implies that the pulse produced by the left phonic lip has, on average, a higher centre frequency than the right one.