Yuka Mishima -- thank_you_for_email_5yuka@yahoo.co.jp
Tokyo University of Marine Science and Technology
Popular Version of Paper 2aAO5
Presented Tuesday morning, May 15, 2012
163rd ASA Meeting, Hong Kong
Dolphins rely on the combination of a variety of vocalizations and vastly better acoustic abilities than humans to communicate with each other or to detect their surroundings and prey in the dark sea. To gain new insights into how dolphins communicate, we create an extremely broadband "dolphin-speaker" prototype capable of projecting dolphins' communication sounds, whistles, burst-pulse sounds, as well as detection sounds such as echolocation clicks.
Dolphins can hear and produce not only low-frequency sounds below 20 kHz, which humans can also hear and produce, but go a step further to high-frequency sounds of up to more than 150 kHz, which are too high for humans to hear (Au, 1993). Not only can dolphins produce tonal sounds like humans, they're capable of vocalizing at a variety of frequencies simultaneously.
Acoustic studies of dolphins that have been done so far focus mainly on recordings of vocalizations and hearing abilities, but relatively few playback experiments have been conducted. There were no speakers that could project from low to high frequencies like dolphins, although some could project the low-frequency sounds (Lang & Smith, 1965, Sayigh et al., 1998, Miksis et al., 2001, Janik et al., 2006, Nakahara & Miyazaki, 2011) or parts of dolphin sounds.
Since playback experiments are a very important part of gaining a better understanding of dolphins' communication and detection abilities, and their receiving system as well, we decided to create our own "dolphin speaker."
Our goal is to develop a dolphin speaker that could project the full range of all of the sounds dolphins make. We succeeded in developing a prototype broadband transducer for an echosounder last year by using new types of piezoelectric elements that had never been used for underwater acoustic transducers (Fig. 1). We applied this technique to the dolphin speaker prototype.
The four green piezoelectric elements are for high frequency sound projection, and the silver element is for low frequency projection. These elements are fixed on an acrylic disk, which separate elements from water. In addition, high-frequency elements are sandwiched from the back with using another acrylic disk. The sizes of each element and the acrylic disk are precisely adjusted to create broadband sounds.
Fig 1. The dolphin speaker prototype.
The frequency response of the dolphin speaker prototype is shown in Fig. 2, which shows the source level vs frequency. The input voltage level was fixed at all frequencies. The dolphin speaker prototype has fairly flat sensitivities between 6 kHz to 170 kHz within about 1 6 dB.
Fig 2. Frequency response of the dolphin speaker prototype.
We conducted playback tests of dolphin sounds in the ocean by using the dolphin speaker prototype. The spectrograms of the "original sounds" of dolphins (the upper diagrams) and the "playback sounds" by using the dolphin speaker prototype (the lower diagrams) are shown in Fig. 3.
The original whistle (see Fig. 3 (a)) is a tonal and long in duration sound. The fundamental frequency was 5 kHz. As seen in the playback sound, not only the dominant frequency but also the change of intensity reproduced. In contrast, burst-pulse sounds (see Fig. 3 (b)) and echolocation clicks (see Fig. 3 (c)) are a series of broadband pulses, but have different inter-pulse intervals (Lammers et al,. 2004).
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Fig 3. Comparisons of the original sounds (the upper diagrams) and playback sounds (the lower diagrams). (a) a whistle, (b) burst-pulse sounds and (c) echolocation clicks.
Compared between the original sounds and the playback sounds, spectrograms are similar, except for the contamination of ocean environmental noise. The results suggest that the extremely broadband dolphin speaker prototype is able to reproduce various types of dolphin sounds.
Our next step is to faithfully playback the original sounds of dolphins by using the dolphin speaker. Once the dolphin speaker is completed it will enable us to playback a variety of dolphin sounds to dolphins, which will help to broaden the research of their acoustic abilities.
References:
Au, W. W. L. (1993). The Sonar of Dolphins (Springer, New York), p. 34.
Lang, T. G., and Smith, H. A. P. (1965). Communication between Dolphins in Separate Tanks by Way of an Electronic Acoustic Link. Science, 150, 1839-1844
Sayigh , L. S., Tyack, P. L., Wells, R. S., Solow, A. R., Scott, M. D., and Irvine, A.B. (1998). Individual recognition in wild bottlenose dolphins: a field test using playback experiments. Anim. Behav., 57, 41-50.
Miksis, J. L., Grund, M. D., Nowacek, D. P., Solow, A. R., Connor, R. C., and Tyack, P. L. (2001). Cardiac Responses to Acoustic Playback Experiments in the Captive Bottlenose Dolphin (Tursiops truncatus). J. Comp. Psychol., 115, 227-232
Janik, V. M., Sayigh, L. S., and Wells, R. S. (2006). Signature whistle shape conveys identity information to bottlenose dolphins. Proc. Natl. Acad. Sci. USA, 103, 8293-8297.
Nakahara, F., and Miyazaki, N. (2011). Vocal exchanges of signature whistles in bottlenose dolphins (Tursiops truncatus). J. Ethol., 29, 309-320.
Lammers, M. O., Au, W. W. L., Aubauer, R., and Nachtigall, P. E. (2004). A comparative Analysis of the Pulsed Emissions of Free-Ranging Hawaiian Spinner Dolphins (Stenella longirostris). in Echolocation in Bats and Dolphins, J. A. Thomas, C. F. Moss, and M. M. Vater, Eds. (University of Chicago Press, Chicago), pp. 414-419.