techniques

In recent years research into the whistle structure of different species of odontocete calls has attempted to identify distinctive individual, pod, or simply species-specific, features, that might assist in acoustic identification and discrimination. So far most of these studies have concentrated on the frequency modulated signal by examining these for characteristic features in the frequency-time domain using FFT analysis. Such analysis has concentrated on the centre frequency, frequency deviation, number of inflection points (frequency reversals), and more significantly the general shape or contour of the call. The Source Level of echolocation signals has also been studied for other reasons but this parameter appears to reflect some body size dependency. This paper discusses the feasibility of using the maximum Source Level of the narrow band social calls (whistles) as an additional cue when attempting to distinguish between species while studying free-ranging animals. The data used for spectral and statistical analysis was recorded from a Dutch fishing research vessel (FRV Tridens) during the 1996 and 1997 CETASEL project sea trials. A single hydrophone was attached to a pelagic trawl net fishing in relatively deep water (100-200 m depth) along the edge of the continental shelf in the Eastern North Atlantic sea areas SW of Eire, through Biscay and towards Finnisterre. Maximum sound pressure levels of social calls were extracted and converted into Source Levels (SL re 1µPa at 1m) using a calibrated 13 kHz pulsed cw pinger (Dukane) to provide a known SL reference. The necessary range information (distance between hydrophone and vocalising animal) was obtained by means of a multi-path echo-ranging method (Kaschner et al 1997). Parallel visual observation in good weather conditions provided the species identification and a time series analysis established the probability of the association between these sighted animals and the acoustic recordings of species whistles. On one occasion a very vocal group of bowriding Common Dolphins Delphinus delphis were recorded simultaneously in air, using a gun microphone directly above them at the bow, and underwater using the hydrophone attached to the trawl at a distance of 518m behind them. The maximum underwater Source Levels of individual signals which were recognisable in both recordings were calculated and compared with the other results in order to test the validity of the method described above. This paper discusses the methodology and limitations of this relatively simple technique but the results achieved to date suggest considerable potential for SL estimation in open sea conditions. Alternative, quieter, platforms than a pelagic trawler are recommended.

Kaschner, K. et al. (1997). Analysis and interpretation of cetaceans sounds obtained by a hydrophone attached to a pelagic trawl. In European Research on Cetaceans - 11. Proc. 11th Ann. Conf. ECS, Stralsund Germany (Eds. P.G.H. Evans) European Cetacean Society, Cambridge

Echolocation signals of dolphins (clicks) consist of short duration and high frequency ultrasonic pulses. Click intervals of dolphins in captivity are less than two-way transiting time of a target range. Click intervals of free-ranging dolphins are thought to provide underwater sensory range of echolocation. A/D converter driven by a data acquisition program on Windows 95 © with a small signal processing circuit (Clicker 45) enabled real-time data acquisition of click intervals. Clicker 45 converted clicks to rectangular signals which duration was 0.5 ms. The amplitude of the rectangular signals were in proportion to sound pressure levels of clicks. When the amplitude of the rectangular signals were more than the threshold level (127 dB re 1µPa), the serial time and the amplitude were measured by the data acquisition system whose accuracy were 5 mV and 100 ms, respectively. Consequently, the click intervals and sound pressure level in click trains could be retrieved automatically. The rectangular signals from Clicker 45 could be recorded by an ordinary band-limited data recorder, so that the present system could be applied not only for a laboratory works but also for an open sea observation of dolphins' sonar.

Studying the vocalisations of dolphins provides an insight into their underwater behaviour whereas visual methods are normally restricted to surfacing behaviour. Apart from whistles, which seem to be produced in a social context, the sonar emissions of dolphins consist of pulsed signals used for different echolocation purposes. By analysing the pulse periodicity, frequency and spectral components of 'clicks' and relating these to video recorded surfacing behaviour it should be possible to extract characteristic patterns which indicate different kinds of dolphin behaviour. Almost all the pulsed emissions recorded during a series of 24 hour long intensive studies suggest behaviour related to foraging. These recordings provide unambiguous conditions where the environmental constraints can be observed and understood. Goodson and Datta (1992) found that recognisable patterns occurred in the repetition rate of the echolocation signals during these sequences. They partitioned these pulse sequences into 4 distinct phases. These sub-classifications have been further examined using an extended data set from the same source. Graphical and statistical analysis of six complete sequences, recorded immediately prior to a visually observed fish capture, allows a better definition of the presence of these foraging partitions. An additional fifth foraging phase was identified. The sequences could be partitioned into separate identifiable segments and characterised as foraging phases. Possibilities exist for automating this analysis for on-line use in the field as well as for the application of this analysis approach to behaviour types other than foraging.

When converting analog signals into the digital domain, steep low-pass filters are required to avoid aliasing. Aliasing is the 'pollution' in the frequency range of interest created by frequencies higher than the Nyquist frequency (half the sampling rate). Good anti-aliasing filters are very expensive and, probably, not strictly necessary when recording music and speech for human listening. On the other hand, when recording/analyzing sounds with unknown frequency con' tents, or with components, including harmonics and unpredictable noises, extending beyond the Nyquist frequency, steep filters are required to avoid aliasing. As DATS are widely used to record animal voices, we made some tests on their anti-aliasing filters to evaluate artefacts possibly present on the recording. Results were compared with those coming from some PC sound boards and from anti-aliasing filters used with signal acquisition instrumentation. All the recorders and sound boards we tested showed different degrees of aliasing occurring in the upper range of their bandwidth. Even if its effect might be considered unnoticeable to human listening, aliasing must be taken into consideration when analyzing recordings or when performing playback experiments.

We have developed a portable, external digital/analogue converter (E-DAC) to get a low cost device with sampling rates up to 600 kHz per channel (16 bit). Two D/A channels are available at the moment, simultaneous data input is scheduled. The attenuation (step size 0.375dB, range 0-90 dB) of the signal takes place after the digital/analogue conversion in order to retain a high signal-to-noise ratio. The D/A-converter operates independently of the main processor to avoid interrupts and timing problems. The standard enhanced parallel port (EPP) is used for data transfer between host and the external device, because it is available at most PCs or notebooks, and the data transfer speed is reasonable. A software program (E-DAC-Control) has been designed to control the output of digitized signals. It will run under Windows 3.x and higher. This computer program allows generation and manipulation of sine waves or sweeps as well as the import/export of modified data files. Additionally, complex programs can be composed, employing different signals, variable repetition rates, pause durations and attenuation levels.

WPLOT is a utility for the display, exploration, and annotation of acoustic data files. The utility is in Borland's Delphi 2 language, so it can be used from any Delphi or C++ Builder program with little modification to the calling program. WPLOT is geared to examining the raw binary data files produced through the typical use of analog-to-digital conversion cards. WPLOT allows the user to view any section of a data file graphically, handling very large files with the capability to separately save displayed intervals. The vertical scale can be changed to correspond with the extreme values produced in sampling. WPLOT displays the in-line distance of echoes received at the hydrophone given a user-selected sampling rate and an on- screen interval. WPLOT also performs little-endian/big-endian byte swapping, so that data files collected on Macintosh or Sparc systems can be properly displayed. The key feature of WPLOT is the ability to annotate data files, producing a companion description file which consists of 2-tuples, associating a sample offset or sample interval with a token. This annotation is critical for the use of offline processing, such as collecting statistics on bioacoustic events or for use in training supervised-learning artificial neural systems.

In dolphins, directional, broad-band pulsed sounds are used for echolocation but due to their high directionality, they are also well suited for addressing intra-specific, social signals. Studies on pulsed communication sounds in free swimming dolphins have so far been restricted to the audible range, not dealing with the directional aspect. Preliminary studies at our facility indicate that dolphin use this option in their intra- specific communication. We have developed a new Pc-compatible datalogger concept for recording and store high frequency pulsed sounds produced by free swimming bottlenose dolphins Tursiops truncatus. The recording unit, called 'MOSART' (MObile Subsurface Acoustic Recording of Transients), is designed to record the highly directional high frequency sounds received by a dolphin, not those produced by it. The 'MOSART' will be attached to the dorsal in by means of a non- invasive saddle pack (Trac Pac). The 'MOSART' has a broad-band hydrophone, and specially developed sound processing electronics and software. The sounds are stored on a 1.8 inch hard disk. Several recording options will be adjustable by means of computer software settings, (e.g. time delay before recording onset, trigger amplitude threshold, high pass filter limit and sampling rate). To maximise the recording time, only a limited and pre-set number of fully sampled clicks will be collected. For the rest of the clicks only a time stamp will be stored. In this way at least 86 minutes of continues click trains, with an average repetition rate of 500 pps, may be recorded. The stored data will be transferred to a PC for analysis. It will be compared with traditional broad band recordings with fixed hydrophones and correlated with video recordings of the dolphin behaviour.

Acoustic censusing of marine mammals is an advancing technique. This presentation will discuss advantages and disadvantages of acoustic censusing compared to visual censusing. When considering using acoustics to describe marine mammal populations, several questions need to be addressed, including type of population estimate (relative or absolute), array design, localization requirements, frequency bandwidth, and species diversity of the sampled population. Major advantages of acoustic censusing include greater detection ranges, fewer environmental limitations, and a complete record of all contact cues. Given recordings of each contact, the signals can be further analyzed for source identification and localization. Acoustic censusing difficulties relate to determining source identity, group size, and detection distance. Fundamental choices such as array design can have major impacts. Finally, the overall advantages of acoustic censusing, particularly when done concurrently with a visual survey lead to useful data. In the recently completed GulfCet I project, acoustic effort occurred along 95% of the survey track, compared to 49% for the concurrent visual survey. Population estimates from the acoustic survey were 316 (265-377) sperm whales and 36,946 (33,512-40,566) dolphins, compared to 313 (192-508) sperm whale and 18,584, (10,268-35,431) dolphins for the visual survey. Subsequent analysis can now be done, for example, on the effects of noise on marine mammals based on signals recorded during the survey.

Cluster analysis is a straightforward approach to classification of sounds, but the distance matrices required for input are problematical. Using a limited number of frequency time coordinates to represent each sound allows efficient handling of large data sets, as well as classification of data not used to define the clusters. Cross-correlation analysis offers the power of complete comparison of sounds, but no way to classify sounds not included in the cross- correlation. We evaluate McCowan's elegant parametric method of comparing dolphin whistles (PCA of 20 equally spaced frequency measurements) for agreement with less efficient but more powerful cross-correlation methods, on an independent data set of bottlenose dolphin whistles. Kmeans clustering of principal components produced different clusters from 3 other methods. Distance matrices generated with McCowan's 20 variables and with cross correlation analysis of time-normalized frequency contours were similar, but distance matrices based on non-normalized contours differed greatly. Our results suggest that McCowan's method might be improved by including duration as a variable in the PCA.

During the past few years, we used acoustic methods to investigate sound communication and to search for the presence and distribution of singing cicadas (Homoptera:  Cicadoidea) in Slovenia, Croatia, Macedonia and S.E. Asia. For detection of high pitched songs of smaller species, e.g. Cicadetta or Tettigetta the use of a bat detector - in our case Ultra Sound Advice 5-25 with the microphone mounted to a Telinga parabola - proved to be very suitable, as reported already at the IBAC conference in Potsdam. We use it mainly in the heterodyne mode with the frequency selector tuned to the lowest frequency. With such equipment it is possible to detect songs of small singing cicadas even in areas with high traffic noise and at a distance of up to about 50 m. In addition to this Telinga microphones Pro 1II and Pro V Science were used for recording in combination with the DAT recorders Sony TCD-D3, -D7, -D10 and Pioneer D-C88. The latter was used mainly in the HS mode in combination with the special version of the Pro V Science microphone, with one mic capsule sensitive also in the ultrasonic range. Such a system enabled us to make recordings in the frequency range of 20 to 44000 Hz. As a result of such investigations in Slovenia, Croatia and Macedonia in the last few years many new data about songs and distribution of single species were obtained. In addition to this, during this survey we found in Slovenia not only Cicadetta montana (Scopoli) as described by Boulard (1995) but, with the aid of the bat detector, another similar and closely related species with a different song, which still has to be identified. In Macedonia this year we were able to find, record and catch on Galicica mountain yet another unidentified species with a very characteristic song, which is also closely related to Cicadetta montana. Furthermore, it was found that the species Cicadatra hyalinata (Bru1le), considered by some authors to be only a variety of C. atra (Olivier), has a different calling song and is therefore most probably a good species. Most interesting is the discovery of Cicadatra persica (Kolenati) in the Radika Gorge, not previously known from the Balcans and detected for the first time a year before during acoustic scanning of this region. The songs of this species were previously not recorded and analyzed. A detailed description of these interesting cicadas and their songs is in preparation.