FUNDAMENTAL FREQUENCY VARIATION OF DOLPHINS WHISTLES. Eduardo Romero-Vivas1 and Braulio Leon-Lopez2. INTRODUCTION. Recordings from ...
SPECTROGRAM DIFFERENCES APPROACH FOR EXTRACTING FUNDAMENTAL FREQUENCY VARIATION OF DOLPHINS WHISTLES Eduardo Romero-Vivas1 and Braulio Leon-Lopez2 1. Centro de Investigaciones Biológicas de Noroeste (CIBNOR). La Paz, B.C.S, Mexico. 2. Sea Mammal Research Unit, University of St Andrews. St Andrews, UK.
INTRODUCTION Studying acoustic communication in the wild usually presents the challenge of detecting signals of interest in noisy environments, which is a common situation for underwater species. Dolphin species are well known for having a complex acoustic repertoire which includes pulsed (echolocation clicks) and tonal sounds (whistles). The latter are regularly omnidirectional in their fundamental frequency components, highly used for social interactions, and therefore, important for study behaviour and ecology of dolphin populations.
METHODS Recordings from two dolphin species: long-beaked common dolphin (Delphinus capensis) and bottlenose dolphin (Tursiops truncatus), were obtained during 2 different surveys on board of a small vessel in La Paz Bay, using a custom made hydrophone [1] connected to a Fostex RD2 192kHz/24 bits digital recorder.
25°N 31°N
Mexico 28°N
24°N
SPECTROGRAM ANALYSIS
114°W
The spectrogram is defined as the squared magnitude of the short time Fourier transform:
24°N
109°w
La Paz 111°W
110°W
Location of La Paz Bay in the Gulf of California, Mexico.
And it is usually displayed as a bi-dimensional plot of time vs. frequency in which amplitude values 𝑺 𝒕, 𝒇 are represented as a range of colors from a user defined palette (Color space). For instance, amplitude values of the spectrogram going from -50 to -140 dB these values would map to a jet Color space as follows:
Giving the familiarly standard spectrogram:
The standard colourmap C is defined by a linear mapping function from an amplitude value to a color within a range:
Giving the difference spectrogram:
𝑪 = 𝑺(𝒕, 𝒇) But the mapping function can be modified to highlight certain characteristics of the signal [2]. We tested a method, based on the differences of contiguous windows of the spectrogram, to detect and identify modulation of the fundamental frequency of tonal sounds. Thus the new colourmap function Cd is defined by: Common dolphin whistle
𝑪𝒅 = 𝑺 𝒕𝟐, 𝒇 − 𝑺(𝒕𝟏, 𝒇)
Common dolphin whistle
RESULTS Following this approach we obtain a simplified spectrogram in which the tonal whistle becomes easier to identify.
Delphinus capensis
Tursiops truncatus
Whistle sequences of Common dolphin (top) and bottlenose dolphin (bottom). In the left the standard spectrogram and to the right the spectrogram based on differences.
CONCLUSIONS The methodology proposed provides a simplified spectrogram in which becomes easier to detect the fundamental frequency of whistles. This approach could be implemented within classification algorithms to simplify automatic detection of tonal sounds for these and other taxa. REFERENCES 1. Romero-Vivas, E & Leon-Lopez, B. (2011).“Construction, Calibration, and Field Test of a Home Made Low Cost Hydrophone System for Cetacean Acoustic Research”. POMA 11, 010001
2. Romero-Vivas E., Viloria L. & Urban J. (2013).”Morphological and gradient power spectrum representations for Bryde's Whale geographical call differentiation” 1er Coloquio Nacional de Acústica, ESIME-IMA, Mexico Dolphin illustrations from www.whaleopedia.org
ACKNOWLEDGEMENTS PRIMMA-UABCS for helping and allowing us to record during their cetacean surveys and Sergio Martinez for the dolphin photographs.
