FY 2007
Acoustic measurement of marine mammal sounds in noisy environments
Mellinger, D.K., J.W. Bradbury, and K.A. Cortopassi
In Proceedings of the Second International Conference Underwater Acoustic Measurements: Technologies Results, Heraklion, Crete, Greece, 25–29 June 2007, 8 pp (2007) |
| Libraries of marine animal sounds often contain long recordings—from towed arrays or autonomous hydrophones—that include marine mammal vocalizations. Searching for these vocalizations within long recordings can be significantly enhanced if they have been previously annotated to indicate times, frequencies, and other characteristics. However, defining optimal search features is a difficult problem. For instance, one may wish to find harbor seal 'roar' vocalizations, which can extend up to 6 kHz, last 3-10 s, and have a broadband, non-tonal sound quality. Which features will best characterize such sounds? Marine recordings made without the recordist identifying a focal animal typically contain vocalizations recorded at low signal-to-noise ratios, and it is essential that measurements of a vocalization be consistent whether the vocalization occurs in high or low background noise. For instance, bandwidth is traditionally measured from a spectrogram: a person indicates lower and upper frequency bounds, and then subtracts the two. However, for vocalizations that fade at higher frequencies, like harbor seal roars, bandwidth measurements made this way can vary by a factor of three from low-noise to high-noise environments. Here we describe measurements, based primarily on Fristrup's "Acoustat" approach, that have consistent values at variable noise levels. We normalize a spectrogram to remove average background noise. We then weight the measurement at each instant by the normalized intensity of the vocalization at that instant, so that louder parts—which are still present in high-noise situations—have the strongest influence on the measurement value. A set of noiserobust measurements have been developed, including measures for duration, bandwidth, amplitude and frequency modulation, tonality (vs. continuous-spectrum), peak frequency and time, etc. These measurements will be used to extract features from the Macaulay Library's marine collection and included in on-line search tools. |
