Some unusual sounds of apparent biological origin responsible for sustained background noise in the Timor Sea

Abstract

Observations of ambient noise in the shallow, tropical Timor Sea north of Australia have shown a predominance of noise of apparent biological origin over much of the audio frequency band. Four of the most commonly occurring sounds which were responsible for the prevailing noise from 100 Hz–2 kHz and which do not appear to have been observed elsewhere, are described. These sounds are quite distinctive in character. Three of the sounds show several harmonics with fundamental frequencies in the range 70–220 Hz, and have durations of 2–4 s. In two of these sounds, the harmonics show a regular modulation of their amplitude, and in one of the sounds, the harmonics rise in frequency by about one octave over the duration of the sound. The fourth sound is nonharmonic and consists of a train of pulses containing four shorter pulses. The mean source levels of the sounds are estimated to lie between 127 and 150 dB re 1 μPa at 1 m.