An electroacoustics playback system was used, with the aim of
ensuring that each fish specimen was exposed to a fairly constant
noise level during the whole test.
25-seconds of boat noise was played back in loop for 5 min with a
20 cm diameter underwater speaker (AQ339 Clark Synthesis; frequency
range 35 Hz–17 kHz), connected to a Sony TCD-D 100 DAT
recorder and a power amplifier. The underwater speaker was the
greatest constraint on the frequency response of the system; for
example, its sensitivity cut-off of 35 Hz removed the noise produced
by the ferry propeller blade rate. However, the maximum hearing
sensitivity to sound pressure of the target species is located at 200 Hz
(C. chromis) and 300 Hz (G. cruentatus) with an upper frequency limit
of 600–700 Hz (Codarin et al., 2009; Wysocki et al., 2009), where the
speaker does not produce distortion in the reproduced signal