Acoustics of fish shelters: background noise and signal-to-noise ratio

Fish shelters (flat stones, shells, artificial covers, etc., with a hollow beneath) increase the sound pressure levels of low frequency sounds (<150 Hz) outside the nest cavity, see Lugli [(2012). J. Acoust. Soc. Am. 132, 3512-3524]. Since some calling males only produce sound when a female is inside the shelter, this study examines the effect of sound amplification by the shelter on signal-to-noise ratio (SNR) in the nest. Background noise amplification by the shelter was examined under both laboratory (stones and shells) and field (stones) conditions, and the SNR of tones inside the nest cavity was measured by performing acoustic tests on stones in the stream. Stone and shell shelters amplify the background noise pressure levels inside the cavity with comparable gains and at similar frequencies of an active sound source. Inside the cavity of stream stones, the mean SNR of tones increased significantly below 125 Hz and peaked at 65 Hz (+10 dB). Implications for fish acoustic communication inside nest enclosures are discussed.