Acoustic threshold in goldfish and its relation to the sound source distance

Abstract

1. 1. Acoustic threshold levels have been obtained for goldfish in classical conditioning experiments, stimulating the fish with pure tones of different frequencies (50–5000 c/s). The threshold intensity for response has been established for different distances (0·1–2·0 m) between the fish and the underwater loudspeaker. 2. 2. Sound pressure thresholds (given in decibels re 1 μBar) for frequencies below 600–700 c/s are strongly dependent upon distance to the sound source, while no significant threshold differences were found for higher frequencies. Tests with an air loudspeaker gave thresholds similar to those obtained with the underwater loudspeaker at 2 m distance. 3. 3. Lowest threshold (−42 to −45 db) are in the 100–1500 c/s range, increasing to −10 db for 5000 c/s and to −6 db (at 2 m distance) or −38 db (at 0·1 m distance) for 50 c/s. 4. 4. The variation in threshold with distance is probably due to the physical characteristics of the sound stimulus. Close to the sound source, i.e. within a distance equal to the wavelength divided by 2π, the near-field water displacement amplitude is considerable. Calculated particle displacement at threshold is roughly the same for all distances for any given frequency and particle acceleration is the same for all distance and frequencies.