1. Sound-pressure levels were measured in the swim bladder of the goldfish using a probe microphone. Measurements of pressure and phase were made relative to the sounds immediately outside of the fish.
2. Results showed that the response of the swim bladder is flat from 50 to 2000 Hz with sound-pressure levels within the swim bladder about 4 or 5 dB below the sound pressure outside of the fish.
3. Phase measurements in the swim bladder were not significantly different from those made outside of the fish.
4. The data indicate that there is little loss between the water and the inside of the fish, indicating that the swim bladder acts to closely couple the fish to the water. This has significant implications since a poorly coupled system would severely limit detection capabilities for sounds and greatly increase the energy needed to get sound into the water during sound production.
5. The fact that there is a flat response over the major portion of the range of auditory sensitivity in goldfish indicates that the swim bladder is most probably not a limiting factor in acoustic sensitivity, at least below 2000 Hz. In addition, the swim bladder, as a poorly tuned or untuned system, does not selectively affect signals around what would be the resonance frequency in a more sharply tuned system.
6. The poor tuning of the swim bladder is important for communication since a portion of the communicatively significant sounds for fishes are broad-band, rapidly repeating pulses. A sharply tuned system cannot respond to these signals while a poorly tuned system is suitable for wide-band and time-locked reception.