Equal-loudness functions describe relationships between the frequencies of sounds and their perceived loudness. This pilot study investigated the possibility of deriving equal-loudness contours based on the assumption that sounds of equal perceived loudness elicit equal reaction times (RTs). During a psychoacoustic underwater hearing study, the responses of two young female harbor seals to tonal signals between 0.125 and 100 kHz were filmed. Frame-by-frame analysis was used to quantify RT (the time between the onset of the sound stimulus and the onset of movement of the seal away from the listening station). Near-threshold equal-latency contours, as surrogates for equal-loudness contours, were estimated from RT-level functions fitted to mean RT data. The closer the received sound pressure level was to the 50 detection hearing threshold, the more slowly the animals reacted to the signal (RT range: 188-982 ms). Equal-latency contours were calculated relative to the RTs shown by each seal at sound levels of 0, 10, and 20 dB above the detection threshold at 1 kHz. Fifty percent detection thresholds are obtained with well-trained subjects actively listening for faint familiar sounds. When calculating audibility ranges of sounds for harbor seals in nature, it may be appropriate to consider levels 20 dB above this threshold.
Bibliographical noteFunding Information:
We thank Lean Hoek for managing the husbandry and research, Tess van der Drift for her help in analyzing the video recordings, and Rob Triesscheijn for making the figures. We thank Bert Meijering (Topsy Baits) for providing space for the SEAMARCO Research Institute. We thank Arie Smink for designing and building some of the electronic equipment. We also thank Nancy Jennings (Dotmoth.co.uk, Bristol, UK), Wim Verboom (JunoBioacoustics), and two anonymous reviewers for their valuable constructive comments on this manuscript. This study was funded by The Netherlands Ministry of Transport, Public Works and Water Management (contact: René Dekeling). We thank Just van den Broek (Ecomare) for making the harbor seals available for this project. The seals’ training and testing were conducted under authorization of the Netherlands Ministry of Agriculture, Nature and Food Quality, Department of Nature Management. Endangered Species Permit FF/75A/2005/048 (contact: Jan van Spaandonk).