Page 25 - Spring2022

Underwater Hearing Thresholds
Defining the average underwater hearing threshold is challenging. One issue is that most human testing has been conducted with just a handful of subjects per exper- iment due to the complexities of testing underwater. Still, human underwater hearing has been tested by at least eight different research teams, building a foundation of information on hearing thresholds in divers. At the same time, the only thing that all the conclusions constructed by these teams have in common is that they are different.
Two methodological approaches are consistent among all the studies. (1) Breath holding by the subjects was done to reduce noise during sound presentations, and (2) underwater hearing was measured as minimum audible field (MAF) audiograms. This means that the subjects are facing a sound projector and detecting and responding to the free sound field to which their heads are exposed. In most cases, this sound field is then calibrated by placing a hydrophone where the location of the head would be to measure the sound level. The calibration procedures are challenging underwater because creating an anechoic (nonreflective) environment is nearly impossible. There is also the problem of removing or limiting environmental noises from the test site.
The first underwater testing of multiple frequencies to measure human hearing was conducted by Ide (1944) of the United States Naval Research Laboratory (Figure
2). The methods were not detailed and the background noise as well as the sound measurements at the swimmers’ heads were not reported, making these data challenging to interpret.
Several studies were then conducted in the 1950s and early 1960s, with threshold results between experiments varying 10-15 dB at each frequency (Figure 2) (Hamilton, 1957; Wainwright, 1958; Montague and Strickland, 1961). Although each team used similar approaches to measure hearing, the likely reasons for these equivocal results are background noise in testing environments, insufficient data on the subjects’ in-air air conduction and bone con- duction thresholds, and challenges with calibrating the sound fields underwater. Thus, although these studies began to map out human underwater hearing abilities, uncertainty remained about the sensitivity and frequency range. This uncertainty was partially alleviated by two research groups that emerged in the late 1960s and early 1970s, both of which advanced understanding of how humans hear underwater.
The first group formed at the Communication Sciences Laboratory at the University of Florida, Gainesville, and was led by Harry Hollien and included John Brandt and Stephen Feinstein among others. In 1967, the group built their Diver Communication Research System (DICORS) to conduct standardized, calibrated testing of human hearing and communication under water (Brandt and
Figure 2. Human underwater hearing thresholds. NSMRL, Naval Submarine Medical Research Laboratory; MAF, minimum audible field. MAF data from International Standards Organization (1993). Figure modified from Al-Masri and Martin (1996).
Spring 2022 • Acoustics Today 25