Page 26 - Spring2022

UNDERWATER HEARING IN HUMANS
Hollien, 1967). This system included a seat to keep a weighted diver in place, transducers (speakers) at known distances, calibrated hydrophones at the location of the diver, and mechanisms for the diver and researchers on the surface to communicate.
The team conducted much of their research at the Under- water Sound Reference Division of the Naval Research Laboratory, Orlando, Florida, test facility at Bugg Spring, which was an extremely quiet, nonreverberant testing environment. Due to the controlled testing environments, the hearing thresholds obtained by this team were long considered one of the gold standards for underwater hearing in humans. Although their data (Brandt and Hol- lien, 1967) were not that different from the data obtained in previous studies (Figure 2), the rigorous testing proce- dures and quiet location supported the accuracy of their results. They also tested whether water depth affected thresholds but failed to find a significant difference of thresholds for depths ranging between 3.7 m and 32 m (Hollien and Brandt, 1969).
The other key research team that worked on underwater hearing starting in the 1960s and lasting through the 1990s was led by Paul Smith of the Naval Submarine Medical Research Laboratory (NSMRL) at the New London Sub- marine Base in Groton, Connecticut. Smith’s efforts in the underwater realm kicked off research that continues at the NSMRL today, covered everything from underwater hearing thresholds to diver aversion to sound, and also produced early recommendations on hearing conservation.
Smith’s (1969) underwater hearing threshold testing was the first to include examination of bone conduction thresholds in air. This was critical because the pathway for bone conduction in air appears to mirror the under- water direct inner ear stimulation. Smith recruited US Navy subjects with normal air conduction and bone conduction hearing as well as some with reduced air con- duction and bone conduction responses. The study was done in a deep, quiet pond (75-80 ft) with the subjects in the middle of the pond at a depth of 4.5 m. Like Hollien’s team, Smith built a platform that housed the diver, trans- ducer, and reference hydrophones all at fixed locations. The underwater hearing thresholds obtained in this study matched those in Brandt and Hollien (1967) (Figure 2), establishing the importance of running experiments in quiet environments. An interesting finding was that the
divers with reduced bone conduction thresholds also had reduced underwater hearing thresholds, further support- ing the direct inner ear stimulation hypothesis.
Following Hollien and Smith, beginning in the 1990s and extending to today, two other groups entered the under- water hearing field. The initial studies were conducted by Mohammad Al-Masri, University of Portsmouth, Ports- mouth, United Kingdom, in 1993 (reviewed by Al-Masri and Martin, 1996) and then carried forward by Parvin and Nedwell (1995) through the rest of the 1990s. These teams built on the lessons learned from previous research, creating as quiet an environment as possible, and reduc- ing the ambient levels in their test tank to ~44 dB re 1 μPa by acoustically isolating the tank from the surround- ing laboratory environment (compared with ~60 dB re
1 μPa in Smith’s [1969] experiments). In addition, the investigators characterized the sound levels in the tank environment so that the level at the diver’s head was as well defined as possible. They also conducted in-air hear- ing tests on all divers (mix of Navy and recreational) to confirm that they had “normal” hearing. All these efforts resulted in underwater hearing thresholds that were sig- nificantly lower (15-20 dB lower at many frequencies) than any measured previously (Figure 2).
The second group beginning to work in this field in the 1990s was out of the NSMRL (Fothergill et al., 2002, 2018). This program reinvigorated the underwater hear- ing research that Smith had started in the 1960s but focused on concerns of US Navy divers being exposed
Figure 3. Diver participating in an underwater hearing test in the NSMRL dive pool.
26 Acoustics Today • Spring 2022