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to low-frequency sonar. The reimagined team was led by Ed Cudahy and gained momentum in the 2000s. Cudahy and his team often conducted hearing studies at the Naval Undersea Warfare Center Newport Dodge Pond Acoustic Test Facility. This testing environment is known for its low ambient noise and minimal reflection except from the surface. Although the NSMRL underwater thresholds were not quite as sensitive as those in the United Kingdom (Figure 2), procedures established by Cudahy’s team are still in use today (Figure 3) as we continue to expand knowledge of underwater hearing in humans.
Between the two groups, over 100 divers (mix of Navy and recreational) were tested, resulting in the largest sample size of divers measured. The underwater thresh- olds from the United Kingdom resulted in significantly lower thresholds detected at many frequencies (Figure 2) and ultimately became the benchmark for underwater hearing thresholds. These measurements still apply to today’s guidance. These increased sensitivities are likely due to the emphasis placed on lowered ambient-noise levels in the testing environments.
Combining the results of the body of research on hear- ing thresholds, we can draw some conclusions. Overall, it appears that there is around a 30-60 dB increase in sensitivity between equivalent air and water thresholds (Figure 2). There is somewhat of a U-shaped threshold curve, with thresholds increasing fairly quickly above 10 kHz. These studies have established average thresholds for a frequency range from 250 Hz to 16 kHz, showing greatest sensitivity between 500 and 1,000 Hz.
In summary, the most important finding from the stud- ies described here is that human hearing underwater is much less sensitive than in air. Many researchers have measured human underwater hearing thresholds. Although the methodologies used were fairly similar, the results vary. Just as in air, measuring hearing in as quiet a place as possible is critical when testing near the threshold of hearing. As researchers learned to minimize ambient noise and refine their techniques, they expanded their knowledge of the range and sensitivity of human underwater hearing.
Where Is That Sound Coming From?
In air, humans use several cues to identify the direction of a sound. Two of the critical cues are interaural time
difference (ITD) and interaural level difference (ILD). The ITD is defined as the time interval between when a sound is perceived by one ear versus the other ear, and
the ILD is the difference in loudness between the two ears. Both features take advantage of the acoustic shadowing provided by the head. After reaching one ear, sound must travel around the head before reaching the other ear. The human auditory system is sensitive enough to process these differences in time of arrival and loudness to determine the direction of sounds. This is a simplified explanation of the process; in actuality, humans use addi- tional cues to refine the ability to determine direction (Middlebrooks and Green, 1991).
When submerged, sound travels through the head instead of going around like it does in air. Furthermore, sound travels about 340 m/s in air and 1,480 m/s in water, and so the sound reaches both ears so close in time that the brain cannot differentiate between arrival times. In combination, these differences effectively eliminate direc- tional cues. Without the directional cues of ILD and ITD, sound should appear to be coming from all directions equally. Humans certainly cannot localize sound in water as effectively as they can in air, but with enough practice, they are not completely lost underwater either.
Feinstein (1973) ran a series of studies measuring mini- mum audible angles (MAAs) for divers to test the ability to discriminate sounds coming from different directions. The MAA is the smallest angular separation at which two sounds are perceived as coming from distinct directions. Once the sounds originate closer to one another than the MAA, the listener perceives the sounds as coming from the same location.
Feinstein (1973) had divers wearing neoprene hoods with holes at the ears sit on a custom-built platform that kept their head in a fixed position. Two speakers were set up in a way that allowed them to be offset from each other by a known angle of separation. The diver would pull one of two ropes to signal if the sound was coming from the left or right speaker. The stimuli were either a 3.5-kHz tone, a 6.5-kHz tone, or white noise. The MAA for each stimulus was 21.5°, 14.5°, and 9.8°, respectively. A second study provided training to the divers by letting them know when they made a mistake. Following the training, the divers improved to 11.3°, 11.5°, and 7.3°, respectively. Feinstein determined that sound localization underwater
Spring 2022 • Acoustics Today 27