Hearing loss depends on the intensity of the sound, the frequency of the sound, and how long the animal is exposed to the sound (duration). Humans exposed to extremely loud sounds for short periods of time (e.g. rock concerts, explosions) experience temporary or permanent hearing impairment. Human hearing damage can also be caused by exposure to moderate levels of noise over long periods of time, as in a noisy work environment. Hearing impairment does not occur if the frequency of the sound to which the animal is exposed is outside the range that the animal can hear.
The softest sound that an animal can hear at a specific frequency is called the threshold of hearing at that frequency. If an animal is exposed to sound below the threshold of hearing, the animal cannot hear the sound.
The animal can accommodate sounds that are above the threshold of hearing until a certain combination of intensity and duration is reached. Above this limit, the animal's hearing threshold will be elevated. This means that the softest sound that the animal can hear is louder than it was before exposure to the sound, or they have lost the ability to hear the softest sounds they could hear before. The shift will be temporary or reversible, and the threshold will return to near normal levels. The loss of sensitivity is called a temporary threshold shift or TTS. As the sound exposure increases yet further, a higher level will eventually be reached at which the threshold shift will be permanent, and the effect is called a permanent threshold shift or PTS. PTS can occur as a result of repeated occurrences of TTS, or it can occur catastrophically as a result of a single exposure to a very intense sound.
Temporary threshold shift (TTS) studies have been conducted with four species of odontocetes (bottlenose dolphins, beluga whales, harbor porpoise and Yangtze finless porpoise) and three species of pinnipeds (harbor seals, California sea lions, and Northern elephant seals). The hearing thresholds of these captive animals are determined using both behavioral and auditory brainstem response (ABR) methods (for more information see Hearing Sensitivity Studies). The TTS investigations introduce sounds at varying frequencies, intensities, and durations to determine sound exposures that cause temporary threshold shifts. (The marine mammals were not exposed to high enough levels to cause permanent threshold shifts (PTS) and completely recovered their hearing.)
One study exposed bottlenose dolphins and beluga whales to signals at frequencies of 400, 3,000, 10,000, 20,000, and 75,000 Hz that lasted for one second. The received levels (estimated at the position of the animals ear) required to cause TTS were generally between 192 and 201 dB re 1 µPa. The data from this study are plotted in the following figure. The highest receive level that was tested at the 400 Hz frequency (193 dB re 1 µPa) did not produce threshold shifts in any of the animals - indicated by the dashed line.
Several studies have shown that exposure to loud sounds for a few minutes to a few hours will cause (TTS) in fishes. The amount of hearing loss appears to relate to how loud the noise is compared to the threshold of hearing at that frequency. A predictable linear relationship was found between TTS and the difference between the sound pressure of the noise and the baseline hearing threshold of the fishes. Loss of sensory hair cells due to exposure to sound has been observed in oscars, a species of cichlid fish, four days after exposure to 1 hour of 300Hz continuous tones at 180 underwater dB. Similarly, the ears of pink snappers exposed to an operating airgun showed damage, with no evidence of repair or replacement being found 58 days after exposure.
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