The concept of acoustic pollution does not necessarily imply a pathology that can lead to acoustic trauma. Any sound at a certain level can result in contamination if it prohibits, or complicates, a good reception of the cetacean echo sonar, or the acoustic communication signals of within a social group.
The contamination levels of a specific sound and its morphological and physiological impact depend on the exposure time and intensity of the received signal.
Trauma associated to noise can result in an either lethal or sublethal impact. The lethal impacts are those that cause immediate death to a subject exposed directly to an intense sound emission.
The sublethal impacts are those cases where the auditory loss is caused by an exposure to perceivable sound, and are called acoustic trauma. In these cases, a sound exceeds the tolerance limit of the ear. Basically, any sound that a mammal can hear can induce, at a certain level, a lesion to the ear, causing a reduction in sensitivity. The minimal level at which a sound (frequency) can be heard is called hearing threshold.
If an individual requires a significantly higher intensity than the normal level for the species, this will be translated as a hearing loss marked by a shift in the threshold level.
Any particular noise at a sufficiently high level will shift the hearing threshold, while other noise at the same level will not cause similar changes.
The question is to know if a received emission produces a temporal or permanent loss.
The mechanism of temporal hearing loss due to a certain frequency and exposure time, is caused by lesions of the inner ear hair cells. The recovery times can vary between a few hours to a few weeks depending on the individual and the source characteristics. However, repetitive exposures to sound sources, without allowing adequate recovery periods, can cause permanent and acute threshold shifts. The duration of a hearing threshold shift has a direct relationship with the duration and intensity of the exposure.
Examination of the cochlear (inner ear) cells can determine the level of change of the hearing threshold and the corresponding frequencies affected.
The difficulty lies both in obtaining fresh ears, extracted immediately after death in order to avoid the effects of autolysis (post-mortem breakdown of organic tissue) that prohibits the correct interpretation of the injuries, as well as in following a complex and rigorous analysis protocol.
Also, the sensitivity at certain frequencies of a cetacean ear can be studied with electro-physiological methods by analysing the evoked potentials registered through the top of the skull. In other words, when an animal hears a sound, its brain registers this vibration through an electrical impulse that can be detected with simple suction cup electrodes. These electrical pulses are called auditory evoked potentials or "Auditory Brainstem Reponses" (ABR), and incorporate a short latency time and duration. This enables examining the hearing sensitivity at frequencies of selected acoustic signals, and establishing the animal's audiogram. In the case of a rehabilitating stranded cetacean, this hearing analysis is fundamental to assess its capacity of correctly using its bio-sonar system, and to evaluate the possibilities of surviving after release.