How Acoustic Habitat Degradation Affects Auditory Perception of Divers in the Indian Ocean Region (IOR)
By Aarohi Kapadia, Dr. (Cdr) Arnab Das
The sounds of the ocean are often used to characterize the uniqueness of the underwater ecosystem. However, the increasing levels of acoustic degradation, especially in the Indian Ocean Region (IOR), are slowly affecting the charm of the majestic marine biodiversity and causing disturbances to different aspects of the undersea world. The growing interest in the maritime industry has resulted in an increase in the anthropogenic activities in the IOR. The most significant repercussion of these activities has been the exponential growth in underwater ambient noise levels. Sound is defined as a vibration and when vibrations cause a change in the ambient pressure, a sound pressure is generated. Noise is expressed as Sound Pressure Level (SPL) measured in decibels (dB). The term ‘ambient noise’ is widely used to refer to the cumulative noise present in a water body, caused by known as well as unknown independent sources of sound.
The increasing levels of acoustic degradation, especially in the Indian Ocean Region (IOR), are slowly affecting the charm of the majestic marine biodiversity and causing disturbances to different aspects of the undersea world.
Underwater noise is often the outcome of various actions such as construction, drilling, maintenance work of underwater structures, explosions, use of sonars, movement of different vessels and many more occupational as well as recreational activities. Sound signals are also often used for underwater research purposes. All these sources of noise have their individual sound pressure levels, directionality and frequencies at which they peak. A broadband noise spectrum is often required to showcase the aggregate noise levels in a region. Table 1 showcases some of the most common sources of underwater noise.
Evidence suggests that underwater noise displays an exponential trend every year. The impact of this rising acoustic degradation on marine mammals has been widely studied and several repercussions to mammalian behaviour and health have been determined. Correspondingly, underwater noise pollution also exerts an influence on humans. Diving, recreational and commercial, is a common activity in the marine industry, characterized as the immersion of body into water either partially or completely. It is a popular sport amongst tourists and an integral part of material sourcing, underwater equipment maintenance, marine research, and naval activities. While the numerous biohazards of diving into contaminated waters are often discussed, the effects of underwater acoustic degradation and auditory deterioration often remain unacknowledged.
While the numerous biohazards of diving into contaminated waters are often discussed, the effects of underwater acoustic degradation and auditory deterioration often remain unacknowledged.
/p>Human ears can detect sounds in the frequency range of 20Hz to 20kHz; however, sound conducts differently in air and water. The exact mechanism of human hearing underwater is debatable; however, a common assumption is that at high frequencies, bone conduction (propagation of sound to the inner ear via bones of the skull) is significant while at low frequencies tympanic conduction (propagation of sound via the outer and middle ear to the inner ear) kicks in. A diver’s headgear also contributes to underwater sound perception. An essential part of the specialized body suits necessary for diving, headgear is generally classified into helmets and hoods. Quite a few divers also dive bare-headed, as indicated in Figure 1. Headgear provides minimal attenuation from ambient noise but plays a greater role in determining the medium of sound propagation to the ears. Since helmets are enclosed structures in which the ears are surrounded by air, noise exposure would be perceived the same way as it would be on land, i.e., in air. On the other hand, hoods are made of breathable material, usually neoprene, and allow for water to be in contact with the ear. The presence of water in the ear canal changes the sensitivity of the ears which alters the mechanism of sound perception. Knowing whether sound perception occurs via conduction through air or water is integral to establishing the extent of hazards to divers.
The biological effects of noise on divers depends on the amount of exposure to noise and its frequency bandwidth.
Overall, the biological effects of noise on divers depends on the amount of exposure to noise and its frequency bandwidth. Generally, physiological effects range from annoyance, temporary dizziness, and disorientation to temporary or permanent hearing loss; in scenarios of severe exposure, divers are at risk of mechanical injuries to various parts of the ear. Several of the possible effects are harmless, in theory, but may cause panic and anxiety in divers, hampering their ability to handle such occurrences. In such cases, diving missions are generally aborted; this can be detrimental to occupational diving operations. Figure 2 summarizes the known effects of severe noise exposure and their subsequent impact on diver performance.
While several global standards define acceptable levels of occupational noise exposure, these legislations only take into account airborne sound perception. Many countries provide guidelines on safe diving practices but don’t recognize the potential underwater noise hazards. Under various occupational health & safety categories, numerous countries elaborate on the standards to be maintained in terms of the equipment required and procedures to be followed during diving activities. The presence of medical personnel and pre-diving medical examinations are also mandated, and in some cases post-diving check-ups are conducted. Divers are required to provide details of pre-existing medical conditions. On completion of a dive, they are expected to report any accidents or discomfort experienced.
The focus of these medical check-ups, is generally on pulmonary and physical discomfort faced by the diver. For occupational divers who have to dive into contaminated waters, directives on possible hazards of the chemical contaminants and large plastic waste found in the water bodies are also often described but associated ambient acoustic degradation is not considered. Another interesting aspect of the existing regulations is that many emphasize on the importance of environmentally sustainable diving practices in accordance with United Nations Sustainable Development Goals, and highlight the need for management of diving practices in a manner that doesn’t cause disturbances to the aquatic environment. This also corresponds with the widespread effort to combat the adverse effects of underwater acoustic degradation on marine mammals; the effects on humans are not commonly taken into account.
The human auditory system is one of the most complex physiological systems and there are various ethical obstacles in the way of observing as well as quantifying damage caused to divers from underwater acoustic degradation. Medical professionals rely on specialized equipment that computes Auditory Brain Stem Responses (ABR) to measure the response of the ears to sound. This equipment tends to be sensitive to artifacts and therefore is not feasible for use underwater. Several previous studies have made use of drifting hydrophones in tow with divers and attempted to generate complementary audiograms to deduce noise levels caused by different machinery and the divers themselves. From a perspective of bioethics, consecutively exposing divers to various levels of noise for experimental purposes would be objectionable.
The human auditory system is one of the most complex physiological systems and there are various ethical obstacles in the way of observing as well as quantifying damage caused to divers from underwater acoustic degradation.
There is a need to initiate a more serious conversation on the impact of acoustic degradation on divers given the subsequent consequences on other stakeholders of the maritime industry, such as the government, blue economy, defence sector, etc. At the Maritime Research Center (MRC), Pune, under the mission statement of our Underwater Domain Awareness (UDA) Framework, we are currently focusing on acoustic habitat degradation in the IOR and working on generating a better strategy to manage diving operations with a reduced risk of auditory biohazards.