Updated: Sep 8, 2020
Article by GWC's board member and scientific advisor, Lenard Milich, PHD.
Imagine the loudest sound you might possibly hear. How about going to see a rocket launch? That would have been, for the Space Shuttle, approximately 120 decibels (dB, a measure of sound intensity) 3 miles from the launch site. If you would have been unfortunate enough to be in the vicinity of the atom bomb dropped on Hiroshima, well, sound meters placed just 100 meters from a nuclear bomb test site registered a peak of 210 dB. The intensity of the sound alone would be enough to kill a human being, so if the bomb doesn't kill you, the noise will. Krakatoa, the volcano that erupted in 1883 between the islands of Java and Sumatra in today’s Indonesia, and which caused extreme winters for the next four years: 180 dB, a blast that could be heard in Australia, 1930 miles away.
Doctors place 115 dB as the threshold at which hearing damage occurs.
So, having put a relative scale on this, consider that seismic exploration surveys, undertaken in the mad search for yet more climate-destructive fossil fuels, generate the loudest human sounds in the ocean short of standing right next to an explosive, can reach more than 250 dB, and be heard for miles — most probably, across entire ocean basins. The low frequency sounds generated by seismic airguns, commonly below 200 Hz in frequency, have been recorded at locations up to 4,000 km from the source, and can ‘blanket’ areas of up to 300,000 km2 with noise. Generally, the dominant frequencies of seismic airguns overlap with those of the communication signals of baleen whales (10 Hz–1 kHz).
These blasts can and certainly do cause hearing loss in marine mammals, disturbing essential behaviors such as feeding and breeding, mask communications between individual whales, and obviously if the whales are nearby, cause massive concussive damage, severely damaging their hearing and pounding their bodies with a sound wave that, sadly, in many cases, will be unsurvivable.
The inner ear of cetaceans works in the same way as that of terrestrial mammals. The differences lie in the inner ear characteristics. The major differences are the number of auditory ganglion cells, the ratio of the number of ganglion cells to the number of hair cells, the size of the auditory nerve, the size of the basilar membrane, and the support of the basilar membrane. Toothed whales have more ganglion cells associated with hearing than terrestrial mammals. Baleen whales have fewer nerve cells associated with hearing compared to toothed whales, but more than terrestrial mammals. Cetaceans also have a lot more ganglion cells associated with each hair cell than do humans and a much larger auditory nerve. All of these adaptations mean that cetaceans engage in more complex auditory processing than do we humans. Unlike humans who learn to use sign language if deaf, whales do not have this option. Damage to these sensitive cilia and nerves will have profound negative consequences. Compromised hearing – a sense so crucial for the whales’ survival – almost assuredly results in their death.
Moreover, cetaceans’ middle ears, like ours, are air-filled. So, loud sounds – which can be thought of as being an intense pressure wave – can cause what is medically termed barotrauma. Significant barotrauma is often associated with permanent complications such as hearing and balance deficits. Imagine being a whale with damaged balance! Furthermore, such intense sounds are likely to have other detrimental physiological effects: human divers exposed to sounds louder than 154 dB in low-frequency ranges are reported to have experienced changes in their heart rates or breathing frequency.
Naval sonar, which can retain an