Let’s first briefly talk about how sound travels underwater. It is said that sound travels best in dense mediums because of the more closely packed molecules. Water, which is far denser than air, helps sound to better. As a result, the speed of sound increases by about four times underwater. Since sound travels at a much faster rate underwater, the direction from which a sound originally came, cannot be easily ascertained.
Types of underwater sounds:
Natural sounds
Natural sounds underwater can originate where the earth’s tectonic plates collide, or be caused by underwater landslides, icebergs breaking off, earthquakes, heavy rains on the water surface and animals
Ocean animals makes noises while swimming, searching for food, sending defense or attack warning signals, sending a mating call, when they are frightened and when they communicate with each other.
Man-Made sounds
Man-made sounds underwater can originate from large freighters in crowded shipping lanes, underwater drilling for oil and underwater explosions, crews laying communications cables and sonar devices used by the military.
How Humans process sounds
Humans perceive sound under water by bone conductivity, which is the vibration of the bones of the skull (Physics). Sound waves enter the ear and strike the tympanic membrane or eardrum, which vibrates the bones of the middle ear. The vibrations are transmitted to the fluid in the inner ear and then vibrate the hair cells that line it. The hair cells are connected by neurons to the auditory nerve, which transmits the signal to the brain. The number of hair cells stimulated determines the volume of the sound and the distribution pattern of stimulated cells determines the pitch of the sound.
Human ears are unable to pick up the frequencies of many of these underwater animal sounds because our hearing threshold is only between 20 and 20,000 Hz. Anthropogenic sounds, such as submarine sonar can be dangerous to divers’ hearing.
A whale’s ear and human ear are similar, save for the fact that a whale’s hearing is 50% more acute than a human’s.
As the picture above shows, the anatomies of both species ears are remarkably similar.
Since submarine sonar and other man-made noises can be dangerous to humans, what effect would you think that it would have on a species whose hearing is 50% more acute than human hearing?
Other points to consider:
It may also be prudent to consider since a whale has evolved to dive for long stretches of time, typically 1-2 hours at a time. In order to do this a whale prepares itself for the dive by spending 10 minutes or more clearing its lungs, blowing a breath in and out every 12 seconds. It’s getting rid of old carbon and loading up with fresh oxygen. Since the whale can and typically does dive deep and has evolved to be able to hold its breath for such prolonged periods of time, it’s safe to assume that it wouldn’t complete such a rigorous preparation for a 30 minute or less dive. Would you prepare for a week for a trip that is only going to take 1 day to make? It be prudent to do so. With that in mind imagine that you have completed your preparation and you make your long anticipated dive, you get 800 feet down in your dive and all of a sudden you are inundated with a wall of sound which appears to be coming from everywhere, so you swim away from the deafening sound.
Since whales navigate using sonar, if they are unable to hear, how are they able to determine the best escape route? They aren't able to do so, which most often results in them beaching themselves as a result. Another thing that one might want to make note of is that when human divers ascend too quickly from a dive, they get what is called the bends. Divers get the bends or decompression sickness when they have been on a deep dive and do not stop in their ascent at regular intervals to decompress.
Small holes are abnormal tissue damage from nitrogen bubbles that form when the animals rise to the surface too quickly.
The worlds waterways are becoming increasingly polluted with sound- man made sound and as a result, I fear that these beaching occurrences will increase.
