06. UNDERWATER NOISE

The Silent World”. This was the title of Jacques-Yves Cousteau’s first underwater feature film in 1956.

However, contrary to this, the underwater world is teaming with noise; more precisely, acoustic waves that sea life recognize, and what we call sound when the human ear decodes the waves.

Sound waves transmit through matter at a velocity that increases with the matter’s density, and decreases as the matter’s elasticity increases. For example, on a planet without an atmosphere, such as the moon, sounds do not propagate. On earth sound moves at a rate of approximately 1,230 km/h / 770 mph at sea level with a temperature of 20°C / 68°F. Due to its higher density compared to air, sound moves much faster through freshwater (also at a temperature of 20°C / 68°F); approximately 5,335 km/h / 3,315 mph.

In seawater, the speed of sound is slightly faster, again due to its increase in density.

Ultimately, the speed of sound in seawater will be affected by pressure, temperature and salinity, and thus changes slightly from location to location. In general terms, the speed of sound in seawater increases by approximately: 3m/sec / 7mph for each additional degree Celsius in temperature, 0.81m/sec / 1.8mph for each increase in salinity by 0.1%, and 0.75m/sec / 1.7mph for each additional atmosphere of pressure.

Like sound, all movement in water produces a pressure differential in the form of a wave. Fish are predisposed to sensing and interpreting these pressure changes via a system of sensing organs called the lateral line. Using these organs, fish are able to decipher if there is danger, or a predator nearby and if so, to appropriately respond. In most cases, divers are not predators, but they do generate “noise” by way of acoustic alarms, excessive movement, the release of exhaust bubbles and/or collisions with other objects.

These examples of a diver’s “noise” warn nearby inhabitants of the diver’s presence, and give them opportunity to appropriately react. Diver’s bubbles commonly cause sea life to stay a fair distance from divers. This is why many underwater photographers and videographers choose to dive using rebreathers that emit little to no bubbles. SNSI offers a rebreather program for divers interested in becoming certified in their use.

It is in a diver’s best interest to imitate fish movement as closely as possible. Doing so helps a diver maintain optimal buoyancy, saves energy, which decreases gas consumption, and increases underwater efficiency. It also cuts down on the “noise” that divers emit into their surroundings, which increases the likelihood of seeing more sea life.

M

Advanced Open Water Diver