Speaker Title - The Propagation of Sound
  Project Information Preparation for the Labs Reception of Sound Propagation through various materials Transmission in WaterConclusions

Transmission Background:

Sonar (SOund, NAvigation, and Ranging) is a technology that was developed for the military detection of submarines. It is also a mechanism that is used by many mammals, such as dolphins and whales, to search for food and for navigation.

Simply put, active sonar uses echoes to locate objects and their shape and movement. The device sends sound waves at regular intervals out into the water. The waves bounce off objects and some of them reflect back to the source of the sound wave.

Low Frequency Active Sonar (LFAS) was developed by the United States Navy to detect foreign submarines. In contrast to passive sonar, by which the Navy 'listens' for the sounds of distant ships, active sonar is a relatively new technology in military respects. LFAS works by emitting intense, low frequency sounds that echo off solid objects in water. The echoes are monitored by Navy ships to determine whether an object is a rock, a school of fish, or a submarine.

The speakers used by LFAS emit sounds as loud as 240dB and at frequencies of about 100 to 600Hz. In humans, exposure to sounds above 140dB cause hearing damage and sounds of 160dB cause instant deafness. Because low frequency sounds travel farther in water than sounds at higher frequencies, a ship operating LFAS can spread noise throughout thousands of square kilometres.

Even though sound acts differently in water than in air, it is fairly obvious that sounds of the magnitude emitted by LFAS is many times louder than the level known to disturb whales and dolphins. Whales and dolphins communicate with sound signals ranging in magnitude from 160 to 190dB.

Research has shown that artificially generated acoustic noise affects whales not only physiologically, but also psychologically. At intensities of 120dB, bowhead and grey whales begin to show avoidance behaviour. Humpback whales show cessation in 'singing' when exposed to 155dB, and sperm and pilot whales stop singing altogether when exposed to intensities of 220dB.

After military exercises involving the use of LFAS, beached whales were found washed up nearby and necropsies showed that they died from severe auditory trauma. In general, loud sounds cause whales to exhibit panic behaviour. Whales, like humans, also have air-filled sinuses and cavities of the middle ear. So when they dive to deeper waters (i.e. higher pressure) or ascend (i.e. lower pressure), they have to equalize the pressure inside their ears. When whales are panicked, they may dive or ascend too fast, and their equalization mechanism cannot work fast enough, and will cause pain, broken eardrums, bleeding in the ear, and death. Also, as loud noises essentially act like pressure waves, they can cause the same problems.

The tympanic bone, which encloses the middle ear cavity, resonates at a relatively low frequency compared to the other bones in the ear — at an estimated several hundred Hertz — as it is bigger than the ossicles. When something resonates at its specific resonating frequency, the amplitude of the vibrations increase rapidly. If the tympanic bone is affected by a sound at its resonating frequency, it is possible that tissue in the middle ear be damaged because of the increase in the contraction and expansion of the air space in the middle ear due to the vibration of the bone.

The tympanic bone is not only thing that resonates. In fact, when the frequency matches the mammal's resonance frequency, resonance would occur within any air space. The most severe of all effects is the resonance of air space around a whale's brain. As the resonance continues, the brain tissue continues to stretch, expand, vibrate, breaking capillaries causing severe internal bleeding, finally killing the whale. Studies done by Dr. Kenneth Balcomb concludes that intensities as low as 180dB, at frequencies of 470 ~ 590Hz can easily cause resonance within whales. The distance that frequencies travel is extremely far; after testing is done close to California, beached whales linked to the testing can be found as far away as Greece.

Resonance is not the only way whale populations are affected. Any man-made low frequency noise of above 120 dB can cause behavioural changes in whale population.

Aside from marine mammals, other species of wildlife may be affected by low frequency noise. The use of LFAS may impair the growth and reproduction of other species such as shrimp, or change the distribution of sharks and threaten sea turtles and fish.

Variables:

Manipulated: Frequency of sound
Responding: Sound intensity loss
Controlled: Distance sound travelled

Extra Materials (In Addition to Materials List):

Constructed lab apparatus
Potassium chloride (KCl);
Distilled water (H2O)
Potassium iodide (KI);
Sodium chloride (NaCl);

Procedure (In Addition to General Procedure):

1. Mix the distilled water, sodium chloride, potassium iodide in proportions similar to real sea water.
2. Follow General Procedure steps 2 to 6.