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SOUND IN WATER

Sound can travel through any medium, such as air or steel or water. As a sonar operator, you are concerned with water as the medium.

 

Illustration of compression.

Illustration of rarefaction.

  How sound waves get started

Imagine an object vibrating back and forth in water. As it moves forward, the particles of water directly in front of it are pressed closer together. Each particle then passes this pressure along to the one ahead of it. Thus a state of compression moves away from the object in all directions.

But when the object moves backward, this pressure is removed and the particles are thinned out. Thus a state of rarefaction follows after each compression.

The vibrating object continues to send out compressions and rarefactions one after the other. Each compression plus - rarefaction is a sound wave.

 

Illustration of waves, with one cycle identified.

Cycle. A single back-and-forth movement of the vibrating object is called a cycle. A single sound wave (as shown in the drawing above) is also called a cycle.

Frequency. The number of cycles per second is the frequency. For convenience, this is sometimes expressed in kilocycles (1000 cycles). A frequency of 16 kilocycles (16 kc), for example, means 16,000 cycles per second.

 
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How we hear sounds

When sound waves reach us through the air, their alternate compressions and rarefactions cause the eardrum to move in and out at the same frequency. By a chain of tiny bones, this vibration is transferred to the inner ear. Here it is changed into nerve impulses, which pass to the brain and produce the sensation of hearing.

But for the sonar operator, the process includes some additional steps. The sound waves in the water must be picked up by some sort of hydrophone and changed into electric currents. These electric currents are then strengthened and sometimes changed, so that they can by heard through the headphones.

Illustration of bell making sound, and an ear hearing it.

Sonic and supersonic frequencies

Illustration of spectrum, sonic below 15,000 cycles; supersonic above 15,000 cycles.

SONIC FREQUENCIES are those below 15,000 cycles per second. They can be heard by the normal human ear. SUPERSONIC FREQUENCIES are those above 15,000 cycles per second - beyond the range of normal human hearing.

Supersonic frequencies must be changed to sonic frequencies before the operator can hear them in his headphones or loudspeaker. Within the sonic range, the pitch of a sound depends mainly upon its frequency. A high-frequency sound has a high pitch; a low-frequency sound, a low pitch. The loudness of a sound depends mainly upon the strength of the compressions. The more powerful they are, the louder the sound. But loudness is also somewhat dependent upon frequency. The normal ear hears best between 1000 and 2000 cycles, and sounds in this frequency range generally seem louder than sounds of extremely high pitch.
 
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How sound waves behave in water

The behavior of sound waves in water is a rather complicated subject. Here are some of the most important features to remember.

Illustration of varied temperature in water. Sound waves travel faster in Warmer Water, Higher Pressure, Saltier Water. Velocity

Sound waves travel very rapidly in water-about 4800 feet per second, as compared with 1100 feet per second in air. Weak sounds and strong sounds, high frequencies and low frequencies, all travel at the same speed. But their speed is affected by the temperature, pressure, and salinity of the water, as shown at the right.

Refraction

The fact that the speed of sound varies, especially with temperature, explains why sound waves are bent out of their normal paths. This bending is called refraction. Usually water is warmer near its surface than at lower depths. As shown in the diagram at the right, the upper part of a sound wave in the warmer water travels faster than the lower part in the colder water. This makes the sound wave bend downward.

Transmission loss

The sound that reaches the hydrophone is very much weaker than it was when it left its source. Two main factors explain this loss during transmission.

1. Spreading. As a sound wave goes out from its source in all directions, it spreads over a larger and larger area. Thus a given amount of sound has to cover an increasingly large space, and it gets thinner and thinner.

2. Attenuation. This term covers the weakening of the sound from a number of other causes. As the water particles move back and forth in the compressions and rarefactions of sound waves, they rub against one another. Some of their original strength is used up in friction. Also, during their travel, sound waves may hit air bubbles, seaweed, fish, the ocean surface, or other obstacles. Some of the strength of the sound waves is absorbed by these obstacles; some is scattered in other directions so that it never reaches the hydrophone.

Most important to sonar is the fact that attenuation is greater with higher frequencies. For this reason, supersonic sounds lose strength more quickly than sonic sounds and therefore cannot travel as far.

 
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Sonic and supersonic sounds from ships

It has already been mentioned that submarines are equipped with sonic and supersonic listening gear. Both kinds are necessary to pick up all the sounds we are interested in hearing.

Propellers generate a wide band of sonic and supersonic frequencies. Consequently, they can be detected with either type of gear.

Ship machinery noises are mainly in the sonic range. Sonic listening is necessary, not only to catch these sounds from enemy ships, but also to locate and identify noises from your own submarine that might give you away to enemy escort vessels.

Enemy echo-ranging, that is, sound signals sent out by enemy escort vessels in searching for submarines, is supersonic, and can be heard clearly with supersonic gear.

Slapping of waves and the sounds of surf pounding on a beach are largely in the sonic range. This is also true of most sounds made by fish and marine animals.

Only with a combination of sonic and supersonic
listening can we be sure of hearing all of these sounds.

 
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