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APPENDIX

Definitions relating to sound and sonar gear

Absorption. The loss of energy by a sound wave when it strikes an obstacle which does not reflect it completely.

Alternating current (A C). Electric current which flows first in one direction and then in the other.

Amplifier. A device which builds up a weak electric current into a stronger one.

Amplitude. The maximum extent of a vibrating movement. With sound, the greater the amplitude, the louder the sound.

Attenuation. The weakening of a sound wave as it travels, caused by losses due to friction, absorption, and scattering.

Audio frequency. Another-term for sonic frequency; that is, below 15,000 cycles per second. Compare Radio frequency.

Audio-frequency-amplifier. An amplifier for use with alternating currents of frequencies less than 15,000 cycles per second.

Background noise. Noise that tends to mask the sounds you want to hear. Chiefly water noise and noise from the receiver itself.

Band filter. An audio filter which suppresses all frequencies except those between a given high and low limit. For example, the band filter on the WCA gear suppresses all frequencies below 600 cycles and above 1,000 cycles.

Beam. A cone of sound waves, such as is used in echo-ranging. Normally, a beam can be obtained only by using supersonic frequencies.

Beat frequency. The frequency obtained by mixing two different frequencies. For example, by mixing 60,800 and 60,000 cycles, we obtain a beat frequency equal to their difference of 800 cycles. This is called heterodyning.

Cavitation. The formation of a series of vacuums when propellers are turning so rapidly that the water does not flow in immediately as the blade passes through. Propeller noise is greatly increased when cavitation occurs.

Compression. The part of a sound wave where the particles are packed together more closely than normal.

  Cycle. A complete sound wave compression-plus-rarefaction.

Decibel. A unit of measurement of intensity of sounds.

Detector. Generally, anything that enables sound to be heard; e.g. the ear. Specifically, in the receiver-amplifier, the detector mixes two supersonic frequencies to give a frequency that can be heard. Also called a "mixer."

Diffraction. The tendency of sound waves to bend around an obstacle in their path and meet somewhere beyond. This accounts for your being able to hear sounds around a corner. (Do not confuse with "refraction.")

Direct current (DC). Electric current that flows continuously in the same direction. A storage battery gives direct current.

Driver. An electrical device used to send a burst of supersonic sound (ping) into the water by means of the projector. In the WCA gear, there are a QB and a QC driver.

Frequency. Number of cycles per second.

Gyrocompass repeater. A device which repeats the movement of the master gyrocompass. The inner dial on the bearing indicator is a gyrocompass repeater dial. The main gyrocompass on a submarine is in the control room.

Heterodyning. The mixing of two frequencies to obtain the beat frequency, which is the difference between them.

Homogeneous water. Water in which the temperature does not change with depth.

Hydrophone. Any device for picking up sound waves from water. It is called a projector only when it also sends sounds out into the water.

Intermediate frequency (IF). The frequency into which the energy entering the superheterodyne receiver is converted in the intermediate stage. In WCA gear, this is 60 kilocycles.

Isothermal. Uniform in temperature. Homogeneous water is isothermal.

Kilocycle (kc). One thousand cycles.

 
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Magnetostriction. Change in size of a metal tube when subjected to an electric current. This principle is used in the JP hydrophone and in the QC and NM projectors.

Modulator. A device which causes a sound to change in pitch continuously. Sometimes used by surface ships in echo-ranging, when an echo is difficult to distinguish.

Negative thermal gradient. Decrease in temperature of water with depth.

Noise level. The volume of background noise heard in the headphones or on the loudspeaker.

Oscillator. A device that produces an alternating current at a particular frequency.

Piezoelectric effect. The development of an electric current when a Rochelle salt crystal changes in size. This principle is used in JK and QB projectors. Compare Magnetostriction.

Positive thermal gradient. Increase in temperature of water with depth.

Projector. The hydrophone portion of QC, QB, and NM gear. Although called a "projector," in submarines it is principally used to pick up sounds. In echo-ranging, it projects the ping into the water.

Quick beating. Bearing of the maximum loudness in sweeping across the target. Also called "Maximum bearing."

Radio frequency. Frequency above 15,000 cycles per second, Compare Audio frequency. Radio-frequency amplifier. An amplifier for use with alternating currents of frequencies higher than 15,000 per second.

Range rate. The rate in knots at which a target's range is changing. Useful in determining the speed of the target.

Rarefaction. The part of a sound wave where the particles are thinned out to less than normal density.

Resonance. The tendency of an object or an electrical circuit to respond well to a particular frequency, but poorly to other frequencies.

  Reverberations. The multiple echoes reflected from the surface, bottom, and many small irregularities in the water. In echo ranging, distinguished from the echo, which comes from the target.

Salinity. The saltiness of water. Normal salinity of sea water is 35 parts of salt per thousand.

Scattering. The loss of energy by a sound wave caused by its striking such irregularities in the water as seaweed, fish, and en trapped air bubbles.

Screen. The antisubmarine escort craft which are protecting a convoy.

Sonic frequencies. Frequencies less than 15,000 cycles per second.

Sound shadow. The region beyond an obstruction where a sound is not heard. See Diffraction.

Split bearing. A bearing obtained by computing the middle point between where the signal comes in and where it goes out in sweeping across the target. Split bearings are used only when quick bearing cannot be obtained.

Sweeping. Turning the projector or hydrophone so that it goes through the entire arc of the target's propeller sounds.

TDC. The Torpedo Data Computer-a device in the conning tower which figures out the correct firing data, using the facts of bearings, ranges, course, and speed.

Transducer. The technical term for what is generally called a "projector." A transducer operates both as a hydrophone (picking up sound vibrations and converting them into electric current pulses) - and as a projector (transforming electric current pulses into sound vibrations and projecting them into the water).

Vibration. A continuous back - and - forth movement, producing sound waves in the medium.

Wake. The ribbon of churned-up water astern of a moving ship or submarine. It consists of small currents, eddies and entrapped air bubbles.

Wave length. The distance from a given point on one compression to the corresponding point of the next compression of a sound wave.

 
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