INTRODUCTION TO SONAR 



Suppose the wavelength is 0.4 foot and the 

 frequency is 12 kHz. What is the velocity? 



0.4 = 



Velocity. 

 12,000 



Expressed another way, velocity = 0.4 x 12,000 = 

 4800 fps. If the wave length and the velocity are 

 known, the frequency can be found in a similar 

 manner. 



0.4 



4800 



Frequency 



QUALITY 



The quality of a sound depends on the com- 

 plexity of its sound waves. Most sounds consist 

 of a fundamental frequency (called the first 

 harmonic), plus severalother frequencies that are 

 exact multiples of the fundamental. The fimda- 

 mental is the lowest frequency component of the 

 sound wave. By combining different fundamentals 

 in suitable proportions, a tone can be built up to 

 a desired quality. Musical tones are produced 

 by regular vibrations of the source; when the 

 source vibrates irregularly, the sound is called 

 noise. By sounding together the proper organ 

 pipes, any vowel sound can be imitated. On the 

 other hand, drawing a fingernail across a black- 

 board creates a noise of a different sort. 



DECIBELS 



Or, frequency = 4800 -^ 0.4 = 12 kHz. 



CHARACTERISTICS OF SOUND 



Sound has three basic characteristics: pitch, 

 intensity, and quality. Together they make up 

 the tone of a sound. With the proper combina- 

 tion of characteristics, the tone is pleasant. 

 With the wrong combination the sound quality 

 degenerates into noise. 



PITCH 



An object that vibrates many times per second 

 produces a sound with a high pitch, as in the 

 instance of a whistle. The slower vibrations of 

 the heavier wires within a piano cause a low- 

 pitched sound. Thus, the frequency of vibration 

 determines pitch. When the frequency is low, 

 sound waves are long; when it is high, the waves 

 are short. 



INTENSITY 



Intensity and loudness often are mistaken as 

 having the same meaning. Although they are 

 related, they actually are not the same. Intensity 

 is a measure of a sound's energy. Loudness is the 

 effect of intensity on an individual, in the same 

 manner that pitch is the effect of frequency. 

 Increasing the intensity causes an increase in 

 loudness, but not in a direct proportion. To double 

 the loudness of a sound requires about a tenfold 

 increase in the sound's intensity. 



Throughout your Navy career as a Sonar 

 Teciinician, you will be using decibels as an 

 indicator of equipment performance. Power output 

 and reception sensitivity of a sonar equipment 

 are measured in decibels, which are used to 

 express large power ratios. 



In the decibel system, the reference level is 

 zero decibel (0 db), which is the threshold of 

 hearing. The pressure level, or signal strength, 

 of underwater sounds is compared to the db 

 level, and is given either a positive or a negative 

 value. Sonar receivers are capable of detecting 

 sounds having a signal strength below the db 

 level. In such instances, the signal is given a 

 minus db value. 



The reason for using the decibel system when 

 expressing signal strength may be seen in table 

 4-1. It is much easier to say that a source level 

 has increased 50 db, for example, than it is to 

 say the power output has increased 100,000 times. 

 The amount of power increase or decrease from 

 a reference level is the determining factor — not 

 the reference level itself. Whether power output is 

 increased from 1 watt to 100 watts, or from 

 1000 watts to 100,000 watts, it still is a 20-db 

 increase. (See table 4-1). 



Examine table 4-1 again, and take particular 

 note of the power ratios for source levels of 

 3 db and 6 db (also 7 and 10 db). It can be seen 

 that to increase a sonar's source level by 3 db, 

 it is necessary to double the output power. As 

 a typical example, if the sonar source level 

 drops from 140 db to 137 db, the sonar has lost 

 half its power. Any 3-db loss, no matter what 

 the source level, means loss of half the former 

 power. 



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