Chapter 4 — PHYSICS OF SOUND 



Medium 



Sound waves are passed along by particles 

 of the material through which they travel. The 

 elasticity of the medium determines the ease, 

 distance, and speed of sound transmission. The 

 greater the elasticity, the greater the speed 

 of sound. The speed of soimd in water is about 

 four times that in air, for example; in steel, 

 it is about 15 times greater than in air. 



Detector 



The detector acts as the receiver of the 

 sound wave. Because it doesn't surround the 

 source of the sound wave, the detector absorbs 

 only part of the wave's energy, thereby usually 

 requiring an amplifier to boost the signal's 

 energy to permit reception of weal: signals. 



THE EAR AS A SOUND DETECTOR 



The limits of human hearing are determined 

 by two interacting physical variables, frequency 

 and intensity, and several other conditions that 

 are dependent on the individual variables (in- 

 cluding age, state of health, attention, and prior 

 exposure). The limits of hearing, however, are 

 normally between 20 and 20,000 hertz for young, 

 healthy persons, provided the upper and lower 

 frequencies are sufficiently intense. For healthy, 

 middle-aged persons, however, the upper limit 

 may lie between 12 and 16 l<Hz, the low-frequency 

 threshold remaining about 20 Hz. For purposes 

 of this text, sounds capable of being heard are 

 called sonics. Sounds below 20 Hz are called 

 subsonics, and those above 15 kHz are known 

 as ultrasonics or supersonics. The term "ultra- 

 sonic" merely refers to acoustic phenomena 

 above the level of human hearing. A 15-kHz 

 vibration might be "ultrasonic" for the average 

 60-year old person. 



Early active sonai* equipments transmitted 

 ultrasonic sounds through the water. Along with 

 other sounds, they received echoes of these 

 ultrasonic sounds and converted them into audible 

 ones. Modern active sonars transmit sounds 

 within the audible range. Because these trans- 

 missions are very high tones, however, the 

 equipment converts them into lower ones better 

 suited for listening. 



The human ear is a good sound detector. 

 It can detect a wide range of frequencies, but 

 does not respond equally well to all of them. 

 Figure 4-2 illustrates the variation in the amount 

 of power that is barely audible to the average 



RELATIVE POWER FOR AUDIBLE SOUNDS 



100 1 000 10.000 



FREQUENCY HERTZ 



100,000 



10,000 



1000 



100 



10 



I 



71.46 

 Figure 4-2. — Frequency sensitivity of the 

 human ear. 



ear at different frequencies. It is evident from 

 this chart that the ear is most sensitive to 

 frequencies in the range from about 1000 Hz 

 to 2000 Hz. The average person finds an 800-Hz 

 note a rather pleasant one to listen to for long 

 periods of time. In underwater echo ranging 

 equipment, the echo frequency commonly is 

 converted to an 800-Hz note. 



Never turn up the volume on sonar equipment 

 so that echo sounds are louder than necessary. 

 One reason is that the ear is not a sensitive 

 detector of relative changes in sound intensity. 

 An increase or decrease of about one-fourth 

 of the total power must take place before the 

 ear notices any difference. 



An intensely loud sound slightly paralyzes 

 the ear, reducing its ability to hear low-intensity 

 sounds that follow immediately. This effect is 

 similar to one you probably have experienced with 

 light. If you look into a very strong light, your 

 eyes are blinded momentarily. 



The ear is a sensitive detector of change in 

 pitch. An average person can tell when sounds 

 differ a few cycles in pitch, even though they 

 cannot detect the change in intensity. This faculty 

 is knoAvn as pitch discrimination. It is a great 

 help in selecting from a background of reverbera- 

 tions a submarine echo of slightly different 

 pitch, that is, one with doppler. 



Here's a summary of your ear's character- 

 istics: 



1. Your ear does not readily detect small 

 relative changes in sound intensity. It is not 

 sensitive to high and lo^ audible frequencies. 



35 



