NATURE OF SOUND 



which was communicated to the air as acoustic 

 energy. 



The foregoing example illustrates the general proc- 

 ess by which sound is generated and detected. A 

 source of sound converts mechanical or electrical 

 energy into energy of vibration and communicates 

 this energy to the surrounding medium as acoustic 

 energy. This acoustic energy travels through the 

 medium to the receiving instrument where it is de- 

 tected. 



1.2.2 Production and Reception 

 of Sound 



Most types of sonar gear produce sound by con- 

 verting electrical energy into acoustic energy and de- 

 tect sound by converting acoustic energy into elec- 

 trical energy. They do this by making use of one of 

 two effects, jnagnetostriction or the piezoelectric effect. 



When certain metals, such as nickel, are placed in a 

 magnetic field, they contract (or expand) in the direc- 

 tion of the field; conversely, when they are subjected 

 to a contracting (or expanding) force they become 

 partially magnetized. Thus, if a nickel rod is made 

 the core of a solenoid and if it is given a permanent 

 magnetization by means of a direct current, then an 

 alternating current passed through the winding will 

 cause the magnetization to increase and decrease 

 with the frequency of the current. As a result, the rod 

 will contract and expand or, in other words, vibrate 

 with the frequency of the impressed current. In this 

 arrangement, electrical energy is converted into 

 acoustic energy which is passed into the surrounding 

 medium. Conversely, if a sound wave hits this instru- 

 ment and causes the nickel rod to alternately expand 

 and contract, the rod will be magnetized and demag- 

 netized rhythmically, thus inducing an electromotive 

 force in the surrounding solenoid. The resulting alter- 

 nating current may be amplified and ultimately re- 

 corded in one form or another. Such a magnetostric- 

 tion transducer may thus be used both as a source of 

 sound and as a receiver of soimd. 



Certain crystals, such as quartz, Rochelle salt, and 

 ammonium dihydrogen phosphate, exhibit the piezo- 

 electric effect. If a slice is cut from such a crystal and 

 if an electric potential difference is applied across 

 such a sKce, the crystal will either contract or expand, 

 depending on which of the two faces is electrically 

 positive. Conversely, if such a slice is compressed or 

 expanded mechanically, the two opposite faces will 

 develop a potential difference. Thus, a piezoelectric 



crystal, or an array of such crystals, may be used as a 

 transducer. If an alternating voltage is applied to the 

 opposite sides of the crystal shce, it will vibrate with 

 the frequency of the applied voltage; and if it is 

 placed in a fluid where the pressure is fluctuating, it 

 will develop a fluctuating emf across its faces. 



Other important sources of waterbome sound are 

 underwater explosions, ships, submarines, waves, 

 underwater ordnance, and biological sources. 



1.2.3 



Propagation of Sound 



Chapters 1 through 10 are concerned with the prop- 

 agation of sound in the ocean. The complexity of this 

 problem is due to the great variabiHty of the mechani- 

 cal properties of the medium in which the propaga- 

 tion takes place, but the basic underlying physical 

 concepts are fairly simple. These principles are dis- 

 cussed in the following sections. 



Direction of Propagation 



Soimd energy is propagated away from the source 

 into a medium. If a single pulse of sound is considered, 

 such as that produced by a sudden explosion, the 

 course of the sound energy in the medium can be fol- 

 lowed by placing a large number of recording micro- 

 phones in the general vicinity and by noting the 

 times at which they show the first response. Each will 

 respond at a slightly different time. Some, placed be- 

 hind obstructions, may not respond at all. 



By using a sufficiently large number of such micro- 

 phones, we can record all those points in space which 

 are reached by the spreading sound pulse at the same 

 time. We shall call the surface on which these points 

 are located a sound front (a better expression will be 

 introduced later). The progression of the pulse in 

 space may then be described by a succession of sound 

 fronts along with the statement of the time at which 

 each front is activated. If the medium of propagation 

 is homogeneous, the perpendicular distance between 

 two sound fronts is proportional to the time it takes 

 the sound pulse to travel from one to the other. In 

 other words, in a homogeneous medium sound travels 

 at a constant speed in a direction perpendicular to the 

 sound front. This direction is called the direction of 

 propagation. 



These simple rules apply only if the sound beam 

 meets no obstructions. If an obstruction is placed be- 

 tween source and microphone, the microphone usu- 

 ally registers some sound, but with a delay indicating 



