ACOUSTIC DISPERSION . Acoustic dispersion is the 

 change of speed of sound with frequency. (9) 



ACOUSTIC ENERGY . That form of energy which is 

 propagated through a medium having distributed 

 mass and elasticity, and which is made manifest, 

 at any point, as modifications in the mechanical 

 strain and stress which would exist at that point 

 in the absence of this energy. (4) 



ACOUSTIC IMPEDANCE . The acoustic impedance of a 

 given surface area of an acoustic medium perpen- 

 dicular, at every point, to the direction of prop- 

 agation of sinusoidal acoustic waves of given fre- 

 quency, and having equal acoustic pressures and 

 equal volume velocities per unit area at every 

 point of the surface at any instance, is the quo- 

 tient obtained by dividing (1) the phasor corre- 

 sponding to the acoustic pressure by (2) the 

 phasor corresponding to the volume velocity. (4) 



ACOUSTIC INTENSITY . The limit approached by the 

 quotient obtained by dividing the power of the 

 acoustic energy being transmitted at a given time 

 through a given area by the magnitude of this area 

 as the magnitude of this area approaches zero. (4) 



ACOUSTIC INTENSITY PER UNIT BAND . The limit ap- 

 proached by the quotient obtained by dividing (1) 

 the intensity of the acoustic energy being trans- 

 mitted through a given area, at a given time and 

 in a given frequency band, by (2) the width of this 

 band as the width of this band approaches zero. (4) 



ACOUSTIC INTERFEROMETER . An acoustic interferome- 

 ter is an instrument for making physical observa- 

 tions upon standing waves. It may be used, for 

 example, to measure velocity, wave length, absorp- 

 tion, or impedance. (9) 



ACOUSTIC OCEAN-CURRENT METER . This device is es- 

 sentially two sing-around velocimeters in which the 

 directions of pulse transmission are opposite in 

 sense. The instrument is oriented so that the 

 direction of acoustic pulse transmission is paral- 

 lel to that of the flow to be measured, and so that 

 the time of pulse translation is greater in one 

 velocimeter than the other. The difference in sing- 

 around frequencies is then proportional to ocean- 

 current velocity. The current flow measurement is 

 independent of variations in the velocity of sound. 

 The instrument contains the necessary electronic 

 circuitry to extract the signal with frequency 

 proportional to flow velocity. (30) 



ACOUSTIC OHMS . Acoustic impedance is measured in 

 acoustic ohms. One acoustic ohm is equal to one 

 gm/cm^ sec, or to one dyne sec/cm-". (4) 



ACOUSTIC PHASE CONSTANT . The acoustic phase con- 

 stant is the imaginary part of the acoustic propa- 

 gation constant. The commonly used unit is the 

 radian per section or per unit distance. (2) 



ACOUSTIC PROPAGATION CONSTANT . The acoustic propa- 

 gation constant of a uniform system or of a section 

 of a system of recurrent structures in the natural 

 logarithm of the complex ratio of the steady-state 

 particle velocity, volume velocities, or pressures 

 at two points separated by unit distance in the 

 uniform system (assumed to be of infinite length), 

 or at two successive corresponding points in the 

 system of recurrent structures (assumed to be of 

 infinite length) . The ratio is determined by 

 dividing the value at the point nearer the trans- 

 mitting end by the corresponding value at the more 

 remote point. (2) 



ACOUSTIC RADIATION PRESSURE . The acoustic radia- . 

 tion pressure is a unit directional steady-state 

 pressure exerted upon a surface exposed to an 

 acoustic wave. Such a steady pressure is usually 



quite small in magnitude and is readily observable 

 only in the presence of very intense sound waves. 

 (9) 



ACOUSTIC RADIOMETER . An acoustic radiometer is an 

 instrument for measuring acoustic radiation pres- 

 sure by determining the unidirectional steady- 

 state force resulting from reflection or absorp- 

 tion of a sound wave at its boundaries. (9) 



ACOUSTIC REFRACTION . Acoustic refraction is the 

 process by which the direction of sound propaga- 

 tion is changed due to variations in the speed of 

 sound in the medium from point to point. Refrac- 

 tion then is due to a nonuniformity of the medium 

 itself. (9) 



ACOUSTIC SCATTERING . Acoustic scattering is the 

 irregular reflection, refraction, or diffraction 

 of a sound in many directions. (9) 



ACOUSTIC SOUNDING . The indirect evaluation of 

 water depth, using the principle of measuring the 

 length of time necessary for a sound wave to travel 

 to the bottom, reflect and travel back to the water 

 surface. (27) 



ACOUSTIC SPECTROGRAPH . An instrument used to 

 analyse the acoustic transmittive and reflective 

 powers of marine life and thermal layers in terms 

 of their effects on particular acoustic frequencies. 



ACOUSTIC SYSTEM MARK 1. MOD 4 . The Acoustic System 

 Mark 1, Mod 4 measures and records very low- fre- 

 quency sounds from the low audio ranges down to 

 static pressures. The pressure-sensitive element 

 of this system is essentially an inductance which 

 is varied by changes in pressure and is contained 

 in one arm of an a.c. bridge located on shore. 

 The bridge is driven by a 1,000 c.p.s. oscillator 

 that has good amplitude and frequency stability. 



An incremental change in pressure applied to 

 the pressure-sensitive element varies the a.c. 

 output of the bridge. This output is combined with 

 an additional output from the balancing network, 

 and after amplification the combined output is 

 demodulated . The output from the demodulator is 

 filtered and appears as a voltage that varies in 

 accordance with the pressure changes on the under- 

 water element. Frequencies below 1.5 c.p.s., after 

 amplification by a d.c. amplifier, are recorded on 

 a milliammeter . This meter also is used to deter- 

 mine the state of balance of the bridge. 



The Acoustic System Mark 1, Mod 4 can operate 

 to a depth of 200 feet and uses cable lengths as 

 much as several miles. This system has been used 

 extensively to obtain wave records over long 

 periods of time off several shorebased stations. 

 (35) 



ACOUSTIC THEODOLITE . An instrument designed to 

 provide a continuous vertical profile of ocean 

 currents from the bottom to the surface in a 

 specific location. 



ACOUSTICS. Acoustics is the science of sound, 

 including its production, transmission, and 

 effects. (1) 



ACRE-FOOT . The amount of water required to cover 

 1 acre to a depth of 1 foot: equal to 43,560 cubic 

 feet. 



ACS. American Chemical Society. 



ACSM. American Congress on Surveying and Mapping. 



ACTINIC . Pertaining to electromagnetic radiation 

 capable of starting photochemical reactions. 



ACTINOMETER . The general name for any instrument 

 used to measure the intensity of radiant energy. 



