112 



USRL TEST STATIONS 



100 TO 800 KC HEAD (WITH 

 STORAGE COVER REMOVED) 



COAXIAL CONDUIT PIPES 



300 TO \ COUPLING NETWORK HOUSING^HYDROPHONE PRE-AMPLIFIEP> 



2200 KC HEAD^ WITHDRAWN FROM HOUSING 



Figure id. High-frequency transducers. 



one measurement on lenses, standing waves were 

 generated between the projector and the first face of 

 the lens and between the lens' faces. 



It can be shown that, if a condition for interfacial 

 standing waves exists and a frequency sweep is made 

 with a sinusoidal source of sound, the response of the 

 hydrophone as a function of frequency will be com- 

 posed of a series of alternate maxima and minima. 

 The frequency difference. A/, between maxima and 

 minima may be shown to be A/ = c /2d, where c is the 

 velocity of sound and d is the distance between the 

 reflecting surfaces. It follows that, if the sound source 

 is composed of a band of frequencies greater than A/, 

 the standing waves will be broken up. This is the pur- 

 pose of the noise generator mentioned previously. 

 On the basis of a 6-kc band for this noise generator, 

 the minimum distance that can be tolerated is 12.5 

 cm. Actually, this figure must be modified by the 

 fluctuation allowed in the recorded data. Another 

 method of eliminating standing waves is to separate 

 the instruments until the normal acoustical losses 

 reduce the reflections to a negligible intensity. 



It is important to note that the above expression 

 for A/ provides an excellent analytical tool for cer- 

 tain observations and gives a basis for analyses of the 

 geometry which is involved in a particular experi- 

 mental set-up. 



Standard Transducers. While the transducers have 

 been described in detail elsewhere, 07 it is best to point 

 out some of their novel features which are definite 

 aids in making measurements at these frequencies. 



The instruments consist of three sections: (1) sound 

 piston and coaxial leads, (2) network housing, and 

 (3) networks. Any one of the three parts is inter- 

 changeable with parts from other instruments. The 

 connections between parts are kept watertight by 

 rubber gaskets. The tops of the pipes containing the 

 coaxial leads have coaxial jacks into which fit coaxial 

 plugs on the bottom of the network assemblies, and 

 the assemblies have coaxial jack outputs for connec- 

 tions to the junction boxes. These connections are 

 made by means of coaxial patch cords. 



The networks serve the purpose of matching the 

 transducers to the transmission lines. There are two 

 such networks available; one is a hydrophone pre- 

 amplifier, and the other is a transformer for use with 

 the projector. The two coupling networks have been 

 described. 



Electroacoustical Measurements 



The technique of measurement with the high-fre- 

 quency system parallels to a great extent those used 

 in the lower frequency systems. However, the fre- 

 quencies and wave lengths involved necessitate a 



