28 



TYPES OF ACOUSTIC MEASUREMENTS 



in decibels versus 1 volt, generated by the unit in a 

 uniform plane-wave, free sound field of reference 

 pressure (1 dyne per sq cm) propagated parallel to the 

 acoustic axis of the hydrophone. 



The selection of the open-circuit voltage has the ad- 

 vantage that, with a few exceptions noted below, it is 

 possible to compute the signal voltage across any load 

 impedance when the open-circuit voltage e,, and the 

 impedance z of a projector or a hydrophone are 

 known. For instance, if the load impedance is z L , the 

 voltage across it is 



z + z L 



For certain types of hydrophones, designed for spe- 

 cial purposes or including a preamplifier of the cath- 

 ode-follower type, it is desirable to state the closed- 

 circuit voltage instead of the open-circuit voltage. 

 The load impedance across which the voltage is meas- 

 ured must be stated in all such cases. 



In order to measure the open-circuit voltage, a very 

 high impedance circuit is required, especially when 

 dealing with crystal hydrophones, which have high 

 impedances themselves. Frequently the measure- 

 ments are made in a closed circuit and then the cir- 

 cuit loss is allowed for. In hydrophones which have a 

 preamplifier associated with them, a small resistance 

 is frequently included in the so-called calibration cir- 

 cuit to permit computing the open-circuit voltage 

 generated by the crystal. Care must be taken in con- 

 nection with the measurement of the response of such 

 hydrophones that the output of the preamplifier is 

 properly terminated. This applies especially to the so- 

 called cathode-follower circuit which is commonly 

 used. 



Formulating a mathematical expression for the re- 

 ceiving response, we have 



R R = 20 log -2 



(10) 



where e g = the generated voltage of the hydrophone 

 in volts, and p = the pressure in the free field sound 

 field in dynes per sq cm. 



Instead of obtaining the open-circuit voltage, the 

 group at the Woods Hole Oceanographic Institution 

 prefer to calibrate their tourmaline gauges in terms of 

 the electric charge on the crystal. This calibration is 

 made by comparing the voltage V P across the ampli- 

 fier input (or output) due to a known pressure on the 



crystal, with the voltage V c across the amplifier input 

 (or output) due to a calibrating voltage V B applied 

 across the amplifier input in series with a known cali- 

 brating condenser C a . The charge on the crystal due 

 to the applied pressure can then be computed from 

 the above mentioned voltages and *he calibrating ca- 

 pacity. If the calibrating condenser C g is in parallel 

 with the crystal when the voltage V P is measured, 



0. 



1'p v„ c g 



From this charge Q and the capacity of the crystal C , 

 the open-circuit voltage e can then be found. 



Q = e g C . 



The determination of the charge instead of the gen- 

 erated voltage is convenient at times because it re- 

 quires only measurements at the input or output of 

 the amplifier, and the results are independent of the 

 length of the intervening cable. 



4.2.2 



Threshold Pressure 



The threshold of a hydrophone or a projector will 

 be expressed in terms of the pressure in a uniform, 

 plane-wave, free sound field propagated parallel to 

 the acoustic axis of the device, in decibels versus refer- 

 ence pressure (1 dyne per sq cm), which produces a 

 signal voltage equal to the inherent noise voltage. 

 This noise voltage is taken in a band width of 1 cycle 

 and the device is supposed to be in a matched, tuned 

 circuit. 



In the following, the significance of the term thresh- 

 old is discussed and the measurements and computa- 

 tions necessary to obtain the threshold pressure are 

 outlined. 



The signal pressure which can be measured with 

 any given device is limited in two directions: over- 

 loading limits it on the upper side, and the noise level 

 limits it on the lower side. The noise may be due to a 

 number of factors, such as the associated preamplifier, 

 pickup in the leads, improper grounding, and con- 

 tacts. When these sources are eliminated there re- 

 mains thermal noise, 7 " which is fundamental and 

 depends only upon the temperature and the fre- 

 quency range covered. The mean square value of this 

 random noise voltage has been determined experi- 

 mentally and theoretically. When reduced to a fre- 



