EQUIPMENT AT MOUNTAIN LAKES 



123 



of the tank and the absence of perfectly absorbing 

 walls. The limitations imposed by the relatively short 

 testing distances and the use of pulsing to make the 

 measurements independent of reflection have been 

 discussed in Chapter 5. 



In actual testing, the optimum values of testing dis- 



HIGH PRESSURE LINE 



GAUGE 

 0-1500 P6I 



.TWO POSITION, 3 WAY VALVES - 



RETURN LINE 



-PORT LOCKS (4 PER PORT) 

 NOTE : 

 PORT LOCKS CLOSE WHEN SIDES "A" ARE CONNECTED TO THE HIGH 



PRESSURE LINE AND SIDES "B" ARE CONNECTED TO THE RETURN LINE 



PORT LOCKS OPEN WHEN SIDES "B" ARE CONNECTEO TO THE HIGH 

 PRESSURE LINE AND SIDES"A"ARE CONNECTEO TO THE RETURN 

 LINE 



Ficurf. 60. Hydraulic system for holding port covers on 

 high-pressure lank. 



HEAT EXCHANGER 

 COUPLED TO 

 FURNACE 



AUTO PRESSURE 

 CONTROL 



Fictjre 61. Hydraulic system of high -pressure tank. Pump 

 No. 1 is used [or filling tank and circulating water 

 through heat exchanger. Pump No. 2 applies pressure to 

 tank when all valves are closed. 



tance and pulse length being somewhat interdepend- 

 ent within certain limits, the distance may be in- 

 creased by using shorter pulse lengths and vice versa. 

 Larger test distances may be used with transducers of 

 good transient response than with sharply resonant 

 ones. A pulse of 1.8 milliseconds is permissible when 

 the test distance is 5 feet because of the bailies which 

 prevent reflections from the sides of the tank. With- 

 out bailies the reflections would be delayed only some 

 0.8 milliseconds. At the maximum distance of 8 feet, 

 the permissible pulse length is 0.4 milliseconds. For 

 many transducers these distances and pulse lengths 

 give results equivalent to those in a free field. Even 

 when the tank will not permit such equivalent cali- 

 brations, it should be possible to observe the relative 

 performance of transducers as functions of tempera- 

 ture and hydrostatic pressure. Observation of these 

 functions is obviously not possible with tests made in 

 the lake. 



The cleaning, rigging, and debubbling preparation 

 of the transducers is identical with that for the free 

 field testing, with the added precaution that the in- 

 struments must be capable of operating at the desired 

 test pressures. The available power is measured with 

 the 30A set, with the transmitter modulator of the 

 pulse system arranged for continuous-wave output. 

 By the nature of the modulator circuit, the same in- 

 stantaneous value of power is maintained when it is 

 switched to the pulsing operation. 



Observation on the cathode-ray oscilloscope [CRO] 

 allows rapid adjustment for the proper pulse length, 

 delay time, and received pulse. The recurrence rate is 

 set so the reverberation from one pulse does not inter- 

 fere with the measurement of the succeeding pulse. 



6.2.7 Nm se and Transient Measurements 



As pointed out in Chapter 5, acoustic noise may be 

 classified, for the purpose of analysis, as continuous, 

 such as thermal or cavitation noise, or as intermittent, 

 such as waves of explosive origin. The method used in 

 measuring continuous noise has been treated under 

 the description of the intermediate-frequency system 

 (15 c— 150 kc) in Section 6.2. None of the methods 

 described previously is suitable for intermittent 

 sound. 



Analysis and Measurement of Transients' 11 



Transients may be considered as composed of sinu- 

 soidal signals having a continuous distribution in 



