EQUIPMENT AT MOUNTAIN LAKES 



101 



Table 1. Test data required for dome investigation. 



PROJECTOR 



HYDROPHONE 



* Transducer used as a receiver only. 



t If the dome is equipped with a removable baffle, tests 

 should be made with (a) dome and baffle, (b) dome alone, and 

 (c) bailie alone, to see how baffle and dome affect noise pickup 

 over a range of angles about the rear of the transducer. 



Studies of Acoustic \Vax>e Signal Generators of 

 Complex Waves. Devices designed to generate acous- 

 tic signals of complex wave forms are divided into 

 two major categories, each requiring a special testing 

 technique. In general, the choice of technique is 

 based on such factors as a continuous signal, recur- 

 rent signal and its rate, and the crest factor (ratio of 

 peak to rms values). 



Obviously, the use of electromechanical equip- 

 ment for the recording of random events is limited by 

 such factors as the speed and dynamic range of re- 

 corder response. Signals that are intermittent, that 

 have low recurrence rates, and those with crest factors 

 greater than 2.5, are usually studied by means of a 

 cathode-ray oscilloscope and high-speed oscillograms. 

 Signals that are continuous or of high recurrence rate, 

 with crest factors not exceeding 2.5, may be studied 

 with the apparatus of system 2. The procedure for in- 

 vestigating such signals with system 2 is stated here- 

 with and may be used for those of periodic or aperi- 

 odic nature. 



The devices are rigged and positioned with the 

 usual procedure for testing projectors. Provision is 



BAFFLE 

 PROJECTOR n HYDROPHONE 



PROJECTOR 



HYDROPHONE 



HYOROPHONE 



Figure 37. Plan view of tvpical test set-ups for studying 

 baffle characteristic's: (A) reference runs. (B) sound trans- 

 mission through battle, (C) threshold signal without 

 baffles. (D) sound reflection by baffle. 



made for the observation and control of the driving 

 power. A standard hydrophone having a substantially 

 uniform frequency response is selected for the acous- 

 tic pressure measurement. In the selection, the maxi- 

 mum instantaneous pressure must be considered in 

 order to avoid overloading of the preamplifier. 



Observation of the crest factor should be made first 

 with the arrangement shown in Figure 38, where the 

 rms of the signal is obtained on the 30A set and the 

 peak amplitude on the cathode-ray oscilloscope. The 

 complex wave signal is then replaced by a sinusoidal 

 one from an oscillator set at a level which gives the 



