The best method for lower losses was to record the 

 reverberation process with a Briiel and Kjaer recorder. 

 Examples of records obtained in this manner are given in 

 figures 5 to 7. The permanent record obtained can be 

 studied in detail later on. Small undulations of the free 

 water surface during the measurements will not be too 

 detrimental, since only the envelope of the signal is meas- 

 ured and the frequency at any moment is immaterial. The 

 relationship between the bandwidth A / in c/s and the re- 

 verberation time t in seconds is expressed by t A/ = 2, 2. 

 This relationship was deduced by assuming that the response 

 curve for the cavity is the same as that of a simple electric 

 resonant circuit. This relationship was checked in many 

 instances, where both t and A/ could be obtained independ- 

 ently and no significant discrepancy was ever observed. 

 The reverberation time, however, has been used exclusively 

 for the data-taking phase. 



A few comments are appropriate in respect to the hum 

 and noise problem. The input voltage to the transmitting 

 hydrophone was approximately 50v. The output voltage 

 measured by the VTVM (fig. 4B) was approximately 5mv 

 as a maximum value, and voltage levels 50 db below this 

 value were measured when the reverberation curve was 

 recorded. Careful shielding and grounding techniques 

 were essential. The tuned resonant circuit shown in fig- 

 ure 4B was necessary to overcome the remaining hum and 

 structural noise as far as possible and to increase the 

 signal sufficiently for the measurements to take place. 



The Q of the electrical tuning circuit in figure 4B was 

 measured independently. It was 60 to 80, depending on 

 the frequency used. This corresponds to a reverberation 

 time of about 1/10 of the shortest reverberation that the 

 equipment could record, and was considered insignificant. 



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