DID YOU KNOW? DTIC has over 3.5 million final reports on DoD funded research, development, test, and evaluation activities available to our registered users. Click HERE
to register or log in.
MICROACOUSTIC SYSTEM ANALYSIS BY THE MEASUREMENT OF FREE-FIELD SOUND SPEED.
NAVAL RESEARCH LAB WASHINGTON D C
An analysis of the microacoustic system was undertaken by means of the measurement of free-field sound speed in water. The water was contained in a 10x5x5 ft cypress tank. A pulse was transmitted from a fixed source to a receiver which was positioned at two different distances along a radius of the source. The distance difference of approximately one meter and the travel time over this distance were accurately measured. Soundspeed measurements were taken at 41 temperatures over a range from 16.8 to 23.10C. A 290-degree spherical cap was used as the fixed source. The movable receiver was a disk transducer whose active element radius was 0.16 lamda at the source resonant frequency of approximately 200 kc. Measurements were also obtained by using 0.63-cm radius disks as source and receiver with the same results. The experimental measurements led to the detection of a large error which exists in the remote determination of large distances using a cathetometer mounted horizontally. This error was eliminated and evaluated by moving along an accurately calibrated bar, allowing distance to be measured directly. Subsequently, a new system was designed which will allow the remote determination of distances accurate to within 0.001 cm. It was found that the ambient temperature at a given depth in the tank remained constant to within 0.01C. for a time sufficient to make the required measurements and that vertical temperature gradients were not present to a significant degree. The free-field value of velocity was determined to be at least 0.2 to 0.6 msec lower than confined field values measured by others. Author