Microacoustic System Analysis by the Measurement of Free-Field Sound Speed

摘要

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 m/sec lower than confined field values measured by others. (Author)