. Basic methods for the calibration of sonar equipment. TRANSMITTING 21 power. Thus, I'onmilalin^ ihe mailuniatical expres- sion for the response, Rr = 20 log -^^ = 20 Ion /; - 10 log P ,. ('5) In eipiation (.S), 20 log p gives the pressure in deci- bels versus 1 dyne per sq cm (1 dyne ]5cr sq cm is called reference pressure) and 10 log P, is ihe power level referred to I ^valt. For actual testing, the practice of expressing power levels in decibds versus 10-"' watt has de\ eloped. It will be noted that 1 meter is chosen lor the refer- ence distance. This, of coinse, does not mean that al
. Basic methods for the calibration of sonar equipment. TRANSMITTING 21 power. Thus, I'onmilalin^ ihe mailuniatical expres- sion for the response, Rr = 20 log -^^ = 20 Ion /; - 10 log P ,. ('5) In eipiation (.S), 20 log p gives the pressure in deci- bels versus 1 dyne per sq cm (1 dyne ]5cr sq cm is called reference pressure) and 10 log P, is ihe power level referred to I ^valt. For actual testing, the practice of expressing power levels in decibds versus 10-"' watt has de\ eloped. It will be noted that 1 meter is chosen lor the refer- ence distance. This, of coinse, does not mean that all calibrations are to be made at tiiat disiaiKe. The ac- tual testing distance will depend on coirsitlcrations ol obtaining wa\es which are suliitienth plane so that spherical wave corrections will not be required either for the projector under test or for the receiving hydro- phone, that is, the testing distance will depend on the size of the instruments and the lrec[uency. Other- wise, the testing distance will be made as short as pos- sible to minimize interference from reflections, etc. The testing distance d will be stated in connection with all tests and the correction C in decibels to dâ = I meter will lie made on the basis of spherical waxes; thus C = 20 log d. (4) A similar consideration applies with respect to the power to be used in the tests. While the response is referred to 1 watt, it xvould ob\ iously be incorrect to make all tests at that power le\el. If the de\ ice is linear, the testing power used is of no consecpience, but if the response varies with power level, then the tests shoidd he made at the actual working le\els used in service and the testing power should be stated. A load characteristic should be furnished showing the relation between acoustic power oiuput (or prcssine on the axis) and available power. Directivity The next item to be measured concerns the distri- bution of the acoustic pressure with direction. This is measured by determining
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