Improvement of reciprocity measurements in acoustical source strength

Reciprocity theory exists in stable, passive, linear dynamical systems, and has been applied to calibrate the microphones and hydrophones successfully. However, the accuracy of this method is doubtable, which limits its use as a widely-used engineering tool. Reciprocity theory has also been applied to measure the strength of sound source. However, compared with the results of the free field method and the reverberation method, the most differences are almost 5dB. This paper is focused on possible factors that affect the accuracy. It is found that the frequency responses of sound source at some frequencies have large peaks or troughs, just like "impulses" in the frequency domain. Besides, it is found that the "impulses" correspond to the coherence functions, and almost all "impulses" appear at the frequencies with low coherent value. The results of the reciprocity method agree with the results of the free field method very well after correcting the results by polynomial fitting.