There are essentially two problems in conventional free-field reciprocity calibration of condenser microphones: 1) an extremely low signal-to-noise ratio at low frequencies, caused by the fact that the microphones are very poor radiators of sound below 500 Hz, and 2) reflections from the walls of the anechoic chamber. The purpose of this PhD study is to examine alternatives to the conventional free-field reciprocity calibration technique. Two experimental techniques for determining the frequency response between two microphones have been studied, a frequency domain method based on pure tone excitation, and the MLS method, which is a time domain method. A computer simulation study of the frequency response between two microphones in a free field has been carried out. The purpose of the simulation study is to examine if the direct field and later reflections from the walls can be separated using time windowing. The corresponding impulse response has been obtained using an inverse Fourier transform. The limitations resulting from the fact that the frequency response because of the poor signal-to-noise ratio is truncated at low frequencies have been studied. The computer simulations are supplemented by experiments using the two different methods mentioned above. The results of the study should improve the accuracy of the methods currently used in determining the free-field correction of condenser microphones. The results may also be applied to simultaneous free-field and diffuse-field calibration of condenser microphones.
|Effective start/end date||01/11/2000 → …|