New methods for transducer calibration: Free-field reciprocity calibration of condenser microphones

Research output: Book/ReportPh.D. thesis – Annual report year: 2003Research

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The unit of sound pressure, the pascal (Pa), is realised by calibrating a condenser microphone in a closed coupler where the sound pressure is uniformly distributed over the diaphragm. When the microphone is placed in a free field, the distribution of sound pressure over the diaphragm will change as a result of the diffraction of the body of the microphone and the load of the radiation impedance onto the impedance of the microphone diaphragm. Thus, its sensitivity will change. In the two cases, a technique based on the reciprocity theorem can be applied for obtaining the absolute sensitivity either under uniform pressure or free-field conditions.

However, some imperfections on the realisation of the free field may invalidate the theoretical background for the application of the reciprocity technique. Specifically, the walls of the anechoic chamber will reflect a portion of the incident energy back to the microphones. Also, under certain conditions the microphones will couple in the free field and the result is a “standing wave” between the diaphragms of the microphones. Additionally, there are some problems related with the measurement set-up. One is the electrical effect known as cross talk, and another is the random error introduced by the finite measurement time.

This thesis describes the application of a time selective technique to the reciprocity calibration of laboratory standard microphones in free field. This technique is used for removing the reflections from the walls of an anechoic chamber, the standing wave between microphones, and electrical noise by manipulating with the frequency response – electric transfer impedance function
– between two microphones and the corresponding impulse response, thus providing a valid realisation of the free field.

The acoustic centres of the microphones have been determined from the cleaned transfer impedance values. The complex free-field sensitivities of the microphones have also been calculated. The resulting complex sensitivities and acoustic centres have been compared to simulated results and proved to be in good agreement. This confirms the reliability of the time-selective technique, even in non-anechoic environments.
Original languageEnglish
Place of PublicationKgs. Lyngby, Denmark
PublisherTechnical University of Denmark
Number of pages193
Publication statusPublished - May 2003

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