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New Measurement Techniques for Portable Listening Devices: Technical Report - October 2013
Diffraction Effects in High Quality Studio Microphones
Microphone enclosures interfere with the approaching sound waves by diffracting and reflecting them to a greater or lesser extent. Since the interference occurs in close proximity to the transducer the relative magnitude of the interference, as "seen" by the transducer, may significantly alter the transducer response. In order to assess the effects microphone enclosures have on the response of their transducers, the directional frequency and transient response of several high quality studio microphones was measured at 5-deg, 15-deg, and 45-deg increments of angular sound incidence. Up to 215 measurements were made for each microphone using a calibrated sound source at a distance of 1.36 m from the microphone diaphragm. Three microphone planes: horizontal, vertical, and diagonal were investigated. The measurements reveal that some microphone enclosures significantly modify the response of the transducer at frequencies above 2 kHz. A discussion of the audibility and desirability of some of the diffraction effects follows. Diffractive and absorptive attachments are installed on microphones to modify their frequency and directional responses without distorting time domain responses. Detailed directional frequency responses and transient responses are measured at every 5-deg and 25-deg of angular sound incidence using impulse techniques to verify the effect of the attachments. These responses may be compared to those of high quality microphones used for the recording of music and speech.
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