Design of a Wideband, Constant Beamwidth, Array Microphone for Use in the Near Field

Directional microphones have been proposed for the removal of room reverberation on the assumption that a properly aimed microphone would pick up the direct path speech energy and reject the reverberant energy. Indeed, highly directive microphones are often employed in "press conference" situations to pick up questions from an auditorium. An array microphone would seem to be an ideal replacement for the "directional" mike in such a situation since it would offer a quick automatic aiming capability and, if so designed, either multiple-speaker monitoring or the ability to correlate multiple reverberant paths from a single speaker. Conventional array microphones, however, suffer from a number of drawbacks which must be considered. If we consider speech in the frequency range 300 to 3000 Hz, then the microphone designer must consider the decade change in frequency (and wavelength). In particular, the beamwidth of the radiation pattern is related to wavelength 1839 and microphone aperture size (microphone dimensions). As wavelength goes down, so does beamwidth. The change in beamwidth over a decade change in wavelength would seem to be unacceptable for this application. If we would like to employ our microphone for small conference room dereverberation, then we must also consider the near-field/farfield transition region of the microphone. Note that at 300 Hz the wavelength of sound is about 4 ft. A microphone with a two-wavelength aperture at this frequency is therefore 8 ft wide.