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2 - MICROPHONE DESIGN ELECTROACOUSTICS

It is always difficult to know where to start in the study of microphone characteristics - indeed it seems sometimes like opening up Pandora’s box even to try! The physical and acoustical principles of design, together with the technical specifications and their measurement, are interwoven with the operational practices of the sound recording engineer, with the final arbiter of course always being the ear. It is clear that frequency response and directivity pattern measurements have pride of place as the main source of published information that can be used to estimate the performance of a microphone. However underlying these measurements are some fundamental design considerations, which form the background to a deeper understanding of the measured characteristics:

• The sound wave propagation around the acoustic obstacle formed by the microphone capsule and housing in conjunction with pressure or pressure-gradient acoustic coupling to the diaphragm
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• The electromagnetic or electrostatic signal transduction process associated with the damping of the fundamental resonant frequency produced by the mechanical vibrating system formed by the diaphragm and its suspension
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• Constant-amplitude or constant-velocity oscillatory vibration

To some extent these considerations are interdependent, but we will briefly look at each process individually, before bringing them together into a more detailed analysis of the frequency response and directivity pattern characteristics of the different types of microphone.

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