TABLE OF CONTENTS This section gives a brief overview of aspects relating to the transducers commonly used to measure sound pressure and vibration; these are microphones and accelerometers.
To measure sound pressure levels or reverberation times in reverberant fields we require an omnidirectional microphone so that the response is independent of the direction of incident sound. Over the building acoustics frequency range we would ideally like to measure sound pressure without altering the sound field due to the presence of the microphone. However, the finite dimensions of a microphone and its acoustic impedance mean that this is only achieved when the wavelengths are large. For small wavelengths, diffraction effects from the microphone become significant.
Microphones are usually designed to give a flat frequency response in a particular sound field; typically a pressure field, free-field, or diffuse (random-incidence) field (Anon, 1996).
In a pressure field, the sound pressure has the same magnitude and phase at all points, when very close to (or flush with) a reflective surface, and within a very small closed cavity such as in a sound level calibrator. In a free-field, waves propagate without any influence from reflecting objects or surfaces, such as in an anechoic chamber. In a diffuse field, there is equal probability of a wave arriving at the microphone from any direction. Although microphones are intended or optimized for one specific field they can sometimes be used with negligible error in other fields.
A diffuse field microphone is specifically designed to give a flat...
