Design and performance of a microprobe attachment for a 12-in. microphone

Abstract

It is often necessary to measure sound fields in confined spaces where minimum disturbance of the sound field is important. In applications where the confinement of the space is extreme such as very small cavities, existing probe microphones are too large. In this paper the probe section of a Brüel and Kjær probe microphone type 4170 is redesigned using smaller diameter probe tubes. New designs (microprobes) are first considered through simulations in the case of probe tubes having inside diameters ranging from 1.25 down to 0.1mm. Several microprobes were constructed and their performance measured and compared to the simulated results. Good agreement was found between the measured and simulated frequency response and sensitivity. Measurements show that a sensitivity of about 0.02mV∕Pa could be obtained from a probe tube with 0.2mm o.d. and 0.1mm i.d. if the length is less than 1cm. The input impedance of the probe orifice is estimated to be greater than 3×1010kgs−1m−4. Viscous resistance and thermal conduction due to the small diameter provides a well-behaved frequency response that is flat within 5dB between 200 and 6000Hz and to within 12dB up to 10000Hz.