Use of Pressure Gradient Microphones for Acoustical Measurements

Abstract

The operation of the pressure gradient microphone is compared with that of the pressure microphone It is shown that the pressure gradient microphone may be used to measure particle velocity in a sound wave. The advantages of the pressure gradient microphone in making loudspeaker measurements, particularly outdoors, are pointed out and experimental data are given for some arrangements which were tried out. The characteristics of the distribution of particle velocity in a complex sound field are studied theoretically and experimentally with a ribbon microphone. A method is described for measuring the energy density in a sound field and some measurements which were taken in complex sound fields in rooms are discussed. It is shown that a combination of three pressure gradient or velocity microphones with a pressure microphone, placed adjacent to each other, may be equivalent in eliminating interference patterns to four pressure microphones placed at distances large compared to the wavelength and with random distribution. A microphone for measuring energy flow in a sound field is described.

A Note on Finite Difference Estimation of Acoustic Particle Velocity

“…Possible applications of energy density sensors include active noise control systems, since minimizing the total energy density at a point in an enclosure is a more e$cient control strategy than minimizing the sound pressure [3]. Because of the directional information, measurements of the particle velocity have also found applications in room acoustics [4], as already suggested in reference [1]. Yet another application is in analyzing large, complicated sources of noise; a small omnidirectional source with a strength that is measured on-line would be very useful in reciprocity experiments for characterizing machinery noise sources [5,6].…”

“…The idea of measuring the total energy density in a sound "eld rather than just the potential energy density (or the squared sound pressure) goes back to the early 1930s [1]; this involves determining the sound pressure and three perpendicular components of the particle velocity. One advantage, demonstrated experimentally in reference [1] and analyzed theoretically in reference [2], is that the total energy density tends to vary less with the position in a reverberant enclosure than the potential energy density. Possible applications of energy density sensors include active noise control systems, since minimizing the total energy density at a point in an enclosure is a more e$cient control strategy than minimizing the sound pressure [3].…”

Generalized acoustic energy density

“…In the early 1930s, Wolff experimentally studied the kinetic energy density as well as total energy density in a room with the use of pressure gradient microphones. 2,3 His results indicated a better spatial uniformity of both the kinetic energy density and total energy density over the potential energy density.…”

“…18 The pressure microphone gradient technique for measuring acoustic energy quantities has been studied and improved over time. 3,4,11,[19][20][21][22] Recently, a novel particle velocity measurement device, Microflown, has been made available to acousticians, 23,24 expanding the methods available to measure acoustic energy density quantities. More and more attention is consequently being devoted to their study and use.…”

Measurement of sound power and absorption in reverberation chambers using energy density

“…12 Since that time, several improved methods have been introduced to estimate particle velocity, allowing kinetic and total energy densities to be measured with greater accuracy and consistency. [13][14][15][16] The introduction of the Microflown™ sensor, a micromachined device that more directly measures particle velocity, has provided additional means to measure energy density up to 20 kHz.…”