GB1048270A - - Google Patents

Abstract

1,048,270. Measuring sound intensity; matching acoustic impedances; microphones. CANADIAN PATENTS & DEVELOPMENT Ltd. Oct. 18, 1963 [Oct. 19, 1962], No. 41213/63. Headings H3U, H4D and H4J. A probe microphone assembly for use in the measurement of sound intensity comprises a probe tube coupled to the microphone via a horn terminated by an acoustic resistance element. Fig. 1 shows the general scheme where the probe tube of length L and uniform crosssectional area S 1 is coupled to the smaller end of an exponential horn of similar area and of length l, the larger end of cross-sectional area S 2 being coupled to a condenser microphone M, which with the coupling cavity has an effective volume V 1 , and to an acoustic resistance R 2 . To prevent the microphone from responding to sounds other than those transmitted by the probe tube the acoustic resistance instead of being left open to the atmosphere is vented into an enclosure V 2 the volume of which is large enough to prevent short-circuiting of the resistance at all significant frequencies. An improved response, however, is achieved by replacing V 2 by an acoustic network comprising an enclosure of volume V 2 a (<V 2 ), Fig. 3, coupled to a much larger enclosure of volume V 2 b via a damped inertance m, the latter forming a Helmholtz resonator which should resonate near the horn cut-off frequency. In a practical embodiment of the invention (Fig. 4) the acoustic resistance is defined by a conical portion 33 in an insert 25 (see also Fig. 5) and an aperture 42 in a cover plate 40 to which the microphone 4 is secured, the distance between 33 and 42 being made adjustable by means of shims 48 when the assembly is clamped by means of screws 45 provided with resilient washers 46. The acoustic resistance is vented into the enclosure, i.e. enclosure V 2 a, defined by the cut-away portions 29 of insert 25 when the latter is clamped inside a cylindrical chamber 20 the flat side 19 of which is provided with an aperture 26 communicating with two relatively smaller apertures 62 (forming the inertance m) communicating with a capacitance member 63 (forming enclosure V 2 b) which may be filled with sound-absorbing material.