GB1423457A - Acoustical impedance matching device in combination with an electro-acoustic transducer - Google Patents

Abstract

1423457 Acoustic matching devices; transducers MOTOROLA Inc 12 June 1973 [28 June 1972] 27941/73 Headings H4X and H4J In sound reproducing apparatus an acoustic impedance matching device 34 (Figs. 2, 6) includes an acoustic chamber 56 acoustically coupled, e.g. by rubber tube 33, to a transducer 29. An aperture 60 in the chamber 56 is for exit of sound waves from the chamber 56 and develops a sound wave pressure in the chamber 56 to provide an acoustic impedance match between the transducer 29 and the outer air. A passage 62 is coupled to the aperture 60 for emission of sound waves. The cross-sectional area of passage 62 increases exponentially until it reaches an output aperture 62. The application is to a radio pager (Fig. 1) where a large acoustic horn 20 is not required, the sound output coming from outlets 16. A shirt pocket clip 11 is provided, together with a switch 15. A housing 10 has a U-shaped cover 17. Housing 10 contains, in addition to the impedance matching device 34 and transducer 29, a printed circuit board 25 of a radio receiver having various components 26 and a battery 27. in a holder 28. Printed circuit board can be removed and replaced in housing 10. The tube 33 has a groove (42) Fig. 7 (not shown) by which it is held captive in the matching device 34. The transducer 29 (Fig. 8) includes a drive unit 25 behind an aperture 36 leading to a spiral passage 32. A cover 31 (Fig. 2) of the transducer 29 has an aperture 30 which in the assembled condition is covered by tube 33.