Patent GB-1479477-A (GB)
Title: High-fidelity moving-coil loudspeaker
IPC: H04R1/28
Filed: 1974-07-26
Abstract: 1479477 Loud-speakers K TSUKAMOTO 26 July 1974 [4 Aug 1973] 33073/74 Headings H4J and H4X One side of the diaphragm of a moving coil loud-speaker is coupled to a tube of constant cross-sectional area throughout substantially the whole of its length by an acoustic transformer comprising a coupling member which forms an air chamber between the diaphragm and the end of the tube. The transforming ratio of the acoustic transformer and the length of the tube are such that the acoustic radiation resistance value of the tube is larger than the vibration reactance value of the diaphragm. The tube has an acoustic resistance imparting device for reducing sound reflections from the ends of the tube. A loud-speaker (Fig. 5) comprising a diaphragm 1 driven by a moving coil 2 is connected by an annular coupling member 4a to an end of a tube 3, thereby forming a front chamber 4b. An acoustic resistance cone 5 is provided at least at the exit end of the tube 3. A baffle 7 is also at this end. If desired the tube may be connected to the baffle by a conical horn (6), Fig. 1 (not shown), to provide an acoustic match between the air column in the tube 3 and the outer air. Preferably the diameter of the tube 3 is much less than the opening of the horn (6) and the angle of the horn (6) is sufficiently large to produce spherical waves. The chamber 4b forms an acoustic impedance transformer, the transforming ratio being defined by the ratio between the surface area of the diaphragm 1 and the cross-sectional area of tube 3. Preferably the cross-sectional area of the tube 3 is approximately one tenth the surface area of diaphragm 1. If desired, the end of the tube 3 may have acoustic matching slits (8), Figs. 3, 4 (not shown), to prevent occurrence of standing waves. Alternatively chambers 10a and 10b damped by sound absorbing bodies 9a and 9b are provided to prevent standing waves and for eliminating a high frequency portion of sound. In another arrangement a slit 8 (Fig. 6b) is at the outer end of tube 3 and two baffles 11 of planar configuration forming between them an angle of 90 degrees are provided on the two sides of the slit, whereby the loud-speaker radiates a spherical sound wave. In another arrangement (Fig. 7, not shown) the tube 3 is provided with concentric or helical constrictions so that it can be bent easily. Its outer end may have a telescopic arrangement of sheaths, gaps between the sheaths functioning as slits (8). Alternatively (Fig. 8, not shown) the outer end may have concentric sheaths with outwardly flared ends, this outer end being formed with a sound absorbing body 9. In another arrangement (Fig. 9a) a terminal device 14 is a flattened member having a slit 8 opening on each of two opposite sides and the slits 8 radiate sound waves. Alternatively the radiating end of the tube (3) has a horn (6), Fig. 10 (not shown), secured to a baffle board (7). A sound absorbing body (15) comprising a porous substance such as porcelain or sintered metal of spherical form, sandwiched between two semi-spherically formed sets, forms an acoustic resistance. In a further arrangement (Fig. 11) the tube 3 is wound around the rear part of a box 16 having a conical horn 19. The outer end 3a is placed near a conical reflecting portion 18. The tube 3, of thermoplastic resin, is embedded in light-weight foamed concrete, asphalt, wax or resin 17. Alternatively the front of a diaphragm 1 (Fig. 12) is used as a direct radiator while the rear is coupled through a coupling device 2 to a tube 3.