Patent GB-1500055-A (GB)
Title: Headphone
IPC: G10H1/04
Filed: 1975-05-14
Abstract: 1500055 Headphones; headphone circuits; acoustic delay devices AKG AKUSTISCHE U KINO-GERATE GmbH 14 May 1975 [17 May 1974 14 June 1974 28 March 1975] 20413/75 Headings H4J and H4R The frequency vibrato of electronic organs is heard through a headphone having smooth frequency-response regions by connecting the diaphragm of the electro-acoustic transducer within the headphone to at least one mechanical delay element having a delay time of not more than 20 ms. An electrodynamic headphone 1 (Fig. 2) has a delay element in the form of a helical spring 2 directly connected at one end to the diaphragm or to its coupling region 3 and at the other end to the headphone housing. Spring 2 passes through a bore 5 in a magnet 6. In a modification (not shown) a spring component within the bore 5 is of thicker wire but of smaller diameter than a spring component outside the bore 5. The spring components are dimensioned so that their wave impedances are equal. Alternatively a transducer 8 (Fig. 3) is mounted at the base of a housing 7 and has a diaphragm 9 facing a large opening covered by a perforated plate having apertures 13. A vibratable diaphragm 11 disposed on the perforated plate has its coupling region 12 secured to one end of a delay element 10 whose other end is connected to diaphragm 9. The periphery of diaphragm 11 is formed with apertures through which sound from diaphragm 9 can travel directly to the ear. A porous or solid filler 7a, e.g. resilient foam plastics, damps cavity resonances and may damp the helical spring 10 by pressing against it. An ear cushion 14 may be of open-cell foam plastics to provide a coupling with the external air. Alternatively some or all of the apertures 13 for conveying direct sound may be selectively closed. Preferably the diameter of spring 10 equals that of the moving coil of the transducer. In another arrangement, (Fig. 4, not shown), a large diameter helical spring (15) is disposed between two smaller-diameter helical springs (16, 17) whose free ends are secured to the front and rear of the housing (7). The junction between the spring 17 in front of diaphragm (21) and the spring 15 behind the diaphragm (21) is at the apex of diaphragm (21). The above embodiments rely on springs excited to compression vibrations. A headphone diaphragm 100 (Fig. 6) is coupled to a helical spring 103 by a pin 106. Spring 103 of unequal lengths on the two sides of the pin 106 is excited to transverse oscillations. Spring 103 is disposed in the groove (107), Fig. 7 (not shown), of a filter 108 of solid material or foam plastics. In another arrangement (Fig. 8, not shown) two such springs (109, 110) are disposed in the groove and each has a separate coupling pin (111, 112) connected to the dome of diaphragm 100. The two springs (109, 110) preferably have different dimensions, mechanical properties and surface irregularities or qualities to give different frequency characteristics. Likewise the two pins (111, 112) have different frequency-dependent characteristics so as to decouple the two springs (109, 110) and divide the frequency range between them. Another embodiment has two transducers 201, 215 (Fig. 9) disposed on a common plate 202. A helical spring acting as the delay element has two parts 203, 210 having a common junction 206 from which a suspension spring 207 extends. Spring part 203 is connected by a coupling element 220 to the diaphragm 211 of transducer 201. The spring ends are enclosed in an elastomer 205. The spring vibrates mainly torsionally but also produces some transverse vibration. Apertures 213, 216 formed in plate 202 may be covered with acoustic resistance material. A potentiometer 218 adjustable by a knob 219 determines the proportion of input signal to each transducer and hence the acoustic power radiated by each transducer. Alternatively the acoustic power radiated may be mechanically adjusted. Local circuit.-Each headphone of a stereophonic pair has two transducers 201, 215 (Fig. 12) connected together by a potentiometer R 1 , R 2 . If the potentiometer sliding contact is moved from one end to the other end a continuous transfer of acoustic power can be made from one to the other transducer and the proportions given by each transducer can be adjusted as required.