A HEADPHONE

Abstract: The headphones (1) comprising a headband (2) with ear speakers (3) each comprising a sound source in the form of a stiff lightweight panel (5) and a vibration exciter (6) for launching bending waves into the panel to produce an acoustic output.

Description: TITLE: A HEADPHONE DESCRIPTION The invention relates to headphones. More particularly the invention relates to headphones comprising bending wave panel -form loudspeakers e.g. distributed mode acoustic radiators of the general kind described in International patent application O97/09842. Conventional headphones employ coherent generally pistonic sound sources. These have significant proximity effects and generally show highly critical placement of the transducer with respect to the entrance of the ear canal . It is difficult to predict and control the actual frequency response on the ear. Known planar headphone designs of the ribbon and electrostatic types tend to be cumbersome and costly. According to the present invention there is provided headphones comprising a sound source m the form of a resonant panel, and transducer means for launching bending waves into the panel to produce an acoustic output. The sound source may be m the form of a distributed mode 5 acoustic radiator. Preferably the headphones comprise a pair of distributed mode acoustic radiators. The or each distributed mode acoustic radiator may comprise a rigid lightweight panel having a cellular core sandwiched between opposed skins. 10 The transducer means for launching bending waves into the radiator, may be m the form of an electrodynamic inertial device. A headphone built using a distributed mode acoustic panel, m addition to the potential for low cost, and 15 lightweight slim construction, offers a more diffuse radiation pattern from a larger radiating area, so reducing the standing wave effects to the head and also reducing proximity problems. Placement on the head is much less critical for good sound quality. 20 For this implementation a thin panel designed to give good distributed mode operation over a wide frequency range is employed, mounted on a resilient suspension which together with the defined lowest bending mode facilitates a near field, audible frequency range of typically 30Hz to 25 20kHz. This is achieved using a compact electrodynamic transducer of low coil inductance. For low cost, or even disposable applications, the distributed mode acoustic panel may be made of a card based reinforced construction driven by a suitable mechanically matched low cost piezo transducer. Similar construction materials may be used for the headband allowing for an ultra lightweight build. The invention is diagrammatically illustrated, by way of example, in the accompanying drawings in which: - Figure 1 is a perspective view of a headphones device; Figure 2 is a cross-section through part of the headphones device of Figure 1 ; Figure 3 is a cross-section, similar to that of Figure 2, through an alternative embodiment of headphones device; Figure 4 is a cross-section, similar to that of Figure 2, through another embodiment of headphones device; Figure 5 is a cross-section, similar to that of Figure 2, through another embodiment of headphones device; Figure 6 is a cross-section, similar to that of Figure 2, through yet another embodiment of headphones device; Figure 7 is a plan view of another embodiment of headphones device; Figure 8 is a diagram showing possible locations for vibration exciters on a bending wave panel, and Figure 9 is a graph of the measured frequency response of a preferred headphones device. In Figures 1 and 2 of the drawings, a headphones device (hereinafter "headphones") (1) comprise a generally inverted U-shaped resilient padded member (2) which forms a headband with ear speakers (3) supported on the free ends of the limbs of the headband, in a generally conventional arrangement. Again generally conventionally, lead wires (4) connect the ear speakers to the output of an audio amplifier, not shown. In accordance with the present invention, each ear speaker comprises a resonant acoustic radiator comprising a stiff lightweight panel diaphragm (5) and a vibration exciter (6) fixed to the diaphragm for launching bending waves into the diaphragm to cause the panel to resonate to provide an acoustic output. International application O97/09842 describes our preferred distributed mode resonant bending wave acoustic radiators . With the implementation, exemplified m Figure 1, each earphone of the pair may be a freely supported resonating panel of physical parameters chosen for modal frequency interleaving with a suitably small exciter located to drive the panel at a predetermined off centre location to couple satisfactorily to the modal distribution. The usual headband serves to locate the headphone assembly on the head and a cable is also present to supply audio signals to each channel exciter, generally a two channel signal, e.g. for stereo. For best efficiency the panel is typically 50mm by 70mm or 50mm by 56mm but may be smaller or larger according to the design constraints, such as price, size, quality and bandwidth and the panel is located more or less centrally on the ear. From Figure 2 it will be seen that the panel (5) is supported m a casing or housing (7), e.g. of moulded plastic by means of a resilient peripheral suspension (8) to define a cavity (9) which is filled with acoustic damping material (10) . The casing is formed with a bulge or extension (11) to accommodate the exciter (6) . The panel (5) may optimally be covered by an acoustically transparent cover or grill. To improve user comfort, a cushion (12) may surround the panel (5) . The casing or housing (7) is shown as closed-backed, but optionally may be open-backed. In Figure 3, there is shown an embodiment of ear speaker 3 m which the housing or casing (7) of the embodiment of Figures 1 and 2 is replaced by our open frame (14) supporting the panel (5) on a resilient peripheral suspension (8) the panel (5) being covered on each side by acoustically transparent covers or grills (13) supported on the frame (14) . In the arrangement of headphones (1) shown m Figure 4, each ear speaker (3) comprises a housing cr casing (17) for supporting a resonant panel (5) and exciter (6) which is generally similar to that of Figure 2 but which is extended on its inner side (15) to enclose the panel (5) and to define an outlet (16) intended to be aligned with the user's ear canal. The housing extension (15) thus forms with the inwardly directed face of the panel (5) a duct (18) by which the acoustic energy from the whole panel is fed to the outlet. In this connection it will be remembered that the output from a bending wave panel is incoherent, unlike the coherent output from a pistonic ear speaker, so that the problems associated with directing coherent sound into the ear canal are avoided. The headphones of Figure 5 comprises an ear speaker (3) having a housing (27) carrying a frame (14) supporting a panel (5) on a resilient suspension (8) with vibration exciter mounted on the panel to drive bending waves into the panel to produce an acoustic output, much as described above. In this case however, the housing (27) forms a duct (18) with the outer face of the panel (5) which terminates m an outlet (19) venting to ambient atmosphere. The duct has the purpose of loading the panel with the acoustic output from the whole outer face of the panel (5) so that the acoustic output from the inner face of the panel to the user's ear is enhanced. In the headphones embodiment of Figure 6, the ear speaker (3) has a housing (37) which is relatively large and which is intended for sealing to the user's ear by means of a cushion (12) to form a closed system. In the headphones of Figure 7 pairs of ear speakers (3) e.g. of the kinds described above are mounted on each end of the headband (2) via support member (20) which is adjustable m attitude relative to the headband. Each ear speaker (3) m the pair is mounted on hinge member (21) which m turn is mounted on the member (20) so that the earpieces may be adjusted m disposition and attitude relative to the user's ear to optimise perceived sound quality. Also the ear speakers can be slid along the support members (20) to vary the attitude of the ear speakers. For this purpose the members (21) are slidable on the member (20 ) . Figure 8 is a diagram showing possible locations for a vibration exciter (6) on a panel (5) , generally as taught m WO97/09842. Near centre locations as indicated by A are the most efficient, but a near edge position indicated at C and as taught in W099/37121 show good perceived sound quality. Research indicates that of the two known methods of driving the local acoustic of the ear namely pressure drive and velocity drive, the velocity method has benefits m terms of the sensation of naturalness and reduced aural fatigue, a feature of longer term headphone listening. The commercial popularity of so called 'open air' headphones such as those made by Sennheiser of Germany testifies to the appeal of velocity designs which do not use a firm pressure sealing to the ear compared with the conventional kind which do require a firm seal. A near central drive to a bending wave panel may be shown to give a combination of pressure and velocity sound energy at the ear entrance. Where higher quality is the objective the efficiency is less important since there is more budget for amplifier power and for magnet strength, and m any case the power level required for headphone is several orders of magnitude lower than those required for loudspeaker use, thus a 1 watt requirement for a perceived loudspeaker sound level may be just lO W for a headphone, it is a feature of this invention to minimise the pressure contribution and maximise the velocity part for a modally interleaved resonating panel. By using an exciter m a marginal or near marginal position, selected for good modal coupling but where the balance or choice for location favours evenness rather than absolute efficiency, the panel may be excited such that predominately bending waves traverse the panel with little or no out of plane contribution, passing smoothly by the ear canal, and thus providing an equivalent to the desired velocity drive for the sound field at the ear. Because the sound field is largely diffuse in nature, due to the complex, near random modal resonances present over the area of the panel, the acoustic coupling of the panel to the ear largely avoids the usual cavity and boundary resonance effects which result from the placement of a conventional coherent pistonic type of speaker element used m headphones. Coherent source driven standing wave modes cannot develop m the region between the headphone and the ear of the new type compared with conventional technology. Many panel materials and constructions may be used while the small size of the panel and convenient mounting/boundary conditions, achieved using a compliant polymer foam suspension, over full or part perimeter. Good results have been obtained with monolithic and composite panel constructions and due to the boundary conditions operative for a relatively small panel, metal can be used for example aluminium alloy from 0.15mm to 0.8mm thick, or copper tin, zinc amongst others. Carbon fibre resin bonded monoliths are also useful 0.8mm down to 0.1mm in additional to various polymers, polycarbonate polystyrene and the like. Efficiency demands are not high and a wide variety of exciters may be used from 9mm pole diameter up to 19mm, depending on the required bandwidth and loudness . Where the exciter is mounted at C, it may be desirable to provide local mass damping of the panel, again as taught in W099/37121. Experiments show that corner position B gives poor results. Figure 9 is a graph of frequency response of a preferred exemplary ear speaker with a panel made from aluminium sheet and with an exciter in the position C of Figure 8 and with mass damping of the panel as taught in W099/37121. The invention thus provides novel headphones which avoid the problems of pistonic devices without the cost penalties of electrostatic headphones.

Claims: CLAIMS 1. Headphones comprising a sound source m the form of a stiff lightweight panel and a vibration exciter for launching bending waves into the panel to produce an acoustic output . 2. Headphones according to claim 1, comprising an opposed pair of ear speakers . 3. Headphones according to claim 1 or claim 2, wherein the rigid lightweight panel is an aluminium sheet. 4. Headphones according to any preceding claim, wherein the vibration exciter is m the form of an electrodynamic mertial device. 5. Headphones according to any one of claims 2 to 4 , wherein the ear speaker comprises a housing defining an acoustic duct. 6. Headphones according to claim 5, wherein the acoustic duct has an outlet intended to align with the ear canal of the user. 7. Headphones according to claim 5 or claim 6, wherein the acoustic duct is intended to load the panel with the acoustic energy from the panel. 8. Headphones according to any one of claims 2 to 7, comprising a headband and wherein the ear speakers are mounted on the headband to be adjustable m attitude relative to the user's ears. 9. Headphones according to claim 1, wherein the stiff lightweight panel is a distributed mode acoustic radiator as taught m WO97/09842 or W099/37121.

Inventors: AZIMA HENRY, COLLOMS MARTIN, MORECROFT DENIS, AZIMA, HENRY, COLLOMS, MARTIN, MORECROFT, DENIS
Applicants: AZIMA HENRY, COLLOMS MARTIN, MORECROFT DENIS, NEW TRANSDUCERS LTD, AZIMA, HENRY, COLLOMS, MARTIN, MORECROFT, DENIS, NEW TRANSDUCERS LIMITED
Application year: 2000
Main classification: G10K
IPC codes: H04R1, H04R5, H04R7