BACKGROUND OF THE INVENTION Most of today's HiFi loudspeakers comprise cone-shaped acoustic transducers acting as point sources. Cone-shaped transducers are very popular because of their ease of manufacturing. It is a disadvantage of point source transducers that they emit sound in all directions, forward, upward, downward, and laterally, thereby generating room reflections. Furthermore, the sound quality perceived by a listening person in a room with such a transducer will change when the listening person moves around in the room. This effect may be reduced by integration of one or more sound transducers with reflectors.

Ribbon transducers and plane magnetic transducers are less common. These transducers have electrical conductors suspended in a magnetic field for emission of sound in response to an ac current flowing through the conductors.

The transducer membrane in a ribbon transducer comprises a ribbon including one or more electrical conductors. Each end of the ribbon is clamped and the ribbon is tensioned in its longitudinal direction. The ribbon is suspended vertically with free side edges whereby acoustic short circuiting occurs around the edges of the ribbon. A membrane in a plane magnetic transducer is glued or adhered on top of or in between magnets generating the magnetic field. Electrical conductors are mounted on the membrane. The membrane has no openings at its side edges so that acoustic leakage and short circuiting are eliminated.

US 4,550, 228 discloses a speaker system with a single housing containing tweeter, mid-range, and woofer transducer units. The tweeter unit consists of an elongated, aluminium ribbon positioned vertically and connected to the top and bottom of a rigid elongated frame. The mid-range transducer is similar to the tweeter transducer in construction except that the edges of the mid-range ribbon are mechanically attached to the frame by foam strips. The woofer transducer consists of an elongated, trapezoidal, corrugated aluminium ribbon attached to a non-vibrating frame. In order to provide a single electrical path, the ribbon is divided into a serpentine path by a series of cuts. The ribbon is tensioned in a transverse direction.

It is a characteristic feature of long and narrow plane magnetic transducers and long and narrow ribbon transducers that they typically act as line sources and that they radiate sound solely in the horizontal direction in cylindrical waveforms so that sound reflections from the floor and the ceiling are not generated.

It is well known to utilize these types of transducers for high frequency transducers commonly known as ribbon tweeters that may be manufactured at a reasonable cost.

Typically, they are small sized and the ear of the listener has to be situated at exactly the same height as the ribbon tweeter.

However, loudspeakers with a very long ribbon tweeter, a very long plane magnetic mid-range, and a very long plane magnetic bass are known, but they are extremely expensive, since the manufacturing process is very cumbersome. Apart from the high manufacturing cost, these types of sound transducers have several desirable acoustical advantages, e. g. the sound pressure is reduced by approximately 3 dB when the listener doubles the distance to the loudspeaker compared to the 6 dB reduction of cone-shaped transducers, and the sound perception in a room is substantially independent of the position of the listener in a horizontal plane in the room.

Further, the membrane on a point source transducer is driven by a coil suspended in a magnetic field making the transducer slower than a ribbon transducer or a plane magnetic transducer without a voice coil.

SUMMARY OF THE INVENTION According to a first aspect of the present invention an acoustic transducer is provided comprising a frame holding at least one magnet for generation of a magnetic field, and a ribbon comprising a material that is attracted by the magnetic field and having at least one electrical conductor, and wherein the ribbon is attached to the magnets by means of the attraction of the ribbon by the magnetic field whereby sound is generated by interaction of the magnetic field and a current flowing through the at least one conductor. The magnets may be permanent or electromagnetic.

According to a second aspect of the present invention, a ribbon is provided for generation of sound waves characterised in that the ribbon comprises a material attracted by a magnetic field, and that the ribbon comprises at least one electrical conductor.

The ribbon may contain iron particles or chrome particles or another material attracted by a magnetic field. The magnetically attracted ribbon may for example be made from

Kynar O or Kapton @ tape or another type of thin film tape with material attracted by a <BR> <BR> magnetic field, such as iron particles, chrome particles, etc. , deposited on the surface of the Kynar (E) or Kapton E) tape etc. It is foreseen that future stronger and lighter ribbon materials will be provided by utilisation of nano-technology for use in transducers according to the present invention.

Preferably, the material attracted by a magnetic field is deposited solely on the parts of the surface of the ribbon situated close to the edges of the tape and covering the magnets when the tape is mounted in the transducer according to the invention in order to lower the weight of the ribbon.

Compared with conventional ribbon transducers, the invention is more robust since the ribbon according to the invention is stronger than conventional ribbons, for example constituted by a thin aluminium foil.

The material of the ribbon may itself be electrically conductive, or, an electrically conducting material may be glued or applied in another suitable way to the ribbon that is attracted by magnetism. The electrically conducting material may e. g. be a cupper foil, silver foil, aluminium foil, or another foil of electrically conducting material. The conducting material may be etched or cut out in one or more conducting paths, e. g. a serpentine conducting path.

In the transducer according to the invention, the ribbon with the at least one electrical conductor is mounted in the frame holding magnets producing a magnetic field. The magnetic force from the north pole and south pole of the magnets holds the ribbon towards the magnets.

This leads to a simplified manufacturing process of the transducer. Compared to conventional plane magnetic transducers, gluing or adhesion of the ribbon membrane to the frame is avoided because of the attachment of the ribbon to the magnets in the frame by the magnetic force and without adhesives. This leads to the further advantage that acoustic leakage and acoustic short circuiting around the edges of the rib bon is avoided. Still further, elimination of adhesives also leads to a simple installation process and possible repair of the transducer according to the invention.

One or both ends of the ribbon are fastened to the frame for connection of the ac transducer drive current to flow through the at least one conductor of the ribbon. When ac current flows through the electrically conducting material of the ribbon, the ribbon will oscillate thereby displacing surrounding air whereby acoustic sound is generated.

Thus, means for fastening of the top and bottom ends of the ribbon are not utilised for

tightening of the ribbon, but establish electrical contact to the at least one electrical conductor of the ribbon so that an ac current may flow in the ribbon according to the invention.

Thus, compared to conventional ribbon transducers, tightening of the ribbon is avoided, since the ribbon according to the invention is attached to the surfaces of the magnets by the magnetic force exerted by the magnets. This leads to the further advantage of lowering the so-called guitar string resonance frequency experienced from conventional ribbon transducers with tensioned ribbons. Further, the absence of a tensioned ribbon makes the transducer according to the present invention less sensitive to temperature fluctuations in the listening environment as compared to conventional ribbon transducers and plane magnetic transducers with tensioned ribbons.

It is a further advantage of the transducer according to the present invention that the transducer substantially does not generate rattle noise. In conventional ribbon transducers, rattle noise may be generated reproducing sounds with a high sound pressure. The rattle noise is caused by the ribbon rattling against the side magnets.

A soft material may be mounted between the magnets and the ribbon attracted by the magnetic force, e. g. a strip of thin cotton textile, a foam strip, a foam rubber strip, etc., so that the ribbon does not contact the magnets directly so that possible rattle noise and acoustic distortion that may otherwise be generated at extremely high sound pressures is substantially eliminated. The soft material may be attached, e. g. by gluing, to the magnets or to the ribbon or to magnetic particles on the ribbon, or, the soft material may be held between the magnets and the ribbon by the magnetic force holding the ribbon towards the magnets.

Due to the attraction of the ribbon by the magnetic field, the ribbon will return very quickly to its starting position on the magnets upon application of an electrical signal resulting in reduction of acoustic distortion and resonances as compared to conventional ribbon transducers and plane magnetic transducers.

The transducer according to the invention also covers a larger frequency range than conventional ribbon transducers because of absence of acoustic leakage around the edges of the ribbon. The transducer according to the invention reproduces sound in substantially all of the audio range.

A listener listening to sound in a room with a transducer according to the present invention will experience a sound image without reflections from the floor and the

ceiling so that the sound image is perceived more clearly and without distortion. The sound image is experienced without loss of mid-range and high frequency details whether the listener is sitting, standing, or moves around in the room substantially independent of the horizontal position of the listener in front of the transducer. Also, the sound pressure is only reduced by 3 dB when the listener doubles the distance to the transducer.

In a preferred embodiment of the invention, the magnets are positioned proximate to each other in the frame, e. g. with a mutual distance less than 19 mm (3/4 inches) so that the so-called cavity-resonance is moved outside the frequency range of audible sound. Hereby, the notch filters required in the cross-over filters of conventional plane magnetic transducers are eliminated in the transducer according to the invention.

Preferably, the transducer according to the invention is mounted in a baffle or housing so that acoustic short circuiting around the transducer is avoided lowering the lower cut-off frequency. The baffle or housing is preferably designed with rounded edges so that diffractions and edge reflections are avoided.

The frame of the acoustic transducer may be open at both sides of the ribbon for dipole operation of the transducer, i. e. for emission of sound in both a forward and a backward direction.

Alternatively, the frame of the acoustic transducer may be closed or damped at one side of the ribbon for mono-pole operation of the transducer, i. e. for emission of sound in a forward direction only.

The flat shape of the transducer according to the invention makes it advantageous for use with flat screens, such as LCD screens, plasma screens, projector screens, etc., for example together with or integrated in a flat television set.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows in perspective a plane magnetic transducer according to the invention without the ribbon, Fig. 2 shows in perspective the plane magnetic transducer of Fig. 1 with the ribbon, Fig. 3 shows a ribbon according to the invention with an aluminium foil, Fig. 4 shows a loudspeaker with baffles and a transducer according to the invention, and Fig. 5 is a plot of measurements of sound emission from the loudspeaker of Fig. 4.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Fig. 1 shows in perspective parts of an acoustic transducer 10 according to the invention without the ribbon. The illustrated plane magnetic transducer 10 comprises a frame 12, in the form of a U-profile, holding a set of magnets 14 with north poles 16 and south poles 18 for generation of a magnetic field. A printed circuit board 20 is provided for attachment of the ribbon. In the illustrated example, the magnets 14 are permanent magnets.

Fig. 2 corresponds to Fig. 1, however the ribbon 22 is included. The ribbon 22 comprises a Kapton @ tape with chrome particles deposited on the parts of the ribbon covering the magnets 14. The boundary 24 between the centre part of the ribbon without chrome particles and the part of the ribbon containing chrome particles and situated close to the edges is indicated. The ribbon 22 is clamped between the printed circuit board 20 shown in Fig. 1 and the printed circuit board 26. The printed circuit boards 20,26 also provide contact to the conductor of the ribbon. In the illustrated embodiment, the same fastening and connection is provided at the opposite end of the ribbon (not shown).

Fig. 3 shows the ribbon 22 with an electrical conductor 28, preferably an aluminium foil, glued onto the Kapton @ tape. The parts 30 of the Kapton @ tape containing chrome particles are also shown. A soft material, preferably a web of thin cotton textile (not shown), may be mounted between the magnets and the parts 30 of the ribbon covering the magnets 14 so that the ribbon does not contact the magnets directly whereby possible rattle noise and acoustic distortion that may otherwise be generated at extremely high sound pressures is substantially eliminated. The soft material may be attached, e. g. by gluing, to the magnets 14 or to the ribbon 22 or to the chrome particles on the ribbon, or, the soft material may be held between the magnets 14 and the ribbon 22 by the magnetic force holding the ribbon 22 towards the magnets 14.

In the illustrated embodiment, the width of the electrical conductor 28 is 16 mm, and the width of the parts 30 of the ribbon containing chrome particles is 16 mm resulting in a ribbon width of 36 mm. The Kapton tí3 tape has a thickness of 20 um, and the aluminium foil has a thickness of 7, um.

The absence of gluing or adhesion of the ribbon membrane to the frame is clearly illustrated. The ribbon 22 is attached to the magnets 14 in the frame 12 by the magnetic force and without adhesives. Thus, the manufacturing process is significantly simplified. This leads to the further advantage that acoustic leakage and acoustic short circuiting around the edges of the ribbon is avoided. Still further, elimination of

adhesives also leads to a simple possible repair of the transducer according to the invention.

Further, the manufacturing process is significantly simplified due to the fact that the ribbon according to the invention is stronger than conventional ribbons that may for example be constituted by a thin aluminium foil.

The simple attachment and electrical connection of the ribbon in the transducer according to the invention also leads to the advantage that the end user may mount and connect the ribbon when installing a loudspeaker with the transducer.

It should also be noted that the means for fastening of the top end and bottom end of the ribbon 22 do not tighten the ribbon, but establish electrical contact to the electrical conductor 28 of the ribbon 22 so that an ac current may flow in the ribbon according to the invention.

Fig. 4 shows a loudspeaker 32 with a transducer according to the invention positioned on a base member 34 and with baffles 36 mounted at its sides to lower the lower cut- off frequency. The baffle is preferably designed with rounded edges so that diffractions and edge reflections are avoided. In the illustrated example, the width of the baffles is 15 cm.

The loudspeaker may also be positioned on a wall or be suspended in a wire.

Fig. 5 is a plot of measurements of sound emission from the loudspeaker of Fig. 4. The relative sound pressure in dB is plotted along the Y-axis, and the frequency in Hz is plotted along the X-axis. The measurements are performed with a second-order 225 Hz high-pass filter, and they are not performed in an anechoic chamber so that reflections from side-walls influence the measurements.

Curve 1 represents on-axis measurements at a level with the transducer. Curve 2 represents measurements 45 ° off-axis at a level with the transducer, and curve 3 represents measurements 90 ° off-axis at a level with the transducer.

Curves 4 and 5 represent on-axis measurements 10 ° below and 10 ° above the ribbon, respectively.

Curve 4 and 5 clearly illustrate that no reflections are generated from the floor or from the ceiling.

The measurements illustrate that a listener listening to sound in a room with a transducer according to the present invention will experience a sound image without reflections from the floor and the ceiling so that the sound image is perceived more

clearly and without distortion. The sound image is experienced without loss of high frequency details whether the listener is sitting, standing, or moves around in the room substantially independent of the horizontal position of the listener in front of the transducer. Also, the sound pressure is only reduced by 3 dB when the listener doubles the distance to the transducer.