LOUDSPEAKER

A coaxial loudspeaker has a first driver producing sound within a first frequency band and a second driver coaxial with the first driver producing sound within a second higher frequency band. The first driver includes a first moveable membrane coupled and moveable relative to a frame to generate sound waves within the first frequency band. The first moveable membrane extends from adjacent a central axis to an outer part above and spaced apart from a rim of the frame, the outer part being resiliently coupled to the rim. The second driver includes a second moveable membrane generating sound waves within the second frequency band. The second moveable membrane is behind or below the first moveable membrane and the resilient coupling is behind the first moveable membrane extending from the rim to the outer part so the surround is not an obstacle to second driver sound waves.

FIELD OF INVENTION

The present invention relates to a loudspeaker and to parts thereof. The invention has particular relevance to electro-dynamic loudspeakers known as moving coil loudspeakers and more especially those of the type commonly referred to as coaxial.

BACKGROUND OF INVENTION

A conventional loudspeaker is an electro-mechanical device (sometimes called a speaker driver) devoted to transduce an electric power signal into air motion, thus producing an acoustic output, i.e. sound.FIG. 1ais a cross-sectional view of a conventional loudspeaker1which for the sake of simplicity of description is symmetrical about a central axis3of symmetry. The loudspeaker1has a solid structure comprising a motor assembly5and a frame7having an outer rim9. A membrane11is connected to the frame7via, at least, two resilient or elastic components: a spider13and a surround15. A part of the membrane11is normally used to seal the motor assembly5from dust and it is commonly called dust cap12.

The motor assembly5is the active part of the loudspeaker, translating the electric power signal coming from an amplifier into motion. The motor assembly5is generally made of several parts including a magnet17, a top plate19, a pole piece21and a voice coil23that is coupled (directly or indirectly) to the membrane11.FIG. 1bshows in more detail the magnet17, the top plate19and the pole piece21which, together, form a magnetic circuit such that a static magnetic field is generated in an air gap25between the pole piece21and the top plate19. The voice coil23is mounted within this air gap25. Due to the presence of the static magnetic field, any voltage applied to the voice coil23translates into an electro-dynamic force that causes the voice coil23to move. As the voice coil is coupled to the moveable membrane11, this movement of the voice coil causes the membrane11to move and to generate sound as a result. The membrane11will hereafter be referred to as the moveable membrane11.

The frame7is generally called a basket. The basket7is attached to the motor assembly5and it supports the moveable membrane11. The outer edge of the basket7, called the rim9, is normally used to host mounting screws which fix the loudspeaker1to a baffle or a cabinet (not shown).

The moveable membrane11is a rotationally curved surface, obtained by rotating a profile around the speaker's central axis3. The moveable membrane11is often referred to as a cone, because of the straight profile typically used in early loudspeaker designs.

Although to maximize the acoustic output (Sound Pressure Level, or SPL) of a driver a large moveable membrane11is desirable, due to several physical constraints, the larger a cone is, the less capable it is of loud (high power) sounds at very high frequencies (greater than about 2000 Hz). This is mainly due to the mass and the consequent inertia of the membrane11itself.

This limitation is normally addressed using more than one driver to achieve an extended frequency response without sacrificing the output power. Each of these (two or more) drivers is then designed to reproduce a specific part of the frequency spectrum. Common names for these drivers are woofer (devoted to reproduce frequencies in between 20 Hz and 2000 or 3000 Hz), midrange (mainly used to reproduce frequencies in between 200 Hz and 4000 Hz) and tweeter (solely used to reproduce frequencies higher than about 2000 to 4000 Hz).

All of these electro-dynamic transducers (drivers) share some or all aspects of the general design and working principle (i.e. motor assembly5) mentioned before in this section, but they also have specific differences which are not relevant to present invention.

A coaxial speaker is a particular example of loudspeaker in which two acoustic transducers (drivers) are mechanically coupled and share the same central acoustic axis3. The resulting device is as compact as a single woofer, but it is capable of covering the entire audible spectrum.

This layout offers benefits over the use of two separate drivers, the most important of which is that the acoustic centers of the woofer and the tweeter coincide which gives an acoustic output that does not suffer from path differences at different frequencies. Whilst this feature could be too subtle to sense when listening to a single speaker cabinet (mono), once the audio system has two (stereo set) or more (5.1, 7.1, 8.1 surround) speakers, this difference in performance is clearly audible.

Prior art coaxial speakers can be classified into two main types—those that use a dome tweeter and those that use a compression driver. The dome tweeter design is simple and cheap to manufacture but its sound output is of lower quality than that of the compression driver design.FIG. 1cshows a typical design of a dome tweeter coaxial speaker. As shown, a dome tweeter40is placed approximatively in the position of the dust cup12, inside the coupler41that couples the moveable membrane11to the voice coil23of the outer woofer driver. The dome tweeter therefore sits in front of the moveable membrane11of the woofer driver.

If a higher acoustic output is desired, then the commonly accepted solution is to use a compression driver rather than a dome tweeter. With the compression driver, the high frequency driver sits behind or below the moveable membrane11of the woofer driver and the membrane11helps to guide and control delivery of the high frequency sound.FIG. 1ddepicts an example of such a compression driver coaxial speaker. The compression driver42has its motor assembly, plus several other parts like the compression driver membrane43, the phase plug44and the horn45. Usual components like the woofer membrane11or surround15are always present. The purpose of the compression driver42is to impose a specific acoustic load on the moveable membrane43, while the horn45and the phase plug44re-align the acoustic wave front produced by the movement of the moveable membrane43and direct it in a predictable way towards the listener. This solution provides a much higher efficiency and a higher sound pressure level at high frequencies, but it is more complicated and costly to manufacture.

The inventors have found that the compression driver coaxial loudspeaker is susceptible to distortions particularly within the high frequency range of the generated acoustic signals; and they aim to modify the design of the loudspeaker to reduce such distortions.

The invention lies in the recognition by the inventors that to keep distortions to a minimum, the profile of the woofer membrane11should follow the profile of the horn45and should not present any obstacle protruding inside the portion of space in front of the moveable membrane11. In particular, the inventors have realized that the conventional design of surround15presents such an obstacle in the propagation path (illustrated by the arrow46inFIG. 1d) taken by high frequencies generated by the compression driver42; and that therefore, a new surround design is needed to reduce this distortion.

According to one aspect the present invention provides a coaxial loudspeaker comprising: a first driver for producing sound within a first frequency band; a second driver mounted coaxially with the first driver for producing sound within a second frequency band that is different from the first frequency band; and a frame; wherein the first driver comprises a first moveable membrane coupled to the frame and arranged to move relative to the frame to generate sound waves within said first frequency band in response to input drive signals, the first moveable membrane extending from an inner part adjacent a central axis of the loudspeaker to an outer part located above and spaced apart from a rim of the frame, the outer part of the first moveable membrane being coupled to the rim of the frame by a resilient surround; wherein the second driver comprises a second moveable membrane arranged to move to generate sound waves within said second frequency band in response to input drive signals, wherein the second driver is supported by the first driver such that the second moveable membrane is located below the first moveable membrane; and wherein the resilient surround sits behind the first moveable membrane extending from the rim of the frame to the outer part of the first moveable membrane such that the surround does not present an obstacle to sound waves generated by the second driver.

Typically, the second driver comprises a horn to guide the sound waves generated by the second driver towards a space in front of the first moveable membrane, and wherein the profile of the horn matches a profile of the first moveable membrane.

The resilient surround may extend from a first edge thereof that is fixed to the rim of the frame to a second edge thereof that is fixed to the outer part of the first moveable membrane. The resilient surround normally comprises one or more rolls that extend between the rim of the frame and the outer part of the first moveable membrane. The resilient surround may extend in a direction that is inclined outwardly away from the central axis from the rim of the frame to the outer part of the first moveable membrane or it may extend in a direction that is substantially parallel to the central axis of the loudspeaker.

The resilient surround may further comprise one or more air permeable walls that extend between the rim of the frame and the outer part of the first moveable membrane.

The first driver typically further comprises a motor assembly that is coupled to the frame and to the first moveable membrane such that in response to said input drive signals, the motor assembly causes the first moveable membrane to move relative to said frame.

The first moveable membrane and/or the second moveable membrane may have a circular, elliptical or oval shaped outer part.

The first driver may be arranged to generate sound waves having frequencies within a frequency range below 3000 Hz and the second driver may be arranged to generate sound waves having frequencies within a frequency range above 2000 Hz. Thus the first driver may be a woofer driver and the second driver may be a tweeter driver.

The surround may have an annular shape formed about the central axis of the loudspeaker and comprises a wall that extends in a direction that is substantially parallel with the central axis or that extends in an outward direction extending away from the central axis.

The invention also provides a method of making a coaxial loudspeaker comprising: providing a first driver for producing sound within a first frequency band; providing a second driver for producing sound within a second frequency band that includes frequencies that are higher than said first frequency band; providing a frame; coupling a first moveable membrane of the first driver to the frame so that the first moveable membrane can move relative to the frame to generate sound waves within said first frequency band in response to input drive signals; arranging the first moveable membrane to extend from an inner part adjacent a central axis of the loudspeaker to an outer part located above and spaced apart from a rim of the frame; mounting the second driver on the first driver so that the second driver is coaxial with the first driver and so that a second moveable membrane of the second driver is behind the first moveable membrane of the first driver; and coupling the outer part of the first moveable membrane to the rim of the frame by a resilient surround so that the resilient surround sits behind the first moveable membrane and does not present an obstacle to sound waves generated by the second driver.

OVERVIEW

As will become clear from the loudspeaker designs described below, the surround15of the coaxial loudspeaker1has been redesigned so that it does not present an obstacle in the high frequency propagation path46. Before describing the invention, it is instructive to consider the design of the moveable membrane11and of the surround15in the loudspeaker shown inFIG. 1d.

Referring toFIG. 1d, once the loudspeaker is placed (fixed) in position, the moveable membrane11can be considered to divide the space in which it sits into two separated portions: an inner portion in which the basket7sits, and an outer portion extending out away from the loudspeaker1. These portions are separated because the moveable membrane11and the surround15are formed of a material that is impermeable to air (otherwise movement of the membrane will not create any pressure waves). Thus the membrane11can be considered to have an internal surface facing the basket7and an external surface facing the outside of the loudspeaker1.

FIG. 1eshows in more detail the structure of the surround15used in the loudspeaker1shown inFIG. 1d. As shown, the surround15is attached in two locations: the first27is at the outer edge of the moveable membrane11and the second29is at the basket rim9. As shown, the surround15sits in a plane31which is perpendicular to the loudspeaker's central axis3. This plane is commonly the front plane of an enclosure housing the loudspeaker, or the plane of a baffle with which the loudspeaker is flush.

As shown inFIG. 1f, the surround15may comprise one or more annular portions, often referred to as rolls33instead of just one roll as shown inFIG. 1e. The number and radius of the rolls33of a standard surround15is directly linked to the maximum excursion of the moveable membrane11(i.e. using a large number of small rolls33or a single roll33with a large radius allows for more movement in the moveable membrane11). A large maximum displacement is desirable, because it impacts directly on the maximum acoustic output (SPL) of the loudspeaker1.

In both cases, the surround15creates a discontinuity in the smooth profile of the moveable membrane11that affects the high frequency acoustic propagation path46. With the large single roll, the surround's profile is substantially protruding inside the volume in front of the moveable membrane11profile, thus contributing to additional reflections of the high frequency acoustic wave. The surround having a large number of smaller rolls will offer a smaller obstacle to the high frequency acoustic propagation path46, but it extends over a larger portion of the cone flare which will be flexing and bending while the membrane11is moving, thus contributing to greater diffraction type distortions.

First Embodiment

FIG. 2ais a cross-sectional view of a coaxial loudspeaker1having a central compression driver42embodying the present invention. The loudspeaker1has a solid structure comprising a motor assembly5and a frame7having an outer rim9. A moveable membrane11is coupled to the frame7via at least two resilient or elastic components: a spider13and a surround15.

As discussed above, the motor assembly5is the active part of the main woofer (lower frequency) driver of the loudspeaker1, translating an electric power signal coming from an amplifier (not shown) into motion of the moveable membrane11. The motor assembly5has several parts including a magnet17, a top plate19, a pole piece21and a voice coil23that is coupled (directly or indirectly) to the moveable membrane11. The magnet17, the top plate19and the pole piece21form a magnetic circuit such that a static magnetic field is generated in the air gap25between the pole piece21and the top plate19. The voice coil23is mounted within this air gap25. Due to the presence of the static magnetic field, any applied voltage to the voice coil23translates into an electro-dynamic force that causes the voice coil23to move. This causes the membrane11to move and generate sound as a result.

The frame7(or basket) is attached to the motor assembly5and it supports the moveable membrane11. The moveable membrane11is typically a rotationally curved surface, obtained by rotating a profile (in this embodiment a curved profile) around the speaker's central axis3, which in this embodiment is also an axis of symmetry.

As shown inFIG. 2a, the compression driver42(higher frequency driver) is supported by the pole piece21of the main woofer driver, such that a moveable membrane43of the compression driver42sits behind or below (relative to the front of the loudspeaker) the moveable membrane11of the woofer driver and the membrane11helps to guide and control delivery of the high frequency sound. The compression driver42also has a motor assembly5′, plus the phase plug44and the horn45. The purpose of the compression driver42is to impose a specific acoustic load on the moveable membrane43, while the horn45and the phase plug44re-align the acoustic wave front produced by the movement of the moveable membrane43and direct it in a predictable way towards the listener. The profile of the horn45is arranged to match the profile of the woofer's moveable membrane11in order to minimize acoustic distortions at the junction between the two.

As shown inFIG. 2a, in this embodiment, the moveable membrane11curves outwardly from an inner part11-1located adjacent the loudspeaker's central axis3to an outer part (or edge)11-2adjacent the outer rim9of the frame7. In this embodiment, the moveable membrane11has a radial extent (measured from the central axis3) that is substantially the same as the corresponding radial extent of the frame7and it curves such that the outer part11-2is above and separated from the rim9. As shown, the surround15(which is generally annular in shape) has a single wall that is fixed at a lower edge15-athereof to the rim9of the frame7and at an upper edge15-bthereof to the outer part11-2of the moveable membrane11. Thus the single wall of the surround15extends (in height) in a direction represented by the arrow39that is substantially parallel to the central axis3of the loudspeaker1.

FIG. 2bis a three dimensional cross-sectional view of the surround15—showing the annular shape of the surround15.

The surround15is formed of a resilient or elastic material, such as rubber (natural or synthetic), vulcanized canvas, coated paper, silicon (for nano-speaker) or foam and has a number of rolls33(in this case two rolls) connected together in series between the rim9and the outer part11-2of the moveable membrane11to aid in the resilience of the surround15.

The moveable membranes11and43and the surround15are all made of materials that are substantially impermeable to air to allow for the efficient generation of sound (pressure) waves as the moveable membranes11,43move. In many speaker designs, the speaker housing (not shown) is designed to separate the inner volume of the loudspeaker1from the external volume of air in front of the loudspeaker. In this way, pressure waves generated in front of the loudspeaker1do not mix and cancel with the inverse pressure waves generated on the inside of the loudspeaker housing.

The design of the coaxial loudspeaker1of this embodiment has the advantage that the surround15does not present any obstacle to the high frequencies sound waves (generated by the compression driver) that travel along the high frequency path46—which helps to minimize sound distortions of the loudspeaker.

Second Embodiment

FIG. 3is a cross-sectional view of a coaxial loudspeaker1according to a second embodiment of the present invention, in which the surround15extends from the rim9to the outer part11-2of the moveable membrane11in a direction represented by arrow39′ that is inclined at an acute angle relative to the loudspeaker's central axis3and to the front plane47of the loudspeaker1. Typically, the surround is inclined relative to the central axis3at an angle of between 5 and 45 degrees. This design of loudspeaker1has a beneficial effect to the acoustic response of the speaker1. This is due to a progressive decoupling of the axial movement of the moveable membrane11with respect to the direction of the elastic force intrinsically present within the surround15. As with the embodiment shown inFIG. 2, as the rolls33of the surround are located underneath the moveable membrane11, the surround15does not present any obstacle to high frequency sounds travelling along the high frequency propagation path46.

Modifications and Alternatives

Two embodiments of the invention have been described above. As those skilled in the art will appreciate, a number of modifications and alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein. By way of illustration some of these alternatives and modifications will now be described.

In the embodiments described above, the elastic surround15was formed from a single wall of material that is impermeable to air. As those skilled in the art will appreciate, the surround15may have multiple walls (two or more), provided that only one of those walls is impermeable to air.FIG. 4aillustrates an alternative loudspeaker design based on the embodiment shown inFIG. 2a, in which the surround15has two walls15-1and15-2that both extend in the direction represented by arrow39which is substantially parallel to the loudspeaker's central axis3. In this embodiment, the inner wall15-2is made of a material that is permeable to air (which is represented in the figure by the dashed outline of the inner wall15-2) whilst the outer wall15-1is made of a material that is impermeable to air. As shown inFIG. 4a, the rolls33of the two walls of the surround15may not be identical, showing different cross-sections and profiles. By making the inner wall15-2of the surround permeable to air, air located in the space between the two walls is not trapped and can freely move as the surround15deforms to accommodate movement of the moveable membrane11. Of course, instead of the inner wall15-2being permeable to air and the outer wall15-1being impermeable to air, the opposite arrangement is possible—with the inner wall15-2being impermeable to air and the outer wall15-1being permeable to air. As before, the rolls33of the surround15are located underneath the moveable membrane, and so the surround15does not present any obstacle to high frequency sounds travelling along the high frequency propagation path46.

FIG. 4billustrates (by way of the arrow50) the way in which the loudspeaker1may be attached to a wall or baffle51, such that the front surface of the loudspeaker1sits flush with the front surface of the wall or baffle51. As shown, the rim9extends beyond the point where the surround15connects to the rim, to provide a fixing point for fixing the loudspeaker to the wall or baffle51.

FIG. 5a, shows a modification to the embodiment shown inFIG. 4ain which a modified surround15is used having two walls15-1and15-2, that are inclined relative to the central axis3. Again, either of the inner wall15-2or the outer wall15-1may be made of a material that is permeable to air (with the other one being impermeable to air) so that air can flow between the air chamber formed between the two walls and the inner volume of the loudspeaker.

The above described embodiments have used coaxial loudspeakers having a generally circular shape. As those skilled in the art will appreciate, the present invention is applicable to a number of different shapes and designs of coaxial loudspeakers. For example, the present invention is applicable to all possible coaxial loudspeaker shapes, including but not limited to circular, elliptical, and oval shaped coaxial loudspeakers.

The invention can be used with other designs of coaxial loudspeaker, including those described in GB 2502189.

As described above, the moveable membrane11is typically a rotationally curved surface, obtained by rotating a profile around the loudspeaker's central axis3. Depending on the profile curve, the surface of the moveable membrane11could be shaped in a number of different ways, offering different mechanical and acoustic properties.

Whilst the coaxial loudspeaker may be sealed into an enclosure or fitted flush with a baffle, it is also possible to have enclosures that are intentionally not sealed. They may have a vent port or employ a horn or a transmission line to enhance some acoustic properties. It will be clear to one of ordinary skill in the art that the present invention could be used in all these enclosure types and other types of speakers such as inverse speakers and the like.