Loudspeaker

Abstract

Sound emanating from the high-frequency diaphragm of a coaxial speaker will diffract into the annular gap between the tweeter unit and the midrange cone. This results in response irregularities. We therefore disclose a loudspeaker, comprising first and second drivers located substantially coaxially with the first driver located centrally and the second driver located concentrically around the first driver, the loudspeaker being bounded at its radially outer side for at least part of its extent by the voice coil former of the second driver and including a spacing between the outermost extent of the first driver and the innermost extent of the second driver thus defining an annular space, the annular space containing a sound-absorbent material. By placing the sound-absorbing material in the annular space, the resonances within this space are damped, thus alleviating their effect. The annular space can have a lower resonant frequency that is below the passband of the first driver. Essentially, instead of minimising the effect of the annular gap by reducing its size and seeking to seal its outer opening, we propose to enlarge the space so that the fundamental resonant frequency it exhibits drops out of the passband of the high-frequency driver and hence out of the frequency range of interest. This both prevents the fundamental frequency of the cavity from being excited, and also allows sufficient room within the space to accommodate a sound-absorbent material to absorb these undesirable resonances.

Claims

1. A loudspeaker, comprising first and second drivers located substantially coaxially with the first driver located centrally and the second driver located around the first driver, each driver having a voice coil former, the loudspeaker including a spacing between the outermost extent of the first driver and the innermost extent of the second driver thus defining an axially-extending space, the space being bounded at its radially outer side for at least part of its axial extent by the voice coil former of the second driver, the axially-extending space being large enough to have a quarter-wave resonant frequency below the passband of the first driver and containing a sound-absorbent material.

2. The loudspeaker according to claim 1 in which the sound-absorbent material extends axially and has a radially outermost edge which is for at least part of its axial extent, at least one of adjacent to the voice coil former of the second driver or bounded by the voice coil former of the second driver.

3. The loudspeaker according to claim 1 in which the space is bounded at its radially inner side for at least part of its extent by a circumferentially-extending solid housing of the first driver.

4. The loudspeaker according to claim 1 in which the space extends rearwardly beyond the voice coil former of the second driver, in which region the sound-absorbent material completely fills the space.

5. The loudspeaker according to claim 4 in which the sound-absorbent material adjacent the voice coil former of the second driver is contained within the space along one edge thereof leaving an air gap remaining adjacent to the voice coil former.

6. The loudspeaker according claim 5 in which the sound-absorbent material is contained within the space along one edge of the outermost extent of the first driver.

7. The loudspeaker according to claim 1 in which the space is bounded at its radially outer side for at least part of its extent by the magnet structure of the second driver.

8. The loudspeaker according to claim 1 in which the space is annular.

9. The loudspeaker according to claim 1 in which the space is concentric around the first driver.

10. The loudspeaker according to claim 1 in which the space has a radius which varies along its axial extent.

11. The loudspeaker according to claim 10 in which the radius varies in a stepwise manner.

12. The loudspeaker according to claim 10 in which the radius is at a maximum adjacent the diaphragms of the first and second drivers.

13. The loudspeaker according to claim 1 in which the sound-absorbent material is one of an acoustic foam, a fabric, an open-cell foam, and a closed-cell foam.

14. The loudspeaker according to claim 1 in which the sound-absorbent material is supported on a former that is fitted to the first driver.

15. The loudspeaker according to claim 14 in which the former comprises a cylindrical section that fits around the first driver.

16. The loudspeaker according to claim 14 in which the former includes circumferentially-outwardly-projecting fingers for supporting the sound-absorbent material.

17. A loudspeaker comprising first and second drivers, each having a voice coil and a voice coil former, located substantially coaxially with the first driver located within the cavity formed by the voice coil of the second driver, the loudspeaker including a spacing between the outermost extent of the first driver and the innermost extent of the voice coil former of the second driver, the spacing being bounded at its radially outer side for at least part of its axial extent by the voice coil former of the second driver, the axially-extending space being large enough to have a quarter-wave resonant frequency below the passband of the first driver and containing a sound-absorbent material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which;

(2) FIG. 1 illustrates a known arrangement of a co-axial loudspeaker;

(3) FIG. 2 illustrates a co-axial speaker design with a resonant cavity;

(4) FIG. 3 shows the frequency-sound pressure response of the speaker design of FIG. 2;

(5) FIG. 4 shows a first embodiment of the present invention;

(6) FIG. 5 shows the frequency-sound pressure response of the speaker design of FIG. 4 vs that of FIG. 2;

(7) FIG. 6 shows a second embodiment of the present invention;

(8) FIG. 7 shows the frequency-sound pressure response of the speaker design of FIG. 6 vs that of FIG. 2;

(9) FIG. 8 shows an isometric view of a former suitable for supporting an acoustic foam element according to the present invention;

(10) FIG. 9 shows a side view of the former of FIG. 8;

(11) FIG. 10 shows a third embodiment of the present invention; and

(12) FIG. 11 shows a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(13) FIG. 4 shows a first embodiment of the invention. This shares several features with the arrangement of FIG. 2, and like reference numerals are used to denote like parts. The embodiment differs from the arrangement of FIG. 2 in that an annular sleeve of sound-absorbent material 60 in the form of acoustic foam has been fitted around the tweeter unit. This sits in the space between the outer trim 62 of the tweeter unit and the voice coil former 23 of the midrange unit, and effectively lines one side of the annular channel 44 from its deepest point 64 up to a point 66 just behind a ledge 68 of the outer trim 62. The ledge 68 thus conceals the sound-absorbent material 60 from view.

(14) Sound vibrations entering into the annular channel 44 will therefore be damped, and thus will have a reduced effect on the loudspeaker response. FIG. 5 illustrates measurements comparing the tweeter according to FIG. 2 but with a rigid card sleeve in the annular space 44 (line 70), and the tweeter of FIG. 4 with the acoustic damping sleeve 46 (line 72). The modification has successfully improved the upper part 74 of the tweeters response. Simulations of the tweeter using a rigid card (line 76) and the tweeter of FIG. 4 (line 78) bear this out; as before the simulations have been displaced by 6 dB for clarity. The odd order harmonics of the quarter wave resonance at approximately 7 kHz and 12 kHz are no longer present in the frequency response of the tweeter with the modification. The primary resonance is lowered in frequency by around 500 Hz.

(15) In this design, the thickness of the acoustic material 60 does need to be carefully chosen so that it does not come into contact with the voice coil former 23 of the midrange driver. Such contact would affect the movement of the midrange voice coil and have an adverse effect on the loudspeaker. FIG. 6 therefore shows an alternative embodiment which addresses this by extending the air path. Referring briefly back to FIGS. 2 and 4, the tweeter unit is supported in place by fitting concentrically within the magnet structure 16, 18 of the midrange unit. The pot 28 has a radially-extending flange 80 which sits on the forward surface of the front annular plate 18 and, behind that, an external screw thread 82 which allows a ring nut (not shown) to be fitted to the rear of the tweeter unit to clamp against the rear face of the magnet structure 16. In the embodiment of FIG. 6, the radially-extending flange 80 is omitted and replaced with a disc 84 of sound-absorbent material. In addition, there is an axially-extending space allowing for a sleeve 86 or sound-absorbent material to be fitted around the pot 28 behind and abutting against the disc 84. As a result, the annular space 44 is considerably extended; instead of ending at the midrange magnet structure 16, 18, it extends inwardly past the rear of the outer trim 62 of the tweeter (in the space occupied by the flange 80 shown in FIG. 4) and continues further axially in a narrower annular shape around the tweeter pot 28. The effect is to extend the air channel 44 (rather than seek to eliminate it) so that it now extends axially to the rear of the midrange magnet structure 16; this both moves its quarter-wave frequency below the output range of the tweeter and also provides space to accommodate the sound-absorbent material 84, 86 so that it is mostly away from the moving midrange voice coil 23, with only the only that part positioned at the same location as the radially outermost edge of flange 80 in FIG. 4 being in the vicinity of the voice coil 23. The sound-absorbent material 84, 86 can fill the extended part of the air channel, thus preventing sound from bypassing the foam. As mentioned above, the volume of the forward axially-extending part of the air channel 44 shown in FIG. 6 which contains no sound-absorbent material can be further reduced by inserting into it a thin axial sleeve of acoustically-permeable material such as paper, perforated card or mesh, taking care that this is not in contact with the moving voice coil 23.

(16) The total length of the air channel in FIG. 6 is now roughly twice as long as the original length in FIG. 2. As a result, the quarter wave resonance is reduced to around 1000 Hz so is no longer in the tweeter's effective passband when crossed over in a loudspeaker system. FIG. 7 shows corresponding simulations and measurements, lines 88 and 90 being the measurements comparing the FIG. 2 and FIG. 4 arrangements respectively, and lines 92 and 94 (respectively) being the corresponding simulations displaced by 6 dB. FIG. 7 shows that the acoustic absorbing material inside the elongated channel has effectively damped the quarter wave resonance and higher harmonics, avoiding response irregularities.

(17) FIGS. 8 and 9 show a preferred form for the tweeter pot 28 of FIG. 4. This both contains the tweeter structure and also supports the sound-absorbing material 84, 86. It comprises a generally cylindrical part 100, with a central bore 102 within the cylindrical part 100 to contain the tweeter structure. The cylindrical part 100 is externally threaded at 104, extending from a rearmost end 106 in order to accept a ring nut to secure the tweeter in place as described above. At a frontmost end 108, the cylindrical part has a retention collar 110 (not shown on FIG. 9) to assist in retaining it in place within the loudspeaker structure.

(18) Immediately behind the collar 110, four fingers 112, 114, 116, 118 extend radially outwardly from the cylindrical part 100, equally spaced at 90 intervals. Each finger is in the form of a rectangular tab that extends radially between to of the radial distance occupied by the disc 84 of sound-absorbent material. The tabs support the disc and allow it to be placed around the tweeter in a stable configuration for assembly of the loudspeaker. The disc 84 may have recesses or rebates formed in it to accommodate the fingers, thus reducing the distortion of the disc 84 around the fingers. Located in the gap occupied by the disc 84, the fingers also stop the ring nut from overtightening the tweeter and crushing the disc 84.

(19) Fingers 116, 118 have elongate grooves extending radially outward from a through hole formed in the fingers 116, 118 adjacent collar 110 to allow wired connections to pass to the high frequency driver.

(20) The sleeve 86 fits around the cylindrical part 100 behind the fingers, and can remain in place due to being a snug fit. Retention of the sleeve 86 is assisted by the screw thread 104 which will provide additional grip.

(21) FIGS. 10 and 11 show alternative examples. Again, in both figures, like reference numerals are used to denote like parts. Both figures show greater detail in relation to the magnet structure of the tweeter and midrange units; thus the midrange unit has a magnet 16 with pole pieces 18 and 18a conveying the magnetic flux to a gap 120 in which the voice coil 122 for the midrange unit is placed, supported by the voice coil former 23 which extends forward to the midrange diaphragm 21. Likewise, the tweeter has a magnet 124 and pole pieces 126a, 126b which define a gap 128 in which the voice coil 36 of the tweeter unit sits.

(22) FIGS. 10 and 11 also show the ring nut 130 which attaches to the rear of the tweeter assembly and tightens against the rear of the midrange unit pole piece 18, securing the tweeter unit in place.

(23) In the example of FIG. 10, the sound-absorbent material 132 is in the same general shape as that of FIG. 6, i.e. an annular disc sandwiched between the pole pieces 126a and 18 of the tweeter and midrange units respectively, with a cylindrical section extending rearwardly from the inner section of the annulus, located around the tweeter body 28. However, in this example the sound-absorbent material is in a single piece 132 rather than two (or more) sections. It may be formed ab initio in this shape, or cut to shape from a larger block of material. A former such as that illustrated in FIGS. 8 and 9 may be used to support the material, or may be set into the material prior to fitting.

(24) FIG. 11 shows an alternative shape of sound-absorbent material 134. It retains the annular disc section 136, sandwiched between the pole pieces 126a and 18 of the tweeter and midrange units respectively. However, instead of a cylindrical section extending rearwardly around the tweeter body 28, there is a second annular disc 138 located behind the first annular disc 136 within a radial slot 140 formed in the midrange pole piece 18. The two discs are joined via a short cylindrical linking section 142. The various elements of the sound-absorbing material 134 are, in this example, in a single contiguous unit, but may of course be made up of several small sub-units assembled together to form the required shape.

(25) Thus, in the example of FIG. 11, the sound path is along the open channel 44, then radially inwardly through the first annular disc 136, then axially through the linking section 142 and, lastly, radially outwardly through the second annular disc 138. Some sound may reflect from the base of the radial slot 140, but it will be reflected back into the sound-absorbent material 134 and is therefore unlikely to escape. This demonstrates that it is the overall path length that is of particular interest, as opposed to the specific shape in which that path is formed.

(26) Thus, the present invention provides a straightforwardly-manufacturable structure that alleviates the problematic resonances caused by the air gap between the two elements of a co-axial loudspeaker. A variety of detailed structures are possible, allowing the solution to be applied to a wide variety of loudspeaker designs, which may differ from those illustrated.

(27) It will of course be understood that many variations may be made to the above-described embodiment without departing from the scope of the present invention.