Loudspeaker bass reflex system

Abstract

An acoustic insert for lining a loudspeaker reflex port, the insert comprising an elongate hollow tube formed of a flexible material, having an outer wall sized and configured to seat within the port and an inner wall extending in the elongate direction, the inner wall having a circumference which varies along the length of the insert, When inserted in a port, the insert alters the port resonance so as to vary the acoustic perfoi mance of the loudspeaker.

Claims

1. An acoustic insert for lining an outer tube fitted to a loudspeaker enclosure to form a loudspeaker reflex port, the insert comprising an elongate hollow tube formed of a flexible, low density foamed polymer material, and having an outer wall sized and configured to seat removably within the outer tube and an inner wall extending in the elongate direction, the inner wall having a circumference which varies along the length of the insert, the reflex port and the enclosure being so dimensioned that air inside the reflex port acts as an acoustic mass, air inside the enclosure behind the driver acts as an acoustic compliance and together the air inside the reflex port and the enclosure form a Helmholtz resonator in the audio bass range.

2. The acoustic insert according to claim I wherein at any point along the length of the insert, the circumference of the inner wall of the insert is substantially symmetric.

3. The acoustic insert according to claim 1 wherein the inner wall of the insert is substantially symmetric in the elongate direction.

4. The acoustic insert according to claim I wherein the insert has two open ends and the inner wall of the insert is narrower at at least one point between the ends than it is towards the ends of the insert.

5. The acoustic insert according to claim I wherein the insert has two open ends and the inner wall of the insert is flared outwardly at at least one of the ends.

6. The acoustic insert according to claim 1 wherein the inner and/or outer wall of the insert, when viewed along the elongate direction, is substantially circular, elliptical, square or racetrack shaped.

7. The acoustic insert according to claim 1 wherein the outer wall of the insert is substantially the same size as an inner surface of the outer tube.

8. The acoustic insert according to claim I wherein the outer wall of the insert is sized so as to be an interference fit inside the outer tube.

9. The acoustic insert according to claim I wherein the insert is the same length as the outer tube.

10. The acoustic insert according to claim I wherein the insert is provided with a lip of greater size than the inner circumference of the outer tube at one end of the insert.

11. The acoustic insert according to claim I wherein the insert comprises an acoustic absorptive material.

12. The acoustic insert according to claim I wherein the inner wall of the insert deflects so as to absorb sound.

13. The acoustic insert according to claim I wherein the length of the tube is substantially the same as the length of the outer tube.

14. The acoustic insert according to claim I wherein the length of the tube is not the same as the length of the outer tube.

15. A kit comprising a plurality of acoustic inserts according to claim 1, of differing dimensions, configuration and/or material.

16. A loudspeaker of the reflex port type comprising one or more acoustic inserts according to claim 1.

17. The acoustic insert according to claim I wherein the foam polymer material is formed with closed cells.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described by way of example, and with reference to the accompanying drawings, in which;

(2) FIG. 1 is a schematic view of a bass reflex loudspeaker with loudspeaker enclosure.

(3) FIG. 2 is a schematic cut away view of an acoustic insert in accordance with the invention in position inside an outer tube forming a reflex port.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(4) FIG. 2 is an isometric, cross-sectional schematic of an arrangement 2 comprising an acoustic insert 6 seated within an outer tube 4 which is fitted to the enclosure of a loudspeaker (not shown). The acoustic insert 6 and the outer tube 4 together form an open ended, cylindrical tube, with a first end 8 opening at the wall of the loudspeaker enclosure and a second end 10 inside the enclosure. The insert flares outwardly at the first and second ends, with a central portion 14 of smaller radius (around the X axis shown) than at the two ends. The first end of the outer tube 4 has a lip 16 which engages with the wall of the loudspeaker enclosure as is known in the art. The insert, which is made of low density foam material, is also formed with a matching lip 18, which seats on the lip 16 when the insert is fully inserted within the outer tube 4; this prevents the insert from being pushed too far inside the outer tube, and instead ensures that the insert 6 is in the correct longitudinal position (along the X axis) relative to the outer tube 4. Also, the lip 18, being outside the loudspeaker enclosure, is easy to grasp if the insert 6 is to be removed. In use, the insert tunes the resonance of the arrangement to attenuate certain frequency ranges (but not others), according to the configuration and dimensions of the inner wall of the insert 6), so as to alter the acoustic performance of the loudspeaker. The resilience of the material from which the insert is formed may be such that in use it deforms slightly and/or the inner wall of the insert deflects and so absorbs sound.

(5) 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. For example, there may be additional means to hold the insert 6 in place within the outer tube 4, as described above; a suitable place for the application of a releasable adhesive is at the interface between the lips 16, 18; if a peelable adhesive is applied to one or other lip, this holds the insert securely in place until, when the insert is to be removed, the insert lip 18 is peeled away from the port lip 16 and the insert can then be pulled out of the outer tube. Other means could be used to releasably connect the lips 16, 18, such as a bayonet fixing, and/or the lip 16 could be separate from the outer tube 4: this would be advantageous, as such a separate lip could then be configured for mounting the arrangement to a loudspeaker enclosure, with different configurations being available for use with different enclosures. Where the foam insert is self-supporting, part of it may protrude beyond the end of the outer tube. The insert 6 is shown with only a single constriction (generally at 14), however there could be two or more constrictions spaced along the X axis, and the longitudinal shape could be like a wave (e.g. sinusoidal). The inner surface of the tube 6 is shown as being longitudinally symmetric (i.e. along the X axis) and concentric, but there may be benefits in some applications for it to be asymmetric (e.g. the “peaks” and “troughs” in a wave-shaped inner surface may not be equally spaced). As stated above, the inner circumference of the insert is symmetrical at any point along the X axis, however it may be any shape, including asymmetric; in general however, the acoustic calculations are simplest for where the insert is cylindrical and/or symmetric/concentric. Those skilled in the art will understand that, for a generally cylindrical insert (as illustrated), the internal diameter may be varied, as well as its length, to give a wider range of tuning frequencies for less variation in depth, and that similar considerations apply to non-symmetric and/or non-concentric inserts. The inner surface of the insert 6, being made of foam, has a certain roughness, however this surface could in some configurations be coated so as to have a smooth surface; alternatively the insert 6 could be formed with a smooth inner surface. For example by moulding the insert in a foamed material the surface of the moulding forms a solid skin which is sufficiently smooth. The insert 6 could be coloured or bear suitable markings to indicate its effect on tuning the frequency response of the loudspeaker (for example, the inserts could vary in shade, with darker hues indicating a “lower” tune and lighter hues a “higher” tune, or they could be numbered according to a list of different degrees of tuning). The insert may advantageously also comprise means for damping longitudinal resonance such as the impermeable, frequency-dependent acoustic leakage path arrangements described in GB2488758 (which provides a frequency-dependent acoustic leakage path in the reflex port tube, such as by having a port conduit acoustically coupling the interior of the enclosure to a region external thereto, the port conduit comprising an acoustic leakage path through a motile part thereof in a direction transverse to a longitudinal axis of the port conduit, the acoustic leakage path having a relatively high acoustic impedance at a first frequency value, and a relatively low acoustic impedance at a second, lower, frequency value; this can be achieved with a plurality of holes in the port tube and an impermeable membrane lining the conduit). The arrangement could be provided with a front flare, or annular facing element which has the dual purpose of clamping the front flange or lip 18 of the insert 6 to the lip 16 and of improving the appearance of the arrangement 2 when assembled. Furthermore, where different variations or alternative arrangements are described above, it should be understood that embodiments of the invention may incorporate such variations and/or alternatives in any suitable combination.