GB2049351A - Loudspeaker unit - Google Patents

Abstract

A partition 14 divides the interior of a cabinet 10 into a lower air chamber 16 in which a bass/midrange speaker unit 12 is provided and an upper air chamber 15 which communicates with the lower air chamber through ports (33) covered with damping material 34. The lower air chamber 16 also communicates with the exterior of the cabinet 10 through ducts 22 and 28 in the manner of a bass reflex enclosure. The upper air chamber 15 essentially constitutes an air pocket which cushions inward movement of a diaphragm D of the speaker unit 12 sufficiently to prevent uncontrolled diaphragm excursion at low frequency resonances, while the ducts 22 and 28 provide good bass extension. In an alternative arrangement, the cushioning effect is obtained using a one-way valve which allows substantially unrestricted flow of air into the air chamber in which the speaker unit 12 is disposed, but which restricts air flow out of the air chamber to the cabinet exterior. <IMAGE>

Description

SPECIFICATION Loudspeaker unit This invention relates to a loudspeaker unit, principally for use in a hi-fi system.

In bookshelf-type loudspeaker units for hi-fi systems, a bass speaker unit is normally used to handle both bass and midrange frequences. If the speaker unit is mounted in a completely closed cabinet (a so-called "infinite baffle"), a damping effect is produced on the diaphragm of the speaker unit: although this effect can be desirable from the point of view of midrange frequencies, it does not permit good reproduction of bass frequencies. The repro duction of bass frequencies is significantly improved by making the loudspeaker unit of bass refex design, but such a design does not permit easy control of the greater diaphragm excursions which occur at low frequency resonances, and in many cases this leads to "booming".The bass reflex design also suffers from the disadvantages that the amount of lowfrequency sound waves which can be radiated in-phase by the reflex port is limited by the simplicity of the port itself, and tuning of the speaker enclosure is often difficult, resulting in a lack of clean bass frequency extension.

It is an object of the present invention to obviate or mitigate the above-mentioned disadvantages.

According to the present invention, there is provided a loudspeaker unit comprising a closed cabinet, a speaker unit mounted on a side wall of the cabinet, an air chamber defined with the cabinet and in which the speaker unit is disposed, port means providing communication between the air chamber and the exterior of the cabinet, and cushioning means arranged to cushion movement of a diaphragm of the speaker unit inwardly of the cabinet when the diaphragm is vibrated at low frequencies.

In this way, unnecessary excursion of the diaphragm induced by back electromotive forces is prevented, so that resonant peaks are kept in check.

The impedance rise is also well controlled.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross-section taken in a central vertical plane of a loudspeaker unit according to the present invention; Figure 2 is a front view of the loudspeaker unit as shown in Figure 1; and Figure 3 is a section taken along the line Ill-Ill in Figure 1.

The illustrated loudspeaker unit comprises generally a closed cabinet 10 having a high frequency speaker unit 11 and a bass/midrange frequency speaker unit 12 mounted on a front-facing side wall 13 thereof. A horizontal partition or platform 14 divides the interior of the cabinet 10 into an upper air chamber 15 in which the speaker unit 11 is disposed and a lower air chamber 16 in which the speaker unit 12 is disposed, the partition 14 being secured to the cabinet along all four sides thereof so that it braces the cabinet 10 internally.

A crossover network 16 for the two speaker units is mounted on the underside of the partition 14 and electrical leads from the crossover network pass through a drilling 17 in the partition to terminals 18 on a back wall of the cabinet. The crossover network is disposed in close proximity to the speaker units 11 and 12, such that short electrical leads can be used to optimise speaker performance. In order to minimise vibration, the crossover network is mounted on rubber, and care is exercised to ensure that its components do not resonate.

The lower air chamber 16 has bitumenised felt panels 19 secured to its bottom, side and rear walls to minimise the effects of spurious air waves on the midrange frequencies. A small amount of damping material is also used around the upper air chamber 15 to control the amplitude of the cabinet resonance.

The lower air chamber is almost completely filled with a damping material 20 of predetermined density, a small air gap 21 remaining between the top of the damping material and the partition 14. The air gap also extends around a diaphragm D of the speaker unit 12 so that an exact amount of resistance is provided against movement of the diaphragm inwardly of the cabinet at its resonant frequency, as will be explained later.

Disposed in the upper air chamber 15 on the central vertical plane P of the loudspeaker unit is an L-shaped duct 22, one arm 23 of the duct being horizontal and opening onto the front wall 13 of the cabinet, the other arm 24 thereof being vertical and opening into the lower air chamber 16 through a port 25 in the partition 14. The duct 22 acts to project forwardly of the loudspeaker unit the bass frequencies produced in the lower air chamber 16 in the manner of a bass reflex enclosure. The arm 23 of the duct is damped by a predetermined amount of damping material 26 (in this case, foam rubber) to provide a correct tone and to complement the arm 23 to the arm 24, thereby giving maximum depth and extension to the bass frequencies.

Also disposed in the upper air chamber on either side of the duct 22 are two further L-shaped ducts 28.

Each duct 28 comprises a horizontal arm 29 which opens onto the front wall 13 of the cabinet and which is considerably shorter than the corresponding arm 23 of the duct 22, and a vertical arm 30 which opens into the lower air chamber through a respective port 31 in the partition 14. These ducts 28 also act in the manner of a bass reflex enclosure to add weight to the bass frequencies. Foam rubber blocks 32 cover the port 31 to tune the ducts 28 and assist in converting out-of-phase base frequency air vibrations in the lower air chamber to in-phase air vibrations projected externally of the cabinet. Thus, the foam rubber blocks act as an acoustical inductance and an air wave resistance. Two such ducts 28 are provided in symmetrical relation with respect to the central vertical plane P of the loudspeaker unit to avoid giving the loudspeaker unit a "handed" characteristic.

The upper air chamber 15 communicates with the exterior of the cabinet through a small hole 27 provided in the back wall of the cabinet. The hole 27 is located on the central vertical plane P of the loudspeaker unit a small distance above the partition 14. In this way, air pressure within the upper chamber 15 can be kept to an absolute minimum in order to obtain a low resonant frequency for the speaker unit 12, since the hole 27 is provided at a point where the air flow takes the least line of resistance.

The upper air chamber 15 communicates with the lower air chamber 16 through a pair of ports 33 in the partition 14, the ports 33 being covered on the underside of the partition by further foam rubber blocks 34. As can be seen to advantage in Figure 3, the ports 33 are disposed symmetrically of the central vertical plane P of the loudspeaker unit. The upper air chamber effectively constitutes an air pocket, with the ports 33 controlling rarefaction in the lower air chamber and the hole 27 in the cabinet back wall controlling rarefaction in the upper air chamber. Thus, a dual rarefraction effect is obtained in which the components integrate with each other to eliminate turbulence and provide a smooth, excellently controlled airflow through the ducts 22 and 28.The air pocket provides a cushioning effect on movement of the diaphragm D of the bassl midrange frequency speaker unit 12 inwardly of the cabinet. This enables diaphragm excursion, particu Marly at its outer periphery, to be closely controlled at lower frequency resonances so that "booming" is avoided or minimised. Furthermore, there is a strong resistance to diaphragm "break-up" minimising spurious sound.

In one particular example, the loudspeaker unit has overall external dimensions of 214 x 122" x 11 -", the cabinet 10 and the partition 14 being made of 9" thick or 11 mm thick chipboard. The high frequency speaker unit is a peerless KO-1 0-DT, and the bassl midrange frequency speaker unit is a 10" Peerless KP-100-WF. The ducts 22 and 28 all have an external diameter of 2for" and a wall thickness of -", and the foam rubber blocks 32 which damp the ducts 28 are 1" thick. The ports 33 are also 22" in diameter, and their damping foam rubber blocks 34 are -" thick.

The hole 27 is 43" in diameter and is located approximately 1" above the partition 14. The bitumenised felt panels 19 used to damp the lower air chamber are 2" thick, as is the foam rubber 26 used to damp the arm 23 of the duct 22. Other dimensions are indicated in the accompanying drawings are as follows: a = 22" b = 81 " c = 10-" d = 3- ' e = 10" (nominal) f=2" The above-described construction has the following further advantages. There are no problems of port/sound cancellation, which is sometimes encountered in conventional bass reflex enclosures.

The resonant frequency of the bass/midrange frequency speaker unit 12 is as close as possible to its "free air" resonance point (26 Hzt, which ensures low fall off of low frequency output. Because the duct 22 is somewhat longer than each of the ducts 28, a continuation of bass extension over a large range of frequencies is obtained.

Subjectively, the loudspeaker unit has a linear sound characteristic, there is outstandingly low cabinet colouration of the sound, bass extension is excellent, and there is clean ambient detail (combining spacious detail resolution) due to careful control of damping and hence the avoidance of unwanted transient distortion effects in the audible frequencies. The loudspeaker unit is of compact design so that it is easily accommodated in domestic surroundings, and has a useful efficiency which suits low-powered hi-fi amplifiers (e.g. 10 watts RMS continuous output into 8 ohms impedance).

In an alternative embodiment (not illustrated), the cushioning effect on inward movement of the diaphragm of the bass/midrange speaker unit is achieved by using a one-wave valve to control the flow of air into and out of the air chamber in which the speaker unit is disposed. The one-way valve allows substantially unrestricted flow of air into the air chamber from the exterior of the cabinet, but restricts air flow from the chamber of the cabinet exterior.

In a further alternative arrangement (also not shown), the air chamber in which the bass/midrange speaker unit is disposed communicates with the exterior of the cabinet through ducts which are similar to the ducts 22 and 28 described above, but whose arms are not joined together so that effectively two separate ducts are formed at right angles to one another and with their ends in juxtaposition. In this case, the one-way valve can be provided in the partition.

Claims (11)

1. A loudspeaker unit comprising a closed cabinet, a speaker unit mounted on a side wall of the cabinet, an air chamber defined within the cabinet and in which the speaker unit is disposed, port means providing communication between the air chamber and the exterior of the cabinet, and cushioning means arranged to cushion movement of a diaphragm of the speaker unit inwardly of the cabinet when the diaphragm is vibrated at low frequencies.

2. A loudspeaker unit as claimed in Claim 1, wherein the cushioning means is provided by a one-way valve which allows substantially unrestricted flow of air into the air chamber from the exterior of the cabinet, but which restricts flow of air from the air chamber to the exterior of the cabinet.

3. A loudspeaker unit as claimed in Claim 2, wherein a partition divides the interior of the cabinet into a first air chamber in which the speaker unit is provided and a second air chamber which communicates with the exterior of the cabinet, and the one-way valve is provided between the first and second air chambers so as to allow substantially unrestricted flow of air from the second air chamber to the first air chamber, but to restrict flow of air from the first air chamber to the second air chamber.

4. A loudspeaker unit as claimed in claim 1, wherein a partition divides the interior of the cabinet into a first air chamber in which the speaker unit is disposed and a second air chamber which forms an air pocket and which communicates with the first air chamber, the port means provides communication between the first air chamber and the exterior of the cabinet, and the cushioning means is provided by the air pocket.

5. A loudspeaker unit as claimed in claim 4, wherein the second air chamber communicates with the first air chamber through at least one port in the partition having a layer of resiliently compressible material thereover.

6. A loudspeaker unit as claimed in claim 4 or 4 wherein the second air chamber communicates with the exterior of the cabinet through a small hole.

7. A loudspeaker unit as claimed in any one of claims 3 to 6, wherein the port means includes at least one port in the partition which is covered by a layer of resiliently compressible material.

8. A loudspeaker unit as claimed in any one of claims 3 to 7, wherein the port means includes at least one duct extending into the second air chamber from the first air chamber and covered by a layer of resiliently compressible material where it opens into the first air chamber.

9. A loudspeaker unit as claimed in any one of claims 3 to 8, wherein the port means includes at least one pair of ducts, the ducts in the or each pair extending into the second air chamber from the first air chamber and the exterior of the cabinet respectively and having their ends in juxtaposition.

10. A loudspeaker unit as claimed in any one of claims 3 to 9, wherein the port means includes at least one L-shaped duct which is disposed in the second air chamber and whose arms open respectively into the first air chamber and onto the exterior of the cabinet.

11. A loudspeaker unit substantially as hereinbefore described with reference to the accompanying drawings.