GB2122051A

GB2122051A - Loudspeaker systems

Info

Publication number: GB2122051A
Application number: GB08215906A
Authority: GB
Inventors: John Julian Price Shelton; Roy Victor George
Original assignee: Goodmans Loudspeakers Ltd
Current assignee: Goodmans Loudspeakers Ltd
Priority date: 1982-06-01
Filing date: 1982-06-01
Publication date: 1984-01-04

Abstract

A casing 10 has an internal partition member 12, forming a front and a rear chamber, 14 and 16, respectively within the casing 10. A baffle 18 has a front loudspeaker 20 mounted therein and partition member 12 has a rear loudspeaker 22 mounted therein, rear loudspeaker 22 facing the rear of front loudspeaker 20. An audio frequency signal 46 is applied to front loudspeaker 20 to provide a drive therefor. A pressure sensitive device 48, mounted within front chamber 14, produces an electrical signal indicative of pressure variation within the chamber. This signal is then applied to rear loudspeaker 22, preferably via a feedback circuit 50 and an operational amplifier 52, so as to drive the rear loudspeaker. <IMAGE>

Description

SPECIFICATION Loudspeaker systems This invention relates to loudspeaker systems and particularly, though not exclusively, to dualunit bass loudspeaker systems.

A loudspeaker of this kind is described in British Patent No. 1 500711 (Tiefenbrun), wherein two loudspeaker diaphragms, positioned one behind the other, are operated in phase by the same audio frequency signal source, so that the pressure of air in a chamber between the two diaphragms remains substantially constant This has the effect of allowing the front loudspeaker diaphragm to operate at its free air resonance frequency, which thus allows its forward acoustical output to be substantially free from colourations which would normally be associated with closed box, reflex or transmission line loading conditions.

However, this known system relies on the loudspeaker diaphragms being closely identical in physical dimensions and acoustic performance, which may impose a practical drawback on the system, wherein the use of two high-quality, relatively expensive loudspeakers would be desirable.

Moreover, in the system described in British Patent No. 1 500 71 1, it seems necessary that the response characteristics between the two loudspeakers are substantially the same and, to the extent that they differ, a curtain of absorbent material is required in the chamber between the loudspeaker diaphragms to absorb any distortion components resulting therefrom.

It is therefore an object of the present invention to provide an improved loudspeaker system in which the characteristics need not be as closely matched as in the system described in British Patent No. 1 500 711.

According to the invention there is provided a loudspeaker system comprising a casing having a wall member which supports a first toudspeaker unit, said unit facing outwardly of the casing and being connectable to a source of electrical signals conveying audio information, the casing also including a partition member which supports a second loudspeaker unit in a position to face the rear of said first loudspeaker unit, a pressure sensitive device mounted in a region defined, within said casing, between said wall member and said partition member, said pressure sensitive device being arranged to generate further electrical signals, and the system also including means for applying said further electrical signals to said second loudspeaker unit.

The present loudspeaker system according to the invention therefore not only provides the advantage shown in British Patent No. 1 500 711 by causing the pressure in the region defined between the wall member and the partition member to remain substantially constant, but also provides an additional advantage wherein substantial differences between the characteristics of the loudspeaker units can be accommodated due to the pressure sensitive device producing an electrical signal, indicative of pressure variations within the chamber, which is used to drive the second loudspeaker.

Consequently, in the present system, a relatively expensive loudspeaker may be employed for the first loudspeaker at the front of the casing, whilst a relatively cheap loudspeaker may be employed for the second loudspeaker at the rear.

The present invention will now be further described by way of example only with reference to the accompanying drawing, the single figure of which shows a sectional view of a loudspeaker casing incorporating the invention.

Referring to the figure, a casing 10 has an internal partition member 12, forming a front and a rear chamber, 14 and 1 6 respectively, within the casing 10. A front-facing baffle 1 8 has a front loudspeaker 20 mounted therein and the partition member 12 has a rear loudspeaker 22 mounted therein, the rear loudspeaker 22 facing the rear of the front loudspeaker 20.

Each loudspeaker consists of a conical diaphragm 24 attached to a cylindrical former 28. The former 28 is attached to a chassis 30 by a resilient suspension 32. A voice coil 34 is mounted on the former 28, and leads 36 are connected between the voice coil 34 and terminals 38 on the chassis 30. To the rear of the chassis a permanent magnet assembly 40 is mounted, which forms an annular flux gap 42 around a central cylindrical pole 44.

An audio frequency signal 46 is applied to the terminals 38 of the front loudspeaker 20. This causes the voice coil 34 of the front loudspeaker 20 to set up a magnetic field which acts in opposition to the magnetic flux in the gap 42, which causes the diaphragm 24 of the front loudspeaker 20 to oscillate in accordance with the source of the audio frequency signal 46.

A pressure sensitive device 48, mounted within the front chamber 14, produces an electrical signal indicative of pressure variations with the chamber. This electrical signal is then passed to a feedback circuit 50, which processes the signal and corrects any effects resulting from phase change, resonance, damping, etc. The output from the feedback circuit 50 is amplified by an operational amplifier 52 and then used to drive the rear loudspeaker 22.

The present loudspeaker system therefore provides a substantially constant pressure within the front chamber 14 and hence assimilates free field loading conditions for the front loudspeaker 20, whilst cancelling out any anti-phase effects.

In the preferred embodiment of the invention, the materials used for the diaphragm of the front and rear loudspeakers are, respectively, a plastics material, such as PVC, and paper, although it will be appreciated that various alternative materials could be utilised.

An advantage of using a paper diaphragm for the rear loudspeaker is that paper has a relatively low inertia and therefore requires less driving power than a plastics material.

The pressure sensitive device is preferably a capacitive microphone or piezoelectric pressure transducer made of either Poly Vinylidene Fluoride (PVDF) or bimorph crystal, although alternative suitable devices could be used. If an integrating microphone is utilised, the amplifier may be required to compensate for the resulting phase difference.

An acoustic wall lining may be provided for the casing to insulate it acoustically from other loudspeakers which may be closely positioned thereto.

The feedback circuit is optional and the omission of the circuit should not prevent adequate operation of the system according to the invention.

Claims

1. A loudspeaker system comprising a casing having a wall member which supports a first loudspeaker unit, said unit facing outwardly of the casing and being connectable to a source of electrical signals conveying audio information, the casing also including a partition member which supports a second loudspeaker unit in a position to face the rear of said first loudspeaker unit, a pressure sensitive device being mounted in a region defined, within said casing, between said wall member and said partition member, said pressure sensitive device being arranged to generate further electrical signals, and the system also including means for applying said further electrical signals to said second loudspeaker unit.

2. A loudspeaker system as claimed in Claim 1 wherein said means includes a feedback circuit for processing said further electrical signals before application thereof to said second loudspeaker unit.

3. A loudspeaker unit as claimed in Claim 1 or 2 wherein said means includes an operational amplifier for amplifying said further electrical signals before application thereof to said second loudspeaker unit.

4. A loudspeaker system as claimed in Claim 1, 2 or 3 wherein a diaphragm for said first loudspeaker unit is made from a plastics material.

5. A loudspeaker system as claimed in any preceding claim wherein a diaphragm for said second loudspeaker unit is constructed from a material having a relatively low inertia as compared with the material from which a diaphragm for first loudspeaker unit constructed.

6. A loudspeaker system as claimed in Claim 5 wherein said material is paper.

7. A loudspeaker unit as claimed in any preceding claim wherein said pressure sensitive device comprises a capacitive microphone.

8. A loudspeaker system as claimed in any one of Claims 1 to 6 wherein said pressure sensitive device comprises a piezoelectric pressure transducer.

9. A loudspeaker system as claimed in any preceding Claim wherein an acoustic insulative lining is provided for the casing.

10. A loudspeaker system substantially as herein described with reference to the accompanying drawing.