GB2218299A

GB2218299A - Artificial mouth

Info

Publication number: GB2218299A
Application number: GB8730346A
Authority: GB
Inventors: Robin Christopher Cross
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 1987-12-31
Filing date: 1987-12-31
Publication date: 1989-11-08
Anticipated expiration: 2007-12-31
Also published as: GB2218299B; GB8730346D0

Abstract

An artificial mouth comprises a loudspeaker (1) having a loudspeaker cone (1a) and a front plate (4) having an aperture (4a) aligned with the axis of the cone. The front plate (4) is provided with a plurality of resonator chambers (5) which open onto that surface (4c) of the front plate facing the interior of the loudspeaker cone (1a). The resonator chambers (5) are tuned to a predetermined frequency, namely to the frequency of the resonant peak that the frequency response of the artificial mouth would have in the absence of the resonator chambers (5). <IMAGE>

Description

ARTIFICIAL MOUTH This invention relates to an artificial mouth, and in particular to an artificial mouth for use in a telephone test set.

A telephone test set is used to measure all the important electrical and acoustical properties of a telephone. In order to carry out these measurements, a telephone test set needs an artificial ear and an artificial mouth to receive and transmit respectively acoustic signals.

An artificial mouth aims to simulate the frequency characteristics of a real mouth as accurately as possible. In order to do this, the voices of a large number of people are measured to derive an average "real mouth" frequency response. There are three main parameters associated with a real mouth, namely: (a)The effective point source, that is to say the point from which the sound coming from a mouth effectively emanates. This controls the radius of the wave front of the emittec sound, ana must be accurately replicated in an artificial mouth.

(b) The acoustic output impedance; and (c) The polar response, that is to say the directicnality of the emitted sound.

Known artificial mouths are capable of simulating these characteristics adequately. They are, however, expensive to make, and they have the aisadvantage of an uneven frequency response curve. This is disadvantageous in some applications, such as speech sensitivity measurement, where a flat frequency response is requireo.

The aim of the invention is to provide an artificial mouth that is relatively inexpensive, and which has a generally flat frequency response.

The present invention provides an artificial mouth comprising a loudspeaker having a loudspeaker cone, and a front plate having an aperture aligned with the axis of the cone, wherein the front plate is provided with resonator chamber means which open onto that surface of the front plate facing the interior of the loudspeaker cone, and wherein the resonator chamber means is tuned to a predetermined frequency.

The predetermined frequency can be chosen to be that of the resonant peak that the frequency response of the artificial mouth would have in the absence of the resonator chamber means. This resonant peak would result from the effect of the aperture in the front plate. The resonator chamber means is, thus, effective to absorb sound energy at this frequency, thereby flattening the frequency response.

Advantageously, the loucspeaker and the front plate are mounted on opposite sides of an annular support plate, the resonator chamber means being positioned within that portion of the front plate which overlies the central opening of the annular support plate.

In a preferrea embodiment, the resonator chamber means is constituted by a plurality of resonator chambers, each of which is tunea to said predetermined frequency.

Conveniently, there are six resonator chambers, each of which is generally cylindrical, the resonator chambers being positioned symmetrically with respect to the axis of the cone.

Advantageously, each of the resonator chambers contains sound-absorbing material. Preferably, the resonator chambers contain different amounts of sound-absorbing material, and acoustic grade foam constitutes the souna-absorbing material.

The artificial mouth may further comprise a closure plate which is positioned within the central opening of the annular support plate and overlying the resonator chamber means, the closure plate being formed with aperture means leading from the interior of the cone to the resonator chamber means. Conveniently, the aperture means is constitutea by a plurality of apertures, each of which leads from the interior of the cone to a respective resonator chamber. Preferably, the closure plate is formed integrally with a horn throat which is co-axial with the cone and the aperture in the front plate.

Advantageously, the front plate is made of aluminium, and that surface thereof remote from the interior of the cone is generally frustoconical having a cone angle of 1300.

The artificial mouth may further comprise a casing fixed to the support plate and defining a rear enclosure surrounding the loudspeaker. The casing may be made of non-foamea polyurethane, and the rear enclosure may contain a predetermined amount of sound-absorbing material such as scrim clad cotton wool.

An artificial mouth constructea in accordance with the invention will now be describeo in detail, by way of example, with reference to the accompanying drawing, the single figure of which is a schematic sectional view of the artificial mouth.

Referring to the drawing, an artificial mouth includes a loudspeaker assembly I mountea on an annular support plate 2 made of 5mm aluminium sheet. The loudspeaker assembly I is designed as a full range unit with an exceptionally good transient performance, and has an aluminium cone la having a diameter ot 50mm. This ensures good heat dissipation, which results in a relatively high figure of 20 watts, continous rating. The loudspeaker assembly 1 is surrounded by a casing 3, which is moulded from a cold-set, two-part, high density plastics material such as non-foamed polyurethane. The casing 3 is formed with four large internal webs (not shown) to reduce flexure of its circular eno plate, the casing being fixed to the support plate 2. The casing 3 defines an enclosure surrounding the loudspeaker assembly.

This enclosure, which has a volume of about C.5 litres, contains sound-absorbing material (not shown) such as scrim clad cotton wool. The provision of the sound-absorbing material helps to reduce standing waves in the rear enclosure defined by the casing 3.

The rear enclosure serves three purposes, namely: (a) It prevents the front ano rear souna waves from combining to zero in a direction normal to the axis of symmetry of the loudspeaker assembly.

(b) It provides a suitable compliance to load the drive units of the loudspeaker assembly 1, thereby preventing large non-linear cone excursions at low frequencies. This, in turn, reduces the distortion produces.

(c) It ensures that the source of sound is limitea to the front opening.

A front plate 4 is fixed to the support plate 2, the front plate having a central aperture 4a, a frustoconical outer face 4t, and a planar inner face 4c. The front plate 4 is made of aluminium, and its central aperture 4a has a ciameter of 20mm. This diameter is important in determining the effective point source of the artificial mouth, and ensures a good match with the target effective point source. The front plate 4 has a cone angle of 0 130 , and this results in a sound pressure distribution around the artificial mouth which is similar to that of a real mouth.

Six 13mm diameter cylindrical chambers 5 are formed in the front plate 4, these chambers opening onto that portion of the planar inner face 4c of the front plate which lies within the aperture of the annular support plate 2. The chambers 5 are fillea with different amounts of DF123 acoustic grade foam (not shown). The chambers 5 are substantially closed off by a closure plate 6 which is integrally formed with a horn throat 7. Each chamber 5 is joined to the interior of the loudspeaker cone la by means of a respective hole 6a formed in the closure plate 6.

The holes 6a each have a diameter of 3mm and a length of 3mm. The horn throat 7 controls the high frequency response of the artificial mouth, and also affects the output impedance and the position of the effective point source.

The chambers 5 constitute Helmboltz resonators, all of which are tuned to 1.2 kHz. This frequency is the same as the resonant peak which would occur in the mid-range response of the response of the artificial mouth if the resonators 5 were not present. This peak would be caused by resonance in the aperture 4a linking the loudspeaker 1 to the outside worlo. The six resonators 5 are effectively in parallel, and the variable amounts of foam they contain result in the resonators having different damping effects. Consequently, the quality factor (Q) of each of the resonators 5 is different. In practice, the Q of each of the resonators 5 is approximately equal to that of the resonant peak to be complemented. The spread of slightly different' Q values helps to ensure that the resonant peak is flattened if this peak wanders slightly under different operating conditions such as temperature, humidity and air pressure. The effect of the resonators 5 is to absorb energy at the resonant peak, thereby ensuring a relatively smooth and flat frequency response for the artificial mouth.

It will be apparent that the artificial mouth described above solves the problem of a peaked frequency response in a simple, cheap and effective way. As the rotation is purely mechanical, there are not electrical or electronic parts which could fail. Consequently, once the artificial mouth is set up and calibrated properly, it should operate reliably and consistently over long periods without any need for attention, and with little risk of faults developing.

Claims

1. An artificial mouth comprising a loudspeaker having a loudspeaker cone, and a front plate having an aperture aligned with the axis of the cone, wherein the front plate is provided with resonator chamber means which open onto that surface of the front plate facing the interior of the loudspeaker cone, and wherein the resonator chamber means is tuned to a predetermined frequency.

2. An artificial mouth as claimed in claim 1, wherein the loudspeaker and the front plate are mounted on opposite sides of an annular support plate, the resonator chamber means being positioned within that portion of the front plate which overlies the central opening of the annular support plate.

3. An artificial mouth as claimed in claim 1 or claim 2, wherein the resonator chamber means is constituted by a plurality of resonator chambers, each of which is tuned to said predetermined frequency.

4. An artificial mouth as claimed in claim 3, wherein there are six resonator chambers, each of which is generally cylindrical, the resonator chambers being positioned symmetrically with respect to tfle axis of the cone.

5. An artificial mouth as claimed in claim 3 or claim 4, wherein each of the resonator chambers contains sound absorbing material.

6. An artificial mouth as claimed in claim 5, wherein the resonator chambers contain different amounts of sound-absorbing material.

7. An artificial mouth as claimed in claim 5 or claim 6, wherein acoustic grade foam constitutes the sound-absorbing material.

8. An artificial mouth as claimea in any one of claims 1 to 7, further comprising a closure plate which is positioned within the central opening of the annular support plate and overlying the resonator chamber means, the closure plate being formed with aperture means leaning from the interior ot the cone to the resonator chamber means.

9. An artificial mouth as claimed in claim 8 when appendant to claim 3, wherein the aperture means is constituted by a plurality of apertures, each of which leads from the interior of the cone to a respective resonator chamber.

10. An artificial mouth as claimed in claim 8 or claim 9, wherein the closure- plate is formed integrally with a horn throat which is co-axial with the cone and the aperture in the front plate.

11. An artificial mouth as claimed in any one of claims 1 to 10, wherein the front plate is made of aluminium, and wherein that surface thereof remote from the interior of the cone is generally frustoconical having a cone angle of 0 13G .

12. An artificial mouth as claimed in any one of claims 1 to 11, further comprising a casing fixed to the support plate and aefining a rear enclosure surrounding the loudspeaker.

13. An artificial mouth as claimed in claim 12, wherein the casing is made of non-foamea polyurethane.

14. An artificial mouth is claimed in claim 12 or claim 13, wherein the rear enclosure contains a predeterminea amount of a sound-absorbing material such as scrim clad cotton wool.

15. An artificial mouth substantially as hereinbefore describea with reference to, and as illustratea by, the accompanying drawing.