WO1995029571A2

WO1995029571A2 - Apparatus including a sound transducer

Info

Publication number: WO1995029571A2
Application number: PCT/IB1995/000215
Authority: WO
Inventors: Bryan David Young; Alan Frederick Dadds
Original assignee: Philips Electronics N.V.; Philips Norden Ab; Philips Electronics Uk Ltd.
Priority date: 1994-04-20
Filing date: 1995-03-29
Publication date: 1995-11-02
Also published as: GB9407819D0; WO1995029571A3

Abstract

A part of an apparatus (14) for receiving human vocal output comprises a first transducer (20) to convert a source sound into an electrical signal, an amplifier (24) to amplify the electrical signal and a second transducer (16) for converting the amplified electrical signal into a sound wave substantially in antiphase with the source sound. The first and second transducers (20, 16) are located close together and the amplitude of source sound which 'spills over' around the apparatus is reduced considerably by the antiphase sound signal. The apparatus has particular application to cellular telephones and walkie-talkies for increasing privacy and reducing disturbance to nearby people.

Description

DESCRIPTION

APPARATUS INCLUDING A SOUND TRANSDUCER

Technical Field The invention relates to an apparatus for receiving human vocal output such as a telephone handset, dictating machine or walkie-talkie.

Background Art

The use of portable radio apparatus and cellular telephones in public places is ever-increasing and the annoyance caused to people nearby and the risk of sensitive conversations being overheard are acknowledged problems. Generally, to operate such equipment successfully a reasonably high level of speech is required but of course such speech is still audible at some distance from the speaker. Even when using conventional telephone handsets in an open plan office environment, the sound overspill can be considerable.

Disclosure of the Invention

An object of the present invention is to reduce the annoyance caused by using such apparatus within earshot of other people. According to the present invention there is provided an appliance for receiving human vocal output, comprising a first transducer for converting a source sound signal to an electrical signal, amplifying means coupled to the first transducer for providing an amplified electrical signal to a second transducer located less than substantially one sixth of a wavelength (at human speech frequencies) away from the first transducer, which second transducer is oriented with respect to the first transducer to provide a sound signal substantially in antiphase with the source sound signal for cancelling a portion of the source sound signal in proximity to the appliance.

The appliance in accordance with the present invention relies on an active cancellation technique. It has been realised that such a technique may be successfully applied to apparatus for receiving human speech, for example a mobile telephone, provided that attention is paid to the amplitude of the cancelling signal and to the relative separation of the first and second transducers. For the best possible cancellation performance, as will be described later, the two transducers should be located as close to each other as possible and ideally at a separation of no more than 10 millimetres. A convenient arrangement places the two transducers on opposite sides of a thin section of the housing of the apparatus and the second transducer is driven out of phase with the source signal either by an inverting amplifier or by reversing the electrical connections to the transducer. The gain of the amplifier may be such that the sound signal provided by the second transducer is substantially at the same amplitude as the source sound signal. The amplifier output may conveniently be used to drive other circuitry in the appliance and so a further output from the amplifier may advantageously be provided. The invention may also be advantageously applied to dictating machines and so on.

Brief Description of Drawings

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows a perspective view from the underside of a mobile telephone made in accordance with the present invention, Figure 2 shows a sectional view through part of the telephone shown in

Figure 1 showing the location of the first and second transducers, and

Figure 3 is a vector diagram to explain the operation of the noise cancelling arrangement.

In the drawings the same reference numerals have been used to denote the same features in the different Figures.

Mode for Carrying Out the Invention

Figure 1 shows a view of the underside of a mobile telephone handset

10 having a main portion 12 which includes a loudspeaker (not shown) for the user to hear the other party to the conversation and a keypad (not shown) as is familiar in such devices. The telephone handset 10 also includes a smaller portion 14 which is at an acute angle to the main portion 12 and has a first transducer or microphone 20 (on hidden top side) and a second transducer or loudspeaker 16 mounted alongside an array of holes in the casing.

Figure 2 shows a side sectional view of the smaller portion 14 of the telephone handset 10. The second transducer 16 is mounted on one side of the 5 handset casing beneath an array of holes to allow the sound wave generated by the second transducer to escape. Directly opposite, the microphone 20 is mounted on the other side of the casing, also beneath an array of holes. Electrical connections to the first and second transducers 20, 16 are shown at 22 and 18 respectively. An amplifier 24 is connected between the two o transducers 16, 20 and may comprise an inverting amplifier although a non- inverting amplifier may be used and the required antiphase separation of the sound input and output be provided by reversing the connections 22, 18 to either the microphone 20 or loudspeaker 16. A further output 26 from the amplifier 24 is coupled to further circuitry (not shown) depending upon the 5 application. Such further circuitry may comprise a transmitter in a mobile telephone or a recording head in a dictation machine.

The degree to which the noise cancelling arrangement reduces the spillover of sound from the user's speech is directly dependent upon the proximity of the two transducers 16, 20. If the distance between them is too o great, the higher frequency components of human speech will not be attenuated effectively because the cancellation signal from the second transducer 16 will not be sufficiently close to antiphase with respect to the spillover signal. In addition, considering the sound field off the axis through the first and second transducers 20, 16, there will be areas of good attenuation separated by areas 5 of constructive interference where the nuisance sound level is actually increased. For 20 dB attenuation of the spillover signal, the phase error between the speech signal and the antiphase cancellation signal must be no more than 5.7° assuming that the two signals have been matched closely for amplitude. If the separation of the two transducers 16, 20 is made to be 5 millimetres, a 0 20dB attenuation in the on-axis direction can be obtained at 955Hz. However the attenuation reduces to 10 dB at 3kHz. This level of attenuation has been found to reduce a quiet conversation to a whisper. A useful spillover sound attenuation has been found to be obtained even if the separation of the two transducers is increased to substantially 10 millimetres. The gain of the amplifier 24 may be such that the amplitude of the sound signal produced by the transducer 16 is substantially the same as source sound signal. Figure 3 shows a vector diagram including a vector 30 of a sound signal to be cancelled and the vector 32 of the ideal cancellation signal. However the separation of the two transducers means that this is not feasible in practice and the actual vector 34 of the cancellation signal will be closer to the vector 30. The angle between the ideal cancellation vector and the actual cancellation vector is shown as θ. If the angle θ is less than 60 degrees then the arrangement will provide a greater or lesser degree of sound cancellation. If θ exceeds 60 degrees then the vectors 34 and 30 will actually sum to a greater resultant than the original vector 30. The separation of the transducing portions (diaphragm and loudspeaker cone) of the two transducers should therefore not exceed one sixth of a wavelength at human speech frequencies.

From reading the present disclosure other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of appliances for receiving human vocal output and component parts thereof and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom. Industrial Applicability

Telephone handsets including cordless and cellular, private mobile radio, dictating machines and walkie - talkie apparatus.

Claims

1. An appliance for receiving human vocal output, comprising a first transducer for converting a source sound signal to an electrical signal, amplifying means coupled to the first transducer for providing an amplified electrical signal to a second transducer located less than substantially one sixth of a wavelength (at human speech frequencies) away from the first transducer, which second transducer is oriented with respect to the first transducer to provide a sound signal substantially in antiphase with the source sound signal for cancelling a portion of the source sound signal in proximity to the appliance.

2. An appliance as claimed in Claim 1 , characterised in that the gain of the amplifying means is such that the sound signal provided by the second transducer is substantially at the same amplitude as the source sound signal.

3. An appliance as claimed in Claim 1 , characterised in that the separation of transducing portions of the first and the second transducers is less than 10 millimetres.

4. An appliance as claimed in Claim 1 , characterised in that the first transducer and the second transducer face opposite sides of an appliance housing.

5. An appliance as claimed in Claim 1 , characterised in that the amplifying means comprises a further output for connection to other circuitry within the appliance.

6. An appliance as claimed in Claim 1 , wherein the apparatus comprises a telephone handset.

7. An appliance as claimed in Claim 1 , characterised in that the first transducer and the second transducer face opposite sides of an appliance housing, and in that the separation of transducing portions of the first and the second transducers is less than 10 millimetres.

8. An appliance as claimed in Claim 1 , characterised in that the first transducer and the second transducer face opposite sides of an appliance housing, in that the separation of transducing portions of the first and the second transducers is less than 10 millimetres and in that the gain of the amplifying means is such that the sound signal provided by the second transducer is substantially at the same amplitude as the source sound signal.