WO2001087004A2 - A loudspeaker incorporating an electromagnetic screen - Google Patents

Abstract

A loudspeaker is provided which has an integrally formed electromagnetic screen (66, 68) or a narrow passage (61) which attenuates radiation passing therethrough. Such a loudspeaker can be driven directly from a switched mode amplifier such as a class D amplifier. This enables a reasonable audio level to be achieved from a low supply voltage.

Description

A LOUDSPEAKER INCORPORATING AN ELECTROMAGNETIC SCREEN AND OPTIONALLY AN INTEGRAL AMPLIFIER

The present invention relates to a sonic transducer, such as a loudspeaker, including an electromagnetic screen. The transducer may optionally also include an amplifier.

Audio reproduction apparatus typically uses a moving coil loudspeaker in order to convert electrical signals into acoustic energy. The coil of the loudspeaker is located in an air gap in which there is a magnetic field produced by a permanent magnet. The coil typically has a resistance of 8 ohms and an inductance of 0.4 mH. The coil is usually driven from a voltage amplifier which takes an audio signal, amplifies it to a larger voltage and then applies this voltage to a loudspeaker coil. Motion of the coil resulting from interaction between the current in the coil and the magnetic field from the permanent magnet causes the coil to vibrate, and this motion is transmitted to a radiating element normally in the form of a conical diaphragm.

There are various types of amplifier which can be used to drive the loudspeaker. Typically, audio amplifier stages have been configured as class B (push-pull) stages. Each of the output transistors operates in its linear region and consequently a significant amount of energy is to be dissipated by each transistor. However, the class B stage is not the most efficient output stage. Greater efficiency could be obtained by using a class D amplifier which uses switching to produce a pulse width modulated wave form. Figure 1 of the accompanying drawings schematically shows a typical class D amplifier employing an H-bridge power circuit. However, persons skilled in the art will understand that a half bridge output configuration may also be employed with one side of the load being driven by the half bridge and the other side of the load being connected to one or both of the power rails via one or more DC blocking capacitors.

In the ideal case, the class D amplifier has an efficiency of 100%.

The circuit shown in Figure 1 has positive and negative voltage supply rails 1 and 2 respectively. The H-bridge output circuit comprises a first MOSFET 3 arranged in series with a second MOSFET 5 between the supply rails 1 and 2. The MOSFETs 3 and 5 have reversed biased body drain diodes 4 and 6, respectively. Similarly a second portion of the H-bridge output stage is formed by a third MOSFET 7 in series with a fourth MOSFET 9 between the supply rails 1 and 2. The third and fourth MOSFETs have associated body drain diodes 8 and 10. The connection between the first MOSFET 3 and the second MOSFET 5 defines a node 12. Similarly the connection between the third MOSFET 7 and the fourth MOSFET 9 defines a node 14. A loudspeaker 16 which is electrically represented by an inductance 18 and a resistance 20 is connected to the node 12 via a first inductor 22 and to the node 14 via a second inductor 24. Additionally a first capacitor 26 is connected between one of the supply rails (in this example the negative one) and the node between the inductor 22 and the loudspeaker 16. Similarly a second capacitor 28 is connected between one of the supply rails and the node between the second inductor 24 and the loudspeaker 16.

Each of the MOSFETs 3, 5, 8 and 10 has its gate connected to a respective input of a MOSFET drive controller 30. The drive controller 30 controls the MOSFETs such that the MOSFETs 3 and 5 conduct in anti-phase, and the MOSFETs 7 and 9 conduct in anti-phase. The conductor 30 receives an audio input signal via an input line 32, converts this to a pulse width modulated format and then generates the drive signals for the MOSFETs in the H-bridge output stage. Circuit configurations for performing the analogue to pulse width modulation conversion and generation of the drive signals for the output transistors are known to the person skilled in the art and do not constitute part of the present invention. The class D amplifier is located within a shielded enclosure 34 and leads into and out of the shielded enclosure pass through feed through capacitors, ferrite beads or the like in order to suppress electromagnetic interference. Figure 2 schematically illustrates a comparison between an audio wave form 32 and the pulse width modulation wave form 34 occurring at the node 12 in Figure 1. A similar wave form (with the audio element in anti-phase) would be seen at node 14. It will be observed that the pulse width modulation wave form contains rapid transitions between the supply rail voltages and that many transitions occur in the time period associated with a single cycle of the audio input wave form. The inductance 18 and resistance 20 of the moving coil loudspeaker serves to filter the pulse width modulated wave form thereby effectively converting it back to an audio signal. The rapid transitions between the supply rails gives rise to the generation of harmonics. For this reason, class D amplifiers can be the cause of unwanted electromagnetic/radio interference. In order to reduce the propagation of interference into the environment, the inductor 22 and capacitor 26 forms a two pole low pass filter. Similarly capacitor 28 and inductor 24 also form a filter. These filters attenuate the harmonic signals thereby preventing electromagnetic interference from being radiated from the output leads. Similarly capacitors extending between the supply rails serve to filter harmonic interference from the supply rails. However these LC filters can attenuate audio frequency components and can also become involved in resonance at audio frequencies causing unwanted peaks in the frequency response characteristics of the amplifier and loudspeaker combination.

If the requirement for filtering out rapid changes in the voltage wave form is removed, and instead emphasis is given to ensuring that a high impedance is presented to unwanted high frequency components of the load current whilst presenting a low impedance to the relatively low frequency audio currents, it is sufficient to include the inductance alone in the load circuit. Thus capacitors 26 and 28 can be removed leaving inductors 22 and 24 solely responsible for the filtering operation. Since the voltage wave form applied to the loudspeaker is a pulse-width modulated wave form which will now produce electromagnetic interference, the screened can becomes of minimal benefit and could be removed.

Clearly, the greater the value of the inductors 22 and 24, the greater will be the attenuation of the high frequency components of the current. However, as the inductance is increased, the supply voltage of the class D amplifier will also need to be increased in order to allow it to achieve the rate of change of current needed to follow the audio signal accurately. Conversely, if the supply voltage is reduced, then the amount of inductance associated with the output stage will also need to be reduced if the amplifier is still to accurately reproduce the input signal. This would mean that the radio frequency interference filtering also becomes reduced.

For minimum power consumption for any given sound output, the amplifier supply voltage should be reduced to a low a value as possible so that switching losses are minimised. There is, however, a lower limit to this supply voltage since sufficient output voltage must be available to drive current through the loudspeaker resistance and achieve the desired current slew rate. The applicants have found that a supply voltage of about 1 volt will allow an 8 ohm loudspeaker to produce a loudness which is adequate for many purposes. A supply of 1 volt can be supplied from a single dry cell. However, with a mere 1 volt supply no additional inductance can be inserted into the load circuit to filter out the switching frequency components otherwise the current to the loudspeaker will not be able to change quickly enough

According to a first aspect of the present invention, there is provided a sonic transducer comprising an electromagnetic screen integrally formed with the transducer so as to attenuate electromagnetic waves emanating from the transducer.

It is thus possible to provide a transducer capable of generating sound, be it at frequencies audible to humans or at higher frequencies, and which includes electromagnetic screening components therein such that the drive signal which is at or which contains harmonics at frequencies which may give rise to electromagnetic interference can be used to drive the transducer. The integral screen attenuates electromagnetic interference.

Preferably the transducer further comprises a drive device (ie an amplifier) as an integral part of the transducer. The driver may be formed of discrete components, or may be provided as part of an integrated circuit. Such an integrated circuit may be a monolithic integrated circuit or a hybrid integrated circuit.

Preferably the drive device includes components operating in a digital manner. Such a component, such as a transistor, is nominally either fully on or fully off. Such a mode of operation minimises power dissipation within the driver. As a result the efficiency of the driver is increased. This has the benefit that either greater amplitude of sound can be obtained from a driver operating at a given supply voltage, and/or the supply voltage to the driver can be reduced.

Preferably the drive device/amplifier includes output transistors arranged in a "H" bridge configuration. Such a configuration enables the voltage applied across the transducer to be reversed, thereby allowing the peak-to-peak drive voltage applied to the transducer to approach 2Vs, where Vs represents the supply voltage of the driver.

Advantageously the driver further includes a converter for receiving an analogue input signal and converting this into a digital representation. The digital representation may, for example, be a pulse-width modulated signal of the analogue input signal.

Additionally or alternatively the converter may be arranged to receive an input signal in a first digital format, and to convert it into the digital representation for driving the transducer.

Preferably the transducer is a loudspeaker.

In conventional designs of moving coil loudspeakers, a coil is held in the air gap of a permanent magnet which comprises a central core which forms one of the poles surrounded by a circular element which forms the other magnetic pole. In order to complete the magnetic circuit, the poles are interconnected by a pole piece. This configuration means that the permanent magnet forms a screen around the moving coil, except in the direction extending towards the front of the loudspeaker. However, the front of the loudspeaker often faces into the environment in order to maximise radiation of sound into the environment, this means that electromagnetic interference can be radiated from the front of the loudspeaker.

Preferably a conductive screen is located between the coil of the loudspeaker and the environment in front of the loudspeaker.

Preferably the central core is extended in a forward direction such that it can become in relatively close contact with a circular flange extending radially inward from the circular element. This creates a relatively narrow annular gap through which a coil support tube can extend. The support tube serves to transmit motion of the coil to the acoustic radiating element of the loudspeaker, which is typically a cone. The narrow gap means that the coil is almost entirely contained within a screened enclosure. This attenuates the radiation passing through the gap to a much smaller value. Additionally and/or alternatively, a flexible conductive element may be formed in the vicinity of the interface between the coil support and the acoustic radiating element. Loudspeakers are often provided with a dust cap and flexible bellows. These can be made of or coated with a conductive material such that these serve to attenuate electromagnetic interference.

According to a second aspect of the present invention, there is provided a moving coil loudspeaker incorporating an amplifier having an output stage operating in a switched mode, the coil of the loudspeaker being directly connected to the output stage.

Preferably the loudspeaker has an electromagnetic screen integrally formed therewith.

The present invention will further be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 schematically illustrates a class D amplifier driving a loudspeaker via two pole filters;

Figure 2 compares an analogue wave form with its pulse width modulated equivalent;

Figure 3 is a cross sectional view of a loudspeaker constituting a first embodiment of the present invention;

Figure 4 shows a modification to the loudspeaker of Figure 3 wherein an amplifier is integrally formed with the loudspeaker;

Figure 5 illustrates a further embodiment of the present invention;

Figure 6 is a circuit diagram showing the electrical arrangement of the embodiments shown in Figures 4 and 5;

Figures 7a and 7b illustrate a conventional loudspeaker design and an extended pole/shortened coil design of a loudspeaker, respectively; and

Figure 8 schematically illustrates a further embodiment of the present invention. Figure 3 illustrates a loudspeaker, generally labelled 36, constituting a first embodiment of the present invention. The loudspeaker comprises a permanent magnet 40. For a loudspeaker orientated as shown in Figure 3, the bottom most surface of the magnet is closed by a circular soft iron plate 42. An upstanding soft iron pole piece 44 extends from the centre of the plate 42 and extends above the upper boundary of the permanent magnet 40. A further circular soft iron plate 46 having a hole formed in the centre extends radially inwardly from the uppermost surface of the magnet 40 and serves to define an air gap 48 in conjunction with the pole piece 44. A voice coil 50 of the loudspeaker is wound on a coil support 52. The voice coil 50 is positioned such that, with the speaker in an undriven state, the centre of the voice coil is substantially centrally located in the air gap 48. The voice coil support 52 mechanically couples the voice coil 50 to the diaphragm 54 of the loudspeaker.

In a conventional loudspeaker, the uppermost surface of the pole piece 44 is substantially aligned with the uppermost surface of the element 46. However in a speaker constituting an embodiment of the present invention, the uppermost surface 56 of the pole piece 44 is extended past the element 46. A metal ring 60 is provided adjacent the end of the pole piece 44 and is positioned and dimensioned such that it co-operates with the pole piece 44 to define a small annular gap 61 through which the voice coil support 52 extends. The metal ring 60 is attached to a cylindrical region 62 of the support frame 64 for the loudspeaker which in turn is connected to the element 46. Thus the ring 60, region 62, element 46, magnet 40, circular plate 42 and pole piece 44 serve to define a screened enclosure 63 which only has a small annular opening 61 therein through which the voice coil support extends.

Any electromagnetic radiation generated as a result of supplying current to the voice coil can only escape through this narrow gap and as such becomes highly attenuated as it passes therethrough. Thus radiation escaping via this route is unlikely to cause interference. Nevertheless, further electromagnetic screening can be provided by forming a dust cap 66 (such dust caps being commonplace in loudspeakers) of a conductive material such as a very thin metal or flexible material impregnated with a conducting material, and connecting this via electrically conductive flexible bellows 68 to the conductive frame 64 of the loudspeaker. Indeed, this combination of flexible bellows and dust cap may be made sufficiently attenuating to allow the ring 60 to be omitted.

The ring 60 is made of a non-magnetic material in order to prevent the ring providing an "magnetic short circuit" for the magnetic flux in the centre pole piece. Alternatively, part of the centre pole piece 44 which extends beyond the region of the voice coil may be made of a non-magnetic but conducting material.

The voice coil 50 is connected to flexible conductors 70 which pass out of the screened enclosure through a hole 72 in one of the walls dividing the enclosure. They then pass into a screened cable 74 for connection to an amplifier. In an alternative arrangement, the wires 70 may pass through the bellows 68 and then immediately enter a flexible screened cable.

Figure 4 illustrates a further embodiment of the present invention in which a class D amplifier 76 is included within the screened enclosure. The amplifier need not be positioned as shown in Figure 4, but may for example be attached to the end plate 42. Electrical power and signals are then provided to the amplifier via a cable 82. The cable 82 may be screened, although this may not be strictly necessary if the amplifier audio input and power supply lines themselves carry little electromagnetic interference.

Figure 5 illustrates the further embodiment of the present invention. A further screened enclosure 90 may be formed on the end plate 42 and an aperture may be formed in the plate 42 such that electrical connections may extend between the voice coil 50 and an amplifier 92 located in the screened enclosure 90. Power supply and signal connections to the amplifier 92 may be made via cables 94. Alternatively, an amplifier 96 may be provided in a screened enclosure 98 extending from or attached to the loudspeaker support element 64. Again, electrical connections to the loudspeaker are made via a cable, which may optionally be screened.

All of the loudspeaker variants disclosed in Figures 3^', 4 and 5 are electrically described by the circuit diagram of Figure 6. The diagram in Figure 6 is similar to that of Figure 1 and like reference numerals have been used for like parts. The capacitor 90 is fitted close to the point in which the power wire enters the screened area 34 (as defined by the body of the loudspeaker) in order to reduce the intensity of electromagnetic omissions carried out of the screened area by the wire.

A loudspeaker constituting an embodiment of the present invention can provide adequate audio output when used with the supply voltage of merely one volt. However, the supply of one volt may be inadequate for pulse width modulation drive circuits. It may therefore be necessary to provide an auxiliary supply of higher voltage to the drive circuit. This can be arranged either by passing a further supply wire through the screened enclosure in order to supply power to the drive circuit or a voltage conversion may be performed. Thus an internal voltage converter may be provided in order to step up the supply voltage as supplied along the wires 1 and 2 to a second voltage suitable for driving the output circuits. As a further alternative, the supply voltage on the rails 1 and 2 may be optimised for driving the output circuits and an internal voltage converter may be used to step down the voltage supplied to other signal processing circuits, for example the analogue to pulse width modulation signal converter. Advantageously the step-up or step-down circuits are implemented by transistors operating a switched mode manner such that heat dissipation is minimised and efficiency is maximised. Suitable converter circuits include the buck and boost converter configurations which are well known in the art and need not be described here further. Indeed, in a further alternative all of the amplifier and/or converter circuits within the loudspeaker may be supplied via a voltage converter.

Although a pulse width modulation strategy has been described for controlling the class D amplifier, this should be interpreted as widely as possible as many other modulation strategies may also be used where the output is switched between 2 (or more) voltage levels. While pulse width modulation wave forms can be used at a fixed switching frequency (this type of pulse width modulation wave form being obtained by comparing the audio signal with a relatively high frequency fixed frequency fixed amplitude triangular or saw-toothed wave) a pulse width modulation wave form may also be created by hysteresis control. In hysteresis control the output is switched between two (or more) voltage levels according to whether the output variable being controlled is above or below the desired value by a predetermined amount. The output variable may be a filtered voltage output wave form or the current wave form in the load. In the case of hysteresis control the switching frequency may vary depending on the magnitude and rate of change of the audio signal.

Pulse width modulation driving methods can be employed whether or not the loudspeaker has any special electromagnetic screening facility. The advantage of this is that there are no filter losses or restriction of audio band width by the filter. In such situations where electromagnetic radiation is not an issue, an ordinary loudspeaker could be used with a Class D amplifier. The amplifier is preferably provided in a shielded can and attached to the loudspeaker in order to reduce cable capacitance which would demand current spikes from the amplifier. However the coil of the loudspeaker could be unshielded.

Figure 7a shows the usual design loudspeakers in which the coil 101 is longer in extension than the gap between the pole pieces (i.e. the annular pole 102 and the central pole 103). This means that as the coil moves backwards and forwards in the air gap, the number of coil turns in the magnetic field does not change substantially and the relationship between force on the coil and current in the coil does not alter substantially with coil position. Also, by extending the coil beyond the poles, the coil can use some of the fringing flux thereby making most use of the permanent magnetic material in the loudspeaker.

An alternative arrangement is shown in Figure 7b in which the extent of the coil 101 is shorter than the gap between the pole pieces. This again has the effect of keeping the number of coil turns in the magnetic field constant as the coil moves. However, the magnetic field has to be maintained in a longer air gap than before and the amount of permanent magnetic material required to do this is increased, thereby increasing the cost of the loudspeaker, so that this design is not popular. However, it will be seen from Figure 7b that this design the coil is less likely to emit electromagnetic interference because it is totally contained within the annular gap between the magnetic poles.

Figure 8 illustrates how one embodiment of the invention resolves the above dilemma. Both the annular pole 46 and the central pole 103 are extended, the annular pole by the ring 104 and the central pole by the pole piece extension 105. Both extensions 104 and 105 are made of conductive material, typically of metal. One or both of these extensions are made of non-magnetic material (for example, brass). If extension 104 is non-magnetic, extension 105 may be achieved simply by making the central pole longer. If extension 105 is non-magnetic, extension 104 can be made of the same material and integrated with the annular pole piece 46. If either or both the extensions 104 and 105 are non-magnetic, the screening of the coil will be extended beyond the length of the coil, while the extensions will not provide an unwanted shunt path for the magnetic field which ideally needs to be concentrated in the gap occupied by the coil. Electromagnetic radiation can only escape through the small annular gap between the pole extensions 104 and 105 . The annular gap need not be of constant width but may be narrower between the extensions 104 and 105 so that the escape of electromagnetic radiation is minimised. A collar or ring 106 of electrically reflective or electrically absorbent material may be added to the coil former close to the surface of the pole extension so that electromagnetic radiation emerging from the annular gap is either absorbed or reflected back into the extension material or the gap.

The electrical connections to the coil 48 are brought from the coil by very flexible wires into the screened cavity 107 where the amplifier may be located with power and audio signal wire emerging though a hole 108 in the back plate 42 of the loudspeaker with suitable screening and filtering to prevent the escape of electromagnetic interference. Alternatively, the flexible wires from the loudspeaker may be brought through the hole 108 in the back plate 42 and into a screened cavity 109 formed by the loudspeaker back plate 42 and a conductive box 110 fixed to the back plate 42. Audio signal connections and power connections can pass through this box by the use of suitably filtered feed-through terminals.

It would be thus possible to provide a loudspeaker which incorporates screening thereby enabling it to be driven in an efficient manner by an amplifier operating in a switched mode, such as is the mode of operation of a class D amplifier.

Claims

A sonic transducer (36) characterised by comprising an electromagnetic screen (56, 60; 66, 68) integrally formed with the transducer (36) so as to attenuate electromagnetic waves emanating from the transducer.

A sonic transducer as claimed in claim 1, further comprising an integral drive device (76).

A sonic transducer as claimed in claim 2, in which the drive device (76) is provided as an integrated circuit.

A sonic transducer as claimed in claim 2 or 3, in which the drive device (76) operates in a switched mode.

A sonic transducer as claimed in any one of claims 2 to 4, in which the drive device (76) includes a class D amplifier.

A sonic transducer as claimed in any one of the preceding claims, in which the transducer (36) is a loudspeaker.

A loudspeaker as claimed in claim 6, in which a voice coil (50) of the loudspeaker is substantially enclosed within a screened container (60, 64, 40, 42, 42, 44).

A loudspeaker as claimed in claim 6 or 1, in which a central pole of a permanent magnet and a screening element having a hole thereon co-operate to define an annular passage (61) through which a voice coil support (52) extends to mechanically couple a voice coil (50) on the first side of the passage to a diaphragm (54) on the second side of the passage.

A loudspeaker as claimed in claim 8, in which the voice coil (50) is set back from the passage.

A loudspeaker as claimed in any one of claims 6 to 9, in which a conductive element (66) is provided adjacent an interface between a loudspeaker diaphragm and a voice coil support. A loudspeaker as claimed in claim 10, in which the conductive element (66) is electrically connected to a screened container surrounding the voice coil.

A loudspeaker as claimed in claim 11, in which the conductive element is a dust cover.

A loudspeaker as claimed in claim 7 or 10 in which the permanent magnet defined the screened enclosure.

A loudspeaker as claimed in claim 6, characterised in that the loudspeaker includes a class D amplifier within a substantially screened enclosure, and wherein the output stage of the class D amplifier is directly coupled to a voice coil of the loudspeaker so as to maximise rate of change of current in the voice coil.

A loudspeaker as claimed in claim 14, wherein the class D amplifier can operate from a supply provided by a single dry cell.

A loudspeaker as claimed in claim 14, wherein the class D amplifier functions with a power supply of substantially 1 volt.

A loudspeaker as clamed in claim 14, characterised by further comprising a voltage converter for supplying power to circuits provided in the loudspeaker.

A loudspeaker as claimed in any one of claims 6 to 17, characterised in that the spatial extent of a voice coil of the loudspeaker is such that it is wholly contained in the annular gap defined by the pole pieces of the loudspeaker.

A loudspeaker as claimed in claim 18, characterised in that extensions (104) are provided on an annular pole of the magnetic so as to enhance electromagnetic screening.