WO2006043219A1

WO2006043219A1 - Loudspeaker feedback

Info

Publication number: WO2006043219A1
Application number: PCT/IB2005/053379
Authority: WO
Inventors: Daniel W. E. Schobben; Okke Ouweltjes
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-10-21
Filing date: 2005-10-14
Publication date: 2006-04-27
Also published as: JP2008518498A; EP1806027A1; KR20070084422A

Abstract

A loudspeaker (1) having a conductive suspension section (3) and two electrodes (5, 6) is controlled by generating a measuring current in the conductive suspension section (3), detecting the voltage thus generated, and using the voltage to control the output power to the loudspeaker. The control method and device may be used for loudspeaker protection, motional feedback and limiting the output power. The conductive suspension section (3) may be part of the rim of the loudspeaker and may be made of conductive rubber.

Description

Loudspeaker feedback

The present invention relates to loudspeaker feedback. More in particular, the present invention relates to a device for controlling a loudspeaker in response to a feedback mechanism.

It is well known to provide feedback in order to control and possibly limit the power of a loudspeaker signal. International Patent Application WO 01/03466 (Aarts et al. / Philips) discloses a loudspeaker protection system in which a resistor is connected in series with the loudspeaker coil. The current through the coil is converted into a measuring voltage that is fed to a peak detector. A microprocessor controls the gain of variable amplifiers in response to the output signal of the peak detector.

Although the loudspeaker protection system of the Prior Art is useful, it suffers from the drawback that it measures the current through the coil of the loudspeaker, that is, the driving current. This current is indicative of the desired movements of the loudspeaker. However, due to physical limitations of the loudspeaker, its actual movements do not always correspond exactly to the driving current. To solve this problem, it has been proposed to mount dedicated transducers on the loudspeaker, such as accelerometers. However, such transducers are relatively expensive. In addition, their mass alters the acoustic properties of the loudspeaker.

It is an object of the present invention to overcome these and other problems of the Prior Art and to provide a device and a method of controlling a loudspeaker that are economical yet effective. Accordingly, the present invention provides a device for controlling a loudspeaker provided with a conductive suspension section, the device comprising: generating means for generating a measuring current in the conductive suspension section, - detection means for detecting the voltage caused by the measuring current in the conductive suspension section, and

- power control means for controlling the power output to the loudspeaker in response to the detected voltage. By providing a measuring current which is passed through the conductive section of the suspension, the resistance of the conductive section may be measured during the use of the loudspeaker. As the conductive suspension section will be flexed and stretched by the movements of the cone, its resistance will be an accurate measure of the actual movement (or "excursion"), more accurate than the driving current. Accordingly, the resistance, or a quantity derived from the resistance, may be used to control the loudspeaker, for example correcting the driving current. To this end, the power control means may be arranged for controlling the signal level of an audio signal fed to the loudspeaker.

In an advantageous further embodiment, the device further comprises a zero crossing detector for detecting zero crossings of the audio signal and an inverter unit for inverting the audio signal in response to any detected zeroes. As each zero crossing typically indicates a sign reversal of the audio signal, the zero crossing detector provides information on the direction of the movement of the loudspeaker cone, thus providing an improved feedback and control.

The device of the present invention may further comprise a power amplifier for amplifying the controlled input signal. However, a power amplifier is not essential and may be external to the device of the present invention.

The device may advantageously comprise filter units for selectively amplifying the controlled input signal. That is, the input audio signal is split into frequency bands, the gain of at least one of these frequency bands being controlled in response to the measured excursion of the loudspeaker.

It is further advantageous if the generating means are arranged for generating a heating current in the conductive suspension section. This allows a substantially constant temperature to be maintained in the suspension of the loudspeaker, thus ensuring uniform loudspeaker characteristics. The heating current could be controlled by a thermostat or other control device sensitive to temperature.

The present invention also provides a loudspeaker provided with a conductive suspension section for use with the device according to any of the preceding claims.

In a first embodiment, the conductive suspension section is comprised in the rim, while in a second embodiment, the conductive suspension section is comprised in the spider. In a further embodiment conductive sections are provided in both the rim and the spider, so as to provide additional feedback.

The conductive suspension section may be made of conductive rubber, for example natural or synthetic rubber containing carbon particles. The present invention also provides an audio system, comprising a loudspeaker having a conductive suspension section and a device as defined above. The present invention additionally provides a method of controlling a loudspeaker provided with a conductive suspension section, the method comprising the steps of: - generating a measuring current in the conductive section, detecting the voltage caused by the measuring current in the conductive suspension section, and controlling the power output to the loudspeaker in response to the detected voltage.

The power output to the loudspeaker may be limited to a maximum value. The method of the present invention may advantageously be used for providing ear protection.

The present invention will further be explained below with reference to exemplary embodiments illustrated in the accompanying drawings, in which: * Fig. 1 schematically shows, in plan view, an embodiment of a loudspeaker according to the present invention.

Fig. 2 schematically shows, in cross-sectional view, the loudspeaker embodiment of Fig. 1.

Fig. 3 schematically shows a first embodiment of a loudspeaker control device according to the present invention.

Fig. 4 schematically shows a second embodiment of a loudspeaker control device according to the present invention.

Fig. 5 schematically shows a third embodiment of a loudspeaker control device according to the present invention. Fig. 6 schematically shows a fourth embodiment of a loudspeaker control device according to the present invention.

Fig. 7 schematically shows a fifth embodiment of a loudspeaker control device according to the present invention. The loudspeaker 1 shown merely by way of non-limiting example in Figs. 1 and 2 comprises a cone 2, a flexible rim 3, a frame 4, a magnet 7, a coil 8 and a spider 9. The flexible rim 3 and the (flexible) spider 9, both of which are connected to elements of the frame 4, constitute the suspension of the loudspeaker.

As is well known, the coil 8 is capable of moving relative to the magnet 7, thus driving the cone 2. Movements of the cone 2 will cause the rim 3 and the spider 9 to flex and stretch. In accordance with the present invention, the suspension is at least partially conductive. In addition, the conductive section of the suspension is provided with electrodes, allowing a current to flow through the conductive suspension section. As the flexing and/or stretching of the conductive suspension section will cause its resistance to vary, the current can be used to measure the resistance and hence the movement of the suspension.

In the embodiment shown, the rim 3 is conductive. Concentric electrodes 5 and 6 are arranged near the edges of the rim 3. Wires 5a and 6a, connected to the electrodes 5 and 6 respectively, allow a current to pass through the conductive rim section.

It will be understood that (part of) the spider 9 can also be conductive, and that suitable electrodes can be attached to the spider so as to provide a conductive suspension section in accordance with the present invention.

In some embodiments, multiple conductive suspension sections may be provided, the resistance of each being measured separately or collectively. For example, both the rim and the spider could be provided with a conductive section, and/or the spider could be provided with two or more conductive sections. By providing separate measuring currents to each conductive section, an even more accurate loudspeaker feedback and control may be achieved. Those skilled in the art will be able to make suitable adjustments to a loudspeaker control device (10 in Figs. 3 - 7) so as to be able to process multiple measuring currents.

In particularly suitable embodiments, the conductive suspension section is given a bias tension, for example by providing an additional spring (not shown), or by suitably dimensioning the spider when the conductive section is located in the rim. This has the advantage that the resistance of the conductive section may both decrease and increase when the loudspeaker is activated, thus providing both the magnitude and the direction of the movement.

The conductive suspension section(s) of the loudspeaker 1 may be made of conductive (natural or synthetic) rubber or suitable conductive and resilient plastics. Such materials may be made conductive, by mixing in, during production, a suitable amount of carbon.

A non- limiting example of a device for controlling a loudspeaker is schematically shown in Fig. 3. The exemplary device 10 comprises a controlled amplifier circuit 11, 12, 13, a power amplifier 14, a voltage source 17 and a feedback branch including a buffer amplifier 16. In the embodiment shown, the feedback branch further comprises an (optional) polarity correction circuit including a zero crossing detector 19 and a polarity inverter 20.

A loudspeaker 1 is coupled to the power amplifier 14. It is noted that the power amplifier is not an essential part of the device 1 and that the device 1 may be coupled to a separate power amplifier. It is further noted that instead of a single loudspeaker, multiple loudspeakers may be used.

The controlled amplifier circuit 11, 12, 13 receives an audio input signal, via the input resistor 11, from the input terminal I. The amplifier 12 is, in the example shown, an op-amp (operational amplifier), the gain of which is determined by the resistors 11 and 13 • and the voltage at the junction of these resistors (point Q). The output signal of the amplifier 12 is fed to the power amplifier 14 which is, in turn, coupled to the loudspeaker 1.

In accordance with the present invention, the loudspeaker 1 is provided with wires 5a and 6a (compare Fig. 1) for measuring the electrical resistance of a flexible suspension section of the loudspeaker. A voltage source 17 causes a measuring current I_m to flow through the conductive suspension section (3 in Figs. 1 and 2) of the loudspeaker 1. The magnitude of this measuring current is determined by the combined resistance of the resistor 18 and of the conductive suspension section (3 in Figs. 1 and 2), and the voltage at the point P will be determined by the instantaneous ratio of these resistances. As the resistance of the conductive suspension section will vary with the displacement of the loudspeaker, the voltage at the point P (the junction of wire 5a and resistor 18) is a measure of this displacement and may be referred to as measuring voltage. A capacitor (not shown) may be used to separate the time- varying (AC) component of the measuring voltage from the constant (DC) component. It is noted that the measuring current I_m does not flow through the coil of the loudspeaker, as is the case in WO 01/03466 mentioned above. As a result, the measuring current I_m measures the actual displacement of the loudspeaker, instead of the desired displacement. In the present invention, the actual displacement of the loudspeaker is measured without using relatively expensive accelerometers or other additional transducers. The measuring voltage at the point P is fed, via a buffer amplifier 16 and a resistor 15, to the point Q which is the junction of resistors 11 and 13, where it controls the gain of amplifier circuit 11 - 13 and thus controls the power of the signal fed to the loudspeaker 1. In this way, a very simple yet effective feedback mechanism is provided. The feedback mechanism of the present invention measures the actual displacement of the loudspeaker without requiring transducers other than a conductive suspension section having two electrodes.

The accuracy of the device 10 can be improved by providing a polarity correction circuit including a zero crossing detector (ZCD) 19 and a polarity inverter (INV) 20. The zero crossing detector 19, which may be known per se, receives the output signal of the buffer amplifier 16. As discussed above, this signal is indicative of the displacement of the (moving parts of the) loudspeaker. A zero crossing in this signal indicates a reversion of the direction of the loudspeaker movement, for example from backward to forward. Upon detecting a zero crossing, the zero crossing detector 19 supplies a suitable control signal to the polarity inverter 20 which then inverts the polarity of the signal. The resulting signal at the point P indicates both the magnitude and the direction of the loudspeaker movement. The polarity inverter 20 may also be known per se. The voltage source 17 or the buffer amplifier 16 may provide a bias voltage to set the zero crossing detection level of the detector 19 at a suitable level. Zero crossing detection is particularly suitable when the loudspeaker 1 operates in a limited frequency band, for example when the audio signal fed to the loudspeaker is the product of an envelope signal and a generator signal having a single frequency, as described in European Patent Application EP 03103398.8 (Aarts et al./Philips). The loudspeaker control device of the present invention may be used for motional feedback (distortion correction of the audio signal) and/or for sound limiting (limiting the output power of the loudspeaker to a maximum value, for example for ear protection). In particular when the loudspeaker 1 is part of a headphone, the device 10 may advantageously be used to limit the sound output power and thus to protect the ear(s) of the user against excessive sound levels. The voltage source 17 may be constituted by a battery or similar element, but is preferably an electronically controlled voltage source, the voltage of which is set to produce a suitable measuring current I_m. Typically, the measuring current will be very small. However, the measuring current I_n, may advantageously used to heat the loudspeaker suspension, which may require a larger current. By slightly heating the loudspeaker suspension, it is ensured that the suspension has a substantially constant temperature. In addition, the flexibility of the suspension can be made more constant, as any heating due to the movement of the loudspeaker will be negligible compared to the heating provided by the current. A modified embodiment of the device 10 is shown in Fig. 4, where the input of the zero crossing detector 19 is connected, via the point R, to the output of the amplifier 12. In this embodiment, therefore, the zero crossing detector 19 detects the zero crossings in the audio signal instead of in the measurement signal, which typically results in a more effective zero crossing detection. To remove any phase errors which may be introduced by the amplifier 14 and the loudspeaker 1, a loudspeaker correction network (LCN) 27 is provided in series with the amplifier 14 (although the LCN 27 is shown to be arranged upstream from the amplifier 14, it may also be located downstream). Such a loudspeaker correction network introduces a suitable phase shift to compensate the phase shift introduced by the loudspeaker and, in the present example, any phase shift introduced by the amplifier 14. The loudspeaker correction network 27 may additionally correct any non-linearities.

Another embodiment of the loudspeaker control device of the present invention is schematically shown in Fig. 5, where minor components have been omitted for the sake of clarity of the illustration. The device 10 of Fig. 5 comprises a controlled amplifier 12 coupled to a power amplifier 14. A loudspeaker 1 coupled to the output of the power amplifier 14 comprises in accordance with the present invention a conductive suspension section (3 in Figs. 1 and 2). An excursion measurement unit (EMU) 23 is coupled to the conductive suspension section of the loudspeaker 1 (compare Fig. 3 where the excursion measurement unit is essentially constituted by the voltage source 17 and the resistor 18). The output signal of the excursion measurement unit 23 is fed to a gain control unit 22 coupled to the controlled amplifier 12. Accordingly, the gain control unit 22 controls the gain (or attenuation) of the amplifier 12 in dependence of the measured excursion (that is, cone movement) of the loudspeaker 1.

The gain control unit 12 may have an upper bound on the gain or attenuation, and may have different adaptation speeds: a rapid gain decrease when the upper bound (maximum power value) is exceeded, and a relatively slow gain increase when the original gain is resumed. Those skilled in the art will be able to devise various suitable control schemes. The gain control unit 22, which may be known jeer se, may be constituted by a microcontroller, a microprocessor or by dedicated digital or analog circuits. The embodiment of Fig. 5 is particularly suitable for loudspeaker protection or ear protection purposes.

The embodiment of Fig. 6 allows gain control per frequency band. A low-pass filter 21a and a high-pass filter 21b are coupled between the input terminal I and the controlled amplifiers 12a and 12b respectively. The output signals of the controlled amplifiers 12a and 12b are coupled to the combination unit (here: adder) 26, which feeds the combined signals to the power amplifier 14. As in the embodiment of Fig. 5, an excursion measurement unit (EMU) 23 is coupled to the conductive suspension section(s) of the loudspeaker 1. In this embodiment, an (optional) excursion correction unit (ECU) 24 is provided for correcting any non-linearities of the conductive suspension section(s) of the loudspeaker(s). The excursion correction unit 24 provides a mapping of the measured excursion into an actual excursion, taking the properties of the conductive suspension section(s) into account. Such a mapping may suitably be provided by a look-up table or similar device. '

The (corrected) excursion signal is fed to a further low-pass .filter 25a and a further high-pass filter 25b, which are coupled to (low frequency, LF) gain control unit (GCU) 22a and (high frequency, HF) gain control unit 22b respectively, which in turn are coupled to the respective controlled amplifiers 12a and 12b. In this way, a gain control per frequency band is achieved. More in particular, the low frequency gain can be controlled virtually independently of the high-frequency gain. This is particularly advantageous as low frequencies typically cause greater excursions than high frequencies. Accordingly, the low- frequency gain may be reduced to limit the excursion of the loudspeaker while the high- frequency gain may be left virtually unaltered. As a result, the overall sound volume is less affected by the output power control. It can thus be seen that in the embodiment of Fig. 6 the input audio signal is split into frequency bands, the gain of at least one of these frequency bands being controlled in response to the measured excursion of the loudspeaker.

The pass-bands of the filters 25a and 25b need not be identical to the pass- bands of the filters 21a and 21b. In particular, the upper limit of the pass-band of low-pass filter 25a may advantageously be twice the upper limit of its counterpart 21a when the measuring voltage derived from the conductive suspension section of the loudspeaker 1 is "rectified", that is, when the measuring voltage has only a single polarity due to the fact that the resistance of the conductive section may in certain embodiments only indicate the magnitude of the loudspeaker excursion but not its direction. Such a "rectified" measuring voltage contains twice the original signal frequency. Accordingly, when the filter 21a has a pass-band of, for example, 0 to 100 Hz, the corresponding filter 25a may in such embodiments have a pass-band of 100 to 200 Hz.

The embodiment of Fig. 7 is similar to the previous embodiments. However, absolute value determination unit 28 and loudspeaker correction network (LCN) 27 have been added, and the operation of the gain control unit 22 is slightly different.

The absolute value determination unit (ABS) 28 determines the absolute value or magnitude of the input signal received at the input terminal I of the device 10. This magnitude, which represents the desired excursion of the loudspeaker, is fed to the gain control unit 22 where it is compared with the measured excursion as determined by the excursion measurement unit 23 and corrected by the (optional) excursion correction unit 24. The comparison of the desired excursion and the actual excursion may be carried out by a division of the signal values. The result of the comparison (here: division) is used to control the gain of the amplifier 12. For example, if the actual excursion is greater than the desired excursion (that is, the signal produced by the excursion correction unit 24 is greater than the ■ signal output by the absolute value unit 28), the gain has to be reduced. This can advantageously be carried out by determining the ratio of the desired excursion and the actual excursion (which ratio, in the present example, is' smaller than 1) and multiplying the present gain value with this ratio so as to obtain a new gain value. The (optional) loudspeaker correction network, which may be constituted by a suitable filter, serves to produce a flat frequency response so as to avoid any phase differences in the control loop constituted by the loudspeaker 1, the excursion measurement unit 23, the excursion correction unit 24, the gain control unit 22 and the controlled amplifier 12. Such a loudspeaker correction network may be known per se. The embodiment of Fig. 7 is particularly suitable for providing motional feedback.

The present invention is based upon the insight that providing a conductive section in the suspension of a loudspeaker allows a very simple and economical loudspeaker feedback mechanism. The present invention benefits from the further insight that a measuring current passing through the conductive suspension section may advantageously be used to heat the suspension and thus to provide a constant temperature.

It is noted that any terms used in this document should not be construed so as to limit the scope of the present invention. In particular, the words "comprise(s)" and "comprising" are not meant to exclude any elements not specifically stated. Single (circuit) elements may be substituted with multiple (circuit) elements or with their equivalents.

It will be understood by those skilled in the art that the present invention is not limited to the embodiments illustrated above and that many modifications and additions may be made without departing from the scope of the invention as defined in the appending claims.

Claims

CLAIMS:

1. A device (10) for controlling a loudspeaker (1) provided with a conductive suspension section (3), the device comprising:

- generating means (17) for generating a measuring current in the conductive suspension section, - detection means (16, 18) for detecting the voltage caused by the measuring current in the conductive suspension section (3), and

- power control means (12, 13) for controlling the power output to the loudspeaker in response to the detected voltage.

2. The device according to claim 1, wherein the power control means (12, 13) are arranged for controlling the signal level of an audio signal fed to the loudspeaker.

3. The device according to claim 1, further comprising a zero crossing detector (19) for detecting zero crossings of the audio signal and an inverter unit (20) for inverting the audio signal in response to any detected zeroes.

4. The device according to claim 1, further comprising filter units (21a, 21b) for selectively amplifying the controlled input signal.

5. The device according to claim 1, wherein the generating means (17) are arranged for generating a heating current in the conductive suspension section (3).

6. A loudspeaker (1) provided with a conductive suspension section (3) for use with the device (10) according to claim 1.

7. The loudspeaker according to claim 6, wherein the conductive suspension section is comprised in the rim (3).

8. The loudspeaker according to claim 6, wherein the conductive suspension section (3) is comprised in the spider (9).

9. The loudspeaker according to claim 6, wherein the conductive suspension section (3) is made of conductive rubber.

10. An audio system, comprising a loudspeaker (1) having a conductive suspension section (3) and a device (10) according to claim 1.

11. A method of controlling a loudspeaker (1 ) provided with a conductive suspension section (3), the method comprising the steps of:

- generating a measuring current in the conductive section,

- detecting the voltage caused by the measuring current in the conductive suspension section (3), and - controlling the power output to the loudspeaker in response to the detected voltage.

12. The method according to claim 11, wherein the conductive suspension section is located in the rim (3) of the loudspeaker (1).

13. The method according to claim 11, wherein the conductive suspension section is located in the spider (9) of the loudspeaker (1).

14. The method according to claim 11, wherein the measuring current is used to heat the conductive suspension section.

15. The method according to claim 11 , wherein the power output to the loudspeaker is limited to a maximum value.

16. The method according to claim 15 used for providing ear protection.