WO2006137010A2

WO2006137010A2 - Thermo-acoustic transducers

Info

Publication number: WO2006137010A2
Application number: PCT/IB2006/051978
Authority: WO
Inventors: Daniel W. E. Schobben; Josephus C. H. Zeegers; Ronaldus M. Aarts; Okke Ouweltjes
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-06-24
Filing date: 2006-06-20
Publication date: 2006-12-28
Also published as: WO2006137010A3

Abstract

A thermo-acoustic transducer device (8) comprises a substantially hollow body (10) in which a thermo-acoustic element (11) is accommodated, and a heating control unit (2) coupled to the thermo-acoustic element (11) for controlling the temperature gradient of the element. A loudspeaker (14) is acoustically coupled to the hollow body (10). The heating control unit (2) may be arranged for being controlled by a control signal produced by a quality control unit (5) so as to control the quality iactor (Q) of the thermo-acoustic device. The quality control unit (5) may be coupled to an audio amplifier (4) to control the quality factor in dependence of properties of an audio signal.

Description

Thermo-Acoustic Transducers

The present invention relates to thermo-acoustic transducers. More in particular, the present invention relates to a thermo-acoustic transducer comprising a substantially hollow body in which a temperature gradient element, typically a so-called thermo-acoustic stack, is accommodated. It is well known to use thermo-acoustic principles to generate or enhance sound. United States Patent US 5 369 625 (Gabrielson), for example, discloses a submersible acoustic generator. The generator comprises a tubular resonator, open at its lower end and closed at its upper end, in which a so-called thermo-acoustic stack is located. A pair of heat exchangers, one of which is heated by a chemical fuel while the other one is cooled by the surrounding water, generate a temperature gradient in the stack. This temperature gradient allows high-amplitude oscillations to be produced in the resonator.

Typical thermo-acoustic generators are capable of producing monotonous sound only, that is, sound having a single frequency, at a substantially fixed sound volume. This has limited the application of thermo-acoustic devices in audio systems. Still, it would be highly desirable to use thermo-acoustic devices in audio systems as they are capable of producing relatively high sound levels.

It is an object of the present invention to overcome these problems of the Prior Art and, particularly, to provide a thermo-acoustic transducer device which may be used advantageously in audio systems.

Accordingly, the present invention provides a thermo-acoustic transducer unit and device comprising a substantially hollow body in which at least one thermo-acoustic element is accommodated, a loudspeaker coupled to the hollow body, and a heating control unit coupled to the thermo-acoustic element for controlling the temperature gradient of the element.

By providing a thermo-acoustic element in a hollow body to which a loudspeaker is coupled, a loudspeaker having a large quality factor (Q) may be obtained. The thermo-acoustic element may significantly increase the quality iactor of the transducer device. It is noted that the quality factor Q is a well-known measure in the fields of electronic and acoustic engineering, and is a measure of the resonance of a device or system.

In the thermo-acoustic transducer unit and device of the present invention, the sound is produced by the loudspeaker, which is acoustically coupled to the hollow body, and as a result a range of audio frequencies may be produced at a relatively high sound level. The thermo-acoustic element is used to enhance the sound produced by the loudspeaker, while in the Prior Art mentioned above the thermo-acoustic element is used to produce sound.

Conversely, the thermo-acoustic transducer unit and device of the present invention may be operated as a resonant unit, the loudspeaker being used to control the resonance, for example to initiate the resonance in the unit.

It is noted that the thermo-acoustic element may be a thermo-acoustic stack known per se, which stack may consist of a set of substantially parallel, spaced plates, preferably arranged in planes parallel to the length of the hollow body. The material of the stack can be porous, although metal may also be used. In a typical embodiment, the thermo- acoustic element comprises one or more heating elements which are in thermal contact with one end of the spaced plates.

It is further noted that more than one thermo-acoustic element may be present in the device of the present invention, for example two or three thermo-acoustic elements could be provided. The loudspeaker is preferably an electro-magnetic loudspeaker but may be replaced with another transducer capable of producing sound, for example an electrostatic loudspeaker or a piezo-electric transducer. More than one loudspeaker or equivalent transducer may be arranged in the thermo-acoustic unit of the present invention.

In a particularly advantageous embodiment, the heating control unit is arranged for being controlled by a control signal. That is, the temperature gradient of the thermo-acoustic element may be controlled, which results in a controlled quality factor of the device. Accordingly, the sound level produced in response to an (electrical) input signal may be controlled.

A preferred embodiment of a thermo-acoustic transducer device according to the present invention comprises a thermo-acoustic unit as defined above and a quality control unit for producing the control signal in response to an audio signal. In this embodiment, therefore, the quality factor of the device may be (directly or indirectly) controlled by the audio signal. It is preferred that the device further comprises an audio amplifier coupled to the loudspeaker and the quality control unit. In this embodiment, the quality control unit receives the audio signal from the audio amplifier and converts this audio signal into a control signal for controlling the temperature gradient of the thermo-acoustic element via the heating control unit.

The quality control unit is preferably arranged for producing the control signal in dependence of signal properties of the audio signal. These signal properties may include the sound level in a particular frequency range or in all frequency ranges, and/or other signal properties, such as relative frequency distributions (that is, the ratio of e.g. bass frequencies and higher frequencies) and music types (e.g. classical music, rock music, pop music, etc.). The quality control unit may be provided with suitable sound analyzers, which may be known per se.

The thermo-acoustic transducer device may further comprise a modulation unit for producing a modulation (or envelope) signal in response to an audio signal. The modulation signal produced by the modulation unit may in some embodiments be fed to the heating control unit so as to produce a heating control signal dependent on the audio signal, in which case the modulation unit may constitute the quality control unit. It is preferred, however, that the modulation unit is coupled to the loudspeaker, in which case the loudspeaker may render narrowband sound modulated by an audio signal. In these embodiments, a sine generator may be arranged to produce a sine wave that may be modulated by the modulation (envelope) signal and then be fed to the loudspeaker. Such embodiments of the thermo-acoustic transducer device of the present invention are particularly, but not exclusively, useful for reproducing bass sound.

In a preferred embodiment, the modulation unit comprises a band pass filter unit for selecting a frequency band of the audio signal, and a detector unit for detecting the envelope of the band-pass filtered audio signal so as to produce the modulation signal. The band pass filter allows a relevant frequency band, such as the bass band, to be selected. The envelope detector produces a suitable modulation signal which is representative of the envelope of the selected audio signal and is subsequently fed to the loudspeaker. The modulation unit may further comprise a generator for generating a signal, preferably a sine wave signal, and a combination unit, preferably a multiplier, for combining the modulating signal and the signal generated by the generator so as to produce a modulated signal. This modulated signal may be fed to the loudspeaker.

The modulation unit may further comprise a low-pass filter unit for low-pass filtering the control signal. This ensures that any undesired frequency components, which may be introduced by the envelope detector, are substantially removed from the control signal.

It is noted that International Patent Application WO2005/027569 (Philips) discloses an audio arrangement in which a single frequency driving signal is produced for a transducer designed to operate at its resonance frequency. The transducer of this known arrangement is a conventional loudspeaker, thermo-acoustic devices are not disclosed in said document.

The present invention further provides a method of driving a thermo-acoustic transducer device comprising a substantially hollow body in which at least one thermo- acoustic element is accommodated, a loudspeaker coupled to the hollow body, and a heating control unit coupled to the thermo-acoustic element for controlling the temperature gradient of the element, the method comprising the steps of:

- feeding an audio signal to the loudspeaker, and

- producing a heating control signal in response to the audio signal so as to control the quality factor of the thermo-acoustic transducer device in dependence of the audio signal.

Further embodiments of the method according to the present invention will become apparent from the description below.

The present invention also provides an audio system, comprising an audio amplifier, and a thermo-acoustic transducer device as defined above. The audio system may further comprise one or more loudspeakers and a sound source, such as a DVD player, a radio tuner, an internet terminal, and/or an MP3 or AAC player.

The present invention will further be explained below with reference to exemplary embodiments illustrated in the accompanying drawings, in which:

Fig. 1 schematically shows a thermo-acoustic transducer unit according to the Prior Art.

Fig. 2 schematically shows a thermo-acoustic transducer unit according to the present invention. Fig. 3 schematically shows a thermo-acoustic transducer device according to the present invention.

Fig. 4 schematically shows a first embodiment of an audio system according to the present invention. Fig. 5 schematically shows a second embodiment of an audio system according to the present invention.

Fig. 6 schematically shows a modulation unit for use with the thermo-acoustic transducer unit of the present invention.

The thermo-acoustic transducer unit 1 ' according to the Prior Art which is shown by way of example in Fig. 1 comprises a substantially hollow body 10 in which a thermo-acoustic element 11 is accommodated, and a heating control unit 2 coupled to the thermo-acoustic element 11 for controlling its temperature gradient. The hollow body 10 may be tubular. In the example shown, the body 10 has a closed end 12 and an open end 13.

The thermo-acoustic element 11 typically comprises a stack of spaced plates and a heating element, one end of each plate being thermally coupled to the heating element so as to provide local heating. The other end of each plate is typically not heated, or may even be cooled, so as to produce a temperature gradient in the stack of plates. As a result of this temperature gradient, a standing wave will be produced in the interior of the body 10: the air within the body 10 will resonate, the required energy being provided by the thermo- acoustic element 11. By setting the heating current produced by the heating control unit 2, the temperature gradient of the element 11 can be set at a suitable, fixed value. The thermo-acoustic transducer unit 1 ' illustrated in Fig. 1 essentially produces a single frequency, that is, its resonance frequency. In order to produce a wider frequency range, the present invention provides a thermo-acoustic transducer unit comprising a loudspeaker. Such a unit is illustrated in Fig. 2.

The thermo-acoustic transducer unit 1 according to the present invention comprises a hollow body 10 in which at least one thermo-acoustic element 11 is arranged. The thermo-acoustic element 11 is electrically coupled to a heat control unit 2 via suitable electrical leads 21.

The hollow body 10, which is preferably tubular and may have any suitable cross-sectional shape, has an open end 13. The other end of the hollow body 10 is terminated by a loudspeaker 14, which may be a conventional loudspeaker known per se.

The loudspeaker 14 produces sound in a certain frequency range determined by the properties of the loudspeaker. The hollow body 10 and the loudspeaker, which are acoustically coupled, together form an acoustic system having a certain quality iactor Q. This quality factor is determined to a large extent by the temperature gradient in the thermo- acoustic element 11. Accordingly, the quality factor Q of the thermo-acoustic unit 1 may be controlled by the heating control unit 2. This allows the unit 1 to have almost any desired quality factor. As the sound level produced by the thermo-acoustic unit 1 depends, amongst other things, on the quality factor, the sound level produced by the unit 1 may also be controlled by the heat control unit 2.

In the embodiment of Fig. 2, all heat produced in the thermo-acoustic element 11 of the thermo-acoustic transducer unit 1 is produced electrically, controlled by the heat control unit 2. It is also possible that part of the heat required is produced by other means, for example by the power amplifier of an audio system. In that case, heat pipes may connect the power amplifier and the thermo-acoustic element.

The thermo-acoustic transducer unit 1 of Fig. 2 may advantageously be used in a thermo-acoustic transducer device comprising further components in addition to the thermo-acoustic transducer unit. An exemplary thermo-acoustic transducer device 8 according to the present invention is schematically illustrated in Fig. 3. The device 8 comprises a thermo-acoustic unit 1 having a hollow body 10 in which a thermo-acoustic element 11 is located and which is terminated at one end by a loudspeaker 14. As in Fig. 2, the heat control unit 2 is electrically coupled to the thermo- acoustic element 11.

The thermo-acoustic device 8 of Fig. 3 additionally comprises an audio amplifier (AA) 4 coupled to the loudspeaker 14 through suitable wires 41. The audio amplifier 4 is also coupled to a quality control (QC) unit 5 which receives an audio signal from the audio amplifier 4 and converts this audio signal into a quality control signal. This quality control signal is then passed on as a control signal for the heat control unit 2. Accordingly, the audio signal produced by the audio amplifier 4 controls the quality factor of the thermo-acoustic device 8 and thereby the sound level produced at certain frequencies.

This allows the device 8 to compensate certain (undesirable) properties of the loudspeaker 14 by properly adjusting the quality factor Q of the device. For example, at higher loudspeaker sound levels the device could have a quality factor Q of approximately 4, while at lower loudspeaker sound levels a quality factor of approximately 2 could be used to avoid a "booming" bass sound. Accordingly, the quality control unit converts the audio signal level, and/or any other suitable audio signal property, into a control signal. The quality factor Q controlled by the quality control unit 5 may be made dependent on the type or style of music, as some music styles may require a lower quality factor for faithful sound reproduction. To provide a music style dependent quality control, the quality control unit may comprise a sound analysis and/or classification unit for classifying sound, in particular for classifying music styles. Such sound classification units are known per se. Alternatively, or additionally, the quality control unit 5 may be made dependent on other properties, for example the (instantaneous) impedance of the loudspeaker 14, or of other loudspeakers (not shown in Fig. 3). Those skilled in the Art know that the (instantaneous) impedance of a loudspeaker is determined by the (incremental) voltage over the loudspeaker divided by the (incremental) current through the loudspeaker: Z₁ = dV/di, where V = voltage, i = current, and Zi is the instantaneous impedance.

Although the quality control unit 5 is shown to constitute a separate unit, in some embodiments the quality control unit 5 is integral with the audio amplifier 4 and/or the heating control unit 2.

A first embodiment of an audio system according to the present invention is schematically illustrated in Fig. 4. The audio system 9 of the present invention comprises a thermo-acoustic transducer unit 1, a heat control unit 2, an audio amplifier (AA) 4, a quality control (QC) unit 5, acoustic speakers 6 and a sound source (SS) 7. The thermo-acoustic transducer unit 1 may be a unit as shown in Fig. 2. The speakers 6, which may be conventional loudspeakers designed for producing mid- and high frequency sound, also receive amplified audio signals from the audio amplifier 4. The audio signals originate from a sound source 7, which may be a CD player, a DVD player, a computer, an internet terminal, an AAC or MP3 player, or any other suitable sound source. In the audio system 9 of Fig. 4, the thermo-acoustic transducer unit 1 allows a very efficient sound production.

In the audio system of Fig. 4, the heat control unit 2 receives its control signal from the quality control (QC) unit 5, which derives this control signal from an audio signal supplied by the audio amplifier (AA) 4. As the thermo-acoustic transducer unit 1 comprises a loudspeaker (14 in Fig. 2), the unit 1 can produce a wider frequency range at a high sound level. Alternatively, or additionally, the unit 1 may be operated as a resonant transducer unit, the resonance being controlled by the loudspeaker.

As explained above with reference to Fig. 3, the quality control unit 5 and the heat control unit 2 define the quality factor Q of the thermo-acoustic transducer unit 1. This controlled quality iactor may be used advantageously to adapt the quality factor to the sound level. This quality factor may be increased to save power, for example when the device is powered by batteries. The quality iactor may further be made dependent on the type or style of music, as some music styles may require a lower quality iactor for faithful sound reproduction. The quality control unit 5 may further be dependent on the (instantaneous or constant) impedance of one or more loudspeakers, such as one or more of the speakers 6, and/or the loudspeaker (14 in Fig. 2) of the transducer unit 1. Suitable connections (not shown in Fig. 4) may for this purpose be present between the loudspeakers and the quality control unit 5.

A second embodiment of an audio system according to the present invention is illustrated in Fig. 5. The audio system 9 of Fig. 5 comprises a thermo-acoustic transducer unit 1, a heat control unit 2, a bass modulation (BM) unit 3, an audio amplifier (AA) 4, a quality control (QC) unit 5, loudspeakers 6 and a sound source (SS) 7. The quality control unit 5 of this embodiment may be identical to the quality control unit 5 of the embodiment shown in Fig. 4.

The bass modulation unit 3 is in this embodiment designed for mapping bass frequencies onto a suitable narrow frequency range of the thermo-acoustic transducer unit 1. The bass modulation unit 3 receives an (amplified) audio signal from the audio amplifier 4 and effectively maps a selected frequency range of the audio signal on another, narrower frequency range. The bass modulation unit 3 is explained in more detail with reference to Fig. 6.

The modulation unit 3 shown by way of non- limiting example in Fig. 6 comprises a band pass filter 31, an envelope detector 32 and a low pass filter 33. A band pass filter 31 receives an input audio signal Si and selects a desired frequency band of this audio signal, for example the bass band. The filtered audio signal is passed to the envelope detector 32, which produces an envelope signal representing the envelope of the filtered audio signal. This envelope signal is then low pass filtered by the (optional) low pass filter 33 to produce a modulation (or envelope) signal which is fed to a combination unit 35. A generator 34 is arranged for producing a signal in a desired frequency range.

This signal preferably is a sine signal. Its frequency is preferably chosen to be substantially equal to a resonance frequency of the thermo-acoustic transducer unit (1 in Fig. 5), although other frequencies may also be used. In some embodiments, the frequency of the generator 34 may be variable and a suitable frequency control signal may be supplied by, for example, the quality control unit (5 in Fig. 5).

The signal produced by the generator 34 is fed to the combination unit 35 where it is combined (preferably multiplied) with the modulation (or envelope) signal to produce a modulated signal S_m. This signal S_m, which is the output signal of the modulation unit 3, is fed to the loudspeaker, as illustrated in Fig. 5. In an alternative embodiment, the modulation (envelope) signal produced by the detector 32 (and, optionally, the filter 33) is fed as a control signal to the heating control unit 2. As a result, this modulation (or control) signal will modulate the temperature gradient in the thermo-acoustic element, and therefore also the sound level produced by the thermo- acoustic unit 1, in accordance with the input audio signal Si. In other words, the sound level produced by the thermo-acoustic unit 1 will vary with the input audio signal Si.

It is noted that the (bass) modulation unit 3 is preferably used for mapping a bass frequency range on a suitable transducer frequency, but is not limited to the bass range. For example, the modulation unit 3 could be used for mapping other frequency ranges, for example from 1 kHz to 5 kHz, onto a narrower frequency range.

The thermo-acoustic transducer unit and device of the present invention may advantageously be incorporated in a bass port of a bass reflex transducer unit (a so-called Helmholtz resonator). The tube typically present in such a bass port then constitutes the hollow body (10 in Figs. 2 and 3) of the thermo-acoustic transducer unit. Other applications of the thermo-acoustic transducer unit and device of the present invention include acoustic alarms, such as fire alarms.

The present invention is based upon the insight that a thermo-acoustic element and a loudspeaker may advantageously be combined in a single acoustic unit to provide a loudspeaker unit having a variable quality factor. 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. In this document, a thermo-acoustic (transducer) device is meant to comprise at least one thermo- acoustic (transducer) unit.

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 thermo-acoustic transducer device (8; 1), comprising:

- a substantially hollow body (10) in which at least one thermo-acoustic element (11) is accommodated,

- a loudspeaker (14) coupled to the hollow body (10), and - a heating control unit (2) coupled to the at least one thermo-acoustic element (11) for controlling the temperature gradient of the element.

2. The device according to claim 1, wherein the heating control unit (2) is arranged for being controlled by a control signal.

3. The device according to claim 2, further comprising a quality control unit (5) for producing the control signal in response to an audio signal.

4. The device according to claim 3, wherein the quality control unit (5) is arranged for producing the control signal in dependence of signal properties of the audio signal and/or loudspeaker properties.

5. The device according to claim 3 or 4, further comprising an audio amplifier (4) coupled to the loudspeaker (14) and the quality control unit (5).

6. The device according to any one of the claims 1 to 5, further comprising a modulation unit (3) coupled to the loudspeaker (14) for producing a modulated loudspeaker signal (S_m) in response to an audio signal (Si).

7. The device according to claim 6, wherein the modulation unit (3) comprises:

- a band pass filter unit (31) for selecting a frequency band of the audio signal (Si),

- a detector unit (32) for detecting the envelope of the band-pass filtered audio signal so as to produce a modulation signal,

- a generator unit (34) for producing a generator signal, and - a combination unit (35) for combining the modulation signal and the generator signal so as to produce the modulated loudspeaker signal (S_m).

8. The transducer device according to claim 7, wherein the modulation unit (3) further comprises:

- a low-pass filter unit (33) for low-pass filtering the modulation signal.

9. An audio system (9), comprising:

- an audio amplifier (4), and - the thermo-acoustic transducer device (8; 1) according to any one of the claims 1 to 8.

10. The audio system according to claim 9, further comprising a sound source (7) and/or at least one acoustic speaker (6).

11. A method of driving a thermo-acoustic transducer device (8; 1) comprising a substantially hollow body (10) in which at least one thermo-acoustic element (11) is accommodated, a loudspeaker (14) coupled to the hollow body, and a heating control unit (2) coupled to the thermo-acoustic element for controlling the temperature gradient of the element, the method comprising the steps of: - feeding an audio signal to the loudspeaker, and

- producing a heating control signal in response to the audio signal so as to control the quality factor of the thermo-acoustic transducer device in dependence of properties of the audio signal.