WO2006075263A1

WO2006075263A1 - Sound detection device and method of detecting sound

Info

Publication number: WO2006075263A1
Application number: PCT/IB2006/050038
Authority: WO
Inventors: Christian Presura
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-01-12
Filing date: 2006-01-05
Publication date: 2006-07-20

Abstract

A sound detection device (100) comprises a first vibratable element (101) and a second vibratable element (102) arranged at a distance from each other and each adapted to vibrate in response to sound (103), a converting arrangement adapted to convert a vibration state of the first vibratable element (101) into a first electromagnetic radiation signal (110) and to convert a vibration state of the second vibratable element (102) into a second electromagnetic radiation signal (111), and an adjustment device (112) adapted to adjust the first electromagnetic radiation signal (110) and/or the second electromagnetic radiation signal (111), such that the sensitivity of the first electromagnetic radiation signal (110) and the second electromagnetic radiation signal (111) with respect to sound (103) originating from an adjustable direction is increased as compared to sound originating from other directions.

Description

Sound detection device and method of detecting sound

FIELD OF THE INVENTION

The invention relates to a sound detection device.

The invention further relates to a method of detecting sound.

BACKGROUND OF THE INVENTION With the advancement of digital audio equipment, there is an increasing need for improvement of the quality of the recorded sound. A recent concept in this context is the development of what is called the microphone array. A microphone array uses a plurality of microphones arranged at different locations to measure a sound wave front. Using the phase information included in this measurement, the location of the sound source (for instance, a human speaker giving a lecture) can be determined. Alternatively, a microphone array system may choose among a small number of speakers speaking at different locations at the same time. A demonstration of such an effect is disclosed in http://www.idiap.ch/pa^es/contenuTxt/Demos/demo5/demo.html.

However, according to the prior art, an array microphone cannot be manufactured in a sufficiently compact and low-cost manner. One drawback results from the large wavelength of sound that may be of the order of meters. In order to discriminate between phases of a sound wave at the location of the different microphones in a measurable and resolvable manner, the microphones have to be placed at considerably large distances from one another. Thus, the microphone array according to the prior art requires much space. JP 59016499 discloses a microphone that amplifies sound coming from a specific direction. However, JP 59016499 requires rotating a microphone to choose a particular direction of sound to be amplified. JP 59016499 thus requires re-orienting the microphone to choose a particular direction.

US 4,559,642 discloses an array of directional microphones having individual directivity patterns which are equally oriented in a given direction. A plurality of controllable delay circuits is connected to be responsive, respectively, to individual signals from the microphones for providing incremental delays to the individual signals and combining the delayed signals for delivery as an output of the apparatus. US 4,559,642 discloses a microphone which can choose the direction from which the sound to be detected originates. This document also discloses control of the phase of an electric signal coming from each individual microphone. However, the system according to US 4,559,642 is very demanding as far as the required space is concerned, because the distance between individual microphones in the array has to be considerably large to achieve a sufficient resolution.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to allow a direction-specific detection of sound in a compact manner.

In order to achieve the object defined above, a sound detection device and a method of detecting sound as defined in the independent claims are provided.

A sound detection device comprises a first vibratable element and a second vibratable element arranged at a distance from each other and each adapted to vibrate in response to sound. A converting means is adapted to convert a vibration state of the first vibratable element into a first electromagnetic radiation signal and to convert a vibration state of the second vibratable element into a second electromagnetic radiation signal. An adjustment means of the sound detection device is adapted to adjust the first electromagnetic radiation signal and/or the second electromagnetic radiation signal in such a manner that the sensitivity of the first electromagnetic radiation signal and the second electromagnetic radiation signal with respect to sound originating from an adjustable direction is increased as compared to sound originating from other directions.

According to a method of detecting sound, a first vibratable element and a second vibratable element are arranged at a distance from each other to vibrate in response to sound. A vibration state of the first vibratable element is converted into a first electromagnetic radiation signal, and a vibration state of the second vibratable element is converted into a second electromagnetic radiation signal. The first electromagnetic radiation signal and/or the second electromagnetic radiation signal is or are adjusted in such a manner that the sensitivity of the first electromagnetic radiation signal and the second electromagnetic radiation signal with respect to sound originating from an adjustable direction is increased as compared to sound originating from other directions. A human user controlling the adjustment can perform the adjustment manually.

Moreover, the adjustment can also be performed automatically, that is to say controlled by software or hardware. In other words, the adjustment of the direction to which the detection of sound is particularly sensitive can be realized by a computer program, that is by software, or by using one or more special electronic optimization circuits, that is in hardware, or in a hybrid form, that is by means of software components and hardware components.

The characteristic features according to the invention particularly have the advantage that a plurality of vibratable elements (such as membranes) can be arranged much closer to one another as compared to the related art. This results from the fact that already a very small difference between mechanical deformations of these vibratable elements in response to acoustic noise generated by a sound source having different distances to the different vibratable elements is sufficient to be resolvable, because the actual detection is carried out by electromagnetic radiation (for instance, optical light) having significantly smaller wavelengths than acoustic waves. By transferring the phase difference of the mechanical oscillation of the membranes into a phase difference of electromagnetic radiation, the resolution is refined due to the different wavelength characteristics of acoustic waves, on the one hand, and optical waves, on the other hand.

In other words, sound has a comparable, large wavelength (typically of the order of meters) so that vibratable elements arranged at a distance of, for instance, one centimeter from one another have a very small phase difference resulting from an excitation with respect to acoustic waves of an acoustic wave source. However, since the system according to the invention teaches to translate these small mechanical phase differences into larger phase differences of electromagnetic radiation signals, a (for example, optical) interferometer has a sufficiently fine resolution to detect these differences with extremely high accuracy. For instance, a mutual analysis (such as interference) of different coherent electromagnetic radiation signals allows detecting sound.

Furthermore, since the adjustment means of the system according to the invention are capable of manipulating the electromagnetic radiation signals in a selectable manner, the phase relation between the different electromagnetic radiation signals reflecting the mechanical deformation of corresponding membranes may be altered, so that constructive interference of these signals can be enforced particularly for sound waves originating from a particular direction. From another point of view, when a preselected phase contribution is added to one of the electromagnetic radiation signals by the adjustment means, constructive interference of the different electromagnetic radiation signals is enabled for sound originating from a sound source which is oriented at a particular angle with respect to the array of the vibratable elements of the sound detection device. The sound detection device according to the invention thus allows preselection of an angle or a range of angles related to a location of a sound source to which the sound detection of the invention is particularly sensitive. In other words, the adjustment means allow filtering sound with respect to the filter criteria of the orientation of a sound source with respect to the membrane array.

A preferred application of the sound detection device according to the invention is a compact microphone that allows choosing the direction from which the sound comes which is predominantly detected. In this description, the term "electromagnetic radiation" particularly refers to waves including an electric field vector and a magnetic field vector and propagating with the velocity of light. The term "electromagnetic radiation" includes quanta having a vanishing rest mass. Particularly, the term electromagnetic radiation includes infrared radiation, optically visible light, and ultraviolet radiation. Electromagnetic radiation used according to the invention should have a wavelength that is preferably smaller than the wavelength of acoustic signals to be detected.

According to an embodiment of the invention, a compact integrated microphone array using laser Doppler effects is provided. A high sensitivity of an interferometer microphone is combined with the sophisticated spatial resolution of a microphone array, which allows resolving angularly an audio signal with a small and compact system. A direction from which detected sound comes can be selectively chosen with a button controlling the adjustment means, for instance, a phase retarder or phase delay element which causes a phase shift between the electromagnetic radiation signals. The sound detection device according to the invention is particularly sensitive to sound originating from a particular direction, without the requirement to mechanically move, shift or rotate the device.

Thus, a phase shift unit may be adjusted in such a way that constructive interference of two electromagnetic radiation beams being reflected from two membranes is (only) enabled for sound coming from a particular direction and exciting the membranes. Controlling the phase delay unit allows adjustment of a direction of a sound source for which the sensitivity of the detector is particularly high and which is therefore "amplified". In other words, sound originating from most directions may only interfere in a random manner, whereas sound originating from a particularly adjustable direction is added coherently in a constructive manner so that a selective amplification of sound coming from a particular angular range is achieved. The system according to the invention may be particularly adapted to detect a sound signal, wherein the system may comprise a first membrane and a second membrane which are oriented relatively close to each other. The minimum distance between the membranes depends on the phase difference being recognizable by an interferometer. Each membrane should have fixed ends. The system may further comprise an interferometer for measuring movements of the at least two membranes, which movements are induced by sound waves. Beam-splitting means may be provided and adapted to split up a light beam of an interferometer into a first light beam and a second light beam. Movements of the first membrane are detectable by means of the first light beam, and movements of the second membrane are detectable by means of the second light beam. Analyzing means may be provided for analyzing the first light beam and the second light beam to reconstruct the sound signal to be detected in a spatial, resolving manner. The system according to the invention may be characterized by adjustment means, for example, a phase retarder (mirror) which may be placed in one of the light beams for choosing a sensitivity for a certain phase difference. The direction of origin of the sound to be detected can be selectively chosen with a button controlling one or more phase retarders, which does not require any mechanical rotation of the microphone.

The sound detection device according to the invention is not limited to only two vibratable elements, but may use more, for instance, ten, vibratable elements. Each element of this type may have an extension of the order of one or two centimeters. Compared to

US 4,559,642, which requires large distances between the vibratable elements (of the order of some ten (10) centimeters) for sufficient angle resolutions, the system according to the invention can be realized with significantly reduced dimensions, because the phase of the sound coming from the vibratable units may be measured, according to the invention, in an optical manner. Thus, a conversion from a mechanical wave system to an electromagnetic wave system is achieved, yielding a refined resolution. A conversion of the information included in the electromagnetic radiation signals into a, for instance, electric system may be carried out at the very end of a detection array or transmission path, so that the signal-to- noise ratio of a transmitted signal is improved. The system according to the invention thus allows an electrical conversion just at the end. The system according to the invention has the advantage that it may be constructed in a compact manner.

One aspect of the invention is related to the provision of a plurality of microphones, each individual microphone generating a mechanical signal. By converting the mechanical information into optically encoded information and by controlling (or modifying) the phase of the optical signal, a certain direction can be chosen. Thus, the phase of an optical signal resulting from a vibration of the membranes may be controlled. This increases the signal-to- noise ratio, because the conversion to an electric signal may be done only once, at the end. The invention allows manufacturing a sound detection device with significantly decreased distances between individual microphones and with microphones each having reduced sizes. According to the invention, it is possible to place the membranes at very close distances from each other (for instance, of the order of millimeters), thus achieving a more compact system. The phase relation between the electromagnetic radiation signals can be manipulated (before or after reflection at the membranes) to carry out some kind of filtering operation, resulting in a significantly amplified detection of acoustic noise coming from a particular direction, whereas acoustic noise coming from other directions may be efficiently suppressed.

According to the invention, light reflected from the different mechanically deformed membranes is brought to interference. Furthermore, the device influences the phase relation between the light signals, which is then added up coherently. As a result, a large amount of light coming from a direction other than the desired one is cancelled out, because a random superposition of these signals is performed. However, sound originating from a particular direction is summed up coherently, which results in a large signal contribution. This desired sound is amplified and emphasized, wherein other sound is suppressed. A possible application of the system according to the invention is related to a lecture hall microphone system that emphasizes sound coming from a human speaker giving a speech in a lecture hall, whereas background noise coming from other directions is efficiently suppressed. This increases the quality of the detected sound.

Another application of the system according to the invention is a sound source (for example, a human speaker) recognition system, wherein the sound detection device according to the invention is used for detecting a direction (by subsequently scanning angles of an angle range with the adjustment means), from which direction the noise is coming, so as to determine a direction with a maximum audio signal. For instance, a human user in an intelligent home environment system may give the oral command "door open". Then, the sound detection device according to the invention may recognize a direction where the user is currently located. From the knowledge of this direction, the system may judge if the user intends the kitchen door to be opened, or alternatively the door of the living room.

Another application of the device according to the invention is the direction-specific recording of audio information. If, for instance, a speaker is interviewed and a camera is directed to the face of the person to be interviewed, the system according to the invention may selectively record sound originating from the direction in which the interviewed person is located.

Referring to the dependent claims, further preferred embodiments of the invention will be described hereinafter. Preferred embodiments of the sound detection device of the invention will now be described. These embodiments may also apply to the method of detecting sound according to the invention.

The adjustment means is preferably adapted to adjust the first electromagnetic radiation signal and/or the second electromagnetic radiation signal in such a manner that the sensitivity of a superposition signal resulting from a superposition of the first electromagnetic radiation signal and the second electromagnetic radiation signal with respect to sound originating from an adjustable direction is increased as compared to sound originating from other directions. Particularly when the first electromagnetic radiation signal is brought to interference with the second electromagnetic radiation signal, the resulting signal reflects contributions of sound coming from the adjusted direction or directions in a stronger manner than sound coming from other directions. This is due to the fact that at least one of these signals is manipulated in such a way that a constructive overlay of the signals is achieved for sound originating from the adjusted direction. Each first vibratable element and second vibratable element may be a membrane.

Such a membrane or diaphragm is an element that is mechanically deformable in the presence of sound signals, i.e. mechanical waves.

Each first vibratable element and second vibratable element may be a membrane having a reflective surface adapted to reflect electromagnetic radiation. Such a reflective surface can be provided in the form of one or more layers on the surface of a flexible membrane, or the membrane itself can be formed from a reflective material. For instance, the membrane can be made of a thin polished metal layer serving as a reflective membrane. Alternatively, a layer of reflective material can be deposited (for instance, by vacuum evaporation) on a vibratable element. The reflection properties may be adjusted to the wavelength of the used electromagnetic radiation.

The converting means may be adapted to generate at least one of the group consisting of infrared radiation, optical radiation and ultraviolet radiation as a basis for the first and the second electromagnetic radiation signal. Electromagnetic radiation in this application is particularly understood to mean any kind of photons, i.e. electromagnetic waves having any appropriate wavelength. For instance, also X-rays or microwaves might be used for the invention. However, infrared radiation, optical radiation and ultraviolet radiation are preferred, because these types of radiation have wavelengths that are appropriate for practical use and allow both a simple construction of the device and a high resolution.

The converting means may comprise an electromagnetic radiation source adapted to generate electromagnetic radiation as a basis for the first electromagnetic radiation signal and for the second electromagnetic radiation signal. Such an electromagnetic radiation source may be adapted to generate coherent electromagnetic radiation or partially coherent electromagnetic radiation (i.e. radiation having a part of the rays with a defined phase relation with respect to each other) as a basis for the first and the second electromagnetic radiation signal. The electromagnetic radiation source is preferably a laser, because a laser (for instance, a laser diode) allows producing coherent light with reasonable effort. "Coherent" particularly means that propagating waves of the radiation have and maintain a defined phase relation over a sufficiently large dimension. The converting means may comprise a beam splitter adapted to split an electromagnetic radiation beam generated by the electromagnetic radiation source into a first beam forming a basis for the first electromagnetic radiation and into a second beam forming a basis for the second electromagnetic radiation. Such a beam splitter may be realized on the basis of a semi-permeable reflection/transmission element, while the transmitted and the reflected partial beams form the two electromagnetic radiation beams. Such a beam splitter allows use of a single electromagnetic radiation source for producing both the first and the second electromagnetic radiation signal.

The adjustment means is preferable realized as a static element. In other words, the adjustment of a direction of sound to which the sound detection device is particularly sensitive does not require any moving parts and can thus be manufactured at low cost and is easy to handle.

The adjustment device of the sound detection device may comprise at least one phase delay element adapted to delay the phase of the first and/or the second electromagnetic radiation signal. By adjusting a phase delay or phase shift between these two signals, a sound source direction can be adjusted to which the sound detection device is particularly sensitive. The phase shift may be set in such a way that a constructive superposition or interference of the two electromagnetic radiation signals is enabled for sound originating from an assigned direction. The at least one phase delay element may be adapted to delay the phase of the first and/or the second electromagnetic radiation signal in such a manner that an interference of the first and the second electromagnetic radiation signal is constructive for sound originating from the adjustable direction.

The sound detection device may further comprise an electromagnetic radiation detecting means for detecting the first and the second electromagnetic radiation signal. Such an electromagnetic radiation detecting means may be adapted to generate an electric detection signal (for instance, an electric current or an electric voltage). At the end of the propagation path, it is preferred that the information contained in the electromagnetic radiation signal beams is converted into an electric signal which can undergo further signal processing. By carrying out this conversion at the end of the path, the signal-to-noise ratio is further increased, because damping of the electric signal is minimized. For instance, the electromagnetic radiation detecting means may be a photodiode, i.e. an element, which, when irradiated with electromagnetic radiation, produces an electric signal having an amplitude which is characteristic of the direction-specific sound signal information encoded in the electromagnetic radiation. The electromagnetic radiation detecting means may be a photodiode that is included in a laser that serves as the electromagnetic radiation source. In other words, the photodiode as detecting means and the laser as electromagnetic radiation source may be integrated into a single unit that saves space and costs and allows an effective generation and detection of the signal. When the photodiode is included in the laser, the detection signal disturbs the laser operation mode; this represents the information that shall be detected.

The converting means may comprise an optical deflection means adapted to deflect the first and/or the second electromagnetic radiation signal between the electromagnetic radiation source and the electromagnetic radiation detecting means. Such a deflection element may comprise at least one lens and/or at least one glass fiber. However, the optical deflection means may, additionally or alternatively, implement any optical element which allows guiding the electromagnetic radiation signals from the source to the respective vibratable element, and from the vibratable element to the detector.

The sound detection device according to the invention may comprise at least one further vibratable element each arranged at a distance from the first vibratable element and from the second vibratable element and each adapted to vibrate in response to sound. The converting means may be adapted to convert a vibration state of each of the at least one further vibratable element into at least one further electromagnetic radiation signal. The adjustment means is adapted to adjust at least one of the electromagnetic radiation signals in such a manner that the sensitivity of the electromagnetic radiation signals with respect to sound originating from an adjustable direction is increased as compared to sound originating from other directions. Thus, three, four, ten or even more membranes may be arranged as a membrane array or microphone array (for instance, arranged in a linear manner or in a matrix-like manner) so that the spatial resolution of the sound direction-sensitive system according to the invention is further increased. These and other aspects of the invention are apparent from and will be elucidated with reference to the non-limiting embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS Fig.l shows a sound detection device according to a first embodiment of the invention,

Fig.2 illustrates the geometry of a sound source and two membranes as vibratable elements,

Fig.3 illustrates a sound detection device according to a second embodiment of the invention,

Fig.4 illustrates a sound detection device according to a third embodiment of the invention,

Fig.5 illustrates a sound detection device according to a fourth embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

The illustrations in the drawings are schematic. In the different drawings, the same reference numerals or signs denote similar or identical elements.

A sound detection device 100 according to a first embodiment of the invention will now be described in detail with reference to Fig.1.

The sound detection device 100 comprises a first diaphragm or membrane 101 and a second diaphragm or membrane 102 arranged at a distance from each other and each adapted to vibrate in response to sound 103 emitted from a sound source 104. The membranes can be made of any suitable material, for instance, mylar, a polymer or silicon. As a consequence of the sound 103 impinging on each diaphragm 101, 102, the first and the second diaphragm 101, 102 are mechanically deformed. Since the distances between each diaphragm 101, 102 and the sound source 104 are different, the phase of the acoustic wave impinging on the diaphragms 101, 102 is different. Consequently, the mechanical deformation of the first diaphragm 101 is different from the mechanical deformation of the second diaphragm 102. A converting means is adapted to convert the mechanical deformation of the first diaphragm 101 into a first optical signal and to convert the mechanical deformation of the second diaphragm 102 into a second optical signal. In the present case, such a converting means comprises a laser 105 that emits a first primary optical beam 106 to a reflective surface 108 of the first diaphragm 101. The laser 105 further emits a second primary optical beam 107 onto a reflective surface 109 of the second diaphragm 102. Due to the different, phase-specific mechanical deformation of the membranes 101, 102, the optical path lengths of the first primary optical beam 106 and the second primary optical beam 107 are different. After being reflected by the reflective surfaces 108 and 109, respectively, the first primary optical beam 106 is converted into a first secondary optical beam 110, and the second primary optical beam 107 is converted into a second secondary optical beam 111. The phase information corresponding to the mechanical deformation of the diaphragms 101, 102 due to the impinging sound 103 is encoded in the different optical path lengths of the two optical beams 106, 110 and 107, 111, consequently in a phase difference between the optical beams 110, 111.

After being reflected by the reflective surface 109 (for instance, made of gold material), the second secondary optical beam 111 is guided through an adjustment means 112 that is implemented as a phase retarder. The adjustment means 112 is adapted to adjust the second secondary optical beam 111 in such a manner that the sensitivity of a superposition signal resulting from a superposition of the first secondary optical beam 110 and the second secondary optical beam 111 to the sound originating from the sound source 104 located at a particular direction is increased with respect to a sensitivity to sound coming from other directions.

As can be seen in Fig.l, interference of the first secondary optical beam 110 and the second secondary optical beam 111 is realized after having passed the adjustment means 112. In dependence upon the selectable phase delay added to the second secondary optical beam 111 by the adjustment means 112, constructive interference of beams 110, 111 is enabled for sound coming from a particular direction. In other words, by setting the phase delay of the adjustment means 112 to a particular value, a direction can be selected from which audio sound comes amplified. Sound waves coming from any other direction are not summed up in a constructive manner and yield a much lower contribution to the detected audio signal than the coherently summed up audio coming from the selected direction. Since the laser 105 is used as electromagnetic radiation source, the optical beams 106, 107, 110, 111 consist of coherent light. For instance, when a human speaker is present at a particular location as a sound source 104, the phase delay of the adjustment means 112 can be selected accordingly, so that sound coming from the speaker is amplified, whereas background noise from the audience is not amplified and thus suppressed. This allows improving the quality of detected sound. Fig.l further shows a control device 113 coupled to the adjustment means 112 and adapted to provide the adjustment means 112 with a control signal encoding the information which phase delay has to be added to the second secondary optical beam 111. The control device 113 can be controlled either manually (for instance, via a graphic user interface) or automatically, for instance, by software (not shown in Fig. 1). For this purpose, the position of the speaker may be detected with an appropriate detecting means, and then the control device 113 may adjust a phase delay value, which phase delay should be added to the second secondary optical beam 111.

The adjustment means 112 is realized as a static element, so that the adjustment of a position of a source emitting sound to be detected can be realized without moving parts. The sound detection device 100 further comprises a photodiode 114 as an electromagnetic radiation detecting means. After bringing beams 110, 111 to interference, the resulting beam impinges on the photodiode 114 that produces an electric signal, which electric signal is characteristic of the optical signal. This electric signal output by the photodiode 114 is characteristic of the detected sound. A scheme 200 will be described with reference to Fig.2, showing the principle of an optical interferometer microphone (similar to the one disclosed in JP 60039614 A) that can resolve very small distances. Fig.2 thus illustrates the principle of the measurement according to the invention.

The scheme 200 shows again the sound source 104 emitting sound 103 to a first membrane 101 and to a second membrane 102. The sound 103 (sound wave front) coming from a sound plane wave encounters the two membranes 101, 102 at different locations. As the sound source 104 is located at an angle α to the normal, the two membranes 101, 102 experience different phases and are thus deformed in a different manner. Since the sound 103 has quite a large wavelength, the difference in phases is quite small when the two membranes 101, 102 are very close together, which is desired in order to have a sound detection device with very small dimensions. An estimation of the phase properties will be presented hereinafter.

According to the scheme 200 shown in Fig.2, reasonable values should be taken, such as α = 30° and a sound wavelength of λ_sound = 1 m. When the two membranes 101, 102 are put very close to each other, for instance, at a distance d = 1 mm, the following phase difference Δφ is obtained:

Δφ /2π = SC/λ_sound = (AC sin α )/λ_sound ~1 mm • 0.5 / 1 m = 0.5 x 10^"3 (1) The definition of the distances BC and AC can be derived from Fig.2. The phase difference Δφ estimated on the basis of equation (1) is very small. When the two membranes 101, 102 oscillate with an amplitude A = I mm, they will not oscillate in phase because of the phase difference. In fact, one will follow the back of the other, at a small distance Δy (resulting from the correlation Δy/(2A)= Δφ/π) of about:

Δy = 2/π A Δφ ~ 2/π (10^"3 m) (0.5- 10^"3) = 1/π μm (2)

In other words, there will be a slight difference between the two membranes 101, 102, of the order of μm. This slight difference is of the order of the wavelength of light, and can thus be measured easily with an optical interferometer.

In order to have a compact and low-cost interferometer, a self-mixing interferometry may be used as described hereinafter.

Fig.3 shows a sound detection device 300 according to a second embodiment of the invention.

The sound detection device 300 has two membranes 101, 102 having fixed ends 301. Furthermore, the path between a laser 105 and the membranes 101, 102 accommodates a first lens 302 and a second lens 303 as optical deflection means adapted to deflect the radiation signals 106, 107 to force them to take a desired optical path. The laser 105 is shown to have a laser cavity.

As can be seen in Fig.3, the light beams 106, 107 coming from the laser 105 are split up into two parts. The first part 106 goes to the first membrane 101, and the second part 107 goes to the second membrane 102.

The light 106, 107 reflected by reflective surfaces of the membranes 101, 102 is then sent back to the laser 105, and the fluctuations and the power of the laser are measured with the photodiode 114, in a similar manner as disclosed in JP 60039614 A. In this respect, reference is made in this application to the disclosure of JP 60039614.

When the path difference between the two membranes 101, 102 is of the order of λ_hght ~ 1 μm, the two beams 110, 111 will constructively interfere, giving a large signal on the photodiode 114. In contrast to this, when the difference is about λi_lght/2, the two beams 110, 111 will destructively interfere, giving a small signal on the photodiode 114. In this case, the system is sensitive to an angle that gives a path difference of λh_gh_t. Of course, the vibration of both membranes 101, 102 gives the signal itself. However, an additional phase retarder 112 placed in the path of one of the two beams 110, 111 can control the phase difference. In fact, the phase retarder 112 acts as a selection button for the angle, choosing the sensitivity for a certain phase difference, and thus for a certain angle. The phase retarder 112 works in a manner that can also be derived from Fig.2. For a single direction to be selected, the phase retarder 112 has to be adjusted in such a way that the signal impinging on the two membranes 101, 102 is in phase. The sound coming from other directions will interfere destructively.

A resolution of the system 300 is given by the multitude of membranes 101, 102 which may be larger than two. Due to the special geometry of the system according to Fig.3, it is possible but not necessary that each membrane 101, 102 has its own phase retarder 112. To simplify the construction, a constant phase change over all membranes 101, 102 can be used, i.e. every two adjacent beams may be provided with a particular phase difference.

Since the phase retardation may have positive and negative values, the direction of the angle (left or right) to which the sound detector is sensitive may be selected as well. The system according to the invention may include more than two diaphragms 101,

102, to achieve a further improved resolution. An embodiment with a detector (photodiode 114), which detector is provided as a separate element from the laser 105, is also within the scope of the invention. However, it is also possible to use a photodiode 114 that is included in a laser 105. A sound detection device 400 according to a third embodiment of the invention will now be described with reference to Fig.4.

In the case of the sound detection device 400, there is a plurality of membranes, namely a first membrane 101, a second membrane 102 and further membranes 403. A phase control matrix 401 comprises elements for allowing a change of the phase properties of light beams 106, 107, 110, 111 propagating between a laser 105 and the membranes 101, 102, 403. For instance, a phase control button 402 for controlling the phase control matrix 401 can be adapted to allow a separate adjustment of each phase delay to be added to light which is reflected by any one of the membranes 101, 102, 403. Alternatively, a constant difference between phase delays provided to neighbouring beams can be set. In Fig.4, d denotes the membrane diameter and is of the order of 1 mm, N denotes the number of membranes 101, 102, 403 and is of the order often, W denotes the width of the phase control and is of the order of 10 mm, f denotes the focal distance of a lens 302 and is of the order of 10 to 20 mm. A sound detection device 500 according to a fourth embodiment of the invention will now be described with reference to Fig.5. The sound detection device 500 differs from the sound detection device 400 in that optical fibers 501 are provided between the laser 105 and the membranes 101, 102, 403 to deflect the respective light beams in such a manner that they are guided between the laser 105 and the membranes 101, 102, 403. It should be noted that use of the verb "comprise" and its conjugations does not exclude elements or steps other than those stated in the claims, and use of the indefinite article "a" or "an" preceding an element or step does not exclude a plurality of such elements or steps. Moreover, elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

1. A sound detection device (100), comprising a first vibratable element (101) and a second vibratable element (102) arranged at a distance from each other and each adapted to vibrate in response to sound (103); a converting means adapted to convert a vibration state of the first vibratable element (101) into a first electromagnetic radiation signal (110) and to convert a vibration state of the second vibratable element (102) into a second electromagnetic radiation signal (111); an adjustment means (112) adapted to adjust the first electromagnetic radiation signal (110) and/or the second electromagnetic radiation signal (111), such that the sensitivity of the first electromagnetic radiation signal (110) and the second electromagnetic radiation signal (111) with respect to sound (103) originating from an adjustable direction is increased as compared to sound originating from other directions.

2. The sound detection device (100) according to claim 1, wherein the adjustment means (112) is adapted to adjust the first electromagnetic radiation signal (110) and/or the second electromagnetic radiation signal (111), such that the sensitivity of a superposition signal resulting from a superposition of the first electromagnetic radiation signal (110) and the second electromagnetic radiation signal (111) with respect to sound (103) originating from an adjustable direction is increased as compared to sound originating from other directions.

3. The sound detection device (100) according to claim 1, wherein each first vibratable element (101) and second vibratable element (102) is a membrane.

4. The sound detection device (100) according to claim 1, wherein each first vibratable element (101) and second vibratable element (102) is a membrane having a reflective surface (108, 109) adapted to reflect electromagnetic radiation.

5. The sound detection device (100) according to claim 1, wherein the converting means is adapted to generate at least one of the group consisting of infrared radiation, optical radiation and ultraviolet radiation as a basis for the first electromagnetic radiation signal (110) and as a basis for the second electromagnetic radiation signal (111).

6. The sound detection device (100) according to claim 1, wherein the converting means comprises an electromagnetic radiation source (105) adapted to generate electromagnetic radiation (106, 107) as a basis for the first electromagnetic radiation signal (110) and as a basis for the second electromagnetic radiation signal (111).

7. The sound detection device (100) according to claim 1, wherein the converting means comprises an electromagnetic radiation source (105) adapted to generate coherent electromagnetic radiation (106, 107) or partially coherent electromagnetic radiation as a basis for the first electromagnetic radiation signal (110) and as a basis for the second electromagnetic radiation signal (111).

8. The sound detection device (100) according to claim 6, wherein the converting means comprises a beam splitter adapted to split an electromagnetic radiation beam generated by the electromagnetic radiation source (105) into a first beam (106) forming a basis for the first electromagnetic radiation signal (110) and into a second beam (107) forming a basis for the second electromagnetic radiation signal (111).

9. The sound detection device (100) according to claim 1, wherein the adjustment means (112) is realized as a mechanically static element.

10. The sound detection device (100) according to claim 1, wherein the adjustment means (112) comprises at least one phase shift element adapted to shift a phase of the first electromagnetic radiation signal (110) and/or of the second electromagnetic radiation signal (111) by adding an adjustable phase shift value to the first electromagnetic radiation signal (110) and/or to the second electromagnetic radiation signal (111).

11. The sound detection device (100) according to claim 10, wherein the at least one phase shift element is adapted to shift a phase of the first electromagnetic radiation signal (110) and/or the second electromagnetic radiation signal (111), such that interference of the first electromagnetic radiation signal (110) and the second electromagnetic radiation signal (111) is constructive for sound (103) originating from the adjustable direction.

12. The sound detection device (100) according to claim 1, comprising an electromagnetic radiation detecting means (114) for detecting the first electromagnetic radiation signal (110) and the second electromagnetic radiation signal (111).

13. The sound detection device (100) according to claim 12, wherein the electromagnetic radiation detecting means (114) is adapted to generate an electric detection signal.

14. The sound detection device (100) according to claim 12, wherein the electromagnetic radiation detecting means is a photodiode (114).

15. The sound detection device (100) according to claims 6 and 12, wherein the electromagnetic radiation detecting means is a photodiode (114) included in a laser (105) adapted as the electromagnetic radiation source.

16. The sound detection device (100) according to claims 6 and 12, wherein the converting means comprises an optical deflection means (302, 303; 501) adapted to deflect the first electromagnetic radiation signal (110) and/or the second electromagnetic radiation signal (111) between the electromagnetic radiation source (105) and the electromagnetic radiation detecting means (114).

17. The sound detection device (100) according to claim 16, wherein the optical deflection means comprises at least one lens (301, 302) and/or at least one glass fiber (501).

18. The sound detection device (400) according to claim 1, comprising at least one further vibratable element (403), wherein each of the at least one further vibratable element (403) is arranged at a distance from the first vibratable element (101) and from the second vibratable element (102) and is adapted to vibrate in response to sound (103); wherein the converting means is adapted to convert a vibration state of each of the at least one further vibratable element (403) into at least one further electromagnetic radiation signal; wherein the adjustment means (401, 402) is adapted to adjust at least one of the group consisting of the first electromagnetic radiation signal (110), the second electromagnetic radiation signal (111) and the at least one further electromagnetic radiation signal, such that the sensitivity of the first electromagnetic radiation signal (110), the second electromagnetic radiation signal (111) and the at least one further electromagnetic radiation signal with respect to sound (103) originating from an adjustable direction is increased as compared to sound originating from other directions.

19. A method of detecting sound, the method comprising the steps of arranging a first vibratable element (101) and a second vibratable element (102) at a distance from each other to vibrate in response to sound; converting a vibration state of the first vibratable element (101) into a first electromagnetic radiation signal (110) and converting a vibration state of the second vibratable element (102) into a second electromagnetic radiation signal (111); adjusting the first electromagnetic radiation signal (110) and/or the second electromagnetic radiation signal (111), such that the sensitivity of the first electromagnetic radiation signal (110) and the second electromagnetic radiation signal (111) with respect to sound (103) originating from an adjustable direction is increased as compared to sound originating from other directions.