WO2004105427A1

WO2004105427A1 - Piezoelectric microphone

Info

Publication number: WO2004105427A1
Application number: PCT/SE2004/000782
Authority: WO
Inventors: Göran EHRLUND
Original assignee: Ehrlund Goeran
Priority date: 2003-05-23
Filing date: 2004-05-21
Publication date: 2004-12-02
Also published as: SE0301541L; SE0301541D0; SE526743C2

Abstract

Piezoelectric microphone (10) with improved audio characteristics. The microphone (10) has a transducer membrane (15) comprising a piezoelectric layer (20) and the improved audio characteristics are achieved because the transducer membrane (15) has an essentially triangular shape.

Description

Piezoelectric microphone .

Field of the invention

The present invention relates to an electro acoustic transducer and more particularly a piezoelectric microphone for transforming vibrations from a vibrating surface, e.g. the resonance body of a musical instrument, to an electrical signal.

Background of the invention ϊ; 'ι ^' Piezoelectric transducers are frequently used to register and produce vibrations.

Such transducers are based on a piezocrystal that produces an electrical signal when deformed along a certain orientation and if an electrical signal is applied to the crystal it will be deformed. Thus, such transducers are designed to register oscillations or vibrations such that they are transformed to deformations of a piezocrystal, whereby the so generated electrical signal reflects the vibrations. Vibration registering piezoelectric transducers or sensors are used in many fields of the" technology and not least in the field of music, on one hand as con- ventional microphones for registering sound waves in air and on the other hand as microphones or pick ups for musical instruments to register oscillations in a instrument cabinet or the like.

There are a number of such piezoelectric microphones for registering oscillations in instruments. Examples of such are Fishman SBT-C, Shatten Design EP-01 and Barcos Berru, of which the first two are shaped as circular plates and the third as a rectangular plate, each having at least .one piezoelectric layer generating the microphone signal. Other examples of known piezoelectric instrument microphones comprise e.g. different types of "stable microphones" that are to be arranged at/ integrated in the stable of a string instrument.

However, piezoelectric microphones are in general considered low quality microphones due to certain obvious shortcomings in the audio performance. All know piezoelectric microphones mentioned above display large variations in frequency response in the audible frequency interval. These variations are mainly caused by pronounced resonance phenomena, e.g. resonance modes of flat membranes that give rise to oscillations with increased alternatively decreased amplitude over a narrow frequency range. Such resonance phenomena give rise to audible shortcomings in the reproduction of the tones from an instrument and the sound is usually regarded as boomy.

Summary of the invention

The object of the present invention is to solve the problem with fluctuating frequency response for piezoelectric microphones. According to the invention the main object is achieved with the piezoelectric microphone according to claim 1.

One advantage with such a microphone is that the audio reproduction is enhanced, because strong local variations in frequency response do not appear, whereby a smoother frequency response is achieved.

Another advantage is that the microphone according to the present invention is considerably less sensitive to feedback, which means that the position of the microphone with respect to loudspeakers is less critical.

Still another advantage is that the microphone according to the invention can be designed as a modular microphone system with a mounting base that enables detachable mounting of the transducer membrane. This embodiment makes it possible to substitute the transducer membrane of the microphone without substituting the mounting base or cables.

Advantageous embodiments of the invention are defined in the dependent claims.

Brief description of the drawings

Fig. la shows a schematic cross sectional view of an embodiment of a microphone according to the present invention.

Fig. lb shows a schematic top view of the microphone in Fig. la, where a-a shows the cross section for Fig. la. Figs. 2a to 2c schematically show alternative shapes of a transducer membrane according to the present invention.

Figs. 3a and 3b schematically show alternatively membrane mounting arrangements according to the present invention.

Fig. 4a schematically shows a modular microphone system according to the present invention.

Fig. 4b schematically shows a cross sectional view of the microphone system of Fig. 4a.

Figs. 5a to 5c show recorded frequency response curves for three different microphones with membranes of different shape.

Fig. 6 schematically shows the recording arrangement used to register the curves in Figs. 5a to 5c.

Detailed description of the invention

Fig. 1 is a cross sectional view of a piezoelectric microphone 10 according to one embodiment of the present invention. The microphone 10 has a transducer membrane 15 comprising a piezoelectric layer 20 laminated to a metal layer 30. The piezoelectric layer 20 is provided with a conducting electrode layer 40 on the surface that is not in contact with the metal layer 30. Electric conductors 50 and 60 are put in electrical contact with the conducting layer 40 and the metal layer 30 respectively and these layers provide that the conductors 50 and 60 are in electrical contact with essentially the whole upper and lower surface of the piezoelectric layer 20, respectively. By this arrangement the sum of the electrical signal produced by the piezoelectric layer 20 over the whole surface is registered when the transducer membrane 15 vibrates.

The electric conductors 50 and 60 are preferably connected with respective layer by soldering or another joining method that guarantees electrical contact between the layer and the conductor. The other ends of the conductors 50 and 60 are put in contact with a reproduction/ recording system (not shown) such as an amplifier or a recording equipment. To guarantee minimum peripheral interfer- ence of the registered signal, the conductors 50, 60 are preferably arranged in the form of a coaxial cable 70.

Further, the transducer membrane 15 comprises an isolating layer 80, e.g. a polymer material such as epoxy or the like, and an electrically conducting shielding layer 90 in electrical contact with the metal layer 30 and thus also the conductor 60, but not with the conductor 50 (because, in that case, the microphone would be short circuited). Hence, the conductor 50 is isolated from the shielding layer 90 with a suitable isolator arrangement, e.g. the conductor 50 is provided with an isolating cover. As shown in Fig. la, the isolating layer 80 also encloses the outer periphery of at least the piezoelectric layer 20, but preferably also the metal layer 30 whereby the shielding layer 90 can surround essentially the whole transducer membrane 15 without short circuiting the piezoelectric layer 20. The shielding layer 90 drastically lowers the susceptibility to interference caused by electromagnetic waves, for the microphone 10.

The piezoelectric layer 20 is made of a suitable piezoelectric material such as naturally occurring crystals cut in suitable directions with reference to the crystal structure. Examples of such naturally occurring crystals are quarts, turma- line and Rochelle salt. The layer 20 can also be comprised of an artificial ceramic layer that has been formed using an electric field to achieve piezoelectric properties. Examples of such artificial piezoelectric layers are lithium sulphate, barium titanate, lead zirconate among others. The metal layer 30 is comprised of a suitable metal with material properties that gives the transducer membrane 15 desired acoustic properties, e.g. brass, bronze, copper, aluminum or the like. The electrode layer 40 is a very thin layer of a conducting material that does not essentially contribute to the acoustic properties. The shielding layer 90 is preferably comprised of a conducting paint that is applied to the transducer membrane using a suitable application method, such as dipping, brush painting or spraying. The mutual material choices for the different layers together with the dimensions of them determines the acoustic properties of the transducer membrane 15, and by adjusting these parameters for one or more of the layers, all layers except the piezoelectric layer 20, the electrode layer 40 and the metal layer may be omitted if desired. Also the metal layer 30 may be omitted and replaced by a second electrode layer.

As is shown in Fig. lb, the transducer membrane 15 has an essentially triangular shape, which has been found to provide notably enhanced audio reproduc- tion. The enhanced audio reproduction is directly noticeable when the microphone 10 is used to register oscillations from a string instrument, e.g. a guitar. The enhancement is basically that the tendency of boominess that known microphones of this type with circular or rectangular membranes exhibits, is not present. Moreover, a noticeable enhancement at high frequencies is provided. These subjective enhancements have also been verified by frequency response recordings, which are discussed in detail below. Moreover, it has been shown that the microphone 10 according to the present invention is very insusceptible to feedback, when compared with known microphones, whereby it does not require the corresponding careful placing of e.g. loudspeakers with respect to the position of the microphone 10.

Herein, the expression essentially triangular shape comprises all types of triangles, even if the disclosed preferred embodiment is an equilateral triangle. Moreover, the expression comprises shapes of the types shown in Fig. 2a and 2b, where 2a shows a triangular shape with concave curved sides and Fig. 2b a triangular shape with convex curved sides. Other possible embodiments comprise triangles with rounded alternatively cut corners, triangles with one or more internal openings, recesses from one or more of the sides and possible combinations of these. An example of a transducer membrane 15 with a through hole is shown in Fig. 2c.

The conventional method of mounting microphones of this type, e.g. to an oscillating panel in an instrument, is by adhesive paste, double stick tape or the like that covers essentially the whole area of the transducer membrane. Using such a mounting the microphone 10 according to the present invention gives noticeable audible enhancements when compared with prior art microphones. However, the largest enhancement of the audio reproduction is achieved when the microphone 10 is supported at three support points 100a, 100b and 100c that are arranged on a surface in the instrument such as schematically shown in Fig. lb. These support points 100a, 100b and 100c can be made of adhesive paste, or another suitable flexible material that easily can be attached to a surface in the instrument and to the transducer membrane. It has further been found that the transducer membrane preferably shall be arranged at a distance from the surface to which it is attached to. This distance is preferably more than one mm, more preferably more than 2 mm and most preferably more than 3 mm. This distance is preferably less than 10 mm, more preferably less than 6 mm and most preferably less than 5 mm. Obviously the preferred distance de- pends on the characteristics of the transducer membrane 15 and for the embodiment that is described in the example below a distance of 3,5 to 4 mm provided the best results.

Figs. 3a and 3b show alternative support arrangements comprised of specially designed support bases 110a and 110b respectively. A simple and robust support base 110a can be formed as a ring 120 with an inner radius that is slightly smaller than the radius of the smallest circle that surrounds the transducer membrane 15. The ring 120 can be formed with a suitable height so that the transducer membrane 15 is supported by its upper surface 130 as is schematically shown in Fig. 3a, or it may be formed as a low ring with three upstanding support means that carry the transducer membrane 15. The ring 120 is preferably made of a flexible polymer material such as rubber or another elas- tomere, but it may also be formed of a stiff material. However, in the later case the support points 100a, 100b and 100c for the transducer membrane 15 may be formed of an elastic material, which in turn are supported by the stiff ring 120. Fig. 3b shows another possible support base 110b with a sub plate 140 and three upstanding support means 100a, 100b and 100c that carry the transducer membrane 15. This embodiment further comprises a cord attachment means 150 designed to relieve the cord 70 with respect to the transducer membrane 15. Further, the cord attachment means 150 allows use of very thin and flexible conductors 50, 60 to transmit the produced signal from the transducer membrane 15 to the cord 70, whereby minimal influence from the conductors 50, 60 can be achieved.

Fig. 4a shows a combination of a transducer membrane 15 and a support base 110c wherein the need for, and hence the mechanical influence from, the conductors 50, 60 has been eliminated. Fig. 4b schematically shows a cross section of the combination along the dashed line b-b in Fig. 4a. By replacing the electric conductors 50 and 60 by electric contact surfaces 160a and 170a that are put in electrical contact with the upper and lower surface of the piezoelectric layer 20, respectively. The electric contact surfaces 160a and 170a are preferably arranged at one and the same corner area as is shown in Fig. 4a and 4b, but they can also be arranged at different corner areas. In this embodiment the support point 100a is provided with two complementary contact surfaces 160b and 170b, respectively, that are in electric contact with the cable 70. By forming the two support points 100b, 100c so that the transducer membrane 15 is detach- ably supported by the three support points 100a, 100b and 100c in a suitable way, a microphone 10, or rather a modular microphone system with exchangeable transducer membranes 15 is provided. In this manner the microphones characteristics can be altered by simply changing the transducer membrane 15, e.g. to upgrade or according to a wish to change to audio characteristics.

Detailed example of an embodiment

A transducer membrane 15 of the type that is shown in Fig. la and lb has been produced using a commercially available piezoelectric "buzzer element" PEL35B29F, available from Schukat, Germany, as a starting point. This element is comprised of a brass plate 30 with a thickness of 0,3 mm and a diameter of 35 mm and a piezoelectric layer 20 with a thickness of 0,2 mm and a radius of 22 mm that is centered and laminated on the brass plate. On top of the piezoelectric layer 20 there is provided an electrode layer 40. It is common knowledge to use buzzer elements of this type as microphones, and it is considered as a very cheap way to create a microphone with decent audio characteristics.

Thereafter, said buzzer element was cut into a triangular form using a suitable tool. Hence, the resulting transducer membrane 15 has a size that is less than the size of the buzzer element. It has been shown that the best result is achieved when the transducer membrane 15 is shaped so that the piezoelectric layer 20 covers essentially the whole surface of the membrane, whereby the size of the transducer membrane 15 is less than the size of the piezoelectric layer on the buzzer element.

When the transducer membrane 15 had been cut out from the buzzer element, the electric conductors 50 and 60 were provided at the upper and lower surface of the membrane, respectively, by soldering. For this embodiment it has shown suitable to apply an epoxy layer with a thickness of about 0,5 mm, which layer moreover is formed so that it surrounds the outer edge of the piezoelectric layer 20 and the metal layer 30 as is shown in Fig. la. As the epoxy layer (isolating layer 80) covers the edge of the piezoelectric layer 20 it is possible to apply a shielding layer 90 that essentially covers the whole transducer membrane 15. As the shielding layer 90 is put in electric contact with the metal layer 30 and the shielding conductor 60 in the coaxial cable that transmits the microphone signal to an amplifier or the like, the microphone 10 is extremely insensible to electromagnetic interference. The microphone 10 produced in this way has been tested with a number of different instruments and, as was mentioned above, it exhibits obvious audio enhancements. At these tests, the microphone was on one hand attached to the instrument in a conventional way with adhesive paste under essentially the whole microphone surface and on the other hand with the three point support according to the above. For all cases, the audio reproduction was clearer and cleaner compared to prior art microphones, and exceptionally good when the three point support was used.

Comparing experiments

In order to verify the enhanced audio characteristics compared to known microphones of the same type, a number of measurements have been performed. Fig. 6 schematically shows the registration equipment that has been designed to achieve equivalent conditions for the different microphones. A conventional loudspeaker element 180 is used to create vibrations that shall be registered. The loudspeaker element 180 is operated by a noise generator, which during the measurements was set to generate pink noise. The loudspeaker element 180 was placed inside a tuned box 190 together with an adapted amount of damping material. A cylinder 200 of a stiff light material was arranged as an extension of the voice coil of the loudspeaker 180, which cylinder 200 serves as vibration platform for the microphone 10 be tested. To avoid resonance problems in the cylinder 200 a cut out was provided in the longitudinal direction.

The microphone 10 to be characterized is placed on top of the cylinder 200, whereby it registers the vibrations produced by the loudspeaker element 180. The signal from the microphone is conducted to a measurement computer and is analyzed using a so called FFT-analysis (Fast Fourier Transform). The test equipment has mainly been designed to give as equivalent measurement as possible for comparing the different microphones, and not to provide absolute measurements .

A number of test series were performed for the different models and Figs. 5a to 5c show examples of measurement results for three different microphones, triangular, circular and rectangular, respectively. Already from a short comparison it is obvious that the frequency response curve for the triangular microphone according to the present invention generally is smoother, i.e. not as strong fluctuations between adjacent frequencies. Both the other microphones show more marked peaks at certain frequencies, which peaks normally appear due to resonance phenomena.

Together with the subjective audio improvements, these measurements unambiguously show the improvement of the audio reproduction that is achieved with the triangular piezoelectric microphone 10 according to the present invention.

Claims

1. Piezoelectric microphone (10) with a transducer membrane (15) comprising a piezoelectric layer (20) characterized in that the transducer membrane (15) has an essentially triangular shape.

2. Piezoelectric microphone (10) according to claim 1, characterized in that the transducer membrane (15) is shaped as an equilateral triangle.

3. Piezoelectric microphone (10) according to claim 1, characterized in that the transducer membrane (15) is shaped as a triangle with one or more curved sides.

4. Piezoelectric microphone (10) according to any of the claims 1 to 3, characterized in that the transducer membrane (15) comprises a metal layer.

5. Piezoelectric microphone (10) according to any of the claims 1 to 4, characterized in that the piezoelectric layer covers essentially the whole transducer membrane (15).

6. Piezoelectric microphone (10) according to any of the claims 1 to 5, characterized in that the transducer membrane (15) comprises at least one isolating layer (80) that covers essentially the whole piezoelectric layer (20).

7. Piezoelectric microphone (10) according to any of the claims 1 to 6, characterized in that the transducer membrane (15) comprises a shielding layer (90) that essentially encloses the transducer membrane (15).

8. Piezoelectric microphone (10) according to any of the claims 1 to 7, characterized in that the transducer membrane (15) is supported by three support points (100a, 100b and 100c).

9. Piezoelectric microphone (10) according to claim 8, characterized in that the support points (100a, 100b and 100c) are located at the corner regions of the transducer membrane (15), respectively.

10. Piezoelectric microphone (10) according to claim 9, characterized in that one of the support points (100a) comprises electric connection points (160b, 170b) for the transducer membrane (15), that the transducer membrane (15) comprises electric connection points (160a, 170a) that are in electric contact with the piezoelectric layer 20, and which are arranged so that they are put in electric contact with the connection points (160b, 170b) when the transducer membrane (15) is supported by the three support points (100a, 100b and 100c).