GB2231235A

GB2231235A - Electroacoustic transducer

Info

Publication number: GB2231235A
Application number: GB8910382A
Authority: GB
Inventors: Mohamed Sami Mohamed Ahmed; Jeffrey Arthur George Knight
Original assignee: Transfer Technology Ltd
Current assignee: Transfer Technology Ltd
Priority date: 1989-05-05
Filing date: 1989-05-05
Publication date: 1990-11-07
Anticipated expiration: 2009-05-05
Also published as: GB2231235B; GB8910382D0

Abstract

A capacitance-type electroacoustical transducer comprises a diaphragm 7 supported on and in registration with a conductive layer 4 formed on a backplate 1. The backplate is formed by a printed circuit board and the surface of the conductive layer is textured. Insulative diaphragm 7 has a conductive layer 6 and is attached to an outer portion 2 of the PCB. A plurality of transducers can be formed on the same side of a single printed circuit board using conventional printed circuit techniques and thus produce transducers of uniform characteristics. One or more preamplifiers can be located on the side of the board opposite to the transducers. <IMAGE>

Description

Electroacoustical Transducer The present invention relates to an electroacoustical transducer having a diaphragm supported on a backplate, and in particular to the surface texture of the backplate and the means of making electrical connection thereto.

Capacitance-type electroacoustical transducers are well known in the art. In conventional transducers, a diaphragm having an insulative layer and an electrically conductive surface has its insulative layer in contact with a grooved, irregular, electrically conductive surface of a substantially inflexible disc or backplate. The periphery of the diaphragm is maintained in a fixed position with respect to the transducer housing and a spring force urges the backplate into tensioning engagement with the diaphragm. The electrically conductive surface (first electrode) and insulative layer of the diaphragm, and the conducting surface (second electrode) of the backplate form a capacitor such that when a d.c. voltage is applied across the electrodes, irregularities in the surface of the backplate set up localized concentrated electric fields in said insulative layer.When an a.c. signal es superimposed on said d.c. bias, the diaphragm is stressed such that oscillatory formulations develop causing an acoustical wavefront which is propagated from said diaphragm. A received acoustical wavefront impinging on the diaphragm produces a variable voltage across the capacitor electrodes. Such a transducer may be used to detect range by measuring the time of flight of a pulse emitted by the transducer which is reflected by the object to be ranged and subsequently received by the transducer.

Conventional transducers are also characterised by connections to the remainder of the circuit of which they form part being made by bonding connecting leads to the backplate and to the conducting surface of the membrane by soldering or the like. The pre-amplifiers which are essential for boosting the signals transmitted and/or received by the transducer are located remotely from the transducer which may result in inefficiencies in maintaining the integrity of signals.

In the case where an array of transducers are used, for example for three-dimensional ranging applications, it is required that several transducers are close together, each having associated preamplification circuitry.

According to the present invention there is provided an electroacoustical transducer comprising: a printed circuit board (PCB) having an insulative substrate and an electrically conductive layer on one surface thereof, wherein the electrically conductive layer includes a first portion and a further portion separated from said first portion by a perimetric channel, and provided with a pre-determined surface texture; a flexible, insulative membrane, supported on said further portion and having an electrically conductive layer on its free surface, the membrane being fixedly attached to said first portion of said electrically conductive layer of said PCB, with the electrically conductive layer of said membrane electrically connected to said first portion of said electrically conductive layer of said PCB; and connection means in contact with each said further portion and said further portion said electrically conductive layer of said PCB. In one embodiment, the printed circuit board (PCB) may be provided with an electrically conductive layer on both surfaces (i.e. a double sided PCB) whereby a transducer formed on one surface of the PCB and tracks providing interconnections for the associated pre-amplification circuitry are printed on the opposite surface of the PCB. Standard photo-resist techniques which are well known in the art may be used to produce both the transducer and the tracks for the pre-amplification circuitry.Connections from the electrode of the transducer to its corresponding pre-amplifier are made by coating bores which extend between the two surfaces of the PCB with an electrically conductive material, one bore being provided adjacent the transducer and one bore adjacent the electrically conductive material surrounding the transducer backplates. Such an arrangement enables each pre-amplifier to be located extremely close to its associated transducer.

Further, the PCB may be provided with a plurality of transducers on one surface, and tracks providing interconnections for the pre-amplification circuitry associated with each transducer may be provided on the other layer. The transducers may be arranged in a linear or two-dimensional array.

The conductive coating on the surface of each backplate may comprise an electrically conductive paint which may be applied to each backplate after the PCB has been etched. The resonant frequency of each transducer,which is determined by the profile of the surface of the backplate, may be selected by varying the surface texture of the electrically conductive paint applied to each backplate, the surface texture being determined by the size of particles and the amount of binder contained in the electrically conductive paint.

Each flexible membrane may be fixedly attached to the PCB along its periphery by means of adhesive, and may be in pre-tensioned condition. Electrical connection of the conductive, free surface of each membrane to the portion of the electrically conductive layer of the PCB which surrounds the transducer or array of transducers may be provided by electrically conductive ink or the like.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows in perspective a PCB transducer having a single backplate etched in the electrically conductive layer of one surface; and Figure 2 illustrates one surface of a PCB having a plurality of backplates forming an array of transducers.

Referring to Figure 1, the PCB transducer is formed from a PCB having an insulative substrate 17, a first electrically conductive layer 1 on one surface of the substrate and a second electrically conductive layer 13 on the opposite surface of the substrate, electrically conductive layers 1 and 13 being formed, for example, from copper.

On the first conductive-layer 1 is etched a channel 3 to form a circular backplate 4. The surface 5 of the backplate 4 is provided with a roughened texture by applying an electrically conductive coating such as electrically conductive paint. The remaining portion 2 of the conductive layer 1 surrounds the backplate 4 and is insulated from it.

A flexible, insulative membrane 6 (shown partially cut away) is fixedly attached to the surface of the conductive layer 1 by means of adhesive portions 8 on the remaining portion 2 of conductive layer 1 adjacent perimetric channel 3, thereby covering backplate 4.

The membrane 6 is provided on its free surface with a flexible, electrically conductive layer 7.

Conductive layer 7 is connected to the portion 2 of conductive layer 1 by means of electrically conductive material 16 such as electrically conductive ink.

The second conductive layer 13 is etched with tracks which provide interconnections between circuit elements forming the pre-amplification circuitry of the transducer, the circuit elements being mounted on the second surface of substrate 17 following the etching of the PCB.

A first bore 9 extends from the free surface of backplate 4 to the free surface of second conductive layer 13. Electrical connection of the backplate 4 to the tracks etched from the second conductive layer 13 is provided by coating 10 (such as a metal applied by conventional plating techniques) on the walls of the bore 9. A second bore 11 extends from the free surface of the second portion 2 of first conductive layer 1 to the free surface of second conductive layer 13. A coating 12, identical to coating 10, on the walls of bore 11 completes the electrically conductive path from conductive layer 7 on membrane 6 to the tracks etched from the second conductive layer 13, via conductive material 16 and second portion 2 of the first conductive layer 1.

Referring to Figure 2, an example of a PCB transducer array is shown wherein an array of transducers, each having the same construction as the transducer illustrated in Figure 1, are provided in a linear arrangement. Such a configuration is facilitated by etching a row of six backplates 4 in the first conductive layer 1 of the PCB. Alternatively, a twodimensional array of backplate may be etched in conductive layer 1, to suit the application for which the transducer array is required. Electrical connections to each transducer element may be made via plated bores 9, the plated bores in turn making electrical connections to tracks etched in the second conductive layer 13 of the PCB which provide interconnections for the pre-amplification circuitry for the array of transducers on the second surface of the PCB.

While single and double-sided PCBs have been described, it is also possible to construct the transducer using a multi-layer board formed using conventional multi-layer manufacturing techniques.

Claims

CLAIMS:

1. An electroacoustical transducer comprising: a printed circuit board (PCB) having an insulative substrate and an electrically conductive layer on one surface thereof, wherein the electrically conductive layer includes a first portion and a further portion separated from said first portion by a perimetric channel, and provided with a pre-determined surface texture; a flexible, insulative membrane, supported on said further portion and having an electrically conductive layer on its free surface, the membrane being fixedly attached to said first portion of said electrically conductive layer of said PCB, with the electrically conductive layer of said membrane electrically connected to said first portion of said electrically conductive layer of said PCB;; and connection means in contact with each said further portion and with said first portion said electrically conductive layer of said PCB.

2. An electroacoustical transducer according to claim 1, wherein: said connection means includes a number of bores extending from a first surface of said PCB to a second surface of said PCB, the walls of said bores being coated with a electrically conductive material.

3. An electroacoustical transducer according to claims 1 or 2, wherein: said PCB includes a further electrically conductive layer on the other surface thereof and said connection means provide electrical connection to tracks formed from said further electrically conductive layer of said PCB.

4. An electroacoustical transducer according to claim 3, wherein: said tracks provide interconnections between circuit elements mounted on the surface of said substrate opposite said first conductive layer, said tracks and said circuit elements forming amplification circuitry, said amplification circuitry amplifying signals received by said transducer.

5. An electroacoustical transducer according to any one of the preceding claims, wherein the surface texture on the further portion is provided by a conductive coating applied thereto.

6. An electroacoustical transducer according to claim 5, wherein: said conductive coating comprises an electrically conductive paint.

7. An electroacoustical transducer array according to any of the preceding claims, wherein: a plurality of electroacoustical transducers are provided on a single printed circuit board, such transducer being in accordance with any one of the preceding claims.

8. An electroacoustical transducer according to any of the preceding claims, wherein: each said membrane is fixedly attached to said first portion of said electrically conductive layer by means of adhesive.

9. An electroacoustical transducer array substantially as described herein with reference to Figures 1 and 2 of the accompanying drawing.