GB2163925A

GB2163925A - Multi-frequency electro-acoustic transducer

Info

Publication number: GB2163925A
Application number: GB08312816A
Authority: GB
Inventors: Bernard Tocquet
Original assignee: Direction General pour lArmement DGA; Etat Francais
Current assignee: Direction General pour lArmement DGA; Etat Francais
Priority date: 1982-05-13
Filing date: 1983-05-11
Publication date: 1986-03-05
Also published as: GB2163925B; FR2570915A1; FR2570915B1

Abstract

A transducer of the Tonpilz type comprises a horn (1), a motor (2) constituted by a piezo-electric stack of ceramics (5) alternating with electrodes 7a, 7b and a counter-mass (3). This transducer can transmit on a first band centred on a low frequency fb which is the fundamental frequency of the whole of the structure, and on a second frequency fh, of the order of three times fb, which is both the intrinsic frequency of the longitudinal mode of vibration of the whole of the structure and one of the fluttering frequencies of the horn alone. One application is the construction of high power sonar transmitters. <IMAGE>

Description

SPECIFICATION Multi-frequency electro-acoustic transducer and method of construction The object of the invention is multi-frequency electro-acoustic transducers intended for equipping sonar transmitters and a method of construction of these.

The technical area of the invention is that of the construction of sonars.

There are known high power sonar transmitters, so-called wide pass band transmitters, which can transmit with great efficiency in a wide band of frequencies, with a level of transmission by SV volts which remain substantially constant over the entire width of the band at about 3db.

A wide band transducer of this type is described for example in French patent application FR 70/ 24.556 (Etat Francais).

To date, the known sonar transmitters, with wide band, can hardly be used with acceptable performances except in bands having a width of the order of an octave, i.e. between a low frequency fb and a high frequency fh substantially equal to twice fb.

It is desirable, for several applications, to use multi-frequency sonar transmitters which can transmit on at least two frequencies fb and fh, such that fh is of the order of three times fb and this result can not be achieved with the known wide-band transducers.

For example high power sonar transmitters which equip reconnaissance ships entrusted with the detection of other ships or an obstacle, generally transmit on low frequency of the order of 3 KHz, since the low frequencies are not very attenuated and therefore have a large range. However, the low frequencies are more sensitive to reverberation than the higher frequencies. Therefore, if the reconnaissance ship is brought to operate near coasts or in sea that is not very deep, it is prefera ble for it to transmit on a higher frequency and for it to use, therefore, a sonar transmitter which al lows it to triple the frequency of transmission.

In the same way it is advantageous for the sonar transmitter to be able to transmit at low frequency to make an exploratory search and that, as soon as it has located a target, it can pass onto a higher frequency, which ensures a better directivity for it and a greater precision and also a greater secrecy which shields it from the enemy's passive listening sonars.

The aim of the present invention is to procure a method of construction of electro-acoustic trans ducers, of the "Tonpilz" type, which can transmit with great efficiency intwo frequency bands sepa rated by an interval greater than an octave, for ex ample in a first narrow band centred on a low frequency fb and in a second narrow band centred on a high frequency fh equal to at least three times fb.

It is recalled than an electro-acoustic transducer of the "Tonpilz" type comprises, at the front end, a metallic part called the horn which is entrusted with transmitting the vibrations to the surrounding medium, at the rear end a metallic counter-mass and between the two a stack of piezo-electric ceramic plates and alternate polarity electrodes, which are inserted between the plates. The stack transforms the electrical signals received by the electrodes into vibrations of the piezo-electric plates and it is called the motor.

The elements composing the structure of a Tonpilz have their particular frequencies which depend on the modes of vibration and it is obviously normal to make the trans-ducer function on frequencies that return into resonance with one of several of the particular frequencies.

Attempts are being made to obtain plane acoustic waves and, to this end, longitudinal modes of vibration are used in which the horn moves as a piston.

A transducer has several longitudinal modes of vibration which correspond to trains of stationary waves, comprising one or several nodes.

The first longitudinal mode, or piston mode, which corresponds to the lowest frequency, or fundamental frequency, corresponds to a system of stationary waves with a single node.

The second longitudinal mode of vibration, or piston mode, corresponds to a train of stationary waves with two nodes.

The horn can vibrate with flexing according to several modes of deformation called modes of fluttering (papillonage). A free rectangular horn having two medial planes of symmetry of the front face has six modes of flexing according to which it deforms in symmetrical or asymmetrical manner with respect to one or the other of these two planes.

To attain the objectives of the invention, i.e. to construct an electro-acoustic transducer of the Tonpilz type which can transmit on a first band centred on a low frequency fb and on a second band centred on a higher frequency fh, such that fh is of the order of 3.fb, first of all one calculates a horn having an intrinsic frequency of fluttering (papillonnage) equal to fh and one thus constructs a Tonpilz unit equipped with this horn having a fundamental frequency equal to fb and a frequency of one of the other longitudinal modes of vibration substantially equal to fh.

An electro-acoustic transducer of the Tonpilz type according to the invention able to transmit on a low frequency fb and on a higher frequency fh, of the order of 3.fb, comprises: a metal or light alloy horn having a fluttering frequency equal to fh and a horn unit, motor and counter-mass, having a fundamental frequency equal to fb and a frequency of one of the other longitudinal modes of vibration substantially equal to fh.

Preferably, it is the frequency of the second longitudinal mode of vibration of the unit which is substantially equal to the fh frequency of one of the fluttering modes of the horn.

The result of the invention is new high power sonar transmitters able to transmit with a good acoustic efficiency on two relatively low frequency ranges, of the order of a few kilovolts, such that the highest frequency fh is of the order of three times the lowest frequency fb.

The sonar transmitters of the invention allow re connaissance ships to be equipped with a sonar which can carry out a general exploration by trans mitting on the lowest frequency fb, with a view to locating a target or an obstacle. When the target has been located, the sonar can follow the latter by passing onto the higher frequency fb, which en sures for it a better directivity and shields it so that its transmissions are not detected by passive listening sonars situated at a greater distance than the target.

The following description refers to the attached drawings which show, by way of non-limiting ex ample, an embodiment of a transducer of the in vention.

Figure 1 is an axial section of a transducer of the Tonpilz type.

Figure 2 is a diagram representing the coefficient of response Sv as a function of the frequency.

Figure 3 represents a quadrant of the structure.

Figure 1 is an axial section of a transducer of the Tonpilz type, of axis x xl, which is composed es sentially of a horn 1, of a piezo-electric motor 2, of a counter-mass 3 and of a sealed housing 4, of bu tadiene or any other equivalent supple material.

The motor 2 is a stack of piezo-electric plates 5 and electrodes 6 which are inserted between the plates and which are connected respectively on two electric conductors 7a, 7b. The plates and the electrodes are threaded on a central rod 8 which joins the horn to the counter-mass. The motor 2 is linked to the housing 4 by a support 9 which is sit uated near the node of the stationary wave which is formed when the whole unit vibrates in piston fashion on the fundamental frequency fb, i.e. the frequency which enters into resonance with the frequency intrinsic in mode 1. In this case there is formed-a system of stationary waves with one sin gle node.

The problem to be solved is the calculation and construction of a transmitting transducer of this type which can transmit with a good coefficient of response both on a low frequency fb, which is generally the fundamental frequency, as well as on a second frequency fh, of the order of three times fb.

To arrive at this result, first of all a horn 1 is cal culated, of traditional general shape, i.e. having a transmitting face of general rectangular shape and a thickness which decreases from the centre to wards the periphery.

The mathematical formula according to the method of finite elements of the dynamic response of a structure such as a horn is known and it is possible to calculate theoretically, with a very good approach to reality, the intrinsic frequencies corre sponding to the different mode of fluttering (papil lonnage) of a horn.

Given a frequency fh that one wishes to obtain, it is possible to calculate and to construct a horn whose intrinsic frequency of one of the modes of fluttering is close to fh.

Detailed theoretical studies of modes of flutter ing of structures comprising a horn associated with a stack of ceramics have shown that the interface between the horn and the ceramic column has a quasi perfect piston movement which is of weak amplitude. Therefore, it is possible to obtain useful information on the modes of fluttering by designing an independent horn disconnected from the rest of the structure and imposing an amplitude of normal displacement identical to all the nodes of the interface.

A modal analysis has been made of a horn alone and of the same horn associated with a stack. This analysis shows that the deformations of the horn associated with the first three modes of fluttering are practically identical in both cases and that the intrinsic frequencies of fluttering are very close.

For example, for the first mode of fluttering one calculates that the theoretical intrinsic frequency is 1923 Hz for the horn alone, 1994 for the horn associated with a stack, and the actual frequency measured is 1806 Hz.

For the second mode of fluttering these same values are respectively 2442 Hz, 2232 Hz and 2020 Hz.

For the third mode of fluttering the respective values are 3899 Hz, 3714 Hz and 3417 hz.

These results show that it is possible to calculate separately, by the method of finite elements, a horn having an intrinsic frequency of fluttering that is close to fh and to be certain that the intrinsic frequency of this same mode of fluttering will remain near the value calculated when the horn is incorporated into a transducer.

So as to obtain an intrinsic frequency of fluttering that is fairly high, a relatively thin horn is constructed, in a light alloy, for example an aluminium or titanium alloy.

Once the horn has been calculated and constructed, one calculates, always by the finite element method, the number of plates 5 that have to be used to construct a motor and a counter-mass 3 such that the whole structure formed by the horn, the stack of ceramics and the counter-mass, simultaneously satisfies the following two conditions.

First of all the intrinsic frequency of the first longitudinal or piston mode of vibration, which is called the fundamental frequency, is substantially equal to the frequency fb. This first condition is easy to realise as one knows perfectly how to construct structures of the Tonpilz type having a fundamental frequency that is close to a predetermined frequency. Then the intrinsic frequency of one of the other longitudinal modes of vibration of the structure is substantially equal to the high frequency fh.

Preferably, one will choose the intrinsic frequency of a longitudinal mode of vibration which leads to a system of stationary waves having two nodes and having an anti node situated on a level with the external face of the horn.

The modal analysis by the method of finite elements of a structure having the form of a transducer of the Tonpilz type allows the calculation of the structures having a fundamental frequency in the first longitudinal mode equal to a predetermined frequency fb, and a fundamental frequency in another longitudinal mode close to a frequency fh.

When the motor is excited on the frequency fb, the whole unit vibrates in piston fashion and returns into resonance on this frequency. When the motor is excited on the frequency fh, the stack and the counter-mass vibrate in piston fashion with two nodes of vibration. But the horn is also deformed with flexing or fluttering and returns into resonance on the frequency fh since it has been calculated in such a way that one of its intrinsic fluttering frequencies is equal to fh, and it has been seen that the intrinsic frequencies of fluttering were practically unchanged when the horn was associated with a stack and a counter-mass.

Figure 2 shows in the abscissae the frequencies comprised between 0 and 12 KHz, and in the ordinates the coefficient Sv which is expressed in decibels and which is equal to twenty times the relationship between the acoustic pressure in baryes and the excitation voltage in volts.

The coefficient Sv has two maxima corresponding to the resonance frequencies fb = 3 KHz and fh = 9 KHz, and two frequency bands are obtained that are well separated from each other, one between 2 and 4 KHz and the other between 8 and 9.8 KHz approximately.

It can therefore be seen that the method of the invention allows the construction of high power sonar transmitters that are capable of transmitting with a good acoustic efficiency and at the same time in a range of frequencies that is three times higher, which is advantageous in countless applications.

This result is obtained by making one of the intrinsic fluttering frequencies of the horn fh coincide approximately with the intrinsic frequency of the whole unit corresponding to the second longitudinal mode of vibration whilst the fundamental frequency of the whole unit gives the low frequency fb.

Figure 3 is a longitudinal section through two perpendicular axial planes which delimit between them a quarter of the structure. In dotted lines there have been shown sections that are perpendicular to the axis x xl at rest, and in solid lines the maximum amplitude of the vibrations of these sections when the structure vibrates at frequency fh. In this case the stack and the counter-mass vibrate in piston fashion with two nodes N1 and N2 and an antinode situated near the external face of the horn. In addition the horn vibrates in fluttering fashion in mode 5 and undergoes a considerable deformation with flexing that is visible on the Figure.

Figure 3 corresponds to a theoretical construction by the method of finite elements.

Claims

CLAIMS 1. Method of construction of an electro-acoustic transducer of the Tonpilz type which can transmit in a first band of frequencies centred on a low frequency fb, and on a second band of frequencies centred on a higher frequency fh, such that fh is of the order of 3 fb, characterised in that: - first of all a horn is calculated having an intrinsic fluttering frequency, when it is considered separately, equal to fh; - and a Tonpilz unit is constructed which is equipped with this horn having a longitudinal fundamental frequency equal to fb and a frequency of one of the other longitudinal modes of vibration substantially equal to fh. 2. Electro-acoustic transducer of the Tonpilz type able to transmit on a first band centred on a low frequency fb and on a second band centred on a higher frequency fh, such that fh is of the order of 3.fb, characterised in that it comprises a horn having a frequency of fluttering that its equal to fh, and a horn unit, motor and counter-mass having a fundamental frequency equal to fb and a frequency of one of the other longitudinal modes of vibration that is substantially equal to fh. 3. Transducer according to claim 2, characterised in that the longitudinal mode of vibration of the unit having an intrinsic frequency that is substantially equal to the frequency fh of one of the modes of fluttering of the horn is a mode comprising two nodes of vibration (N1, N2) and an antinode on a level with the external face of the horn. New claims or amendments to claims filed on Superseded claims all New or amended claims:- all

1. A method of construction ot an electroacoustic transducer, of the Tonpilz type, which can transmit in a first band of frequencies centred on allow frequency fb, and on a second band of frequencies centred on a higher frequency fh, such that fh is of the order of at least 3 fb, wherein the method comprises: - firstly, calculating a horn having an intrinsic fluttering (papillonnage) frequency, when the horn is considered separately, equal to fh; - and, secondly, constructing a Tonpilz unit which is equipped with said horn and which has a longitudinal fundamental frequency equal to fb and a frequency of one of the other longitudinal modes of vibration substantially equal to fh.

2. An electro-acoustic transducer, of the Tonpilz type, which is able to transmit on a first band centred on a low frequency fb and on a second band centred on a higher frequency fh, such that fh is of the order of at least 3.fb, the transducer comprising a horn having a frequency of fluttering (papillonnage) that is equal to fh, and a horn, motor and counter-mass having a fundamental frequency equal to fb and a frequency of one of the other longitudinal modes of vibration that is substantially equal to fh.

3. A transducer as claimed in claim 2, wherein said longitudinal mode of vibration having an intrinsic frequency that is substantially equal to the frequency fh of one of the modes of fluttering (papillonnage) of the horn, is a mode comprising two nodes of vibration, and an antinode coinciding with an external transmitting face of the horn.

4. A transducer as claimed in any of claims 2 and 3 wherein fh is substantially 3 fb.

5. A method as claimed in claim 1, wherein said longitudinal mode of vibration having an intrinsic frequency that is substantially equal to the frequency fh of one of the modes of fluttering (papillonnage) of the horn, is a mode comprising two nodes of vibration, and an antinode coinciding with an external transmitting face of the horn.

6. A method as claimed in any of claims 1 and 5, wherein fh is substantially 3 fb.

7. A method as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

8. An electro-acoustic transducer as claimed in claim 2 and substantially as hereinbefore described with reference to the accompanying drawings.