WO1997050077A1

WO1997050077A1 - Acoustic transducers using a working fluid

Info

Publication number: WO1997050077A1
Application number: PCT/GB1997/001684
Authority: WO
Inventors: James Edward Stangroom; Rex Walton
Original assignee: Er Fluid Developments Limited
Priority date: 1996-06-22
Filing date: 1997-06-20
Publication date: 1997-12-31
Also published as: GB9613150D0; AU3548497A; EP0906611A1

Abstract

The invention relates to the use, in an acoustic transducer (1) of an ER fluid as the working fluid; to an acoustic transducer (1) using an ER fluid as its working fluid; to the use of an ER fluid as the working fluid of a vibrator; to a vibrator employing an ER fluid as its working fluid.

Description

ACOUSTIC TRANSDUCERS USING A WORKING FLUID

Introduction

This invention is concerned with acoustic transducers, i.e. devices which convert

electrical signals into mechanical vibrations in air, water or other media.

A sound, i.e. a series of successive waves of compression and rarefaction in a

compressible medium, can be produced by two main methods:-

a) By vibrating a surface at the desired frequency. This is by far the commonest method.

In standard units, the surface is usually a papier mache cone, which is light and stiff in

compression, but other shapes and materials can be used. For the remainder of this

application, the term "plate" will be used,

b) By allowing pulses at the desired frequency to escape from a continuous supply of the

medium at elevated pressure: sirens are the chief example.

Under a), there are three main methods commonly used to move the plate: -

(i) A light coil is attached to the plate, and this is suspended in a radial magnetic field,

normally produced by a permanent magnet. When a current is passed through the coil,

it gives rise to an axial force, so the plate moves in accordance with the magnitude and

direction of the current.

(ii) The plate may be moved electrostatically. With realistic fields, the force and

displacement that can be achieved by this method are severely limited, so it is normally

used only for high frequencies.

(iii) A stack of piezo-electric elements may be used to move the plate. This method can

produce quite large forces, but the amplitude is relatively low. The response is poor at

low frequencies, but very good at higher frequencies, so this is the method of choice for underwater acoustic transducers used for sonar, which work at about 20kHz.

At present . only method (i) above can deal with the lower acoustic frequencies, below

approximately 200 Hz. The sound intensity produced is governed by the area of the plate and

the amplitude of its movement, and both of these are effectively limited by the force available.

The acceleration required to achieve an oscillation at a given frequency and amplitude is

proportional to the square of both parameters. The force required is proportional to the

oscillating mass, so the latter is a key consideration. The mass of a plate of constant thickness

will be proportional to its area, and in practice a larger plate will require additional stiffening.

The overall result is that the acoustic output of a transducer at low frequencies is governed by

the force that can be produced.

The force produced in an electromagnetic transducer is governed by the field, the

performance of the permanent magnet, and the current through the coil, which is determined in

turn by heat dissipation. Both of these are approaching the limits of existing materials, so it

seems unlikely that electromagnetic transducers will greatly improve on their present practical

limit on the mass of the oscillating element which is approximately 500g.

A basic object of the present invention is to provide an improved acoustic transducer

overcoming the limitations of prior art proposals and thereby increasing the output of low to

medium frequency acoustic transducers.

Accordingly, a first aspect of the present invention is directed to the use of an Electro-

Rheological (ER) fluid as the working fluid of an acoustic transducer.

Another aspect, of independent significance is directed to an acoustic transducer

employing an ER fluid as its working fluid.

Yet another aspect of independent significance is directed to the use of an ER fluid as the working fluid of a vibrator.

Yet another aspect of independent significance is directed to a vibrator employing an ER

fluid as its working fluid.

The invention thus makes a radical departure in the field of transducers and vibrators by

employing ER fluids, which are concentrated suspensions of finely divided solids in oily base

liquids which solidify, progressively, reversibly and virtually instantaneously, in an electric field.

It is also well known that if ER fluids are pumped between two parallel fixed plates to which

a voltage may be applied, the assemblage functions as a high-speed valve, and bi-directional

actuators, in which such valves are combined with well-known hydraulic principles, have been

described. However, the design of such actuators has largely followed conventional hydraulic

practice, and, as a result, the frequency range of such units is limited. More recently, high

frequency versions of these actuators have been the subject of another Patent Application, and

it has been shown that these can be operated successfully up to at least 5kHz, making them

suitable for use as acoustic transducers in the low to medium frequency range.

The technical advantages of ER vibrators as acoustic transducers are as follows:-

1. Since they operate on hydraulic principles, there are virtually no limits on the force that

can be applied.

2. The main source of energy is the pump which operates continuously and need not be

modulated. Hence, only the electrical control power need be modulated. This

represents a considerable saving in cost, space and weight on conventional electro¬

magnetic practice which requires equipment to modulate the whole of the electrical input.

Another aspect of this advantage is that the main power input need not be electrical. The

pump may be driven hydraulically, pneumatically or even by a self-contained engine. 3. As in all hydraulic-type devices, waste heat is carried away in the working fluid and may

be removed elsewhere in the circuit.

In one embodiment, ER vibrators are used directly to move a plate similar to that used

, 5 in present loudspeakers. However, since the improved performance characteristics of the

vibrators will remove existing limitations on mass, these plates can be made much larger than

at present, and other constructional materials may be used.

In another embodiment, an ER vibrator is used to operate a high speed valve mechanism

which allows pulses of air to escape from a pressurised reservoir. This aspect of the invention

10 thus replaces the perforated disc used in conventional sirens with a high-frequency controllable

valve. Although ER vibrators are ideal for this task, insofar that the mass of the moving parts

is not critical, this aspect of the invention is not limited to this alone. Providing the valve itself

is correctly designed, other actuators, such as electro-magnetic, electrostatic or piezo-electric

could also be employed.

15 Various aspects of the invention will now be described in greater detail, by way of

examples, with reference to the accompanying drawings, in which:-

Figure 1 is a diagrammatic layout of a loudspeaker; and

Figure 2 corresponds to Figure 1 but shows a siren.

In both Figures, like reference numerals are employed for like components.

20 A high-speed ER actuator is indicated at 1 through which ER fluid is circulated via outlet

and return lines 8 and 9 by a pump 2 running continuously at constant speed. A high voltage

source 3 supplies high voltage signals to the ER actuator 1 via heavily insulated cables 10, which

source 3 is, in turn, controlled by a low voltage source 4, for example a standard audio-amplifier.

A cone 5, similar to a cone in a standard loudspeaker, serves to transfer the mechanical movements of the output of the ER actuation 1 to the surrounding medium, e.g. air. However,

due to the increased thrust capability of the ER actuator 1 compared with a standard electro¬

magnetic loudspeaker, this can be made larger and heavier than standard loudspeaker cones.

In Figure 2 a pump or blower 6 runs continuously, whilst a valve 7 is operated by the ER

actuator 1 which modulates the output of the valve 7 in response to the signals from the high

voltage source 3.

Claims

CLAIMS:

1. The use. in an acoustic transducer of an ER fluid as the working fluid.

2. An acoustic transducer, using an ER fluid as its working fluid with means to apply,

in a controlled manner, an electric field to the ER fluid.

3. An acoustic transducer as claimed in Claim 2, comprising a pump for displacement

of the ER fluid around a flow circuit.

4. An acoustic transducer as claimed in Claim 3, wherein the pump is continuously

operable.

5. An acoustic transducer as claimed in Claim 3 or Claim 4. wherein the pump is driven

hydraulically, pneumatically or even by a self-contained engine.

6. An acoustic transducer as claimed in any one of Claims 2 to 5, comprising means to

modulate electrical control power providing the electric field.

7. The use of an ER fluid as the working fluid of a vibrator.

8. A vibrator employing an ER fluid as its working fluid.

9. A loud speaker comprising an ER vibrator as defined in Claim 8, operable directly

on a movable loud speaker plate.

10. A siren comprising an ER vibrator as defined in Claim 8, operable on a high speed

valve mechanism which allows pulses of air to escape from a pressurised reservoir.