US2923782A - Loudspeakers - Google Patents

Description

' Filed May '24. 1956 Fell 19-60v J, N. ARMSTRONG- ETAL LOUDSPEAKERS 2 Sheets-Sheet 2 IIVIIINRS: JOHN I. msmm EIIIZ 8. HOLT! Mtornoyl United States Patent LOUDSPEAKERS Tohn Noel Armstrong, Caterham, and Heinz Siegfried Wolif, London, England, assignors to National Research Development Corporation, London, England, a corporation of Great Britain Application May 24, 1956, Serial No. 587,100

Claims priority, application Great Britain May 31, 1955 20 Claims. (Cl. 179-113) This invention relates to a device for modulating the rate of flow of a gas.

According to the invention there is provided a device for modulating the rate of flow of a gas comprising a helix of resilient material placed in the path of the said flow so that gas passes through the space between adjacent turns of the helix, and means for cyclically extending and contracting the helix in an axial direction, within the elastic limit of the material of which the helix is made, whereby the cross sectional area of the said space is cyclically increased and reduced respectively.

According to the invention there is further provided an electro-acoustic transducer of the type in which a stream of gas under pressure is constrained to pass through a valve having an aperture of variable admittance; wherein the valve consists of a helix of resilient material, wherein the aperture consists of the space between adjacent turns of the helix and with means for axially extending and contracting the helix within the elastic limit of the material of which the helix is formed, to increase and reduce the said space respectively, under control of an alternating electric current.

The invention is applicable to electro-acoustic transducers of the type Where a stream of air or other gas passes through a flow-controlling valve oscillated under control of audio frequency electric currents and the embodiments of the invention described below relate to such applications. The invention is not, however, confined to such applications and may be used to" control the fiow of a liquid, for instance for under-Water signalling, the oscillations applied to the helix valves are not necessarily confined to frequencies within the audible range, for instance ultrasonic frequencies may be used provided that such frequencies do not exceed the maximum frequency to which a helix of any practicable material can respond, and the exciting force for oscillating the helix need not be of electrical origin, for instance a sound actuated diaphragm or mechanical vibrator could be used.

The invention will be more readily understood from the following description of the said embodiments illustrated in the accompanying drawings in which:

Fig. 1 shows, in section, a first embodiment of the invention.

Fig. 2 shows, in section, a second embodiment of the invention.

Fig. 3 shows, in section, a third embodiment of the invention.

Fig. 4 shows, in section, a fourth embodiment of the invention.

Fig. 5 shows, in section, a fifth embodiment of the invention.

Fig. 6 shows, in section, a sixth embodiment of the invention.

Fig. 7 shows, in section, a seventh embodiment of the invention.

Fig. 8 shows, in section, an eighth embodiment of the invention.

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Fig. 9 shows, in cross section, part of the embodiment shown in Fig. 8.

Figs. 10 and 11 are perspective sketches of two components of the embodiment shown in Fig. 8.

Fig. 12 shows, in section, a ninth embodiment of the invention.

Fig. 13 shows, in section, a tenth embodiment of the invention.

In Fig. 1 a helix 1, which may be made of metal wire, is gripped at one end in the end of a tube 2, the other end of helix 1 being attached to a speech coil 3 mounted for axial movement in an annular air gap in a pot magnet system 4 of the conventional moving coil loud speaker Pot magnet 4 is supported on tube 2 by supports 5 spaced at intervals around the helix 1, being attached to a collector ring 6 clamped to the end of tube 2.

A compressor supplies an air stream which is introduced into the free end of tube 2 and which finds an escape path, through the convolutions of helix 1 to the open air, which can be varied in cross sectional area by axially contracting or extending the helix 1. Audio frequency currents are applied to coil 3 which operates like the speech coil of a normal moving coil loudspeaker and-moves axially in the air gap of pot magnet 4, being centred in this air gap by any conventional centring device. The excursions of coil 3 extend or contact the convolutions of helix 1 to modulate the air stream which issues in all directions from the outside of the helix.

In Fig. 2, the air stream is introduced into the inside of the helix through a central bore 7 in the centre pole or pot magnet 4, the fixed end of the helix 1 being mounted on a plug 8 suspended by spider arms 9 on the outer ring of a cup-shaped member 10 fixed to the outer annular pole piece of pot magnet 4 and surrounding the helix 1. The modulated air stream is directed by member 10 in a direction axial of the helix and emerges through the spaces between the spider arms 9. The plug 8 may extend within the helix and may be of conical or flared form to deflect the air stream radially through the convolutions of the helix.

Fig. 3 shows an arrangement similar to Fig. 2 but the member 10 is extended to form a short horn, the member 8 also extending away from the helix to give the correct flare rate to the annular space through which the air stream emerges.

In this figure, the driving coil 3 is sandwiched between two helices 1 and 1, the latter being a dummy helix surrounding the centre pole of the pot magnet. These two helices can be wound so that their adjacent turns make contact and when mounted with the coil between them, tension may be applied to open spaces between the turns to produce the desired mean spacing. Means (not shown) may be provided for adjusting this tension so that the spacing of the turns may be adjusted for maximum efiiciency. This arrangement enables the spacing to be accurately set and although the working helix is stressed the system is mechanically balanced by the opposing stress in the dummy helix 1.

Fig. 4 shows an arrangement similar to that of Fig. 2 but the pot magnet system is duplicated and both ends of the helix are controlled by moving coils 3 and 3'. Air streams may be introduced simultaneously through central bores 7, 7' in the central poles of pot magnets 4, 4', these pot magnets being mounted together by struts 5 corresponding to supports 5 in Fig. 1. Alternatively only one of the pot magnets may have its centre pole bored out for introduction of the air stream, the other being solid.

Fig. 5 shows an alternative arrangement in which the air stream is applied to the outside of the helix 1 which is surrounded by a chamber 11 mounted on the front face of pot magnet 4 and having in its end wall 12 remote from the pot magnet a hollow central boss 13 the inner end of which provides a mounting for the fixed end of the helix 1 and the outer end of which forms a throat to which can be attached a horn 14. The air is introduced through a pipe 15 into chamber 11 and passes radially inward through the convolutions of the helix, passing thence down the interior thereof and through boss 13 and horn 14.

In Fig. 6 an arrangement similar to that of Fig. is shown but here the air stream outlet from the helix is through a bore 7 in the centre pole of pot magnet 4 which has a hollow boss at the rear for attachment of horn 14. The boss 13 in chamber 11 is not hollow in this case and may be extended inwards into the helix and may be conical or flared (as 8 in Fig. 2) to deflect the modulated air stream towards bore 7.

In Fig. 7, two concentric helices 8, 8' are used.

The arrangement is basically that of Fig. 2 but without the cup-shaped member 10.

Two pot magnets 4, 4 are mounted face to face after the manner of Fig. 4, by struts such as 5 in that figure, which are not shown in Fig. 8 to simplify the drawing. The air gap of 4 is of smaller diameter than that of 4. A centre post 16 within the hollow centre pole of 4' provides a mounting for the fixed end of the inner helix 1, the other end of which is fixed to coil 3 working in the air gap of pot magnet 4. The outer pole piece of pot magnet 4 provides a mounting for the fixed end of outer helix 1' the other end of which is fixed to coil 3' working in the air gap of pot magnet 4.

Pot magnet 4 has a tubular centre pole with an annular passage 7' betwen its inner surface and the centre post 16, both of which are mounted on the back plate of 4 which is perforated to permit of access to the said annular passage for an air stream applied through a pipe 15' fixed to a boss 17' on the outside of the back plate of 4'. Air so introduced passes to the annular space between the concentric helices 1, 1 and passes radially outwards through the convolutions of outer helix 1'.

Pot magnet 4 has a bore 7 through its centre pole to permit passage of an air stream, applied through a pipe 15 attached to a boss 17 on the outside of the backplate of 4, to the inside of helix 1, this air stream escaping radially through the convolutions of helices 1 and 1' in succession.

Coil 3 is supplied with audio frequency currents in the upper end of the range required to be reproduced and 3 with currents in the lower frequency end of that range.

Pipes 15, 15 can be supplied from a common air stream source through a common equalising chest 18.

In this embodiment, the air stream emerging through helix 1 is again modulated at lower frequencies on passing through helix 1 which may lead to the generation of spurious frequencies.

The embodiment shown in Fig. 13 which is similar in many respects to that shown in Fig. 7, provides independent outlets from the two helices which are mounted end-to-end on a common annular mounting member 42, having protruding suspension lugs 44 spaced around its periphery which are secured by studs 43 screwed into the front face of the outer pole piece of pot magnet 4. Pot magnet 4 is also supported by means of similar lugs 45 threaded over studs 43. As shown in the drawing the positions of 4 and 42 in relation to 4 are determined by spacer tubes threaded over the studs, the assembly being secured by nuts 46.

Air may be supplied to both helices from one end, through pipe 15', the smaller helix 1 receiving its air supply through the central aperture in the common annular mounting member 42.

To ensure a correct distribution of the air supply between the two helices a tube 47 is provided, fixed at one end within the central aperture in mounting member 42, and projecting into the air entry duct 7 through the middle of the centre pole of pot magnet 4'. Having first determined by means of adjustable shutters or the like the optimum distribution of the air flow between the two helices, this distribution can be ensured, in a production design by shaping the end of tube 47 which lies within duct 7 in any convenient way, for instance by thickening its inside or outside contours, or both, so as to leave passages leading to the two helices proportioned so as to provide the optimum air flow distribution found by experiment. The end of tube 47 within duct 7 should be formed to provide an eddy-free smooth fiow of gas to the helices.

In the arrangements illustrated in Figs. 1 to 7 and 13, the helix is actuated at one end and fixed at the other. A displacement produced by the driving means will be propagated down the helix towards the fixed end at a finite propagation speed which will depend on the density and modulus of elasticity of the material of which the helix is made. If the unwound length of the helix is of the order of half a wavelength or less, in the helix material, of the highest frequency to be reproduced, the phase lag in the transmission of displacements along the axial length of the helix will not cause cancellation of the modulation of the air stream. This places limitations on the diameter of the helix and its axial length, which may be to some extent avoided if the material of the helix is such that the speed of propagation of a displacement axially along it is equal to or greater than the speed of sound in air. As the speed of propagation along the helix depends on its unwound length a helix of any given diameter of any given material has a maximum number of turns per unit length for the latter criterion to be satisfied and the thickness of the unwound material must be such that, when wound to a helix of the given diameter with the said maximum turns per unit length, or any chosen lesser number of turns, there is left a mean interturn spacing sufficient to ensure eflicient modulation of the air stream.

This inter-turn spacing in the rest condition of the helix should be of the order of half the maximum axial excursion of the driving means to secure modulation of the air stream though in practice a wider spacing is advisable to avoid rattling due to adjacent turns coming into contact, and to ensure linearity since conditions of viscous flow would come into play if the turns approached too close to one another on the contraction half cycle of the helix.

The fixed end of the helix is preferably mounted in some energy absorbent material to prevent the reflection of waves back along the helix towards the driving means. Alternatively the helix can be extended to such a length that the waves are dissipated by reason of the internal resistance of the material. Such an extension of the helix should preferably be blanked off from access and egress of the air stream, and may be supported in a manner such that free axial movement of the helix is impeded over the extension portion; for instance the extension may be embedded in rubber sponge or threaded over a loosely fitting mandrel. If the mounting of the extension is too rigid there will be reflections, if the mounting is too loose the extension may sag and rattle against some fixed part of its surroundings, or the energy may not be completely absorbed and be reflected from the end of the helix extension.

In an alternative arrangement the driving force is applied uniformly down the length of the helix. This may be achieved either by passing the audio frequency currents down a helix made of non magnetic conducting material and immersing it in a unidirectional magnetic field or by making the helix of magnetic material and immersing it in an alternating magnetic field. Figs. 8, 9, 10 and 11 illustrate the former method and Fig. 12 the latter method.

In Fig. 8, there is a pot magnet 4 with concentric tubular extension pole pieces 19 and 20 mounted on the front of the pot magnet and defining an annular air gap between them. The helix is made of electrically conducting material with an insulating coating and is embedded and centred in this gap at the end nearest to the pot magnet 4 and at the other end is supported on a resilient washer 21 which is held by its outer margin between a flange 22 in the end of pole extension 20, and a clamping ring 23, and substantially seals the said gap at this end. Sectors of 19 and 20 are cut away at 24, 25 as shown in Fig. 9 to leave segments a through which the air stream can escape from the interior of pole extension 19 through the convolutions of the helix to the outer air. The remaining segements b serve for the establishment of the magnetic field against which the helix reacts when a current is passed through it to produce axial displacement of the helix. The two ends of the helix are connected to pigtail connecting wires 26, 27 to which the audio frequency currents are applied.

The air stream is introduced through a pipe 15 into the open end of pole extension 19. The apertures 24 and 25 are of convenient axial length to permit a maximum length of the helix to be effective without unduly weakening the structure of 19 and 20 and the form of the latter is more clearly shown in the perspective views of Figs. 10 and 11 respectively. Alternatively 19 and 20 can be perforated, care being taken to avoid holes of such a size as to cause whistles.

With this arrangement, the turns of the helix are individually acted on by extending and contracting forces caused by interaction of the alternating fluxes arising from the audio frequency currents flowing in the helix and the unidirectional flux in the annular air gap. There is thus no problem of phase differences between the opening and closing of the path through the interstices between the turns as between one end of the helix and the other provided that the forces in question are distributed along the length of the helix according to the movement required to open equal spaces between adjacent turns. In this connection it should be noted that the excursion of the turns must increase in amplitude from the fixed end of the helix towards the free end. This involves higher velocities of the turns at the free end as compared with the fixed end and correspondingly higher accelerating forces are required. If the flux density along the gap is uniform there will be a tendency for part of the higher accelerating force required at the free end of the helix to be provided by excess accelerating force applied to turns nearer the fixed end. This borrowed accelerating force will be transmitted along the helix and tend to set up standing waves or phase differences therein. One solution of this problem is to grade the intensity of the magnetic field along the length of the annular air gap so that it is most intense at the free end of the helix. Various ways of doing this are available the simplest of which is to widen the air gap towards the fixed end of the helix. Alternatives would be to increase the circumferential width of the air stream exit apertures in the pole pieces towards the fixed end of the helix so as to reduce the available pole piece material or to reduce the radial thickness of the pole pieces towards the fixed end of the helix so that they would be more nearly saturated magnetically towards that end.

It is in some respects more convenient to have the fixed end of the helix remote from the pot magnet 4 and the centring washer for the free end accommodated within the interior of the pot magnet 4. This lends itself more readily to arrangements giving higher gap flux densities nearer to the free end of the helix because this end is nearer to the magnet.

To avoid any tendency for the coil to sag and touch the pole pieces, they may be surfaced with velvet or the like so that the turns of the helix ride on the top of the pile.

Fig. l2 shows an arrangement in which the helix is made of magnetic material and is analogous to the iron diaphragm of a telephone receiver in a magnetic system having a fixed speech coil and a permanent magnet to provide a bias flux.

The magnetic system comprises a tubular permanent magnet 28 seated on a back plate 29 in the centre of which is a cylindrical centre pole 30 around which is a bobbin 31 wound with the speech coil 32 which can fill the annular space between the magnet 28 and the centre pole 30 and locate the latter centrally within the former. An outer extension pole piece 20 rests upon the end of the magnet 28 remote from back plate 29. Pole piece 20 is tubular and has a small inwardly extending stepped flange 33 which cooperates with a washer 34 of magnetic material trapped between pole piece 20 and magnet 28, to grip the outer margin of a resilient washer 21 which supports the free end of the helix 1 centrally within the magnetic system. The end of pole piece 20 remote from magnet 28 has an inwardly extending flange 35 the inner rim of which is tapered. A tubular extension pole piece 19 is seated upon centre pole 30 at the end thereof remote from the back plate 29. Sectors are removed from 19 to leave air stream escape passages 24 in the tubular walls thereof.

A retaining plate 36 of non-magnetic material, is seated on the outer surface of flange 35 of pole piece 20 and has an inwardly extending central tubular boss 37 the outer surface of which is tapered and co-operates with the inner tapered rim of flange 35 of 20 to grip the fixed end of helix 1. The inner surface of boss 37 is a close fit over the end of pole piece 19 and exerts an axial pressure thereon to hold it in place upon centre pole 30.

Helix 1 surrounds pole piece 19 which is tapered on its outer surface with its smallest diameter at the end remote from centre pole 30. In the drawing this taper is much exaggerated. The whole assembly is clamped together by bolts 38 which lie in grooves in the outer surfaces of 20 and 28 and pass through holes in 29 and 36.

Ignoring for the moment the effect of permanent magnet 28, when current flows in winding 32, flange 35 and the helix assume one magnetic polarity and pole piece 19 assumes the opposite polarity so that each turn is attracted towards the areas of greater field intensity surrounding pole piece 19 in regions nearest to centre pole 30. The helix is thus extended.

But for the permanent magnet 28, the helix would be extended whatever the direction of the current flowing in coil 32, and frequency doubling would result. The magnet 28 however provides a steady flux tending to extend the helix, which must be pre-stressed to hold its adjacent turns together to an extent such that the permanent pull due to 28 opens the spaces between the turns to the desired mean size. Fluxes due to alternating currents in coil 32 then either augment or reduce the attracting flux between the helix and 19.

As mass is no detriment in the fixed coil 32 it can have a sufliciently high impedance to be connected directly in the anode circuit of the output valve of an amplifier so as to carry the DC. component of the anode current which will produce the required steady biassing flux enabling magnet 28 to be dispensed with.

Alternatively the helix itself can be made of magnetically hard material and be magnetised.

The taper on the working surface of pole piece 19 provides the graded flux as between the fixed and free ends of the helix previously referred to.

With this arrangement it is convenient to apply the air stream to the chamber enclosed by 20 and to let it escape after passing inwardly through the helix via the passages 24 in 29 and thence axially of the internal bore in 19, emerging from the free end thereof, as indicated by the arrow 39.

The air stream entry pipe 15 delivers into a hole 40 in the cylindrical outer wall of 20.

The centre pole 30 may have a flared conical extension 41 extending into the bore of 19 to act as an air stream deflector.

In the figures described above the magnetic systems are shown in diagrammatic form to facilitate comparison with well known loudpseaker magnetic systems. It is to be understood however that in a production model more sophisticated magnetic systems would be used.

It is important to provide an air stream which is free from any audible modulation received before it reaches the helix. To this end, the passages carrying the air stream to the helix may be packed with acoustic damping material, and any constrictions should be of stream-line form.

It is also important to avoid any eddy tones or noises due to the flow of the air stream past the turns of the helix. To minimise this the turns of the helix may be shaped to present a stream-line form, to air passing radially of the helix.

To match the device to the load represented by the surrounding air where no horn is used it must have low impedance characteristics involving a relatively large mean volume of air released through the helix, at relatively low pressure and with a relatively large percentage modulation. This calls for a relatively wide spacing between adjacent turns of the helix in its rest position and a relatively large extension and contraction to be provided by the driving means. The provision of the necessary large excursion of the helix presents no great difficulty as compared for instance with the speech coil of a moving coil loudspeaker where the mass of a large diaphragm has to be moved against an air load, since there is only the mass and stiflness of the helix itself to be overcome, the radiation energy being provided by the air stream.

An added advantage of the invention when no horn is used is that there is no problem of out-of-phase radiations (as from the back of a direct-radiating diaphragm for instance) so that bulky battles, reflex cabinets and the like can be dispensed with. With careful design to avoid deleterious diflraction efiects, the device can be made to operate as virtually a spherical sound source over a wide band of frequencies.

In the embodiments described the helix has been represented as cylindrical but it is to be understood that it may taper axially and/or may be non-circular and the term helix, where used in this specification is to be interpreted as including these variants.

As indicated above, the helix may be directly controlled by mechanical means to provide a mechanical/ acoustic translator amplifier. For instance it may be controlled by a diaphragm directly vibrated by sound waves which may be received-say-via an acoustic conduit from a mouth piece, thus providing a pneumatic speech amplifier, or the helix may be coupled to a resonant body such as a gong to amplify the sound output therefrom. Such alternatives are within the scope of the invention.

Although the illustrative embodiments take the form of generators of sound waves in a gas such as air, the invention as has already been indicated, is applicable to the generation of sound waves in a liquid, for instance for underwater transmission, in which case a liquid stream would replace the gas stream of the described embodiments and similar arrangements to those illustrated could be used with such consequential changes to dimensions to compensate for the different density and relative incompressibility of the liquid medium as compared with a gas medium as are within the competence of those skilled in the art to devise.

We claim:

1. An acoustic device for modulating the rate of flow of a gas therethrough comprising a housing having inlet and outlet means for the passage of gas through said housing, a helical member of resilient material having adjacent turns in close axial proximity so as to define a space between such turns, means to support said helical member in said housing in the path of the flow so that gas is constrained to pass radially through the said space,

and means for cyclically extending and contracting the helix in an axial direction within the elastic limit of the material of which the helix is made, whereby the cross sectional area of the said space is cyclically increased and reduced respectively.

2. An electro-acoustic transducer of the type in which a stream of gas is constrained to pass through a valve having an aperture of variable admittance, comprising a helix of resilient material having adjacent turns in close axial proximity so as to define a space between such turns, such space constituting the said aperture, and comprising means, under control of an alternating electric current, for axially extending and contracting the helix within the elastic limit of the material of which the helix is made to increase and reduce the said space respectively, whereby the admittance of the aperture constituted by such space, is varied.

3. A device according to claim 1 in which the means for extending and contracting the helix is an electromechanical transducer.

4. A device according to claim 3 in which the electromechanical transducer comprises a cylindrical coil mounted for axial movement in an annular gap between a central pole piece and a surrounding pole piece forming parts of a magnetic circuit adapted to produce a radial magnetic field across said gap.

5. A device according to claim 3 in which one end of the helix is fixed and the other end is connected to the transducer.

6. A device according to claim 3 comprising two electro-mechanical transducers one connected to one end of the helix and the other to the other end of the helix, with electrical connections to the two transducers such that when a current of a given polarity is applied over the said connections to the transducers they move the two ends of the helix in opposite directions axially of the helix.

7. A device according to claim 1 in which the gas, the flow of which is to be modulated, is introduced into the interior of the helix and in which the outside of the helix is surrounded by a deflecting member adapted to permit the gas outside the helix to flow only in one direction generally parallel to the axis of the helix.

8. A device according to claim 7 in which the said deflecting member provides a passage, for flow of gas from the helix, which increases progressively in cross section in the direction away from the helix.

9. A device as claimed in claim 8 in which a tapered plug member is mounted centrally within the deflecting member and has an end of larger diameter adjacent to and forming a fixed mounting for one end of the helix, and an end of smaller diameter remote from the helix.

10. A device according to claim 1 comprising a chamber surrounding the outside of the helix, adapted for the entry of the gas the flow of which is to be modulated, and an apertured end wall providing a fixed mounting for one end of the helix with the aperture located coaxially with the helix to provide a passage for the outflow of gas from the interior of the helix.

11. A device according to claim 10 comprising a horn and a mounting therefor adapted to position the throat of the horn in communication with the said aperture.

12. An acoustic device for modulating the rate of flow of a gas comprising a helical member of resilient material, placed in the path of said flow so that gas passes through the space between adjacent turns of said helical member, means for cyclically extending and contracting said helical member in an axial direction within the elastic limit of the material of which said helical member is made, whereby the cross sectional area of the said space is cyclically increased and reduced respectively, said means for extending and contracting said helical member being an electro-mechanical transducer, comprising a cylindrical coil mounted for axial movement in an annular gap b tween a central pole piece and a surrounding pole piece forming parts of a magnetic circuit adapted to produce a radial magnetic field across said gap, said center pole defining the inner margin of said annular magnetic gap being in the form of a tube communicating with the interior of said helical member.

13. An acoustic device for modulating the rate of flow of a gas comprising a helical member of resilient material, placed in the path of said flow so that gas passes through the space between adjacent turns of said helical member, means for cyclically extending and contracting said helical member in an axial direction within the elastic limit of the material of which said helical member is made, whereby the cross sectional area of the said space is cyclically increased and reduced respectively, said means for extending and contracting said helical member being an electro-mechanical transducer, comprising a cylindrical coil mounted for axial movement in an annular gap between a central pole piece and a surrounding pole piece forming parts of a magnetic circuit adapted to produce a radial magnetic field across said gap, said center pole defining the inner margin of said annular magnetic gap being in the form of a tube communicating with the interior of said helical member, and means for introducing said gas, the flow of which is to be modulated, along said tubular center pole to the interior of said helical member.

14. A device according to claim 4 with a horn mounted so that the throat of the horn is in communication with the end of the said tubular centre pole remote from the helix and with means for introducing the gas the flow of which is to be modulated, to the outside of the helix so that it flows through the horn after modulation by the helix.

15. A device according to claim 1 comprising helices of difierent diameters with means for extending and contracting the helix of smaller diameter at frequencies in one range and for extending and contracting the helix of larger diameter at frequencies in another and lower range.

16. A device as claimed in claim 15 in which the smaller helix is located within and substantially coaxial with the larger helix.

17. A device as claimed in claim 16 with means for introducing gas, the flow of which is to be modulated, to the annular space between the two helices and, separately, to the interior of the smaller helix.

18. A device according to claim 1 comprising two substantially coaxial tubular apertured pole pieces mounted one within the other, means for applying a magnetic field across the annular gap between them a mounting for locating the helix in the said gap coaxial with the pole pieces, the helix being of electrically conductive material, means for causing a varying electric current to flow through the turns of the helix and means for constraining the flow of gas which is to be modulated to pass through the apertures in the pole pieces and the space between adjacent turns of the helix.

19. A device according to claim 1 comprising two pole pieces, means for attaching one end of the helix to one pole piece mounting means for the other pole piece adapted to locate a part of it adjacent to the other end of the helix, the helix being of ferro-magnetic material, and means for applying an alternating magnetic flux across the two pole pieces of a configuration such that it embraces at least part of the helix and applies axial forces thereto.

20. An electro-acoustic transducer as claimed in claim 2 in which the dimensions of the helix and the material of which it is made are such that the speed of propagation of wave motion axial of the helix, applied to an end of the helix is substantially equal to the speed of propagation of pressure waves in the fluid.

References Cited in the file of this patent UNITED STATES PATENTS 1,766,612 De Forest June 24, 1930 1,815,552 Eckhardt July 21, 1931 2,768,235 Knoblough Oct. 23, 1956 2,810,021 Brownscombe Oct. 15, 1957

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