GB2085694A

GB2085694A - Balanced armature transducers

Info

Publication number: GB2085694A
Application number: GB8031747A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1980-10-02
Filing date: 1980-10-02
Publication date: 1982-04-28
Also published as: GB2085694B

Abstract

An electro-acoustic transducer of the balanced armature type, for use as a telephone earpiece or microphone, has its magnetic circuit formed by two parallel magnetic material yokes (1, 2), between which is located a speech coil (9). A ceramic permanent magnet (5) is located between the ends of the yoke at one end of the transducer. At the other end the yokes are angled inwards (4) to define a short narrow gap in which a flat reed-like armature (10) of ferro-magnetic material is secured. This is held in place with spacers (11) of a weakly magnetic material e.g. nickel, to facilitate welding. The armature extends through the speech coil (9) and into another narrow gap defined by inward-angled portions of the yokes. This, the main gap, is of relatively large area. Between the main gap and the speech coil a drive rod (7) is secured, preferably not by welding to avoid distorting the armature, and this drive rod drives a conical diaphragm. <IMAGE>

Description

SPECIFICATION Telephone receivers This invention relates to an electro-acoustic trans ducer of the balanced armature type.

A known example of a balanced armature trans ducer is the so-called rocking armature transducer currently used in the earpiece of the standard British Post Office telephones. This, which follows the prin ciples of our British Patent No. 606012 (J. S. P.

Roberton - 18), has a drive unit which consists essentially of an E-shaped magnetic structure, the central leg of which is a permanent magnet and the outer legs of which carry the operating coils. The armature is a plate-like structure pivotted about the central leg and which as it vibrates due to speech currents in the coils drives the diaphragms via a drive rod. Thus the functions of the armature and the diaphrams are separated andthe alternating speech flux does not pass through the magnet.

However, both the alternating and the steady fluxes share paths in the yoke and the armature.

An object of the invention is to provide a trans ducer of the balanced armature type which is an improvement on known transducers such as the type referred to.

According to the present invention, there is pro vided an electro-acoustic transducer of the balanced armature type, which includes two yokes of ferro magnetic material which extend parallel to each other and between which is a speech coil, a perma nent magnet between the ends of the yokes at one end of the transducer, and a ferro-magnetic arma ture of flat-strip like form mounted by one of its ends between the ends of the yokes at the other end of the transducer, wherein at the other end of the trans ducerthe yokes are angled inward to produce a short and narrow gap between which the end of the armature is secured, wherein between the magnet end of the transducer and the speech coil the yokes are angled inwards and then extend parallel to their length and are then angled outward to define a main gap which of larger area than the first-mentioned gap and is a narrow gap, wherein the armature extends from its said securing point through the speech coil and into the main gap, and wherein the armature is coupled by a drive rod to a diaphragm so that when the armature is driven it drives the diap hragm via the drive rod, the arrangement being such that speech currents in the coil vibrate the armature and hence the diaphragm or that speech waves inci dent on the diaphragm vibrate the armature and induce currents in the speech coil.

An embodiment of the invention will now be described with reference to the accompanying draw ing, in which Fig. 1 is a sectioned view of a trans ducer embodying the invention, Fig. 2 is a top-plan view of the transducer of Fig. 1, and Fig. 3 is a plan view of a metallic blank from which one of the two yokes ofthe transducer of Fig. 1 is made.

On the accompanying drawing, the dimensions of one example of a transducer embodying the invention have been shown, from which it will be seen that the figures are on an enlarged scale. The actual dimensions shown are those for a transducer which could be a direct replacement in a telephone earpiece for a rocking armature transducer. The upper and lower yokes 1,2 are identical piece-parts, each of which is a blank such as shown in Fig. 3 bent to the shape shown in Figs. 1 and 2. The blanks are of a suitable ferro-magnetic alloy. The yokes are shaped with inwardly bent portions such as 3 and 4, of which the portions 3 of the two yokes define as relatively long active pole gap. The other gap is a short back gap.

The magnet 5 is a ceramic permanent magnet, and is between the left-hand ends of the yokes, poled as shown. Note that the active gap has a short magnetic path length of 1 mm and a large area, of 36 sq mm.

Each yoke has a hole such as 6 via one of which holes passes a drive rod 7, which drives the transducer's diaphragm (not shown). Note that the hole is not strictly necessary in both yokes, but from the production aspect it is convenient for the yokes to be identical.

Between the widened out portions of the yokes is a coil former 9 on which the speech coil is wound: terminals for this coil are conveniently mounted on extensions of the cheeks of the coil former. The space between the yokes in which the coil former is located is longer than the former length so that its position can be adjusted if necessary before it is fixed in place.

The armature 10 is a flat thin reed, 0.8 mm thick by 6 mm wide by 24 mm tony, clamped by nonmagnetic spacers 11 in the back gap. This back gap also has a short magnetic path length, 1 mm, but is of relatively small area, 9 t,o 12 sq mm. This armature passes freely through the eye of the coil, and through the centre ofthe active pole gap. It also carries the drive rod 7 which is suitably secured to it. Note that the armature 10 is accessible from the sides so that the pole air gaps can be adjusted.

Since the active pole gap is larger in area, by two or three times, then the back gap, most of the steady flux passes across it so that there is little left to pass along the length of the yokes to the back gap. Hence the parts of the yokes near to the magnet may be near to saturation, while the rest of the yokes carries that value of steady flux needed for optimum AC permeability.

Little of the AC flux due to the speech coil passes through the magnet since its reluctance, and the reluctance of the parts of the yokes near it can be greaterthan the reluctance to AC of the active pole gap. As the magnet is close to the pole gap, some of its leakage flux adds to the fringing flux of the gap: to exploit this the end of the armature projects through the gap towards the magnet.

The flat reed armature is virtually in the neutral magnetic plane for the steady flux, so very little of it passes along its length, its cross-sectional area being 4.8 sq mms. The main flux crosses the arma The drawings originally filed were informal and the print here reproduced is taken from a later filed format copy.

ture in the pole gap, where it passes through a grea ter area, 25 to 36 sq mm. This enables it to be made of a higher permeability material, which saturates at a lower flux density, so that its negative stiffness due to magnetic pull weakens enough to prevent it hold ing itself poled.

As can be seen from the drawing, the blank for the yoke is widened at the magnet end: this widened portion becomes a flange which stiffens the yoke near the pole gap. If the spacers used to define the back gap 11 are a weakly magnetic alloy they should be compatible with the yoke and armature, and enable final fixing to be done by welding.

The pole gap face and back gap face are coplanar, and are parallel to the magnet sealing. The air gap shown at the pole gap is 0.1 mm, and may provide some damping resistance due to air viscosity.

To ensure that the flatness of the reed like arma ture, which can be maintained during any annealing needed, is not spoilt on assembly of the drive rod 7, this latter should be stuck or clipped to the armature, rather than being welded or soldered. If the armature is truly flat, the amount of adjustment needed is minimised.

The magnet is not attached to the yokes, being held in place by slight depressions in the yokes until magnetised. Note that although the magnet in the example shown is of a ceramic, a metallic magnet can also be used.

To consider the magnetic structure again, it will be noted that to a very large extent the AC and DC flux paths have been separated. As will be seen this per mits the use of relatively high DC flux without upset ting the AC efficiency.

The AC permeability of a soft magnetic material rises from its initial value to a blunt maximum at a DC induction of around 0.5 Tesla. As the DC flux increases further, the AC permeability falls, more rapidly as saturation is approached. For good effi cieney the product of the square of the DC flux and the AC flux must be maximised. Thus a high AC permeability enables the magnetising ampere-turns in the coil to induce a high AC flux in the active gap.

With the arrangement described above, the parts of the yoke which carry the DC flux due to the magnet can be near to saturation without regard to the AC permeability. Hence the DC flux can be very large without spoiling the device's response to AC flux.

The above indicates why the yokes are widened at -the magnet end:this allows the yokes to carrythe high DC flux to all of the main gap and thus stiffens the yoke nearthe main gap where the magnetic pull is considerable. The space provided beyond the coil former is to allow adjustment of the reed armature so that its free end can be set to the most sensitive position in the main gap.

The yokes can be made of such material as a grain-oriented silicon steel, using some ofthe increased efficiency to reduce cost or difficulties in manufacture.

Although the transducer which has been described herein is intended for use as a telephone earpiece it wilt be understood that transducers embodying the invention can also be used as mic- raphoneS.

Claims

1. An electro-acoustic transducer of the balanced armature type, which includes two yokes of ferromagnetic material which extend parallel to each other and between which is a speech coil, a permanent magnet between the ends of the yokes at one end of the transducer, and a ferro-magnetic armature of flat strip-liked form mounted by one of its ends between the ends of the yokes at the other end of the transducer, wherein at the other end of the transducer the yokes are angled inward to produce a short and narrow gap between which the end of the armature is secured, wherein between the magnet end of the transducer and the speech coil the yokes are angled inwards and the extend parallel to their length and are then angled outward to define a main gap which is of larger area than the first-mentioned gap and is a narrow gap, wherein the armature extends from its said securing point through the speech coil and into the main gap, and wherein the armature is coupled by a drive rod to a diaphragm so that when the armature is driven it drives the diaphragms via the drive rod, the arrangement being such that speech currents in the coil vibrate the armature and hence the diaphragm or that speech waves incident on the diaphragms vibrate the armature and induce currents in the speech coil.

2. An electro-acoustic transducer as claimed in claim 1, wherein the drive rod is secured to the armature at a point between the main gap and the speech coil, and extends substantially at right angles to the armature through a hole in one of the yokes.

3. An electro-acoustic transducer as claimed in claims 1 or 2, wherein at its securing point the end of the armature is spaced from the two yoke ends by spacers of weakly magnetic material, being secured thereat by welding.

4. An electro-acoustic transducer of the balanced armature type, which includes two yokes of ferromagnetic material which extend parallel to each other and between which there is a speech coil, a source of direct current magnetic flux between the ends of the yoke at one end of the transducer, and a ferro-magnetic armature of flat strip-like form mounted by one of its ends between the ends of the yokes at the other end of the transducer, wherein between the first-mentioned ends of the yokes and the speech coil the yokes are so shaped as to define a narrow main gap which is of relatively large area, such that the direct current flux due to the source and the alternating current flux due to the speech coil are conveyed via separate magnetic flux paths,4 wherein the armature extends from the point at which it is secured to the yokes through the speech coil and into the main gap, and wherein the armature is coupled by a drive rod to a diaphragm so that when the armature is driven it drives the diaphragm via the drive rod and vice-versa, the arrangement being such that speech currents in the coil vibrate the armature and hence the diaphragm or that speech waves incident on the diaphragm vibrate the armature and induce currents in the speech coil.

5. An electro-acoustic transducer of the balanced armature type substantially as described with referpence to the accompanying drawing.