GB2134745A

GB2134745A - Electro-acoustic transducer

Info

Publication number: GB2134745A
Application number: GB08303131A
Authority: GB
Inventors: Frank Taylor; Norman William Tester; Malcolm Roger King; Trevor Burton
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1983-02-04
Filing date: 1983-02-04
Publication date: 1984-08-15
Also published as: SG40087G; GB8303131D0; IE840162L; AU569275B2; NZ206899A; AU2394284A; GB2134745B; IE55102B1; HK76187A

Abstract

In electro-acoustic transducers, especially for use in a telephone subscriber's instrument, to reduce the low frequency response of the transducer without increasing distortion one or more holes or tunnels (11) are provided through the diaphragm support member (5), the holes or tunnels coupling the air behind the diaphragm with the air in front. Such holes or tunnels can also be formed by a groove or grooves in the support member, the diaphragm rim then acting in effect as a lid for the or each such groove. The holes or tunnels do not have to be parallel to the plane of the diaphragm, and the idea is applicable to types of transducers other than the moving coil type. <IMAGE>

Description

SPECIFICATION Electro-acoustic transducer This invention relates to electro-acoustic transducers, and especially to such transducers for use in telephone subscriber's instruments.

Such transducers often need a-reduced sensitivity at low frequency to prevent low frequency noise from interfering with the required signals, and in the case of telephone transducers both transmitters (microphones) and receivers (earphones) may require such low frequency sensitivity reduction. The usual way to produce the drop in sensitivity is to make a small hole through the diaphragm. At low frequencies, air can pass through the diaphragm, so little energy is transferred between acoustic and electrical forms. As the frequency increases the acoustic mass of the hole increasingly impedes the flow of air and the attenuation is reduced. However, the hole increases low frequency distortion, particularly in the region at which the reduction begins. This effect occurs both with transmitters and receivers, but the main problem is for receivers where the diaphragm movements are larger.This suggests that the problem is due to the high particle velocity of air flowing through the hole, which must be small (diameter of the order of 0.7mm) to give the required frequency response. Thus the flow becomes turbulent and the relationship between the pressure across the hole and the flow of air through the hole is nonlinear. The distortion effect may also be partly due to relative velocity between the air and diaphragm which, because diaphragm is itself in motion, is greater than through a stationary hole.

An object of the invention is to provide a transducer in which the above difficulties are at least minimised.

According to the invention there is provided an electro-acoustic transducer of the type having a movable diaphragm supported at its outer periphery by a diaphragm support, wherein one or more holes or tunnels are provided in the diaphragm support, the or each said hole or tunnel coupling the air in front of the diaphragm with the air behind the diaphragm.

Thus we have one or more holes or tunnels through the member or members supporting the diaphragm, which holes or tunnels are defined by slots formed by the diaphragm and grooves in the surface of the diaphragm support member. As the path for the air to travel through these holes or slots is longer than that though the thickness of the diaphragm, they have a larger total crosssectional area than the diaphragm hole to produce the same low frequency cut. The larger area results in lower relative air velocities, as does the fact that the holes are stationary. The longer holes or tunnels have greater acoustic resistance, which reduces distortion.

We have found that the harmonic distortion is reduced by these means from 7% to less than 1.5% in a preferred embodiment of the invention.

There follows a detailed description with reference to Figures 1 and 2 of the accompanying drawings of two implementations of this invention. Both are moving coil types of the sort described in our Application No. 8203650 (F.

Tailor eft al 7-7-3-1-1-25), but the invention is also applicable to other types of transducer, e.g.

electro-magnetic or piezo-electric.

The first form is shown in Figure 1. It consists of a moving coil transducer arrangement having a magnetic assembly formed by the magnet 1, ring 2, inner pole 3 and outer pole-piece 4. The air gap for the coil is between the ring 2 and the inner pole 3. The magnetic assembly is mounted in a support ring 5 and is held in place by the rear cover 6. The diaphragm 7 supports the coil 8, which is movable in the annular gap between ring 2 and inner pole-piece 3. The transducer is completed by terminals 9 and front cover 10. In practice acoustic resistance may be inciuded to produce a smooth frequency response, but the means of providing this is not shown.

Further details of such a transducer will be found in the above-mentioned application. Instead of the normal hole through the diaphragm, the volumes on either side of the diaphragm are connected by holes 11 through the support ring 5.

Only one such hole is shown but more may be provided if this is found to be desirable.

In an alternative, and preferred arrangement, see Figure 2 the hole is replaced by one or more slots 12 formed by grooves in the upper surface of the support ring 5 and the diaphragm 7. This method is preferred because when the support ring 5 is a thermo-plastics moulding the manufacturing tooling is simpler and the part less expensive.

Both versions have the diaphragm attached to the support ring by adhesive or welding. The invention may also be applied when the diaphragm is clamped in place. The grooves or radial holes are then needed in the clamping members on each side of the diaphragm. The use of holes at right angles to the plane of diaphragm is also possible.

The invention can also be applied to a moving coil transducer in which the diaphragm rim is clamped between two members: however, in this case the holes or grooves or slots are provided in both of those members.

Claims

1. An electro-acoustic transducer of the type having a movable diaphragm supported at its outer periphery by a diaphragm support, wherein one or more holes or tunnels are provided in the diaphragm support, the or each said hole or tunnel coupling the air in front of the diaphragm with the air behind the diaphragm.

2. A transducer as claimed in claim 1, in which the or each said hole or tunnel is formed within the diaphragm support.

3. A transducer as claimed in claim 2, in which the or each said hole or tunnel has its axis substantially parallel to the plane of the diaphragm.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Electro-acoustic transducer This invention relates to electro-acoustic transducers, and especially to such transducers for use in telephone subscriber's instruments. Such transducers often need a-reduced sensitivity at low frequency to prevent low frequency noise from interfering with the required signals, and in the case of telephone transducers both transmitters (microphones) and receivers (earphones) may require such low frequency sensitivity reduction. The usual way to produce the drop in sensitivity is to make a small hole through the diaphragm. At low frequencies, air can pass through the diaphragm, so little energy is transferred between acoustic and electrical forms. As the frequency increases the acoustic mass of the hole increasingly impedes the flow of air and the attenuation is reduced. However, the hole increases low frequency distortion, particularly in the region at which the reduction begins. This effect occurs both with transmitters and receivers, but the main problem is for receivers where the diaphragm movements are larger.This suggests that the problem is due to the high particle velocity of air flowing through the hole, which must be small (diameter of the order of 0.7mm) to give the required frequency response. Thus the flow becomes turbulent and the relationship between the pressure across the hole and the flow of air through the hole is nonlinear. The distortion effect may also be partly due to relative velocity between the air and diaphragm which, because diaphragm is itself in motion, is greater than through a stationary hole. An object of the invention is to provide a transducer in which the above difficulties are at least minimised. According to the invention there is provided an electro-acoustic transducer of the type having a movable diaphragm supported at its outer periphery by a diaphragm support, wherein one or more holes or tunnels are provided in the diaphragm support, the or each said hole or tunnel coupling the air in front of the diaphragm with the air behind the diaphragm. Thus we have one or more holes or tunnels through the member or members supporting the diaphragm, which holes or tunnels are defined by slots formed by the diaphragm and grooves in the surface of the diaphragm support member. As the path for the air to travel through these holes or slots is longer than that though the thickness of the diaphragm, they have a larger total crosssectional area than the diaphragm hole to produce the same low frequency cut. The larger area results in lower relative air velocities, as does the fact that the holes are stationary. The longer holes or tunnels have greater acoustic resistance, which reduces distortion. We have found that the harmonic distortion is reduced by these means from 7% to less than 1.5% in a preferred embodiment of the invention. There follows a detailed description with reference to Figures 1 and 2 of the accompanying drawings of two implementations of this invention. Both are moving coil types of the sort described in our Application No. 8203650 (F. Tailor eft al 7-7-3-1-1-25), but the invention is also applicable to other types of transducer, e.g. electro-magnetic or piezo-electric. The first form is shown in Figure 1. It consists of a moving coil transducer arrangement having a magnetic assembly formed by the magnet 1, ring 2, inner pole 3 and outer pole-piece 4. The air gap for the coil is between the ring 2 and the inner pole 3. The magnetic assembly is mounted in a support ring 5 and is held in place by the rear cover 6. The diaphragm 7 supports the coil 8, which is movable in the annular gap between ring 2 and inner pole-piece 3. The transducer is completed by terminals 9 and front cover 10. In practice acoustic resistance may be inciuded to produce a smooth frequency response, but the means of providing this is not shown. Further details of such a transducer will be found in the above-mentioned application. Instead of the normal hole through the diaphragm, the volumes on either side of the diaphragm are connected by holes 11 through the support ring 5. Only one such hole is shown but more may be provided if this is found to be desirable. In an alternative, and preferred arrangement, see Figure 2 the hole is replaced by one or more slots 12 formed by grooves in the upper surface of the support ring 5 and the diaphragm 7. This method is preferred because when the support ring 5 is a thermo-plastics moulding the manufacturing tooling is simpler and the part less expensive. Both versions have the diaphragm attached to the support ring by adhesive or welding. The invention may also be applied when the diaphragm is clamped in place. The grooves or radial holes are then needed in the clamping members on each side of the diaphragm. The use of holes at right angles to the plane of diaphragm is also possible. The invention can also be applied to a moving coil transducer in which the diaphragm rim is clamped between two members: however, in this case the holes or grooves or slots are provided in both of those members. Claims

4. A transducer as claimed in claim 1 or 2, in which the or each said hole or tunnel has its axis substantially at right angles to the plane of the diaphragm.

5. A transducer as claimed in claim 1, in which the or each said hole or tunnel is a groove in the diaphragm support member which is closed or partly closed by the peripheral rim of the diaphragm.

6. A transducer as claimed in claim 1, in which the rim of the diaphragm is clamped between two members which together form said diaphragm support, the holes or slots being provided in both of those members.

7. An electro-acoustic transducer, substantially as described with reference to Figure 1 or Figure 2 of the accompanying drawings.

New claims filed on 6 December, 1983.

New or amended claims

8. An electro-acoustic transducer of the type having a movable diaphragm supported at its outer periphery by a stationary support member, wherein one or more holes or tunnels are provided in the support member, the or each said hole or tunnel having its axis substantially parallel to the plane of said outer periphery and coupling the air in front of the diaphragm with the air behind the diaphragm, and wherein the presence of said one or more holes or tunnels reduces the sensitivity of the transducer to low frequency noise without introducing low frequency distortion.

9. An electro-acoustic transducer of the moving coil type, which includes a diaphragm unit including the diaphragm and coil, and a magnet unit including a magnet and a plurality of polepieces so located as to define an annular air gap in which the coil is located, wherein the diaphragm is made in two parts one of which is a substantially circular central part whose outer periphery carries the coil, and the other of which is a substantially annular outer part attached to the coil-carrying part of the central part of the diaphragm, so that the coil is supported in the air gap by both parts of the diaphragm, wherein the outer part of the diaphragm is supported at its outer periphery by a stationary support member, wherein one or more holes or tunnels are provided in the support member, the or each said hole or tunnel coupling the air in front of the diaphragm with the air behind the diaphragm, and wherein the presence of said one or more holes or tunnels reduces the sensitivity of the transducer to low frequency noise without introducing low frequency distortion.