GB1604167A - Microphone units - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO MICROPHONE UNITS (71) We, A.R.D. TECHNICAL ASSISTANCE AND ENGINEERING SERVICES INTERNATIONAL AN STALT, of P.O. Box 34 613, 9490 Vaduz, Liechtenstein, a body corporate organised under the laws of Liechtenstein, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a microphone unit, in particular, a microphone unit arranged so as to operate with substantial freedom from echoes and noises, such as ambient and field noises, which is an improvement and modification of the microphone unit described and claimed in British Patent Specification 1487847.

Except in relatively unusual circumstances, a microphone unit will be exposed in use not only to incoming sound vibrations which it is desired to convert to an electrical output signal, but also to sound and/or mechanical vibrations, as from the engine of a vehicle in which the unit is mounted, which should provide no part of this output.

In some instances for example where a microphone unit is used in a crowd or in proximity to machinery, substantially complete elimination of unwanted sound vibrations is essential if the unit is to provide an output which can be understood when converted to sound through a loudspeaker.

The invention is accordingly concerned with the provision of a microphone unit including features whereby the effects on the electrical output of unwanted incoming sound - noise and echo - are largely if not completely eliminated.

The invention provides a microphone unit comprising a housing mounting a pair of substantially identical electro-acoustic transducers each having a sound-responsive element, the housing extending between the elements and also providing two oppositely extending ducts of which one extends from one of the sound-responsive elements to a main sound inlet aperture and the other extends from the other sound-responsive element and is closed at its end remote therefrom, and further sound inlet apertures in the housing providing acoustic communication from outside the housing to each side of each sound-responsive element.

The housing is preferably tubular, with the sound responsive elements extending transversely thereof.

The other sound inlet apertures between the sound-responsive elements preferably extend through the tubular housing wall on each side of each transducer, substantially transverse to the housing axis. The elements may be parallel, whilst the transducers can be physically separated by a baffle.

The unwanted sound vibrations reach the sound-responsive element of each transducer directly, through air, and also by microphony, that is, by way of the structure of the microphone and the housing and other mounting of the transducer or transducers.

The effect of airborne noises on a microphone is reduced by arranging that the two sides of its diaphragm are exposed substantially equally to such noises, so that the resultant of such noises in the movement of the diaphragm and thus in the signal output is substantially zero.

The effect of unwanted noises reaching the transducers through the associated supporting and mounting structure can be dealt with by making the structure of a large number of parts of three or more different materials and preferably arranging that a part of one material is not adjacent a part of the same material. The structure is thus rendered substantially acoustically nonconductive or "dead". Alternatively or in addition, two identical microphones arranged as described can be electrically connected so that the combined output is substantially free of sound components due to noise. When used in association with one or more loudspeakers, as in a loudspeaking telephone or public address system, the outputs of the two microphones can instead be analysed if required to separate out a substantially pure noise or unwanted component for cancellation purposes as described in British Patent 1 458 663. The invention then provides combined acoustic and electronic noise cancellation techniques to deal with both direct and indirect, airborne noise and the mechanical vibration channel by which random external noises and echo can impose on the output signal obtained from a microphone.

The invention will be more readily understood from the following illustrative description and accompanying drawing of an embodiment in accordance therewith. In the drawing: Figure I is a schematic sectional side view of a microphone unit; Figure 2 is a partial schematic sectional view of a housing for use in the unit of Figure 1, and Figure 3 is a like view of another form of housing for use in the unit.

Referring to Figure 1, there is illustrated therein a microphone unit 10 comprising a tubular housing 12 open at one end 13 and closed at its other end 15. The housing 12 contains two electro-acoustic transducers 14, 16 for use as microphones. The transducers 14, 16 can be of the moving coil or any other suitable kind. The transducer 14 is shown, by way of example, as being a moving coil microphone having a substantially planar diaphragm 18 whilst the transducer 16 is shown as a microphone of the same general kind but having a conventional conical diaphgram 20. It will be understood however that in practice the two microphones employed in any microphone unit embodying the invention will be substantially identical for reasons of symmetry. The diaphragms of the electro-acoustic transducers extend at right angles to the axis of the housing 12.

The housing which may be of circular or other desired cross-section has two annular partitions 22 on which the tranducers are mounted, and an internal baffle 24 which physically separates the two transducers.

Alternatively, the baffle 24 may be provided with one or more apertures 30 or the baffle 24 may even be omitted. To ensure access of random sound to the inner sides of the transducer diaphrams, one or more apertures 26 are provided in the housing wall and, if desired, in partitions 22 at 28. If desired, means may be provided to permit selective adjustment of the effective area of the various apertures, such means being preferably readily operable from outside the housing.

The microphone unit is intended to be used with the open end 13 thereof directed to the user's mouth; and may be additionally shaped or provided with a handle to facilitate this. The nearer microphone thus has one side directly exposed to the wanted sound signals. To minimise exposure of the other microphone to this wanted sound, which may enter the housing 12 through the apertures 26, one or more external baffles 32 can be provided on the housing. By this construction, it is seen that wanted sound, e.g. sound from the speaker holding the unit 10, is allowed to reach the transducers 14 and 16 effectively only via the main sound inlet aperture provided by the open end 13 of the housing 12. If the baffle 24 is provided, and if it does not have apertures, such as apertures 30, therein, the wanted sound wil effectively impinge on the transducer 14 only. Unwanted sound, however, will enter the housing 12 through the apertures 26, which are located on both sides of both transducers 14 and 16, so as to provide substantially no net effect on either transducer. Very good noise cancellation is thereby achievable with the unit 10.

Additionally, the electrical outputs of the transducers 14 and 16 may advantageously be connected together in subtractive relationship, thereby enhancing the noisecancelling properties of the unit 10. To this end, the outputs of the coils of the transducers 14, 16 are connected substantially, in out of phase relationship, through potentiometers 34, so that the output of each microphone can be selectively adjusted. The output of the microphone unit is obtained from terminals 36 and will be understood to be the differences between the microphone outputs as taken from the potentiometers 34.

The ends of the housing 12 extend outwardly beyond the transducers and, if required, may provide loading chambers for the transducers, which may but need not be identical.

Sounds incident upon the microphone unit 10 can be resolved into sounds parallel to the planes of the diaphragms 18, 20, which sounds can be regarded as noise or unwanted sounds, and sound perpendicular to the diaphragm 18 which can be regarded as wanted sounds. The former sounds will bring about impulses on the two diaphragms which are identical in all respects, the distance between them being comparable to the wavelength of the sounds. It has been found in practice that the distance should not be greater than 1/4 of the wavelength of the frequency concerned or 1/4 of the wavelength of the highest frequency to be handled by the unit. The forces produced are then substantially equal. For best results of course the distance should be kept smaller than this.

Incident sounds moving parallel to the diaphragms then produce a movement of each diaphragm which is theoretically zero and which is normally negligibly small. The electrical signals resulting from any such negligibly small movement are added out of phase, leading to substantially complete cancellation of any electrical output due to them.

Sounds received through the open housing end 13, resolved into the direction perpendicular to the diaphragms, apply their maximum force to the diaphragm 18 first encountered. If the baffle 24 is provided, the diaphragm 20 of course remains unaffected by this sound and the output from the diaphragm 18, receiving this wanted signal, is substantially unchanged by the sound travelling parallel to the diaphragm. If there is no baffle between the two diaphragms, the diaphragm 20, which does not directly receive the incoming wanted sound, moves in sympathy with the diaphragm 18 because of the vibrations of the diaphragm 18 conveyed through the air between the two diaphragms.

In use, the microphone unit 10 will be effectively echo-cancelling. Desired sound, for example, from a speaker holding the unit so that he directly faces the transducer 14 will be fully reproduced in the electrical output. The potentiometers 34 permit adjustment of the unit to compensate for departures thereof from exact symmetry, electrical or structural, about the central transverse plane of the housing 12.

This form of unit will be seen to be free of focussing, sound guidance, or reflecting means such as tunnels or baffles acting between the exposed diaphragm sides and the extension of the unit.

The structure of the housing, the nature and dimensions of the various parts of the unit theoretically do not affect the performance of the unit, but are preferably chosen so as to assist as much as possible the elimination of the effects of noise and echoes. Thus the housing is preferably built up in a manner similar to that illustrated in Figure 2 which shows a tubular housing made from a plurality of rings or like inner and outer diameter arranged in co-axial alignment to form a tube. The rings are made of different materials A, B, C each having different sound conductive properties. Thus one ring may be formed of metal, the next made of glass fibre and the third of rubber or a hard or soft plastics material. Sound transmitted through any one ring to the next is very substantially attenuated at the interface and the whole structure is effectively acoustically "dead" or non-microphonous. Instead of repeating the sequence of, say, metal, glass fibre and plastics rings, it is preferred instead to use for the next three rings the same three materials but in a different order, again as shown in Figure 2. The next three rings can be arranged in a further different border. In this way each ring has on either side of it a ring of different material. Obviously this aspect of the invention is not confined to the use of three materials only, to the materials mentioned above, or to any particular sequence in which the rings are arranged.

It will be appreciated that a tubular microphone housing built up as described will have almost infinite resistance to sound conduction in the axial direction. Sound conductivity within each ring radially of the housing will however be no more reduced than if the housing were constructed of an integral piece of the material of the ring. In order to obtain the required high sound attenuation characteristics in the radial direction also, the housing can be formed not simply of axially adjacent single rings but of axially adjacent concentric rings, preferably at least three in number and again of different materials, as shown in Figure can 3.

The invention can of course be applied to hearing aid equipment to eliminate instability and inadequate operation due to acoustic feed-back which takes place in such equipment mainly through microphony and mechanical vibrations.

The invention can of course be embodied in various ways other than as specifically described without departing from the scope thereof.

WHAT WE CLAIM IS: 1. A microphone unit comprising a housing mounting a pair of substantially identical electro-acoustic transducers each having a sound-responsive element, the housing extending between the elements and also providing two oppositely extending ducts of which one extends from one of the sound-responsive elements to a main sound inlet aperture and the other extends from the other sound-responsive element and is closed at its end remote therefrom, and further sound inlet apertures in the housing providing acoustic communication from outside the housing to each side of each sound-responsive element.

2. A microphone unit as claimed in claim 1, in which the housing is tubular and the sound-responsive elements extend transversely of the housing.

3. A microphone unit as claimed in claim 1 or 2, in which the sound-responsive elements have a common axis, the unit being substantially symmetrical about the axis.

4. A microphone unit as claimed in any

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

Claims (19)

**WARNING** start of CLMS field may overlap end of DESC **. are then substantially equal. For best results of course the distance should be kept smaller than this. Incident sounds moving parallel to the diaphragms then produce a movement of each diaphragm which is theoretically zero and which is normally negligibly small. The electrical signals resulting from any such negligibly small movement are added out of phase, leading to substantially complete cancellation of any electrical output due to them. Sounds received through the open housing end 13, resolved into the direction perpendicular to the diaphragms, apply their maximum force to the diaphragm 18 first encountered. If the baffle 24 is provided, the diaphragm 20 of course remains unaffected by this sound and the output from the diaphragm 18, receiving this wanted signal, is substantially unchanged by the sound travelling parallel to the diaphragm. If there is no baffle between the two diaphragms, the diaphragm 20, which does not directly receive the incoming wanted sound, moves in sympathy with the diaphragm 18 because of the vibrations of the diaphragm 18 conveyed through the air between the two diaphragms. In use, the microphone unit 10 will be effectively echo-cancelling. Desired sound, for example, from a speaker holding the unit so that he directly faces the transducer 14 will be fully reproduced in the electrical output. The potentiometers 34 permit adjustment of the unit to compensate for departures thereof from exact symmetry, electrical or structural, about the central transverse plane of the housing 12. This form of unit will be seen to be free of focussing, sound guidance, or reflecting means such as tunnels or baffles acting between the exposed diaphragm sides and the extension of the unit. The structure of the housing, the nature and dimensions of the various parts of the unit theoretically do not affect the performance of the unit, but are preferably chosen so as to assist as much as possible the elimination of the effects of noise and echoes. Thus the housing is preferably built up in a manner similar to that illustrated in Figure 2 which shows a tubular housing made from a plurality of rings or like inner and outer diameter arranged in co-axial alignment to form a tube. The rings are made of different materials A, B, C each having different sound conductive properties. Thus one ring may be formed of metal, the next made of glass fibre and the third of rubber or a hard or soft plastics material. Sound transmitted through any one ring to the next is very substantially attenuated at the interface and the whole structure is effectively acoustically "dead" or non-microphonous. Instead of repeating the sequence of, say, metal, glass fibre and plastics rings, it is preferred instead to use for the next three rings the same three materials but in a different order, again as shown in Figure 2. The next three rings can be arranged in a further different border. In this way each ring has on either side of it a ring of different material. Obviously this aspect of the invention is not confined to the use of three materials only, to the materials mentioned above, or to any particular sequence in which the rings are arranged. It will be appreciated that a tubular microphone housing built up as described will have almost infinite resistance to sound conduction in the axial direction. Sound conductivity within each ring radially of the housing will however be no more reduced than if the housing were constructed of an integral piece of the material of the ring. In order to obtain the required high sound attenuation characteristics in the radial direction also, the housing can be formed not simply of axially adjacent single rings but of axially adjacent concentric rings, preferably at least three in number and again of different materials, as shown in Figure can 3. The invention can of course be applied to hearing aid equipment to eliminate instability and inadequate operation due to acoustic feed-back which takes place in such equipment mainly through microphony and mechanical vibrations. The invention can of course be embodied in various ways other than as specifically described without departing from the scope thereof. WHAT WE CLAIM IS:

1. A microphone unit comprising a housing mounting a pair of substantially identical electro-acoustic transducers each having a sound-responsive element, the housing extending between the elements and also providing two oppositely extending ducts of which one extends from one of the sound-responsive elements to a main sound inlet aperture and the other extends from the other sound-responsive element and is closed at its end remote therefrom, and further sound inlet apertures in the housing providing acoustic communication from outside the housing to each side of each sound-responsive element.

2. A microphone unit as claimed in claim 1, in which the housing is tubular and the sound-responsive elements extend transversely of the housing.

3. A microphone unit as claimed in claim 1 or 2, in which the sound-responsive elements have a common axis, the unit being substantially symmetrical about the axis.

4. A microphone unit as claimed in any

preceding claim, in which the soundresponsive elements are parallel.

5. A microphone unit as claimed in any preceding claim, in which each transducer is mounted on an annular partition within the housing.

6. A microphone unit as claimed in claim 5, in which each partition has one or more apertures providing acoustic communication between both sides of the respective transducer.

7. A microphone unit as claimed in any preceding claim, in which the apertures are selectively adjustable.

8. A microphone unit as claimed in claim 7, in which the apertures are selectively adjustable from outside the housing.

9. A microphone unit according to any preceiding claim, in which the further sound inlet apertures between the soundresponsive elements extend through the housing substantially transversely thereof.

10. A microphone unit as claimed in any preceding claim, in which the soundresponsive elements are spaced apart by approximately one quarter of the shortest wavelength of a desired sound input.

11. A microphone unit as claimed in any preceding claim, in which the housing has a baffle physically separating the transduers from one another.

12. A microphone unit as claimed in claim 11, in which the baffle has one or more apertures providing acoustic communication between the transducers.

13. A microphone unit as claimed in any preceding claim having means connecting the electrical outputs of the transducers in subtractive relation.

14. A microphone unit as claimed in claim 13 having variable resistance means between the transducers and output terminals of the unit for selective adjustment of the contributions of the transducer outputs of the output of the unit.

15. A microphone unit as claimed in any preceding claim, in which each soundresponsive element is planar or generally planar.

16. A microphone unit as claimed in any one of claims 1 to 14, in which each sound responsive element is conical.

17. A microphone unit as claimed in any preceding claim, in which the housing comprises a plurality of housing elements each of one of three or more different materials, no element having a surface thereof in abutment with an element of like material.

18. A microphone unit as claimed in any preceding claim having one or more external baffles on the housing.

19. A microphone unit substantially as hereinbefore described with reference to the accompanying drawing.