US2165123A - Contact microphone - Google Patents

Description

July 4, 1939.

. s. BALLANTINE CONTACT MICROPHONEA Filed Aug. 13, 1957 .SOKAQ k als o0 Patented July 4, 1939 UNITED STATES PATENT OFFICE Application August 13,

14 Claims.

'Ihis invention relates to improvements in contact microphones of the moving-armature type, as for example the throat microphone described in my copending application Serial No. 74,201, filed April 13, 1936 now Patent No.. 2,121,781.

In this type of microphone, the mechanical vibrations of the source are communicated to an armature and cause it to vibrate in relation to a fixed pole piece, thus varying the magnetic flux and generating an electrical voltage between the terminals of a suitable coil. The sensitivity of the device (defined as the voltage generated per unit velocity of the armature) depends upon a number of factors, for example: 1) number of turns on the coil, (2) strength of steady magnetization produced either by a permanent magnet or by D. C. current through the coil, (3) reluctances of the magnetic materials; and (4) length of the air-gap between armature and pole piece.

The pressure with which the microphone is applied to the source of vibrations will cause a steady deilection of the armature, and the Working air gap and sensitivity will depend upon this pressure. For many uses to Which a contact microphone may be put, as for example its use as a throat microphone, this increase in output with applied pressure is a serious disadvantage. In the case of a throat microphone, it is very dinicult to maintain initially applied or steady pressure at a constant value because of variations due to movements of the head, etc.

The term steady pressure, as employed in the specification and claims, indicates the pressure of engagement of the microphone and the source of mechanical vibrations when the source is at rest and, as just stated with reference to throat microphones, this steady pressure may vary during the use of the microphone.

In my experimental Work With the throat microphone, I have discovered another effect which makes this increase of microphone response with pressure doubly undesirable. This is the effect of compression of the tissues of the neck in increasing their ability to transmit the sound vibrations from the thyroid cartilage, or voice box. The output of the microphone will increase as the pressure of application increases due to this effect alone, even though the microphone sensitivity Were not affected by pressure per se. Hence, in the case oi the throat microphone there are tvvo effects that operate to cause undesirable variation in output with pressure.

Objects of this invention are to provide contact microphones of the moving armature type, and methods of operating such microphones, that 1937, Serial No. 158,992

are characterized by a reduction in the variation in output with applied pressure. An object is to provide throat microphones of the moving armature type in which the output decreases with applied pressure, thus compensating or tending to compensate for the increase in transmission efficiency of the tissues oi the neck as the microphone pressure is increased. More specically, an object is to provide throat microphones of the moving armature type in which the reluctance of the magnetic circuit and the coercive force of the permanent magnetization are so related as to produce saturation or incipient saturation in the magnetic circuit, thus precluding an increase in output with a decrease in the air gap.

These and other objects and advantages of the invention Will be apparent from the following specification when taken with the accompanying drawing in which:

Fig. l is a curve showing the variation in sensitivity With length of air gap in a typical contact microphone of the moving armature type;

Fig. 2 is a side elevation of the principal elements of a throat microphone embodying the invention;

Fig. 3 is a curve sheet showing the relation of magnetic force to magnetic flux in the iron parts of the magnetic circuit; and

Figs. 4 and 5 are curve sheets illustrating the relation between output and air gap for certain embodiments of the invention.

The relation between microphone output and length of air gap for a typical contact microphone of the moving armature type is illustrated by curve A of Fig. 1 The magnitude of the output is quite small, particularly in the case of the throat microphone, and the design practice developed in other branchesof the telephone receiver and microphone arts Would indicate that the gap should be made as short as possible. 'I'he output for an operating point corresponding to about 0.0015 inch is almost double the output that is obtained when the gap is increased to about 0.003 inch.

A simpliiied contact microphone construction conforming to this invention, as shown in Fig. 2, has a movable armature I forming a part of the magnetic circuit that includes the pole piece 2, connecting bar 3 and permanent magnet 4. A coil 5 of ine Wire is mounted on the pole piece 2 and has terminals 6 for connection to a transmission line. One end of the armature l is secured to the magnet 4 and the other end carries a button 'l for engagement With a source of Vibrations 8 that is, in the case of a throat microphone, the throat of the operator. Vibrations imparted to the armature I by the source 8 produce variations in the length of the air gap 9, thus varying the magnetic flux through the pole piece 2 and consequently inducing electrical voltages in the coil 5.

In accordance with this invention, the operating characteristics of the microphone .are such as to reduce or to eliminate the variation of output With changes in contact pressure. Means for predetermining or for adjusting the operating characteristics are illustrated in Fig. 2 as the notch IIJ in armature I for establishing a predetermined ux density at the air gap; the clamp bolt II for securing the pole 2 in desired adjustment on bar 3 to determine the length of the air gap; and the adjustable magnetic shunt I2 that is slidably mounted on bar 3. The manner in which one or more of these elements may be used to control the operating characteristics Will be clear from the following explanation.

The output voltage e of a system such as shown in Fig. 2, for a given velocity 1J of the armature, can be expressed as:

Where n is the number of turns on the coil, qb is the total magnetic flux and .1: is the displacement of the armature. For a given number of turns on the coil the sensitivity is proportional to the factor dqs/d, that is to the change in flux for unit displacement of the armature. Now if the air gap be reduced in length, as for example by a steady deflection of the armature produced by the pressure of contact against the throat, the reluctance of the air gap is reduced and more magnetic ux flows through the magnetic circuit. At the same time the change in reluctance (and also the change in iiux) for a given displacement of the armature increases, thus increasing the sensitivity, as shown in Fig. 1. This, as pointed out above, is the conventional method of operation of the movable armature type of microphone and is undesirable in the case of contact microphones.

According to this invention, the magnetic circuit is so designed that, With the normal air gap, it is in a state of incipient saturation; whereby the decrease in air gap reluctance Which results from a further decrease in the air gap isY automatically compensated or overcome by the increase in reluctance of the iron parts of the magnetic circuit due to the onset of saturation. The total reluctance thus tends to remain constant and the sensitivity likewise tends to remain constant. The Way this automatic compensation by saturation occurs may be illustrated graphically as follows.

Let us assume that the magnet 4 produces a certain steady magnetization force I-Io. Let H=f() represent the relation between ux qa and magnetic force H in the ferrous parts of the magnetic circuit. Then the variation of flux with air gap can be found graphically as shown in Fig. 3. The curve OA represents H :f(q or the characteristic of the iron parts of the magnetic circuit. Now H0:Ra+f( Where Ra is the reluctance of the air gap (zlength/area), hence qa is given by the intersection of the curve OA and the curve BC Which represents Hin-Raga As the length of the air gap varies, due to vibration of the armature, this line BC Will Vary between the extremes represented by the dotted lines BC, BC. The change in ilux will follow the small incremental ellipse a, b and the voltage output Will be proportional to Aqb. As the air gap is decreased by pressure against the throat or other source the angle e, Fig. 3, increases and the operating point on curve OA moves from point D into the region of saturation E as shown by the dashed lines B1', B1, B1. It will be seen that the slope of the (p, I-I curve OA of the magnetic circuit is now less than before due to saturation, and Aqa and the output voltage for a given armature displacement remains constant or decreases. We may place the operating point D above the point of inflection F at the upper knee of the curve, or just below it, so as to obtain substantially constant output as the air gap is further decreased, or may, by taking the operating point further into the region of saturation, actually obtain a decrease in output with decreasing air gap.

According to the conventional method of operation, the `air gap would be shortened as much as possible in order to secure maximum sensitivity, As typical operating point would be as shown in Fig. 1. I depart from this procedure and widen the air gap to a value such that deflection of the armature Will not cause the complete closure of the gap at the maximum pressure of application likely to be encountered. This entails a deliberate sacrice of sensitivity but is desirable in view of the advantages that may be obtained when the magnetic circuit is designed or adjusted to exhibit the desired saturation characteristics.

The desired magnetic saturation can be brought about in practice in several Ways: (l) by increasing the strength of the permanent magnet by the use of alloys having a high coercive force, for example, Alnico; (2) by decreasing the thickness of the armature, or decreasing the cross-section of the magnetic path at some other point, and (3) by the use of high-permeability materials e. g., nickel-iron alloys) in some part of the magnetic circuit, for example in the armature. The'second method is illustrated in Fig. 2, the desired decrease in section being produced by a constriction at groove I where saturation takes place.

If the design permits, the adjustment of the coercive force of the permanent magnet offers a very convenient method of obtaining a proper degree of saturation. An alternative method is to select a suitable alloy for the armature. In this connection, I have carried the invention into use by substituting a nickel-iron alloy such as Allegheny electric metal for soft iron or silicon steel as an armature material. For any given magnetization, the adjustability of pole piece 2 and magnetic shunt I2 permit control of the degree of saturation.

The types of output versus air gap characteristic which I obtain by this method are illustrated by curves I3, Irl of Figs. 4 and 5. In Fig. 4, the operating point is at A (normal air gap=0.0035 in.) and further reduction in the air gap by pressure against the source results in practically no change in sensitivity. When the output characteristic is of the type shown by curve I4 of Fig. 5, a reduction in air gap by pressure results in an actual decrease in output due to saturation of the armature. The latter characteristic is especially adapted to offset the increase of transmitting eiiiciency of the neck with increasing pressure When the device is to be used as a throat microphone. It will be seen that both characteristics represent considerable improvement over the conventional prior art characteristics shown in Fig. 1.

Curve I 3 of Fig. 4 is an actual experimental curve taken on an electromagnetic microphone of the type described in my copending application Serial No. 74,201. In this case, the armature was of silicon steel .014 thick and the permanent magnets were 55g long made of Alnico. 'I'he magnets were magnetized after assembly by the application of 4.6 amperes through a 100 turn coil. Curve I4 of Fig. 5 was obtained with an armature .014" thick of Allegheny electric metal (nickel-iron alloy) using Alnico magnets of the same dimensions.

A characteristic of the type of Fig. 4 has the further important practical advantage of reducing variation in output due to setting of the air gap during manufacture as it renders the setting of the initial air gap much less critical.

As an example of the benefit thus derived, I

may say that out of 1000 microphones incorporating this principle recently manufactured only 4 (0.4 per cent) were rejected because of varie-.tion in output.

According to the conventional method of operation, the air gap would be shortened as much as possible in order to secure maximum sensitivity. A typical operating point would be as shown in Fig. l. I depart from this procedure and Widen the air gap to a value such that deflection of the armature will not cause the complete closure of the gap at the maximum pressure of application likely to be encountered. This entails a deliberate sacrifice of sensitivity but is desirable in view of the advantages that may be obtained when the magnetic circuit is designed or adjusted to exhibit the desired saturation characteristics.

It is to be understood that the iield or normally constant magnetization in the magnetic circuit may be supplied either by a permanent magnet, by a steady current through the coil, or other means and that Where the adjustment of permanent magnetism is spoken of in the following claims it is contemplated that this adjustment can be carried out by adjusting the dimensions of a permanent magnet or its magnetization, or equivalently by adjusting the current through the coil.

I claim:

l. The method of maintaining the sensitivity of a Contact microphone substantially independent of changes in the length of the air gap in the magnetic circuit that result from variations in the steady pressure with which the microphone is applied to a source of mechanical vi'- brations, the magnetic circuit including a permanent magnet in series with the air and an armature that is vibrated by the source to alter the flux in the magnetic circuit, said method comprising adjusting the normal air gap and the coercive force of the permanent magnet to produce incipient saturation of the magnetic circuit thereby to prevent an increase in sensitivity when the microphone is pressed against the source of vibrations so strongly as to reduce the air gap below its normal value.

2. The method of maintaining the sensitivity of a contact microphone substantially independent of changes in the length of the air gap in the magnetic circuit that result from variations in the steady pressure with which the microphone is applied to a source of mechanical vibrations, the magnetic circuit including a permanent magnet in series with the air gap and an armature that is vibrated by the source to alter the flux in the magnetic circuit, said method comprising adjusting the normal air gap and the coercive force of the permanent magnet to produce saturation of the magnetic circuit thereby to prevent an increase in sensitivity When the microphone is pressed against the source of vibrations so strongly as to reduce the air gap below its normal value.

3. The method of maintaining the sensitivity of a contact microphone substantially independent of changes in the length of the air gap in the magnetic circuit that result from variations in the steady pressure With which the microphone is applied to a source of mechanical vibrations, the magnetic circuit including a permanent magnet in series with the air gap and an armature that is vibrated by the source to alter the flux in the magnetic circuit, said methd comprising magnetizing the permanent magnet to provide a coercive force at the normal air gap which places the operating point of the magnetic circuit above the point of inection of the magnetic flux-magnetic force curve of the magnetic circuit, thereby to prevent an increase in sensitivity when the microphone is pressed against the source of vibrations so strongly as to reduce the air gap below its normal value.

4. The method of maintaining the sensitivity of a contact microphone substantially independent of changes in the length of the air gap in the magnetic circuit that result from variations in the steady pressure with Which the microphone is applied to a source of mechanical vibrations, the magnetic circuit including a permanent magnet in series with the air gap and an armature that is vibrated by the source to alter the flux in the magnetic circuit, said method comprising magnetizing the permanent magnet to provide a coercive force that at the normal air gap places the operating point of the magnetic circuit at the upper level of the magnetic flux-force curve of the magnetic circuit, thereby to prevent an increase in sensitivity when the microphone is pressed against the source of vibrations so strongly as to reduce the air gap below its normal value.

5. The method of maintaining the sensitivity of a contact microphone substantially independent of changes in the length of the air gap in the magnetic circuit that result from variations in the steady pressure with which the microphone is applied to a source of mechanical vibrations, the magnetic circuit including a permanent magnet in series with the air gap and an armature that is vibrated by the source to alter the flux in the magnetic circuit, said method comprising adjusting the coercive force of the permanent magnet so that at the normal air gap the magnetic circuit is operated in the region cf saturation of the magnetic circuit, thereby to prevent an increase in sensitivity when the microphone is pressed against the source of vibrations so strongly as to reduce the air gap below its normal value.

6. In the operation of a throat microphone of the moving armature type comprising a magnetic circuit including an air gap, the method of reducing the effect on the output of the increase of transmission eiiiciency of the neck tissues with increasing pressure of application, which comprises adjusting the air gap to a normal value Which prevents complete closure of the air gap at the maximum pressure of application, and adjusting the coercive force of permanent magnetization to produce incipient saturation in the magnetic circuit, whereby a decrease in output is produced by further reduction in the air gap.

'7. In the operation of a throat microphone of the moving armature type comprising a magnetic circuit including an air gap, the method of reducing the effect on the output of the increase of transmission eiiciency oi the neck tissues with increasing pressure of application which comprises adjusting the air gap to a normal value which prevents complete closure of the air gap at the maximum pressure of application, and adjusting the coercive force of permanent magnetization to produce saturation in the magnetic circuit whereby a decrease in output is produced by further reduction in the air gap.

8. In the operation of a throat microphone of the moving armature type comprising a magnetic circuit including an air gap, the method of reducing the effect on the output of the increase of transmission eliciency of the neck tissues with increasing pressure of application which comprises adjusting the air gap to a normal value which prevents complete closure of the air gap at the maximum pressure of application, and adjusting the reluctance of the magnetic circuit to produce therein (incipient) saturation whereby a decrease in output is produced by further reduction in the air gap.

9. In the operation of a throat microphone of the moving armature type comprising a magnetic circuit including an air gap, the method of reducing the effect on the output of the increase of transmission efficiency of the neck tissues with increasing pressure of application which comprises adjusting the air gap to a normal value which prevents complete closure of the air gap at the maximum pressure of application, and adjusting the reluctance of the magnetic circuit to produce therein saturation whereby a decrease in output is produced by further reduction in the air gap.

10. In the operation of a throat microphone of the moving armature type including a magnetic circuit having an air gap, the method of reducing the effect on the output of the increase of transmission eciency of the neck tissues with increasing pressure of application, which method comprises increasing the air gap to a normal value which prevents complete closure of the air gap at the maximum pressure of application, relating the reluctance of the magnetic circuit and the coercive force of permanent magnetization to produce therein incipient saturation, whereby a decrease in output is produced by further reduction in the air gap.

11. In a throat microphone of the moving armature type comprising a magnetic circuit including an air gap, the method of reducing the effect on the output of the increase of transmission eiiciency of the neck tissues with increasing pressure of application which comprises adjusting the air gap to a normal value which prevents complete closure of the air gap at the maximum pressure of application, and adjusting the reluctance of the magnetic circuit and the coercive force of permanent magnetization to produce therein saturation, whereby a decrease in output is produced by further reduction in the air gap.

12. A contact microphone comprising, in combination, an armature adapted to be vibrated by vibrations of a source, a ferrous magnetic circuit including said armature and an air gap between said armature and a pole piece, the length of the air gap being aiected by the pressure under which said microphone is held in contact with said source of vibrations, a coil embracing said magnetic circuit and having a pair of terminals for connection to an external electrical circuit, and magnetizing means operative at the normal air gap to place the magnetic circuit in a state of incipient saturation, whereby the output remains substantially constant with variations in the air gap due to variations in the pressure under which said microphone engages said source of vibrations.

13. A contact microphone comprising, in combination, an armature adapted to be vibrated by vibrations of a source, a ferrous magnetic circuit including said armature and an air gap between said armature and a pole piece, the length of the air gap being affected by the pressure under which said microphone is held in Contact with said source of vibrations, a coil embracing said magnetic circuit and having a pair of terminals for connection to an external electrical circuit, and magnetizing means operative at the normal air gap to place the magnetic circuit in a state of saturation, whereby the output remains substantially constant with variations in the air gap due to variations in the pressure under which said microphone engages said source of vibrations.

14. A contact microphone comprising, in combination, an armature adapted to be vibrated by vibrations of a source, a ferrous magnetic circuit including said armature and an air gap between said armature and a pole piece, the length of thev air gap being affected by the pressure under which said microphone is held in contact with said source oi vibrations, a coil embracing said magnetic circuit and having a pair of terminals for connection to an external electrical circuit, and a permanent magnet of such strength that with the normal air gap the magnetic circuit is in a state of saturation, whereby the output decreases with variations in the air gap due to variations in the pressure under which said microphone engages said source of vibrations.

STUART BALLANTINE.

CERTIFICATE OE CORRECTION., Patent No. 2,165,125. July LL, 1959.,

' STUART BALLANTINE..

It is hereby certified that error eppears in the printed specification of the above numbered patent requiring correction as follows: Page L1, first colmrin, line 29, claim 8, for (incipient)" read incipient; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the 'Patent Office.

Signed and sealed this 8th day of August, A. D. 1959,

Leslie Frazer, (Seal) Acting Commissioner of Patents.

CERTIFICATE OE CORRECTION., Patent No.. 2,165,125. July in, 1959n STUART BALLANTINE n It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page L| first column, line 29, claim 8, for "(incipient)" read incipient; and that the said Letters Patent should be read with this' correctionv therein that the same may conform to the record of the case in the 'Patent Office,

Signed and sealed this 8th day of August, Aa 1).,'1959o Leslie Frazer, (Seal) Acting Commissioner of Patents.

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