US1834051A - Microphone - Google Patents

Description

L. DE FOREST Dec; 1, 1931.

MICROPHONE Filed Oct. 2, 1929 awuwntoz 1 ea De Ff'eof' @351 5 6 Game/1 s.

Patented Dec. 1, 1931 UNITED STATES PAETENT OFFICE LEE DE FOREST, OF NEW YORK, N. Y ASSIGNOR TO GENERAL TALKING PICTURES COR- PORATION, OF NEW YORK, N. Y.,

A CORPORATION OF DELAWARE mcnoPHoNE Application filed October 2, 1929; Serial No. 396,642.

This invention relates in general to microphone pick-up devices for translating sound waves into varying electrical currents.

One of the objects of the invention is to obtain a microphone devicewhich is at once small, free from carbon noises, sensitive and not subjectto the difliculties normally encountered in securing and maintaining perfect insulation between electrodes as is fre- 1o quently the case with microphones of the carbon or electrostatic types. Another object of this invention involves a microphone having an incandescent cathode and a movable anode responsive to sound to waves.

Another object of this invention is to provide a microphone of the above type in which the cathode operates either in a vacuum or a gaseous atmosphere either above or below atmospheric pressure.

A still further object of this invention is to provide a microphone which possesses a lower impedance than impedance of an electrostatic microphone.

Other objects of this invention involve features of construction, gases employed as well as pressures, and other characteristics,

i all as will be described in greater detail below. 9

This invention resides substantially in the combination, construction, arrangement and relative location of parts all as will be described in further detail below.

Referring to the drawings 0 Figure 1 is a longitudinal cross-sectional view through one form of microphone employing the principles of this invention;

Fig. 2 is an end elevational view of this device; a

Fig. 3 is a plan view of a modified form of cathode;

Fig. 4 is a side elevational view of the modified cathode with some parts. in cross-section; 7

Fig. 5 is another form of microphone operating at greater than atmospheric pressure;

Fig. 6 illustrates still another form operating below atmospheric pressure;

Fig. 7 illustrates still another form of the device operating below atmospheric pressure with a modified form of anode;

Fig. 8 is a cross-sectional view taken on the line 8-8 of Fig. 7.

As is well known the many forms of microphones now in use have various undesirable characteristics peculiar to each type. For example, the carbon microphone and the electrostatic microphone are sources of undesirable noises which reduce their efficiency of operation; the electrostatic t pe whose undesirable noises are in part proper insulation in their construction, and in the carbon microphone due to the presence of the high resistance carbon contacts. The microphone of this invention possesses a lower impedance than those generally used and, therefore, permits the use of a long cable be ue to im-' tween it and the device to which it is connected, which is not true of a microphone of electrostatic type, for instance, it does not operate well with a long cable connection such as is normally required, for example, in making talking motion pictures. These known .forms of microphones, particularly the electrostatic types, are seriously afi'ected by moisture conditions. The device of this invention is not so affected. The microphone on this invention is also particularly adapted for small constructions which is highly desirable in some uses.

For a better understandin of the inven tion and its advantages re erence will be made to the drawings. In general the microphone of this invention may be said to be of the two-electrode vacuum tube type. The form of a device shown in Figure 1 comprises a metal c linder 1 having a. chamzfered edge to which is welded the lass member 2 at the joint indicated at 2 to orm a hermetic seal. In accordance with resent welding practice the tube 1 is pre erably of copper. Byreducing the edge of the tube to make it thin the glass portion may be welded thereto in a manner now well known in the welding art. The glass portion 3 is provided with a re-entrant stem or tube 4.- which is pressed to gether at the point 5 while hot to hermetically seal the tube at the point where the wires enter the vessel. The glass portion of the container may be evacuated and sealed as at the points 6. The end of the tube 1 is closed by the metal cap 7 which is of thin material and forms a hermetic seal with the tube 1. Supported on stiff wires 9 is a cathode 8 adapted to be heated to incandescence and preferably coated with an electronically active oxide coating. The wires 9 are connected to the supporting wires 10 which are sealed in the brace 5 and to which are connected the lead wires 12. The cap 7 forms the anode of the device and may be made of thin copper, nickel, tantalum, duraluminum, or other suitable material pressed in the form of a cap. The wires 12 are connected across the variable resistance 13 to the current source 14 which is used to energize the cathode 8. The casing 1 is provided with a binding post 18 which is connected by a wire to one terminal of the primary 17. of a suitable transformer. The other terminal of the primary is connected through the current source 16 by wire 15 to one of the wires 12.. The secondary 19 of the transformer is connected by the wires 20 to any device to be actuated by the currents produced by the microphone, such as the input of an amplifier. The tube 4 is shown by this form of the device as extending beyond the brace 5 to help to support the cathode 8 and its supporting wires 9. The complete vessel formed by the tube 1, glass member 3 and cap 7 may be exhausted preferably to a high degree of-va'cuum and sealed off. The device will, of course, operate when exhausted even to a low degree. In certain cases it is pref.- erable to employ some gas pressure other than air such as that.provided byplacing a globule of mercury within the container and exhausting it to the pressure of mercury vapor. In addition to or in lieu of the mercury one of the noble gases such as argon,

krypton and zenon, either pure or in any desired mixture thereof may be employed. Although the mercury tends in time to become amalgamated with the copper tube the device still retains the qualities of a mercury'vapor device. When the pressure within the device isbelow atmospheric pressure the thin diaphragm or anode 7 tends to bend inwardly into a slightly concave form and puts the diaphragm under considerable tension. For good results the diaphragm is initially secured to the tube 1 under tension which tension is enhanced by the atmospheric pressurethereon. This tension in the diaphragm materially reduces the natural period of vibration. In one desirable form the period of vibration of the diaphragm is well above one six thousandths of a second. In preferable form the cathode 8 is placed close be hind the anode 7 at a distance of the order of one-sixteenth of an inch.

When sound waves are directed against the dlaphragm they impart their vibrations thereto to alternately move the diaphragm closer to and further from the cathode member thereby periodically altering the impedance of the device by andin accordance with the impinging sound waves. This action results in corresponding linear changes in the electronic or ionic currents passing between the cathode and ,theanode. These electronic shown at 8 in Figs. 3 and4. To prevent sagging of the spiral it may be supported on the disc 21 of insulating material such as mica. The mica disc may be supported from the brace 5 by means of the support wires 9 and additional support wire such as the wire 22. It is, of course, apparent that the cathode may be made in many forms without departing from this invention.

Animportant advantage of this construction of microphone is that it maybe made extremely small. Thus its overall dimensions may be of the order of one inch across and two to three inches in length. Such a small microphone maybe readily concealed and-is hence very well adapted for use in the making of talking motion pictures since it may be easily located close to the person speaking without it being observable in the picture when thrown on the screen.

In the forms of the device shown in Figs. 5 and 6 the diaphragm 7 is made of rubber or rubberized fabric instead of metal and hermetically sealed to the tube 1. The form shown in Fig. 5 is intended to operate at greater than atmospheric pressure and the tube 23 connected thereto to the valve 24 is employed to supply air or other gases alone or in combination at greater than atmospheric pressure to the device. In this case the diaphragm will convex outwardly. In the form of device shown in Fig. 6 the device is operated below atmospheric pressure and, as a result, the diaphragm will convex inwardly. Since a diaphragm of this type might permit leakage in time the tube 23 which contains valve 24 is provided to correct the pressure condition within the device from time to time as it becomes necessary. In the case of a diaphragm of this nature its inner surface must be metallized to provide a conducting surface which will, of course, be in contact with the tube 1 to which one connection is made. Instead of metallizing the inner surface of thediaphragm the construction shown in Figs. 7 and 8 may be employed. In this case the anode comprises a supporting metallic ring 25 for a mesh 28. The anode is supported by the wires 26 and 27 from the for the anode connection. In the operation of the forms of device shown in Figs. 5 to 8 inclusive, the vibration of the flexible diaphragm in accordance with impinging sound 5 Waves produces alternate compression and a 'rarefaction of the gas within the chamber regardless 'of its pressure relative to the atmosphere. These changes of pressure within the device control the amount of ionic current passing from. the cathode to the anode and hence the impedance of the device.

It will be apparent from the foregoing disclosure that I have embodied certain principles of construction and operation in a new form of microphone device which principles may be embodied ,in other physical forms without departing from the spirit and scope of this invention. I do not, therefore, desire to be strictly limited to the form of disclosure 20 given for purposes of illustration but rather to the spirit and scope of the appended claims.

What I seek to secure by United States Letters Patent is:

1. A microphone device of the type described, comprising a glass portion and a metal portion hermetically sealed together, a

conducting flexible portion hermetically sealed to the metal portion to form an anode and a cathode within said vessel supported adjacent the flexible portion, said vessel being exhausted.

2. A microphone device of the type described, comprising a glass portion and a metal portion hermetically sealed together, a

conducting flexible portion hermetically sealed to the metal portion to form an anode and a cathode within said vessel supported adjacent the flexible portion, said vessel con- 40 taining a mixture of gases. I

3. A microphone device of the type described, comprising a metal portion and a glass portion hermetically sealed together,

said glass portion having a reentrant stem, a

' flexible diaphragm hermetically sealed to the metal portion to form a closed vessel and a cathode supported on said reentrant stem adjacent said flexible diaphragm, said flexible diaphragm acting as a sound responsive anode with respect to said cathode.

4. A microphone device of the type d scribed, comprising a metal portion and a glass portion hermetically sealed together, said glass portion having a reentrant stem, 9. flexible diaphragm hermetically sealed to the metal portion to' form a closed vessel, a

cathode supported on said reentrant stem ad-" jacent said flexible diaphragm, said flexible diaphragm acting as a sound responsive anode with respect to said cathode,-current leads extending through said reentrant stem 's for said cathode, and; means connected to said leads for energizing said cathode.

5. A microphone device of the type described comprising an exhausted vessel having a portion thereof of flexible non-conducting material mounted under tension and having its surface exposed to the interior of the Vessel metallized, said diaphragm forming an anode, and a hot cathode electrode within said vessel. A

6. In a. microphone device of the type described the combination comprising an exhausted vessel having a portion-of its wall made up of a rubber diaphragm, said diaphra m being metallized on its surface expose to the interior of the vessel to provide an anode and a cathode within said vessel mounted adjacent the anode.

In testimony whereof I have hereunto set my hand on this 30th da of Sept., A. D. 1929.

IYEE on FOREST.

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