US2400281A - Electromechanical signal translating apparatus - Google Patents

Description

May 14, 19

L.. J. AAhlpnalasolw 2,406281 ELECTRO-MECHANICAL SIGNAL TRANSLATING APPARATUS v 2 Sheets-Sheet 1 Filed Oct. 31, 1940 :Summon lie-:Zlzirsam May 14, 1946. l.. J. ANDERSON ELECTRO-MECHANICAL SIGNAL TRANSLATING APPARATUS Filed Oct. 51, 1940 2 Sheets-Sheet 2 Mar/v.; YJ, C

Snnentor (Ittorneg Patented May 14, 1946 ELECTROMECHANICAL SIGNAL TRANSLAT- ING APPARATUS Leslie J. Anderson, Haddonfield, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application October 31, 1940, Serial No. 363,632

vs Claims.

invention to provide an improved signal translating device of this type which is particularly useful as a microphone for picking up and translating speech with good efliciency.

Another object of my present invention is to provide an improved signal translating device of the type set forth in which the mass of the moving parts is reduced to a minimum.

A further object of my present invention is to provide an improved signal translating device as aforesaid which, when used as a microphone, will not be appreciably subject to the disturbing effects of wind and low frequency noises which may be present.

It is also an object of my present invention to provide an improved reversible signal translating device as described above which is relatively simple in'construction and highly efficient in use.

In accordance with this invention, I employ a sound powered telephone unit and couple to the diaphragm thereof an acoustic network so proportioned that a substantially flat frequency response is obtained over the desired range. The diaphragm is coupled through a, coupling of small mass to a suitable reversible electro-mechanical unit, such as a magnetic motor or generator, as the case may bel the diaphragm being mounted in a casing having a front plate and a back plate and being spaced from said plates whereby an air space is provided both in front of and behind the diaphragm. Both of the aforementioned plates are provided with one or more openings which constitute acoustic paths between said air spaces and the exterior of said casing, the openings being preferably covered by silk or other suitable material constituting an acoustical resistance, and one or more of the openings in the back plate behind the diaphragm may be covered by an enclosure providing a, small chamber. Coupled to the back plate of the diaphragm casing is a housing which eneloses the reversible unit and the aforementioned enclosure, the openings in said back plate affording communication between the air space behind the diaphragm and the interior of said housing. A tubular member extending into the housing from one of the walls thereof affords communication between the air in the housing and the atmosphere. lFor best results, I have found that the following general constants are desirable:

(a) The length and diameter of the tube which connects the atmosphere with the interior of the housing is preferably so chosen that the mass of air therein resonates with the compliance of the air in the housing at the lowest frequency at which transmission is desired.

(b) The resonance of the mass of the complete moving system consisting of the diaphragm, the armature of the electro-magnetic unit, and the coupling member therebetween with the compliance of this system is placed as high in frequency as will allow the desired low frequency response.

(c) The compliance. of the volume of air 'in the space behind the diaphragm, that is, between the back surface of the diaphragm and the back plate of its casing, is made small enough so that, in combination with the mass and compliance of the moving system, resonance will be obtained at the highest frequency desired.

(d) The compliance of the resonator volume, or the volume of air in the small chamber or chambers behind one or more of the openings in the back plate of the diaphragm casing is preferably so chosen that, in combination with the compliance of the air in the space between the back ofthe diaphragm and the back plate of its cas- Vlili Ain the front wall of the diaphragm casing and the compliance of the volume of air in the space between this front wall and the front surface of the diaphragm are then selected to resonate at the high frequency end, This has the effect of removing the excessive peak otherwise obtained.

(g) The values of the acoustical resistance behind the one or more openings in the front plate of the diaphragm casing, the acoustical resistance behind the aforementioned tubular member, the acoustical resistance behind the one or more open- .ings in the back plate of the diaphragm casing, and the acoustical resistance behind the chamber covered opening or openings in the back plate of the diaphragm casing are then all adjusted or selected to obtain the desired frequency response, which is preferably assumed to be as at as possible.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description of one or more embodiments thereof, when read in connection with the accompanying drawings, in which Figure 1 shows a central sectional view of one form of my invention,

Figure 2 is a sectional view taken on the line II-II of Figure 1,

Figure 3 is an enlarged, detail, sectional view of a portion of the coupling member between the diaphragm and the armature,

Figure 4 is a fragmentary sectional view taken on the line IV--IV of Fig. 2,

Figure 5 is an electrical wiring diagram showing the electrical analogue equivalent to the acoustical system of the structure shown in Figures 1 and 2, and

Figures 6 to 9, inclusive, are response curves showing how the output characteristics vary by proper selection of the values of mass, compliance and resistance as discussed above.

Referring more particularly to the drawings, wherein similar reference characters designate corresponding parts throughout, there is shown, in Fig, 1, a, diaphragm I within a casing 3 having a front plate 5 and a back plate 1, the diaphragm being suitably clamped at its periphery within the casing 3 in spaced relation to the walls 5 and 1 thereof whereby to provide a front air space 9 between the front casing wall 5 and the front surface of the diaphragm I and a back space II between the back wall 1 and the rear surface of the diaphragm I. The front wall 5 of the diaphragm casing is provided with one or more openings I 3 for establishing communication between the air space 9 and the atmosphere, and the back wall 1 is provided with a plurality of openings I5 which are disposed symmetrically about the diaphragm axis and serve to establish communication between the back air space II and the ex` terior of the casing 3. In the particular modification disclosed, only one opening I3 is shown in the front plate 5 and the three openings I5 are shown in the back plate 1, but it will be obvious that any desired number of openings may be employed in either of the plates 5 or 1 for establishing communication between the interior and the exterior of the casing 3 on both sides of the diaphragm I. If desired, there may also be provided a perforated disk or grid I1 behind the front plate 5 to protect the diaphragm I from accidental injury. The openings I3 and I5 are all covered by a layer of foraminated material I9, such as silk, constituting an acoustic resistance to the passage of sound waves through the openings I3 and I5, the silk layer associated with the opening I3' being shown immediately behind the grid I1 in the particular modification shown in the drawings.

The back plate 1 is formed with an internally threaded, hollow extension 23 with which another opening 25 through the back plate 1 communicates, a plug 21 being threaded into the extenslor- 23 to form a cavity or chamber 29 behind the opening 25, and a layer of silk I9 being also pi--Iced behind the opening 25 similar to the bpenings I5. A coil spring 3l in the chamber 29 serves `to hold the associated silk layer I9 against the opening 25 when the plug 21 is screwed into position.

The casing 3 is preferably molded of a suitable plastic material and has secured to its back plate 1 a reversible electro-mechanical unit 33 for cooperation with the diaphragm i. The unit 33 includes an operating coil 35 between a pair of pole pieces 31 to which flux is supplied by a pair of magnets 38, one of the pole pieces being preferably molded integrally with the back. plate 1, and the other being molded integrally with a supporting member 39 which may be of the same material as the casing 3, the pole faces ci the polepieces 31 being suitably ground after the molding operation. Thereafter, the supporting :member 39 is mounted on the back plate 1 by means of a plurality of screws or the like 4I, a pair of washers 43' being employed on either side of an armature 45 to accurately center 'the armature in the air-gaps between the pole pieces 31.

The armature 45 constitutes the active element of the electro-mechanical device 33 and is coupled to the diaphragm I by a pair of light, telescopic members 41 and 49, the formezI of which is provided wthan enlarged head 41u and the latter of which has an enlarged head 49a, as clearly shown in Fig. 3. The diaphragm i is provided with an opening Ia of a diameter sub stantially equal to the internal diameter of the member 41J and the member 49 fits through the opening la and is slidably received within the tubular member 41, the area of the diaphragm immediately surrounding the opening Ict being clamped between the heads 41a and 49a. Frei erabiy, the pin or central member 49 is coated with a suitable cement prior to insertion into the tube 41, and its head may be heated with a soldering iron to cause the cement to bond the members 41 and 49 firmly to each other and to the diaphragm I. The coupling tube 41 and thc pin 49 are preferably made of aluminum, for lightness, and also preferably pass through a rubber seal '50 on the back plate 1,

By forming the reversible unit 33 and the coupling member 41-49 between its armature 45 and the diaphragm I in the manner above de scribed, there is provided, first, a vibiatory system constituted by the armature 45, the coupling member 41-49, and the diaphragm I which is of minimum mass, a condition essential to the high efficiency of my improved translating device. Furthermore, mounting the pole pieces on the back plate 1 and the supporting member 39 and thereafter assembling them in the manner heretofore described after grinding the two pole faces of each of the pole pieces 31 along a common plane not only insures an air gap of desired width, but avoids setting up strains in the armature which, 'in conventional telephone units, causes an initial set to take place in the armature with a tendency to throw it initially oil center and thus introduce distortion. Also, the use of a synthetic resin or other plastic material for the case 3 and the supporting member 39 in the manner described eliminates the proximity of metal parts near the varying flux, and thus eddy current losses are eliminated, while grinding of the pole piece faces after they have been molded in place assures proper lining up of the pole faces, so that, with the aid of the washers 43, accurate adjustment of the air gaps can be maintained.

Secured to an annular nange 3a on the back plate 1, as by means of a clamping ring I, is a housing 53 in which the translating device 33 and the extension 23 andv its associated plug 21 are all enclosed. The housing 53 is preferably provided with an opening 55 in the wall thereof, a tubular member'51 being fitted into the opening 55 and extending radially inwardly of the housing l53. As in the case of the openings I3, I5 and 25, a layer of silk or the like I9 may be placed over the inner end of the tubular member 51.

Figure 5 shows an electrical wiring diagram corresponding to the'acoustical system heretofore described. If, in the acoustical system, it is assumed that Cris the compliance of the air in the housing 53,

Cz is the compliance of the air in the back space C3 is the compliance vof the air in the resonator chamber 29y C4 is the compliance of the air in the space 9,

C5 is the compliance of the whole moving or vibratile system consisting of the diaphragm I, the armature 45 and the couplingtherebetween I1-49,

M1 is the mass of air in the tube 51,

M2 is the mass of air in the openings I5,

M3 is the mass of air in the opening 25,

M4 is the mass of air in the opening I3,

M5 is the mass of the entire vibratile system,

R1 is the' acoustic resistance provided by the silk screen at the end of the tube I9, v

R2 is the acoustic resistance offered by the silk screen behind the openings I5,

R3 is the acoustic resistance offered by the silk screen behind the opening 25, and

R4 is the acoustic resistance offered by the screen behind the opening I3 (or the grid I'I) as the case may be,

then the relation of the system will be as shown in the electrical analogue of Fig. 5.

It will be obvious,` from an inspection of Fig. 5, that the response characteristic-of the system can be altered by changing the'vaiues of the various inductances, resistances, or capacitances. I have found that, for a speech microphone, the following general constants are desirable:

(a) By proper selection of the length and internal cross-section, or diameter, of the tube 51, the mass of air therein, as represented by the inductance M1 in Fig. 5, can be made to resonate with .the compliance C1 of the air inthe housing 53 at the lowest frequency at which transmission is desired. In one particular microphone, this frequency is approximately 200 cycles. It has been found that this cut-ofi frequency detracts very little from the intelligibility of speech and greatly reduces the effect of wind and low frequency noise which may be present. 1

(b) The resonance of the mass M5 of the whole vibratile system with the capacitance C5 of this system should be made as high in frequency as will allow the desired low frequency response.

(c) The compliance C2 of the air in the space II is made so small that, in combination with the mass and compliance of the vibratile system, M5 and C5, respectively, resonance will be obtained at the highest frequency which it is 'desired to transmit.

(d) The value of the capacitance C3 of the air in the resonator chamber 29 is preferably so chosen that, in combination with the compliance C2 of the air in the space II, the compliance C5 of the vibratile system, and the mass zol M5 of the vibratile system, resonance will occur at a frequency which is about midway between the highest and lowest frequencies which it is desired to transmit by the unit. Y

(e) The mass M3 of air in the opening 25 and the compliance C3 of the air in the resonator v chamber 29 are also made to resonate at the highest frequency which it is desired to transmit.

(f) The mass M4 of air in the opening I3 and the compliance of the air in the space 9 are then made to resonate at the high frequency end to remove the excessive peak which would otherwise be attained.

(g) The values of the acoustic resistances R1,

R2, Rs and R4 are then adjusted to obtain the desired frequency response, which, in the particular case illustrated, is assumed to be as flat as possible.

Fig. 6 shows a response curve for the system without the resistances R1, R2 and R3, the various peaks at the frequencies f1, f2, f3 and f4 resulting from the various portions of the system designated by the corresponding masses, compliances and resistances appended to the individual peaks. By adding the resistances R1, R2 and R3 of appropriate value, the response curve of Fig. 7 will result. It will be noted that the output of the system is then represented by a much flatter response curve throughout the range up to frequenoy f3, the peak at frequency f4 still remaining. However, the peak at the frequency f4 can be reduced considerably (as in Fig. 8) by properly selecting the values of M4 and C4, that is, by making the opening I3 and the air space 3 of proper dimensions, and the response curve may-be made substantially flat, nally, by proper selection of vthe values of M2 and Rz to provide a response such as shown in Fig. 9.

In practice, I have found that it is very desirable to have the openings Iii formed in the back plate 'I behind the central portion of the diaphragm i, which executes a nearly piston-like motion. Furthermore, asv pointed out heretofore, the openings iii should preferably be uniformly and symmetrically spaced around 'the Vcenter of the diaphragm. Ir the openings I5 are not placed symmetrically with respect to the center of the diaphragm, or if they are formed-in the plate 1 behind a region adjacent the periphery of the diaphragm, it has been found that the diaphragm has a tendency to rock which, in turn, produces rather erratic response curves. For the foregoing reasons, therefore, it is essential that the openings i5 be properly located.

Although I have shown and described but a y single embodiment of my invention, it will be relation to each of said walls whereby to provide front and back air spaces, respectively, in front of and behind said diaphragm, signal translating means exterior to saidcasing and including an active element, means coupling said element to said diaphragm for movement in unison, and a housing coupled to said back casing Wall and enclosing said translating means, said front casing wall having at least one opening extending therethrough and aiording communication between said front space and the atmosphere, said back casing Wall having at least one opening extending therethrough and affording communication between said back space and the interior of said housing, and said housing having at least one opening extending through a Wall thereof and affording communication between the interior of said housing and the atmosphere.

2. The invention set forth in claim A1 characterized in that each of said openings is covered by a foraminous material constituting an acoustic resistance to the passage of sound waves therethrough.

3. The invention set forth in claim 1 characterized by the addition of an open ended tubular member extending inwardly of said housing through the opening therein whereby communication between the interior of said housing and the atmosphere is through said tubular member.

4; The invention set forth in claim 1 characterized by the addition of an open ended tubular member extending inwardly of said housing through the opening therein whereby communication between the interior of said housing and the atmosphere is through said tubular member, and characterized further in that said tubular member is of such length and internal crosssectional area that the mass of air therein resonates with the compliance of the air in said housing at substantially a predetermined low frequency such that said low frequency will be substantially the lowest frequency that said apparatus will translate.

5. The invention set forth in claim l characterized by the addition of an open ended tubular member extending inwardly of said housing through the opening therein whereby communication between the interior of said housing and the 4atmosphere is through said tubular member, and characterized further in that the openings in both said casing walls and the inner end oi said tubular member are all covered by a foraminous material constituting an acoustical resistance to the passage of sound waves therethrough.

6. In signal translating apparatus, the combination of a casing having a front wall and a back `least one opening extending therethrough and aiording communication between said iront space and the atmosphere, said back casing wall having at least two openings extending therethrough, at least one of said last named openings affording communication between said back space and the exterior of said casing, means on said back wall behind the other of said openings therein providing a closed chamber therebehind, said last named opening affording communication between said chamber and said back space, and a housing coupled to said back plate and enclosing said translating means and said chamber providing means, said housing also having an opening extending through a wall thereof and ailording communication between the interior oi said housing and the atmosphere.

'i'. The invention set forth in claim 6 charactcrized by the addition of a tubular member extending through said housing'wall opening into the interior of said housing whereby communication between the interior of Said housing and the atmosphere is through said tubular member.

Ei, The invention set forth in claim f5 characterized by the addition of a tubular member extending through said housing wall opening into the interior oi said housing whereby communication between the interior of said housing and thc atmosphere is through said tubular member, and characterized still further by the addition of a cover of foraminous material over all of the openings in each of said casing plates and over the inner end of said tubular member, said ioraminous covers constituting acoustical resistances to the passage of sound waves through said openings and through said tubular member, and the values of said acoustical resistances being such as to give a substantially at response characteristic to said apparatus.

LESLIE J. ANDERSON.

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