US2509224A - Electroacoustical transducer - Google Patents

Description

y 1950 M. 1.. GAYFORD 2,509,224

ELECTROACOUSTICAL TRANSDUCER Filed Sept. 16, 1944 3 Sheets-Sheet 1 IO 25 I9 24 I3 I ventor NuunELLnwREncEQRX iimkv 7 By Altorhey May 30, 1950 ELECTROACOUSTICAL TRANSDUCER Filed Sept. 16, 1944 M. L. GAYFORD F/GB.

3 Sheets-Sheet 2 Inventor M \umEL Lnwamcx. Gavin?) y [tome y 1 y 1950 M. 1.. GAYFORD 2,509,224

ELECTROACOUSTICAL TRANSDUCER Filed Sept. 16, 1944 3 Sheets-Sheet 3 5000 7000 IOOOO FIGS.

Q I n mentor m hcnnELLeunmcE M WW A Home Patented May 30, 1950 UNITED STATES TENT OFFICE ELECTROACOUSTICAL TRANSDUCER Application September 16, 1944, Serial No. 554,407

In Great Britain September 28, 1943 2 Claims.

The present invention relates to electro-acoustical transducers of the electro-magnetic moving coil type which may be either microphones or receivers.

The invention will be presently explained in terms of two embodiments illustrated in the accompanying drawings inwhich:

Fig. 1 shows a side elevation of a moving coil microphone sectioned along the lines BB of Fig. 2;

Fig. 2 shows a top view sectioned along the lines A-A of Fig. 1;

Fig. 3 shows an enlarged view of the diaphragm and magnetic circuit of the microphone to show the details more clearly;

Fig. 4 shows a plan view of the magnet employed in the microphone;

Fig. 5 shows a sectional view of another type of microphone according to th invention; and.

Fig. 6 shows output response curves for moving coil microphones in which the abscissaeare the frequencies in period per second (pzs) and the ordinates are output levels in decibels referred to an arbitrary zero level. Curve A is for a microphone according to the invention, and curve B is for a microphone of the unimproved type to show the nature of the irregularity corrected by the invention.

In order that the significance of the invention may be clearly appreciated, an account of the present state of the art will first be given.

A considerable amount of attention has been paid in recent years to the design of moving coil microphones and receivers in order to improve their fidelity of response over a wide frequency range. Instruments of this type are described in the following two articles in the Bell System Technical Journal:

(it) Oct. 1931, page 565, "Moving coil telephone receivers and microphones by E. C. Wente and A. L. Thuras.

(1)) July, 1936, page405, A non-directional microphone by R. N. Marshall and F. F. Romanov.

and also in the following article in the Journal of the Society of Motion Picture Engineers of the U. S. A.:

(0) Dec. 1931, page 9'77, A moving cOil microphone for high quality sound reproduction by W. C. Jones and L. W. Giles.

These instruments gave fairly good results, and employ a now common form of diaphragm of aluminium alloy consisting of a domed central portion with a flat annular surrounding portion so designed that the dome moves substantially as a piston. The diaphragm carries alight cylindrical coil which moves in an annular magnetic field created between a central domed pole piece and a :cc-axial outer pole-piece. The space between the diaphragm and the central domed pole-piece.

forms a chamber to which is connected a narrow passage formed by the coil slot. The arrange ment constitutes efiectively a Helmholtz resonator of which the coil slot and connecting passages constitute the neck. The constants of this resonator have to be carefully chosen so that a suitable resonance frequency is obtained,'and appropriate acoustic resistance is introduced in the neck to damp the resonance. This acoustic resistance is obtained by connecting the. coil slot to the relatively large hollow space inside the case of the instrument by very narrow passages formed either by slits in metal parts or in the meshes of fabrics such as silk. British Patent specification No. 333,579 for example, describes one method of obtaining this acoustic resistance, and various other Ways of achieving th same result have been since proposed. It may be now regarded as a quite usual practice to provide some form of acoustic resistance connection between the coil slot and the main instrument case.

Instruments designed along these lines have generally been found to have an undesirable irregularity in the response characteristics, in the middle and upper part of the useful frequency range, which is attributable to the fact that the resonance frequency of the chamber behind the diaphragm is too low, and it has not been found practicable to raise it sufficiently with the already known designs. Curve 3 of Fig. 6 illustrates the nature of this irregularity, and may be compared with curve A which is for an instrument corrected according to the invention.

The principal object of the present invention is to remedy the dimculty byreducing the effective mass associated with the neck of the Helmholtz resonator behind the diaphragm so that the resonance frequency may be raised by the desired amount. This is done by providing an additional acoustic resistance connection between the chamber behind the diaphragm and the relatively large space inside the case of the instrument by means of a passage or passages through the central pole-piece. This provides an additional neck to the Helmholtz resonator with a corresponding effective mass in shunt with the mass provided by the previously mentioned coil slot connection. This will be explained more fully later on.

The relatively large space inside the case has a negligible stiffness effect except at the lowest audio frequencies (for example, below about 150 pzs). The frequencies which are of interest in the present invention, as already explained, are the middle and upper frequencies of the range. When therefore, it is stated that the space inside the case has a negligible stiffness effect, the qualification just explained is supposed to be understood, and is immaterial in the circumstances of the invention.

Attention is directed to the arrangement shown in Fig. 9 of the article by Wente and Thuras and also in the article by Jones and Giles, and described also in British patent specification No. 382,668. This figure shoWs a small resonant chamber in the dome of the central pole-piece connected to the chamber behind the diaphragm by a narrow slit or passage. The purpose of this small chamber is to correct the response characteristic of the instrument for an unexpected irregularity, shown in curve A of Fig. 3

of the article by Jones and Giles, the reason for V which was not very clearly understood, but which is now believed to be due to a resonance caused by the diaphragm breaking up and oscillating in higher modes than the fundamental so that its central domed portion ceases to vibrate substantially as a piston. This particular effect is a sharp irregularity at the upper end of the frequency range and is easily distinguished from the much smoother and more extensive irregularity, shown in curve B of the accompanying Fig. 6, which is caused by the low resonance frequency of the chamber behind the diaphragm. This sharp irregularity can be avoided by proper care in the design and manufacture of the diaphragm so that the small chamber is not required, and it is not employed in the embodiments of the present invention.

Reference has been made to this arrangement because of its apparent similarity to the principal feature of the present invention, in order that the distinction may be clearly appreciated. The present invention is concerned with means for raising the resonance frequency of the chamber behind the diaphragm and not with difiiculties associated with imperfect performance of the diaphragm, which are better dealt with by paying careful attention to its design and. construction.

According to the invention, therefore, there is provided an electro-acoustio transducer comprising a case having therein a hollow space of negligible stiffness effect, a diaphragm carrying a coil adapted to move in a slot across which is established a magnetic field, a substantially closed resonating chamber behind the diaphragm, and an acoustic resistance connection between the chamber and the hollow space through a path which does not include the slot.

The invention also provides an electro-acoustic transducer comprising a case having therein a hollow space of negligible stiffness effect, a magnet having a central pole-piece and an annular pole-piece, the two pole-pieces being separated by a narrow slot, a diaphragm carrying a circular coil adapted to move in the slot, a substantially closed resonating chamber behind the diaphragm, and an acoustic resistance connection through the central pole-piece between the chambar and the hollow space.

According to another aspect, the invention consists in an electro-acoustic transducer comprising a diaphragm carrying a coil adapted to move in a narrow slot, magnetic circuit means for establishing a magnetic field across the slot, a substantially closed resonating chamber behind the diaphragm communicating with the slot, an enclosing case having therein a hollow space of negligible stiffness effect, an acoustic resistance connection between the slot and the hollow space, and a further direct acoustic resistance connection between the chamber and the hollow space.

The two embodiments of the invention will now be described. Although both the embodiments are microphones, the same principles and constructions are applicable to moving coil receivers, which differ from microphones only in so far as they must be designed for handling higher powers, and usually also to work into a different acoustic impedance.

Figs. 1, 2 and 3 show an electromagnetic microphone of the moving coil type which is generally similar to that illustrated in the article by Marshall and Romanov. It has spherical case I in the upper part of which there is mounted a circular magnet 2 having a central pole 3 and a surrounding annular pole A top View of the magnet is shown in Fig. 4 and indicates that the annular pole is interrupted by four slots 5 providing communication between the annular gap 6 and the interior of the main case I of the microphone. Fixed to the central pole 3 is a circular disc l forming one pole-piece, and fixed to the annular pole t is a corresponding annular pole-piece 8. Between the two pole-pieces is the coil slot 9. These details are most clearly seen in Fig. 2.

The diaphragm is of well known type, and comprises a central domed portion H), a substantially flat periphery H and an intermediate corrugated annular portion l2. It is clamped between the pole-piece 8 and an annular shoulder 13 on the inside of the case I (see Fig. 1). The case in the neighbourhood of this shoulder is fragmentarily shown in Fig. 2. The moving coil [4 is fixed to the diaphragm and is centrally located in the coil slot 9.

A domed plate I5 is fixed on the central polepiece 1 just below the domed portion of the diaphragm. This plate is perforated with a number of holes l6, and a disc l? of silk or gauze covers the under surface of the plate. A cylindrical channel l8 passes right through the centre of the pole-piece l and magnet 2, and forms a wide communication between the space [9 under the plate l5 and the inside of the case I.

The coil slot 9 communicates with an annular space 23 closed by an annular plate 2i perforated by a ring of holes 22, the upper side of which is covered by an annular sheet of silk or gauze. This provides the usual acoustic resistance connection between the coil slot 9 and the inside of the case through the gap 6 of the magnet, which, as already stated, communicates directly therewith through the slots 5.

The magnetic circuit is bolted to a plate 33 by four screws 32 (of which only one is shown in Figs. 1 and 2) located in the slots 5 of the magnet 2.

By means of the perforated plate l5 and the channel it, a connection is established between the chamber behind the diaphragm and the inside of the case in addition to the connection through the coil slot. This additional connection illustrates the principal feature of the invention. Preferably the channel is is stuffed with lambs wool or the like to damp out any propagation effects in the channel.

As an alternative to the perforated plate [5, two such plates fitting closely together one in-.

side the other with the holes registering, may be employed, with the silk sandwiched between them. This provides a possibly better method of fixing the silk which may not stick very satisfactorily to a curved surface.

In accordance with Fig. 4 of the article by Wente and Thuras quoted above, let S1 be the stiffness of the air in the chamber behind the diaphragm, and let M1 and R1 be the total eifective mass and acoustic resistance, respectively, of all the narrow passages which constitute the neck of the Helmholtz resonator. Then it is well known that the resonance frequency is given by:

It has already been stated that in the microphones or receivers of the type under considera tion it has been found to be impracticable to raise the value of F insufficiently high. This is principally because R1 and M1 are not independent, and by means of the coil slot path alone it has not generally been found possible to obtain a suitable value of R1 without associating therewith a value of M1 which is too large. By means of the extra connection according to the invention a further mass is provided which is effectively in parallel with the mass associated with the coil slot connection, and thereby M1 is considerably reduced. It is, of course, understood that R1 will also be changed, but by adjusting the acoustic resistance associated with the coil slot in known way it has been found that a suificiently low value of M1 can be obtained according to the invention with a suitable value of R1 at the same time.

The remaining features of the microphone shown in Fig. 1 are well known and for the most part are described in the quoted articles. Thus 23 is the base equalising tube which forms an appropriate connection between the inside and outside of the case. 24 is the front grille covered with silk for providing appropriate acoustic resistance for damping the resonance of the chamber in front of the diaphragm, according to usual practice. spherical in form in order to obtain non-directional properties in accordance with the principles explained in the article by Marshall and Romanov. The polar equalising screen is shown at 25.

Fig. 5 shows another embodiment of the invention comprising a moving coil microphone intended for a special purpose where a much smaller frequency range had to be covered. In this case it was found preferable to close up the coil slot entirely so that there was no communication with the inside of the case, and to provide all of the mass M1 and acoustic resistance R1 by means of a connection through the centre of the magnet in much the same way as in the embodiment previously described. In this way the desired amount of acoustic resistance is obtained with a smaller value of M1 than by means of the coil slot connection.

In Fig. 5 the magnet N12 is a plain hollow cylinder, the central pole-piece IQ! being a smaller coaxial cylinder with a flange at the lower end making contact with the magnet. The annular pole-piece Hi8 surrounds the upper end of the pole-piece IEI'I, the coil slot IllB being formed between them. The magnet and polepieces are fixed together as a unit by screws I26, and the space I2! between is filled by means of a solidifying compound such as styrene, so as to seal off the lower end of the coil slot.

The case of the microphone is made The domed dia hragm I28 isof similarl-type to that shown-inFig. 2 and isclampedbetWEfin the polerpiece H38 and the perforatedfrontplate 1 I293. The whole is secured inside a cylindrical case it: spun over a closing plate I30.

The relatively largehollow space I3I' insided the pole-piece I0! is closed at the upper endbyr a flat perforated plate H5 covered with a sheet.

of silk Hi. There is then formed behind the. diaphragm a resonating chamber I34 which .is much larger than the corresponding chamber in the previously described embodiment, since there.

is no dome mounted on the pole-piece IN. A

larger chamber is required in this case since the resonance frequency desired is much lower.

The silk covered perforations in the plate H5 provide the neck for the Helmholtz resonator, and introduce the desired mass and acoustic resistance.

The hollow space I3! may be stuffed with lambs wool or the like if necessary in order to avoid propagation effects.

In either of the embodiments which have been described, it is preferable to use for the magnet a material which has been heat treated and/or cooled in a magnetic field to predetermlne the direction in which it is to be subsequently magnetised. This increases the efiiciency of the ma netic material and enables the maximum flux to be obtained in the coil slot. The use of such a magnet accounts for the fact that in Fig. 6, curve A is at a higher level than curve B.

It will be seen that the main feature of the invention whereby a central connection is established between the chamber behind the diaphragm and the inside of the case may be employed whether or not a connection is also provided between the coil slot and the interior of the case. It is to be noted that this central connection has another advantage particularly applying to the embodiment described with reference to Figs. 1 to 4.. Owing to the fact that the acoustic resistance is introduced very close to the centre of the diaphragm by the domed plate mounted on the central pole-piece, any tendency for the dome to break up and to oscillate in a higher mode through lack of rigidity of the diaphragm dome is largely prevented due to the local damping of the corresponding resonances by this acoustic resistance. The action is similar to the damping action of the backing plate holes commonly pro vided close behind the diaphragm of a condenser microphone.

What is claimed is:

1. An electro-acoustic transducer including a diaphragm, a coil carried thereby, magnetic circuit means for establishing a magnetic field across a space having the form of a slot, said coil being positioned to move in said slot, a substantially closed resonating chamber positioned behind said diaphragm communicating with said slot, a case enclosing a hollow space of negligible stiffness effect, an acoustic resistance connection passing between said slot and said hollow space independently of the chamber, a further direct acoustic resistance connection passing between said chamber and said hollow space, and said magnetic circuit means comprisin a central pole-piece and an annular pole-piece spaced therefrom by said slot, and a domed acoustic resistance plate mounted on said central pole-piece so as to form part of the boundary of said chamber behind said diaphragm.

2. An electro-acoustic transducer including a case inclosing a relatively enlarged hollow space to be of negligible stiffness efiect, a magnet lo cated in said case and having a, central pole piece and an annular pole piece, said two polepieces being separated by a narrow gap, a diaphragm carrying a coil positioned to move in said gap, a substantially closed resonating chamber positioned behind said diaphragm, said central pole piece being formed to establish an acoustic resistance connection between said chamber and said hollow space independently of the gap and said annular pole piece being interrupted by slots to provide communication between said annular gap and said hollow space independently of said chamber.

MICHAEL LAWRENCE GAYFORD.

8 REFERENCES CITED UNITED STATES PATENTS Number Name Date 1,952,357 Blumlein et a1. Mar. 27, 1934 1,954,966 Thuras Apr. 17, 1934 1,964,606 Thuras June 26, 1934 2,084,944 Cornwell June 22, 1937 2,095,373 Speer Oct. 12, 1937

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