US2194070A - Sound translating device - Google Patents

Description

March 1940. e. M. GIANNINI I SOUND TRANSLATING DEVICE Filed Sept. 2, 1936 Patented Mar. 19, 1940 UNITED STATES PATENT OFFICE SOUND TRAN SLATING DEVICE of Delaware Application September 2, 1936, Serial No. 99,019

4 Claims.

This invention relates to energy translating devices and especially to devices for translating sound from or into other forms of energy. More particularly the invention relates to microphones, loud speakers and telephonic apparatus and the method of making the same.

One object of my invention is to provide a simple and inexpensive telephonic apparatus for detecting, amplifying and reproducing sound.

In the accompanying drawing I have shown a preferred embodiment of my invention and various modifications thereof. These I have chosen with a View to explaining the invention and the best manner of embodying it in practical commercial applications. It should be understood, however, that neither the accompanying drawing nor the description given herewith are intended to be exhaustive or limiting of the invention, but on the contrary are intended by explanation and illustration to so fully disclose the invention and the principles involved therein and the application of the invention under varying circumstances that others skilled in the art will be enabled to apply the invention in numerous other embodiments each as may be best adapted to the conditions and requirements of any particular circumstances.

Figure 1 is a view in longitudinal vertical section of one of the speaker-microphone units for translating sound energy into electrical energy and vice versa.

Figure 2 is a similar view of a modified form of speaker-microphone unit.

In Figures 1 and 2 I have shown an energy translating unit for converting sound energy into electrical energy and vice versa and particularly adapted to serve both as a microphone and as a speaker unit.

As shown in Figure 1 the unit comprises a folded horn with a parabolic cavity, that is to say, with a passage from the throat 10 to the mouth II which approximately follows the law S=S0(X+K) 2 where S is the open transverse area within the passage tangential to the center of the wave front of sound passing through the horn, So is the open transverse area at the throat l0, X is the distance along the line of propagation of the sound wave through the passage and K is a constant determining the flare of the parabole.

As shown in the drawing, the speaker unit comprises a permanent magnet having a central pole piece I4, 2. bottom disc member I3, and annular member 12, and an upper annular pole member IS with the voice coil l6 which is attached to the diaphragm I1 fitted with only a few thousandths of an inch clearance between the center pole piece l4 and the annular pole piece l5.

These magnetic parts I prefer to make of the strongest possible magnetic material, for example, the well known magnetic alloy of iron, aluminum, nickel and cobalt known commercially as Alnico. By use of a very strong magnet I have made possible the high efficiency horn microphone and speaker with matched impedances in the compact form shown.

The diaphragm I1 is pierced at its center and a projection [8 at the center of the pole piece l4 is accurately fitted into this hole in the diaphragm whereby to position the diaphragm so that the voice coil I6 is accurately held in a clearance position. Securing the diaphragm thus at the center as well as at the periphery increases the stiffness while keeping a low mass, which is important for best efiiciency.

At its outer periphery the diaphragm I! is clamped between the ring l9, secured to the top of the pole piece l5, and the baflle 20, which is designed to provide an enclosed air space over the top of the diaphragm ll and communicating with the mouth of the horn l0.

Onto the back of the member I3 is secured, through an insulating disc 2| a small electric lamp 22 which is controlled by the same switch which controls the main circuit of the device. Thus the lamp is lighted and serves as a signal when the device is operative to warn that it is transmitting sounds which fall upon it.

This unit as just described is received in the cup member 24 and a rubber ball 25 is placed between the member l3 and the bottom of the cup 24. The horn member 26 is then screwed into the cup 24 and its inner shoulder 2'! engages the member 20 compressing the entire unit against the rubber ball 25 so as to produce a secure, tight and non-rattling engagement of the unit in the assembly.

The rubber spacer units 28 are next fitted over the ends of the horn member 26, the outer member 29 fitted over the entire assembly and against the spacers 28, and finally, the mouth member 30 with the soft rubber spacer balls 3! in position therein is screwed onto the end of the member 29 so that the inner parts are securely clamped between the soft rubber spacer members 3| and 28 and the entire assembly is thus held with the annular space which constitutes the outer part of the horn accurately determined by means of these spacer members 3| and 28.

It will be observed that the mouth member 30 .tothe center of the wave front.

extends beyond the end of the inner member 24, and thus, although the open area in the passage increases, the member 30 may be curved inwardly over the ends of the member 24. In this way, in particular, it has been possible to streamline the entire unit so that, although the unit is placed on a desk or in other position where it is subjected to wind or other substantial air currents, these will produce a minimum disturbance in the air within the mouth ll of the energy translating unit.

The surfaces of the members 24, 26, 29 and 30 which form the serially communicating passage of the horn from the throat iii to the mouth I! should, as in other horns, be as smooth as possible. This is particularly important, however, at the turns, that is to say, where the sound wave changes direction from the inner passage through the member 26 to the outer peripheral passage between the members 26 and 29. The shaping of the member as at this turn is also of great importance since it serves substantially to preserve the continuity of expansion of the passage according to the single parabolic law. These requirements are met most advantageously by molding these parts from hard rubber or a resin plastic.

In designing such a horn I have found that the sound wave is changed from the simple circular Wave front to the annular wave front facing in the opposite direction according to Hughes principle of redevelopment of a Wave from a small orifice and thus in designing the expansion in area around the turn over the end of the member 26 the open transverse area should be considered at each point as the surface tangential Tins surface, of course, throughout the turn will represent a conical surface, or, at one position, a cylindrical surface whereas within the main passages the surfaces will be substantially a plane.

Although I have stated that the expansion of the passage follows approximately a single law representing a parabolic curve it is not essential that this be followed exactly; and, in fact, in the embodiments shown in the drawing, in order to simplify the design and manufacture, the horn is made as a series of conically expanding passages whlch approximate the parabolic expansion, The acoustic effect obtained by this close a proximation is very nearly the same as that ob tained by following precisely the parabolic law.

It should be understood, moreover, that the horn is not necessarily of the parabolic type. This has been chosen particularly because it lends itself to the streamline form as shown and is, therefore, most desirable for use on a desk or other locations where it may be subjected to substantial air currents. Other types of horns, however, may be used, as for example, the conical type where the expansion in open transverse area of the passage from the throat to the mouth follows a simple conical law: S=So (X-I-K), or it may follow an exponential law: S=S0 E WXJFK); S, 50, X and K having the same values as already discussed above, 6 being the base of natural logarithms and m being a constant.

In Figure 2, I have shown in section an exponential horn of this latter type. In this figure the magnet, the diaphragm and the voice coil may be substantially the same as in Figure 1, and are received in the cup 35 with an elastic ball 3.5a corresponding to the ball 25 used in Figure l. The member 20a, however, in this case is extended beyond the annular ring 19a onto the shoulder 36 of the outer member 29a and is secured thereto by means of the screws 37, thus serving to hold the entire unit within the cup 35. The upper surface of this member 20a is curved smoothly to form one side of the annular space constituting the first of the serially communicating passages of the folded horn, the other side of which is determined by the under surface of the member 2%. This member 24a is held in the annular ridge 38 of the member 29a by means of the soft rubber spacers 3 la. One side of each of these spacers extends into a hole in the ridge 38, as clearly shown in Figure 2, and the opposite side of which has a hole in which it is engaged by a screw secured in the member Ma.

The outer surface of the annular ridge 38 and the inner surface of the peripheral part of the member 2411 are shaped relative to one another so as to form a continuation of the passage beginning at the throat liia in the member 20a continuing substantially the same law of expansion and again the inner surface of the outer peripheral part of the member 29a and the outer surface of the peripheral parts of the member Z ta are shaped relative to one another so that the annular passage between them continues to expand with approximately the same exponential law.

In this form, as in the parabolic form shown in Figure 1, it is important that the surfaces which define the horn passages should be highly polished and especially at the turns over the upper edge of the ridge 38 and over the peripheral edge of the member Zita because of the deep pressures which occur.

In the form shown in Figure 2, the outer peripheral edge is not carried beyond the member E ia, because the exponential horn does not lend itself so well to streamlining. By suitable choice of the constants n and k and carrying the edge of the member 29a just beyond the back of the member Me an approximately streamlined form may be given to the horn without use of excess material for fairing.

Referring again to Figure l, I have, as already mentioned above, provided a lamp 22 controlled by a switch which is operated simultaneously with the switch by which the energy translating unit is connected into a. transmission circuit. Over this lamp 22 is provided a crystal or lens 32 preferably of colored glass. Thus the light appearing through this crystal or lens gives warning that the device is connected and will transmit any conversation or other sound occurring within hearing distance of the device.

Instead of using a special crystal or lens 32, the entire cup 22 or all of the parts 24, 25, 29 and 30 may be made of transparent material, preferably one of the transparent plastic materials such as the polystyrol plastics known as trolitol" or iper trolitol, the transparent urea-aldehyde plastics, or cellulose derivatives, etc. This also has advantage from an aesthetic viewpoint, as the reflection and refraction from the several interior surfaces of the horn produce pleasing effects and the source of the sound-translating device is made visible.

What is claimed as new is:

1. An energy translating unit comprising a magnet having central and peripheral pole pieces separated by a narrow annular air gap, a diaphragm, a coil secured to the diaphragm and positioned in said annular air gap, a spaced ring on the peripheral pole piece adapted to provide a bearing surface for the edge of the diaphragm and to enclose a chamber between the diaphragm and the pole pieces, an annular member adapted to fit said spacer to clamp the diaphragm therebetween and having an inwardly extended portion spaced from the diaphragm and having a mouth therein, a tubular member having a portion adapted to form an abutment for said annular member and having its central passage connected to the mouth of said annular member, a cap adapted to fit over said magnet and to form a continuation of the outer surface of the tubular member, a spacer of elastic, acoustically dead material adapted to transmit pressure from said cap to said magnet whereby the magnet, spacer, diaphragm and annular member may all be clamped against the tubular member by said cap, an outer tubular member having an open mouth at one end over said cap and a closed end over but spaced from the open end of the inner tubular member whereby a continuous passage is formed from the diaphragm through the tubular members to the mouth, and spacers of acoustically dead elastic material between the open end of the inner and the closed end of the outer tubular members and between the open end of the outer tubular member and said cap, whereby the parts are all held clamped together and in operative relation.

2. The combination as defined in claim 1 in which the exterior form of the entire unit is streamlined whereby disturbance of sound waves by external movement of air thereover may be minimized.

3. An instrument for translating energy to or from sound which comprises a first and second hollow streamlined body, each of said bodies having in one end only a single opening into its interior, a diaphragm having one surface only communicating with the opening in said first body and mounted therein, and said first body held in suspension entirely within said second body by means of resilient cushions to form an air passage between the interior of said second body and the exterior of said first body for passage of sound and positioned so that its opening faces the closed end of said second body.

4. In a microphone speaker unit, an inner body having a tear-drop streamlined shape, a diaphragm mounted Within said body near the Wider portion thereof, the narrow end thereof having an opening extending toward the front surface of said diaphragm, an outer hollow body having a tear-drop streamlined shape, said outer body having an opening into its interior at the larger end thereof, and rubber cushions suspending said inner body within said outer body so that the open end of the inner body faces the closed end of the outer body to form an air passage between the inner surface of said outer body and the outer surface of said inner body for passage of sound around said inner body.

GABRIEL M. GIAN'NINI.

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