US2064316A - Electroacoustic device - Google Patents

Description

Dec. 15, 1936. H. F. OLSON ET AL I ELECTROACOUSTIC DEVICE Filed Dec. 14, 1952 2 Sheets-Sheet 1 INVENTORS #4511) 0150 BY FfiA/YK MASS/1 ATTORNEY H. F. OLSON ET AL ELECTROACOUSTIC DEVICE Dec. 15, 1936.

14, 1952 2 Sheets-Sheet 2 JL Filed Dec.

. r arm 0 fl/ m INVENTORS HARRY F OLSU/V mA/v/r mas/4 A'TTCSRNEY Patented Dec. 15, 1936 UNITED STATES PATENT OFFICE ELEGTROACOUSTIC DEVICE Application December 14, 1932, Serial No. 647,132

6 Claims.

This invention relates to electro-acoustic devices and has for its principal object the provision of an improved electro-acousticdevicewhichhasa substantially constant response over a comparatively wide range of operating frequencies.

Another object of the invention is the provision of a telephone receiver which produces at the ear a substantially constant response.

A further object of the invention is the provision of an improved electro-acoustic device of the type wherein a single element functions both to carry the electrical current which actuates the device as well as generating the acoustic pressure.

A further object is the provision of an improved telephone receiver of the ribbon type.

While the improved device herein described possesses particular utility in connection with testing high fidelity transmission systems and monitoring in broadcasting stations, sound picture and phonograph recording and the like, it will be apparent that it has important advantages in other cases where extreme accuracy of response is desirable. As utilized in practice, its maximum variations in response between 30 to 6,000 cycles is about 2 /2 db. When utilized as a receiver, its response above 6,000 cycles is purposely made to drop 7 db. from the maximum at 8,000 cycles in order to compensate for an increase in the ear sensitivity resulting from standing waves being set up in the ear cavity. From 8,000 to 10,000 cycles, its response is substantially constant.

The invention will be better understood from the following description when considered in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

Referring to the drawings:

.Fig. 1 is an exploded view of a device constructed in accordancce with the invention.

Fig. 2 is a sectionalized perspective view of the mechanism of the device,

7 .Fig. 3 illustrates the device as it appears with the front of the casing removed, Fig, 4 is a perspective view of the device,

Figs. 5 to Tare explanatory diagrams for illustrating the effect of various constructional features on the response of the device at different frequencies, and

Fig. 8 illustrates a modified embodiment wherein means are provided for varying the size of-the cavity at the rear of the diaphragm or vibratory. member.

The device includes a magnetic yoke provided with, pole shoes [I and I2 and arranged to receive between its polar extremities an insulation member I 3 which forms a support for the operating mechanism and also provides a cavity It at the rear of the vibratory diaphragm I5. Interposed between the diaphragm I5 and the cavity or chamber M is a member It. This member serves to partition off the enclosure behind the vibrating diaphragm into two cavities. Several small holes are placed into the member I6 of such dimensions that their inertance is such that resonance is obtained with the stiffness of the air in the cavity l t at the same frequency that the vibrating diaphragm and its air chamber resonate, and acts to smooth out the response curve as shown in Fig. 6 and to be described later. Several small holes are placed into the member It instead of one larger hole in order to increase the resistive component and thereby obtain a broader resonance.

The diaphragm I 5 is mounted on a strip II preferably made of empire silk about .002 inch thick and cut away at its center in order to decrease the efiective mass of the vibrating system. The diaphragm I5 is electrically insulated from the pole shoes II and I2 and is clamped at one of its ends between members I8 and I9 and 3.1; the other of its ends between members 20 and Electrical leads 22 and 23 connected to the opposite ends of the ribbon I5 either through the clamping members I8 and 20 or in other suitable manner are provided for supplying operating current to the device.

In the assembly of the apparatus, the magnetic structure In is molded into a bakelite shell 24 which serves to form this structure into a rigid unit and to provide a suitable exterior surface for the main body of the device. It will be apparent that the pole shoes I I and I2 are clamped to the insulation member I3 by means of screws 25 and that the non-magnetic bridging members I9 and M are insulated from the pole shoes II and I2 by the insulation washers 26 and 21 and by the insulation collars and Washers interposed between the bridging members I92I and the screws 28 by which these members are clamped to the pole shoes. As previously indicated, the ends of the ribbon I5 are clamped to the terminal blocks is and El by means of members I8 and 20, suitable means such as members 29 and 30 being provided for clamping the edges of the empire silk support I! to the pole shoes. As indicated by Fig. 4, an ear cap 3| is provided at the front of the casing in which the working parts are mounted and a suitable support 32 is provided. The

' the left.

ear cap has been made of molded rubber having an undercut projecting lip which easily conforms to the contour of the ear when placed against it, thus serving to prevent leakage of air between the vibrating diaphragm and the ear.

From what has been said, it will be apparent that the device consists fundamentally of a magnetizing unit, a diaphragm mounting and an ear cap. It is similar in some respects to the highquality microphone of the ribbon type but differs therefrom in other respects.

In the case of the microphone, the ratio of the velocity of the conductor to the pressure in the sound wave must be independent of frequency to obtain a system in which the sensitivity is independent of frequency. The mechanical system in the microphone is mass controlled, that is, the mechanical impedance is proportional to the frequency. To maintain constant velocity of the ribbon, the acoustic system is designed so that force available for actuating the ribbon is proportional to the frequency over the working range of the microphone. In the case of the receiver, we must supply constant sound pressure to the ear cavity for constant voltage applied to the receiver. If weassume that the ear cavity presents a constant acoustic capacitance to the receiver, the ratio of amplitude of the ribbon to the applied voltage should be independent of the frequency. It is well known that the impedance of the ear cavity presented to the receiver is not a pure capacitance, particularly at the higher frequencies, due to standing wave systems between the receiver and portion of the ear cavity, and also due to absorption which results in a resistive component. However, these factors Vary from person to person. For this reason it seems that a receiver in which the ratio of amplitude to the applied voltage is independent of frequency meets the actual conditions about as accurately as any other characteristic.

In order that the pressure in the ear cavity shall be independent of the frequency for constant voltage applied to the receiver, the ratio of the amplitude of the ribbon diaphragm to the applied voltage must be independent of the frequency. This is equivalent to stating that the system must be stiffness controlled. There are a multitude of ways in which this may be accomplished. The discussion which follows takes into consideration one system which will satisfy the requirements outlined above.

A system employing a ribbon diaphragm and and acoustic capacitance is shown in Figure 5 at The equivalent circuit of this system is shown in Figure 5 at the right. The velocity of the ribbon is given by expression.

where F: force due to the current,

m=mass of the ribbon,

r= mechanical resistance,

V C 2 s2 p: density of air A: area of the ribbon, p: velocity of sound V=volume of the cavity, to: 211') f: frequency The amplitude in terms of the velocity X= The amplitude response of the system is shown at the center of Figure 5; It will be seen that the excessive response at the resonance frequency is objectionable. To maintain constant amplitude in this system, either the resonant frequency must be located outside the operating range, or a large amount of resistance must be used in the circuit and the resonant frequency placed near the upper limit of the operating range. The primary disadvantage of either of these systems lies in their relative insensitiveness.

In general in any system designed to yield uniform response efiiciently over a large frequency band, it is necessary to resort to more than one resonant circuit. A system which accomplishes this result is shown in Figure 6. It is this type of system which has been adopted for the ribbon receiver.

In this system, the velocity of the ribbon is given by Y F: force dueto current,

m=mass of the ribbon,

r1=damping resistance of the ribbon, C1=compliance of first cavity,

C2: compliance of second cavity,

r2: resistance of the aperture, m2: equivalent mass of the aperture to: 21rf frequency The amplitude in terms of the velocity is We may adjust the constants of the system so. that the amplitude per unit force will be practically independent of the frequency. The amplitude for a certain set of constants is shown at the center of Figure 6.

It was found very important to prevent all leakage of air from the front to the back of the ribbon in order to preserve the low-frequency sensitivity of the receiver. An aluminum ribbon .00025" thick and 13/64 wide cemented along its edges to a mounting of empire silk .002" thick comprises the vibrating system. A window is cut out in the silk behind the ribbon in order to decrease the effective mass of the diaphragm.

In order that the entire surface of the ribbon will vibrate in phase, it is necessary to corrugate it. Satisfactory corrugations are formed by pressing the mounted ribbon on a group of parallel wires .014" diameter, spaced 35 to the inch; the axis of the ribbon being kept parallel to the wires.

When the ribbon is clamped in its mounting it is installed without stretching. The controlling stiffness of the system is obtained by the enclosed volume of air behind the ribbon. With a fiat piece placed across the bottom face of the pole pieces, an enclosure of 0.42 cc. remains behind the ribbon.

This stiffness resonates with the ribbon at about 8000 cycles. In order to eliminate the excess response at resonance and increase the sensitivity, a second air chamber having a volume of .38 cc. is coupled to the first chamber thru 5 holes .012" diameter, drilled thru a brass plate 1/64" thick.

The frequency-response characteristic of the receiver is shown in Figure 7.

The necessity for preventing air leakage from the front to the back of the ribbon has already been mentioned. It is equally important that there is no air leakage from the ribbon to the atmosphere when it is placed against the ear. In order to have a good seal between the ear cap and the ear, the cap was made of rubber molded into the shape shown. In assembling the ear cap to the receiver, a layer of sealing compound composed of wax and oil was smeared over the pole pieces before fastening the cap. This compound effectively prevents leakage between the ear cap and pole shoes.

The two ribbons in the receivers making up the head set are connected in series and a transformer is provided for stepping up the impedance to 2000 ohms. The resistance of both ribbons in series is .065 ohms; therefore, it is necessary to use fairly heavy leads from the receivers to the transformer in order to prevent loss of power in the cable. Four feet of parallel, finely-stranded cable equivalent to #10 B. & S. gauge comprises the phone. cord to which the transformer is attached. A satisfactory transformer has been made having 10 turns of #16 enamelled copper for the primary and 1600 turns of #36 enamelled copper for the secondary (the primary is wound between two halves of the secondary in order to reduce the secondary capacity). The core consists of a stack of small nicoloi laminations having an outside dimension 1 x 1 The absolute sensitivity of each receiver at 1000 cycles has been measured as 1.3 bars into a 4 cc. cavity per milliwatt electrical input.

As will be readily understood, the advantages of the invention are not limited to the specific structure disclosed but may be realized with different modifications of this structure. Figure 8, for example, illustrates a modification wherein the size of the cavity at the rear of the diaphragm I5 may be adjusted by means of a rotatable control member 33. By controlling the size of the cavity behind the diaphragm, the resonant frequency of the system may be adjusted to any value within the limits of motion of the member 33. This construction provides a receiver having a very high output at any single frequency within the range of the unit. Other modifications of the invention will be apparent in view of the foregoing description.

We claim:

1. An electro-acoustie device including means for producing a magnetic field, an insulating support impervious to air mounted in said field and provided with a central aperture, a stiffness controlled diaphragm arranged on said support to cover said aperture, and electrical leads connected to said diaphragm.

2. An electro-acoustic device including means for producing a magnetic field, means arranged to form a chamber at one side of said field, a flexible insulating strip mounted in said field and provided with an aperture, and a stiffness controlled diaphragm coordinated with said strip to form a seal over the opening of said chamber.

3. An electro-acoustio' device including means for producing a magnetic field, an empire silk strip mounted in said field and provided with an aperture, a diaphragm arranged to cover said aperture and provided with corrugations extending transversely of said field, and means supporting rigidly the diaphragm at the ends of the corrugations.

4. An electro-acoustic device including means for producing a magnetic field, longitudinally corrugated aluminum diaphragm having a thickness of substantially .00025 inch mounted in said field, means forming an acoustic chamber having an opening at one side of said diaphragm,

and a flexible insulating support combined with said diaphragm to form a seal to said opening whereby the operation of said device is stiffness controlled.

5. An electro-acoustic device including means for producing a magnetic field, a ribbon diaphragm corrugated transversely of and mounted by its ends in said field, an air chamber mounted on one side of said ribbon diaphragm, a second similar chamber separated from the first by a partition, and holes in said partition.

6. An electro-acoustic device including means for producing a magnetic field, an empire silk strip mounted in said field and provided with an aperture, a diaphragm arranged to cover said aperture and provided with corrugations ex.- tending transversely of said field, means supporting the diaphragm at the ends of its corrugations and means arranged to form a chamber at one side of said diaphragm and said chamber divided into two parts by a partition provided with fine holes.

HARRY F. OLSON. FRANK MASSA.

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