US3435143A

US3435143A - Ribbon microphone

Info

Publication number: US3435143A
Application number: US476290A
Authority: US
Inventors: Charles P Fisher
Original assignee: Individual
Current assignee: Individual
Priority date: 1965-08-02
Filing date: 1965-08-02
Publication date: 1969-03-25
Anticipated expiration: 1986-03-25

Description

March 25, 1969 c. P. FISHER 3,435,143

RIBBON MI CROPHONE Filed Aug. 2, 1965 Sheet of 2 i INVENTOR CHARLES P. FISHER ATTORNEYS March 25, 1969 c. P. FISHER RIBBON MICROPHONE Sheet Filed Aug.

EMITTER FOLLOWER AMPUFIER INVENTOR CHARLES P HS BY, I

FIG?

ATTORNEYS CYCLES PER SECOND IOOO United States Patent Ofitice 3,435,143 Patented Mar. 25, 1969 3,435,143 RIBBON MICROPHONE Charles P. Fisher, Framingham, Mass. (473 Washington St, Wellesley, Mass. 02181) Filed Aug. 2, 1965, Ser. No. 476,290 Int. Cl. H04m 1/00; H04r 9/00, 9/08 US. Cl. 179-1 13 Claims ABSTRACT OF THE DISCLOSURE In a ribbon microphone, the width of the ribbon, the cross sectional area of the poles, and the separation between the poles is varied along their common length thereby maintaining saturation of the poles and a uniform magnetic field in the ribbon gap. A feedback circuit is used to reduce the bass sensitivity of the transmission.

The present invention relates in general to electroacoustical transduction and more particularly concerns an improved ribbon microphone characterized by good frequency response and directivity while providing a relatively noise-free output signal capable of being transmitted over relatively long distances without picking up undesired noise components. Yet, the assembly is compact and uses relatively few components.

A ribbon microphone typically comprises a very low mass conductor supported in a magnetic field. This conductor, or ribbon, responds to the pressure differential between its two faces to provide an electrical signal representative of the velocity component of an incident sound wave. The magnetic field is typically furnished by one or more permanent magnets and delivered to the region of the ribbon by soft iron poles. These poles act as baflles establishing the pressure differential to which the ribbon responds. At high frequencies, sound energy is diffracted around the poles to reach the back of the ribbon in phase opposition to the sound energy incident upon the front of the ribbon and produces cancellation efiects. Ribbon microphones are thus limited in high frequency response by the Width dimension of the poles. Since the electrical signal for a given sound pressure level is proportional to the product of magnetic field density, ribbon length and ribbon velocity, it is desirable to have a strong magnetic field surrounding the vibrating ribbon. But dense magnetic fields call for relatively massive poles, a requirement apparently in conflict with the requirement for good high frequency response; namely, that the poles be slender. Increasing the length of the ribbon and the poles does increase signal, but when the length approaches a wavelength at sound frequencies, off-axis response of the microphone exhibits a sharp decrease from cancellation effects due to opposing signal phases near the ends of the ribbon.

The ribbon itself is a mass responsive to the moving viscous air medium. Its own velocity will therefore be less than that of the medium. If the ribbon mass is decreased, for example by making it thinner, its velocity will be a higher percentage of that of the sound wave in the surrounding medium. At the limit, when the mass is zero, this percentage would be one hundred percent. Practical considerations limit the thinness of the ribbon material. Using the usual material, aluminum, it has been found that if the ribbon thickness is reduced below about 0.1 mil, the resistance increases faster than the velocity squared so that the signal power output decreases.

An especially severe limitation resides in the saturation density of the soft iron used in the poles. This density could be made available in the gap embracing the ribbon,

0 but the required thickness of the poles would be so great 7 as to set a relatively low top limit on frequency response.

Accordingly, it is an important object of this invention to overcome the difficulties enumerated above and provide an improved ribbon microphone characterized by a wide frequency response, satisfactory sensitivity, the provision of a relatively low noise output signal relatively easy to transmit over relatively long distances with relatively little additional increase in noise, all embodied in a relatively compact physical structure.

According to the invention, a vibratable ribbon means is situated in a magnetic field provided by opposed poles. This apparatus is supported in a housing about the size of a conventional microphone which also includes amplifying means responsive to the electrical signal provided by the ribbon means to provide an output signal relatively free from noise at a low impedance level suitable for transmission over relatively long distances. Preferably, switchable feedback means are provided intercoupling the amplifying means output and the ribbon means so as to roll olf the effective bass response of the system and reduce the sensitivity of the system effectively to bass signal sources close to the ribbon means.

In a preferred form of the invention the width of the ribbon, the cross sectional area of the poles and the separation between the poles is varied along their common length so as to maintain saturation of the poles and approximately uniform magnetic field density in the gap immediately adjacent to the ribbon means. The resulting pole configuration comprises conical tapered poles which converge at their small ends and a ribbon means tapered similarly to a norrow width at one end and considerable width at the other so that its overall resistance is relatively low.

According to theoretical consideration believed to be applicable to the objects of this invention, profiles of the opposed pole faces are preferably of parabolic outline for their cross section at all points to be precisely in proportion to the flux requirement. In practice they have been formed as truncated cones with extra material being left on the small ends with the discrepancy between desired and actual flux density being minor from the practical standpoint. An important useful effect of this type of construction is that at the small end of the poles, the baflle effect is negligible so that the off-axis response above 15 kilocycles is much better than that predicted for pole faces of uniform and larger cross section. Practically, good response and directivity are still provided at 20 kilocycles.

Numerous other features, objects and advantages of the invention will become apparent from the following specification when read in connection with the accompanying drawing in which:

FIG. 1 is a perspective view of a typical microphone system according to the invention;

FIG. 2 is an enlarged perspective view of the ribbon and magnetic field force structure;

FIG. 3 is an exploded view of the means for fastening the narrow end of the ribbon;

FIG. 4 is a side view of the structure shown in FIG. 2;

FIG. 5 is a top view taken through section 55 of FIG. 4;

FIG. 6 is a combined block, pictorial, schematic-circuit diagram of a preferred embodiment according to the invention; and

FIG. 7 shows the response change due to the switchable bass roll-0E means embodiment shown in FIG. 6.

With reference to the drawing and more particularly FIG. 1 thereof, there is shown a microphone system according to the invention suspended by the spring 11 and U-shaped bracket 12 so that sound incident upon the acoustically transparent cage 13 surrounding the ribbon assembly is transduced into an electrical signal relatively free of noise and presented at a convenient impedance level for transmission over relatively long distances without picking up appreciable undesired noise o-ver output cable 14.

Batteries

15 and 16, which provide power for the internally contained amplifying means, are shown mounted on the top of housing 17 to permit convenient battery replacement.

Referring to FIG. 2, there is shown a perspective view of a preferred embodiment of the ribbon means and source of magnetic field. A U-shaped permanent magnet comprising the two

sections

21 and 22 comunicates with the left pole piece 23 and the right pole piece 24 which terminate in a left bafile 25 and a right baffle 26, respectively. Baffles 25 and 26 define the air gap which decreases in width as a function of increasing height in which the correspondingly shaped ribbon 27 vibrates. The long edges 31 and 32 of ribbon 27, are closely adjacent to but spaced from bafiles 25 and 2-6, respectively, so that they embrace essentially the maximum amount of flux in the gap while being free to vibrate without rubbing against

baffles

25 and 26. The wide bottom edge 33 is clamped in lower support 34 which in turn is supported with its top surface against

pole pieces

23 and 24. The upper narrow edge of ribbon 27 is similarly clamped in horizontal member 35 by means including a brick 38 secured to member 35 by means including the screws 39 residing in tapped holes 40, shown more clearly in FIG. 3.

Horizontal supporting member 35 is sandwiched between a pair of left insulating washers 36 and right insulating washers 37 and a pair of left nuts 41 and a pair of right nuts 42 which mate with threaded left and right studs 43 and 44, extensions of left vertical support 45 and right vertical support 46, respectively. By adjusting the

nuts

41 and 42, the tension in ribbon 27 may be adjusted. Conductors '46 are arranged as a rectangular loop to prevent sensitivity to stray fields because the ribbon bisects the loop.

Referring to FIGS. 4 and 5, there is shown a side view of the structure shown in perspective in FIG. 2 and a top view through section 5 of FIG. 4, respectively. In these figures the reference numerals of FIG. 2 identify corresponding elements.

A pair of conductors 46 are soldered to clamping means 35 and 38 and provide a very low resistance path from the upper edge of ribbon 27 to the transformer input.

Referring to FIG. 6, there is shown a combined pictorial block schematic-circuit diagram of a preferred system according to the invention having switchable bass roll-off to compensate for the peak in bass response usually exhibited by a ribbon microphone when in the near field of a bass signal source. A transformer 47 has its primary 51 connected between right pole face 24 and conducting leads 46 to transformer-couple the low level signal developed across ribbon 27 into the input of transistor amplifier 51. The amplified signal from amplifier 51 is applied to emitter follower 52 to provide the amplified signal on output line 53 at a convenient impedance level so that the output signal may be carried by output cable 14 (FIG. 1) over relatively long distances without undesirable pickup. Batteries and 16 supply power to

amplifiers

51 and 52 when switched on by double pole-single throw switch A feedback

network comprising resistors

54 and 55 and capacitor 56 with the parameter values indicated functions with switch 57 closed to provide the bass rolloif shown in FIG. 7 when the resistance of ribbon 27 is approximately 0.5 ohm.

In a specific embodiment of the invention the ribbon was made of aluminum .08 mil thick, 1% inches long and tapering from .200 inch at the bottom to .07 inch at the top. Bafiies and 26 were of cobalt-iron with circular cross section at the bottom .33 inch in diameter, 1 inches long and .13 inch in diameter at the top.

Pole pieces

23 and 24 were made of mild steel material.

Magnets

21 and 22 were made of Almco material and weighed a total 4 of 36 ounces. Amplifier 51 was a P65 type Philbrick amplifier with a gain of 20 and emitter follower 52 was 21 P6 6 type Philbrick emitter follower amplifier. Battery 15 was +15 volts and battery 16 was 15 volts. Transformer 47 was an audio transformer with a turn ratio of :1.

With a sound pressure of one microbar, the voltage at ribbon 27 was 7 microvolts; at the secondary of transformer 47, 1.25 millivolts; and at

output

53, 25 millivolts, making the total voltage gain essentially 4,000.

It is evident that those skilled in the art may now make numerous modifications of and departures from the specific embodiment described herein and the specific dimensions and parameters set forth without departing from the inventive concepts. Consequently, the invention is to be construed as limited solely by the spirit and scope of the appended claims.

What is claimed is:

1. Electroacoustical transduction apparatus comprising vibratable ribbon means,

means including a pair of opposed pole means for providing a magnetic field,

means for supporting said vibratable ribbon means between said opposed pole means and in said magnetic field, the width of said vibratable ribbon means, the cross sectional area of each of said opposed pole means and the separation between said opposed pole means varying along a length common to said vibratable ribbon means and said opposed pole means so as to both maintain saturation of said pole means and establish approximately uniform magnetic field density in the gap between said opposed pole means immediately adjacent to said vibratable ribbon means,

means responsive to vibration of said vibratable ribbon means for providing a low level electrical signal, and

amplifying means responsive to said low level signal for providing an output signal relatively free from noise at a low impedance level suitable for transmission over relatively long distances.

2. Electroacoustical transduction apparatus in accordance with claim 1 and further comprising:

feedback means intercoupling the amplifying means output and said vibratable ribbon means for rolling off the effective bass response of said apparatus and reducing the sensitivity of said apparatus effectively to bass signal sources close to said vibratable ribbon means.

3. Electroacoustical transduction apparatus in accordance with claim 1 wherein said means responsive to said low level signal includes a conductor connected to said vibratable rib bon means and forming a generally rectangular loop that is substantially bisected by said vibratable ribbon means with the long sides of said loop substantially parallel to the length of said vibratable ribbon means.

4. Electroacoustical transduction apparatus in accordance with claim 1 and further comprising:

transformer means for coupling said low level electrical signal to said amplifying means.

5. Electroacoustical transduction apparatus in accordance with claim 1 wherein said opposed pole means comprise conical tapered pole elements which converge toward one another at their small ends so that the separation between said small ends is less than that between the large ends of the latter elements and said vibratable ribbon means is tapered similarly to a narrow width at the end near said small ends and larger width at the other while being characterized by relatively low resistance between its narrow width end and its larger width end.

6. Electroacoustical transduction apparatus in accordance with claim 5 and further comprising:

feedback means intercoupling the amplifying means output and said vibratable ribbon means for rolling off the eifective bass response of said apparatus and reducing the sensitivity of said apparatus effectively to bass signal sources close to said vibratable ribbon means.

7. Electroacoustical transduction apparatus in accordance with claim 5 wherein said means responsive to said low level signal includes a conductor connected to said vibratable ribbon means and forming a generally rectangular loop that is substantially bisected by said vibratable ribbon means with the long sides of said loop substantially parallel to the length of said vibratable ribbon means.

8. Electroacoustical transduction apparatus compris- 111g:

vibratable ribbon means,

means including a pair of opposed pole means for providing a magnetic field,

means for supporting said vibratable ribbon means between said opposed pole means and in said magnetic field,

the width of said vibratable ribbon means, the cross sectional area of each of said opposed pole means and the separation between said opposed pole means varying along a length common to said vibratable ribbon means and said opposed pole means so as to both maintain saturation of said pole means and approximately uniform magnetic field density in the gap between said opposed pole means immediately adjacent to said vibratable ribbon means.

9. Electroacoustical transduction apparatus in accordance with claim 8 wherein said means responsive to said low level signal includes a conductor connected to said vibratable ribbon means and forming a loop that is substantially bisected by said vibratable ribbon means.

10. Electroacoustical transduction apparatus in accordance with claim 8 wherein said means responsive to said low level signal includes a conductor connected to said vibratable ribbon means and forming a generally rectangular loop that is substantially bisected by said vibratable ribbon means with the long sides of said loop substantially parallel to the length of said vibratable ribbon means.

11. :Electroacoustical transduction apparatus in accordance with claim '8 wherein said opposed pole means comprise conical tapered pole elements which converge toward one another at their small ends so that the separation between said small ends is less than that between the large ends of the latter elements and said vibratable ribbon means is tapered similarly to a narrow width at the end near said small ends and larger width at the other while being characterized by relatively low resistance between its narrow width end and its larger width end.

12. Electroacoustical transduction apparatus in accordance with claim 11 wherein said means responsive to said low level signal includes a conductor connected to said vibratable ribbon means and forming a loop that is substantially bisected by said vibratable ribbon means.

13. Electroacoustical transduction apparatus in accordance with claim 11 wherein said means responsive to said low level signal includes a conductor connected to said vibratable ribbon means and forming a generally rectangular loop that is substantially bisected by said vibratable ribbon means with the long sides of said loop substantially parallel to the length of said vibratable ribbon means.

References Cited UNITED STATES PATENTS 2,093,664 9/ 1937 Swickard. 2,493,819 1/1950 Harry. 2,495,809 1/ 1950 Graham. 3,130,374 4/1964 Beres et a1. 179-1.2 3 ,293,3 64 12/ 196 6- Richter 179-1 .2

WILLIAM C. COOPER, Primary Examiner.

R. P. TAYLOR, Assistant Examiner.

US. Cl. X.R. 179-138, 119