GB2113041A - Loudspeaker diaphragm - Google Patents

Abstract

A dome-type moving coil driver for a loudspeaker has a diaphragm (28) with a dome (32) and an annular surround (34), the surround producing substantial radiation at frequencies below the active range of the dome. The surround (34) is of folded annular shape and its outer diameter is greater than 2.3 times its inner diameter. The dome is of diameter at least 1.5 inch (3.8 cm) and is of either partial spherical or parabolic shape. <IMAGE>

Description

SPECIFICATION Dome-type moving coil driver for loudspeaker and diaphragm therefor This invention relates to loudspeakers having moving coil, permanent magnet drivers, also referred to as electrodynamic loudspeakers. More specifically, the invention relates to dome-type drivers, and to the design of diaphragms for such drivers. As used herein the term "driver" refers to an individual speaker and the term "loudspeaker" refers to a combination of drivers organized as an integrated system.

In the attempt to provide loudspeaker systems for radiating sound effectively throughout the range of sensitivity of the human ear, commonly assumed to be 20 Hz to 20 KHz, it is common to provide two or more drives in a loudspeaker system, each driver being adapted for radiating sound primarily within only a portion of that range.

Electronic crossover networks operate to distribute the electrical audio output power to the drivers so as to acheive a composite response, as measured in decibels of acoustic output per unit of electrical energy, which is as nearly uniform over the full sensitivity range as possible. In the design of such systems, attention must be given to the dispersion of acoustic energy, with wider dispersion throughout the entire range being generally preferred. in addition, attention must be given to the physicai durability of the loudspeakers, and partcularly the diaphragms which generally have extended surface areas and are made of thin sheet material. Also, harmonic distortion must be considered.

In the design of such multiple driver systems, a pivotal decision is the choice of the primary low frequency radiator or "woofer". A common choice is a cone driver. The low frequency response of such drivers depends in general on the diameter of the cone, and the high frequency response has an inverse relationship to that diameter. As with cone speakers generally, the dispersion of radiated sound is limited by the expression (1)A=wd where d equals the diameter of the cone and A is the wave length below which the dispersion becomes more directional. The choice of the diameter thus determines both the low frequency response characteristics of the system and the degree of dispersion.In any case, it is frequently desired to augment the response of the system at frequencies above the radiating capability of the woofer by the use of a "tweeter", with or without an intermediate or mid-range speaker.

Dome-type drivers are now well-known.

Examples include the constructions shown in U.S.

Patent No. 3,328,537 issued June 27 1967 to Hecht, and U.S. Patent No. 4,190,746 issued February 26, 1980 to Harwood et al. Because the diaphragms of these drivers are convexly curved toward the direction in which sound is radiated they have excellent dispersion at high frequencies.

Thus the combination of cone-type woofers with dome-type drivers of appropriate dimensions for higher frequency response serves to maintain a dispersion of sound more nearly comparable to that of the woofers, as compared with the combination of cone-type woofers with cone-type tweeters. However, dome-type drivers of conventional construction, as shown in the above patents for example, if designed to provide adequate response at the highest frequencies for which the system is designed, do not provide sufficient response in the middle frequencies above those at which the woofer is effective. This defect may require the inclusion of a third or mid range driver in some cases.Coversely, if a dometype driver is designed for a response that is adequate to compensate for the cutoff of the woofer in the mid-range frequencies, the high frequency response is often severly limited and a third driver or tweeter must be employed to extend the high frequency response.

By reason of the foregoing considerations, an important object if this invention is to provide a driver having substantial mid-range as well as high frequency response, and which has a substantially wider dispersion characteristic than cone-type drivers.

Another object of the invention is to provide a driver of the foregoing type having a durable structure, capable of handling substantial acoustic power with minimal fatique and breakup.

A further object is to provide a driver of the foregoing type having low distortion in the midfrequency range.

A still further object is to provide a driver constructed of materials that have uniform properties that are relatively unaffected by humidity, changes or extreme of temperature, and fatique.

With a view to accomplishing the foregoing objects, this invention features a dome-type driver configuration having a diaphragm comprising a dome section and a folded annular surround section that is so constructed as to radiate substantial acoustic power at frequencies below the range that the dome section could effectively radiate if suspended by a surround of conventional form.

The surround according to this invention has an annular attaching means that is fixed in relation to the permanent magnet structure, the location of this attachment being spaced a substantial distance from the voice coil at which the dome and surround are joined. That is, the diameter of the diaphragm as measured at the annular attachment to the fixed magnet structure is substantially greater than the diameter of the dome section and voice coil. Preferably the ratio of the former diameter to the latter is greater than 2.25.

The dome section itself may have a partial spherical shape, or it may have a parabolic shape, the latter providing an extended high frequency response as compared with the former.

By a proper choice of materials for the diaphragm structure, especially materials having adequate internal damping, it is possible to provide a mid-range loudspeaker having a smooth frequency response characteristic with a large dynamic range, as well as an extended frequency response characteristic.

By way of example, two embodiments of the invention will now be described with reference to the accompanying drawings, in which: FIGURE 1 is an elevation in diametral section of a first, preferred embodiment of the invention.

FIGURE 2 is a view in plan of the embodiment of FIG. 1.

FIGURE 3 is an elevation in diametral section of a second embodiment of the diaphragm having a parabolic dome.

FIGURE 4 shows frequency response characteristics for drivers, used for purposes of comparison.

FIG. 1 illustrates a preferred form of dome-type driver according to this invention, designated generally at 1 2. This comprises a permanent magnet structure of conventional form, including a pole piece 14 having an integral bottom plate portion 16, a permanent magnet or magnets 1 8 and a top plate 20, these parts being suitably joined as by adhesive in a conventional manner.

The magnet or magnets establish a permanent magnet flux in an annular gap 22. A mounting plate 24 is suitably attached to the top plate 20, and a clamping plate 26 is suitably secured to the plate 24 for clamping a diaphragm 28 thereto. The diaphragm is attached to a voice coil 30 which vibrates to radiate acoustic energy developed by the flow of current in the coil.

The diaphragm 28 comprises two sections, namely, a come section 32 and a surround section 34. In the illustrated and presently preferred embodiment these two sections are vacuum formed or otherwise fabricated from one sheet of material, thus being one integral body.

Alternatively, the two sections may be formed from separate sheets of material, the outer periphery of the dome section and the inner periphery of the surround section being attached together by any suitable means such as adhesive.

The references herein to the dome section and the surround section being "joined" together are intended to include both the preferred, integral embodiment and the alternative embodiment; and the reference to the surround section being "folded" mean that it has a curved shape in cross section as shown in FIG. 1 The curvature of this shape is preferably substantially circular.

At the juncture of the dome and surround there is attached a cylindrical coil form 36 which supports the voice coil 30 within the gap 22. The diaphragm 28 also has a flange 37 which is secured between the plates 24 and 26. Preferably, the dome section and surround section are imperforate and have the illustrated configurations in all sections on planes that pass through the axis of the driver, as indicated by FIG. 2. Holes 38 are provided in the plate 26 for securing it by screws to the plate 24.

Terminal 40 are secured to the plate 26 and connected by leads 42 and 44 to opposite ends of the voice coil 30.

A A capsule 46 comprising a body of fiberglass, foam plastic or fibrous mass is adhesively secured to a surface of the pole member 14 beneath the dome 32, and serves to absorb acoutic energy radiating from the bottom surface of the latter, thereby minimizing reflections from the member 14 that would otherwise produce interference with the acoustic energy radiating directly from the dome surface.

The gap 22 as referred to herein comprises both the space between the form 36 and the member 1 4 and the space between the voice coil 30 and the top plate 20. This gap is substantially filled with "ferrofluid", which is liquid containing suspended magnetic particles and is conventionally used in loudspeakers to act as a heat conduit from the voice coil to the plate 20.

In the structure of FIGS. 1 and 2, the relative dimension of the dome and surround section are chosen to cause the surround surface to act as a radiator of substantial acoustic power. To illustrate the importance of dimensional considerations, FIG. 1 has been labeled with letters "a" and "d" to represent the edges of the outer periphery of the surround 34, letters "b" and "c" to represent the points of juncture between the dome and the surround, letters "e" and "f" to represent the points on the surround that are furthest spaced from the plane of the annular attachment of the surround between the plates 20 and 24, and the "g" to represent the point on the dome furthest removed from that plane. The letters "h", "i" and represent points in the same plane on the normals thereto passing through the points "g", "e", and "f", respectively.The following dimensions apply to a presently preferred embodiment.

Dimension Inches ad 4.6 bc 2 ei = fj 0.47 gh 0.63 It is noted that the ratio "ad" to "bc" is 2.3. In practice, the above dimensions may be varied in magnitude and relative value, depending upon the frequency range of useful response to be achieved and the power handling capability of the materials used. In order to obtain a sufficiently low midfrequency output, with a structure that is stable enough for adequate power handling with the available methods and materials of diaphragm construction, the diameter of the dome at its periphery, and the inner extremity of the surround, must be at least approximately 1.5 inch (3.8 cm).

Also, in any case, the ratio ad/bc is at least 2.3.

Also, the ratio ei/ab is preferably at least 0.33.

The materials of which the diaphragm is constructed are of great importance in achieving the desired power handling capacity without breakup, ability to withstand various conditions of humidity, heat and cold, and resistance to fatigue failure. The presently preferred diaphragm material for both the dome and surround is a talcfiltered polypropylene sheet material. Other diaphragm materials that may be employed are vinyl film upon which a layer of aluminum has been vacuum deposited, and polycarbonate film.

FIG. 3 illustrates a second embodiment of the diaphragm according to this invention. This embodiment is the same as that of FIGS. 1 and 2 except for the shape of the come in cross section.

A dome 48 of parabolic section is formed integrally with an annular folded surround 50 of substantially the same form as the surround 34, or is attached to an annular surround of the same shape, the diaphragm having a flange 52 for annular attachment in a fixed relation to the permanent magnet components. A coil form 54 is attached to the juncture of the dome and surround, and supports a helical voice coil 56. The parabolic shape of the dome extends the high frequency end of the response curve as explained more fully below with reference to FIG. 4.

FIG. 4 illustrates typical frequency response curves for three taken for purposes of illustration.

A solid line 58 represents the response of the embodiment of FIGS. 1 and 2 having a partial spherical dome. A broken line 60 represents a portion of the response curve of a prior art dome driver have the same dome dimensions as FIG. 1, but having a typical surround as used in the prior art. In the prior art, surrounds are used merely to suspend or support the dome. Such surrounds may be of the same material as the dome or of a different material, and have relatively small dimensions in the radial direction with reference to the axis of the loudspeaker. The curve 60 represents an embodiment having a dimension "ab" of one-quarter inch or less and having no appreciable sound radiating capacity.The curves 58 and 60 merge at a frequency slightly above 100 Hz, and at higher frequencies the curves are substantially coincident by reason of the common characteristics of the domes.

FIG. 4 also has a broken line 62 which represents a driver like that of FIG. 1 but having the diaphragm of FIG. 3. This curve shows an extended high frequency response, merging with the solid line 58 at approximately 10,000 Hz. At lower frequencies the parabolic dome embodiment has the same characteristics as the dome embodiment of FIGS. 1 and 2.

An examination of FIG. 4 shows that the embodiments of both FIGS. 1 and 3 provide a substantial improvement over the prior art dome configuration by reason of the extended low frequency response. Taking the region of substantially "flat" response as representing zero decibels, it may be assumed for example that the effective frequency range of the driver includes those frequencies that are less than six decibels below the zero value. On this basis, the conventional dome driver has a useful range of about 650 to 10,000 HZ, the partial spherical dome driver according to this invention has a range of about 300 to 10,000 Hz, and the parabolic dome speaker according to this invention has a range of about 300 to 15,000 Hz.

In certain embodiments of the parabolic speaker, the high frequency response may be further improved, with the upper limit of useful response reaching close to the limiting value of 20,000 Hz, thereby eliminating the necessity of incorporating a separate tweeter in the system.

In the practice of this invention, it is preferred that the points on the surround furthest removed from the plane of annular attachment thereof, such as the points "e" and "f" of FIG. 1, are closer thereto than the point on the dome furthest removed from such plane, such as the point "g".

This criterion will ensure a better dispersion of acoustic energy over a wider range of frequencies, compared to cone-type speakers of equal dimensions.

One of the advantages of the present invention, as previously noted, is the durability of a typical dome driver, designed to provide an adequate range of frequency response for use as the mid range driver in a multi-driver loudspeaker system.

That is, the configurations of the dome and surround, when chosen for the desired frequency response, are such that with the use of the preferred materials, diaphragm breakup is minimized. Also, the preferred materials enable the driver to perform as designed under varying condition of humidity and temperature.

It will be understood that although the configurations of the domes in the illustrated embodiments are herein described as partial spherical and parabolic, in practice there may be variations in the shapes due to conditions of fabrication. Empirical methods may be employed to determine the exact effects of such varations on overall driver performance. However, such variations do not represent departures from the spirit or scope of this invention

Claims (14)

1. A moving coil driver for a loudspeaker having, in combination, permanent magnet means having pole pieces defining an annular gap, a diaphragm comprising a dome section having a diameter of at least 1.5 inch (3.8 cm) at its periphery and a folded annular surround section, the dome section being joined at its periphery to the inner extremity of the surround section and the outer extremity of the surround section having annular attaching means fixed in relation to the permanent magnet means, and a coil attached to the diaphragm at the junction of the dome and surround sections and extending into the gap, the diameter of the surround at said attaching means being greater than 2.3 times the diameter of the coil.

2. The combination according to claim 1, in which the dome has substantially partial spherical shape.

3. The combination according to claim 1, in which the dome has substantially a parabolic shape.

4. The combination according to claim 1, in which the surround has a substantially circular curvature in any diametral cross section of the diaphragm.

5. The combination according to claim 1, in which the maximum distance of the surface of the dome section from the plane of said annular attaching means is greater than the maximum distance of the surface of the surround section from said plane.

6. The combination according to claim 4, in which the maximum distance of the surface of the surround section from the plane of said annular attaching means is at least one-third the distance from the coil to said annular attaching means.

7. A diaphragm for a moving coil driver comprising a dome section having a diameter of at least 1.5 inch (3.8 cm) at its periphery, and a folded annular surround section joined at its inner extremity to the periphery of the dome section and having an outer extremity adapted for annular attachment, the diameter of said outer extremity being greater than 2.3 times the diameter at said inner extremity.

8. The diaphragm according to claim 7, in which the dome section and surround section comprise a single integral body.

9. The diaphragm according to claim 7 or claim 8, in which the dome section has substantially a partial spherical shape.

10. The diaphragm according to claim 7 or claim 8, in which the dome section has substantially a parabolic shape.

11. The diaphragm according to claim 7 or claim 8, in which the surround has substantially a circular curvature in any diametral cross section of the diaphragm.

12. The diaphragm according to claim 7 or claim 8, in which the maximum distance of the surface of the dome section from the plane of said outer extremity is greater than the maximum distance of the surface of the surround section from said plane.

13. The diaphragm according to claim 7 or claim 8, in which the maximum distance of the surface of the surround section from the plane of said outer extremity is at least one-third the distance from the juncture of the surround and dome sections to said outer extremity.

14. A moving coil driver substantially as hereinbefore described with reference to and as shown in the accompanyinq drawings.

15 A diaphragm for a moving coil driver substantially as hereinbefore described with reference to and as shown in Figure 1 or in Figure 2 of the accompanying drawings.