US3866710A

US3866710A - Horn loudspeakers

Info

Publication number: US3866710A
Application number: US302752A
Authority: US
Inventors: Mario Cesati
Original assignee: Individual
Current assignee: Individual
Priority date: 1972-11-01
Filing date: 1972-11-01
Publication date: 1975-02-18
Anticipated expiration: 1992-02-18

Abstract

The specification describes horn loudspeakers including electromechanical transducer means comprising a diaphragm having a solidly oscillating surface in opposition to another surface spaced from the diaphragm at an interval which is minimal in a location in particular at the center of the diaphragm, and which progessively increases towards the edge of the diaphragm, this interval forming the first part of a passage which progresses outside said edge within the rigid walls of the horn, the said diaphragm, another surface and walls being arranged to provide a passage substantially uninterrupted, having no sharp modifications of direction and cross-sectional area, and whose cross-sectional area progressively increases from said location to the mouth of the horn.

Description

United States Patent I [191 Cesati [4 1 Feb. 18,1975

[76] Inventor: Mario Cesati, 16 Via De Vitalis,

Brescia, Italy [22] Filed: Nov. 1, 1972 [21] Appl. No.: 302,752

[52] US. Cl 181/159, 181/152, 181/192 [51] Int. Cl. Gl0k 11/10 [58] Field of Search 181/27 R, 31 R, 31 B, 159,'

[56] References Cited UNITED STATES PATENTS 1,792,655 2/1931 Norton 181/27 R 1,907,723 5/1933 Bostwick v 181/27 R 2,058,555 10/1936 Betts et a1. 181/27 R 2,194,070 3/1940 Giannini 181/27 R 2,956,636 10/1960 Bocrsma 181/31 R HORN LOUDSPEAKERS Primary ExaminerStephen J. Tomsky Attorney, Agent, or FirmMichaeI S. Striker [5 7] ABSTRACT The specification describes horn loudspeakers including electro-mechanical transducer means comprising a diaphragm having a solidly oscillating surface in opposition to another surface spaced from-the diaphragm at an interval which is minimal in a location in particular at the center of the diaphragm, and which progessively increases towards the edge of the diaphragm, this interval forming the first part of a passage which progresses outside said edge within the rigid walls of the horn, the said diaphragm, another surface and walls being arranged to provide a passage substantially uninterrupted, having no sharp modifications of direction and cross-sectional area, and whose crosssectional area progressively increases from said location to the mouth of the horn.

9 Claims, 14 Drawing Figures PATENTEU 1 81975 SHEEI 2 OF 4 HORN LOUDSPEAKERS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to horn type loudspeakers and particularly horn loudspeakers improved as specified below so as to provide particular advantageous features as compared to other well-known loudspeakers.

2. Prior art Loudspeakers as understood in the broadest meaning of their term are well-known and the existing comprehensive technical and patent literature of the art makes an extended description of prior art technique superfluous. It may be considered in principle that loudspeakers fall into two classes, viz. directed irradiation and horn loudspeakers. In'the former, the essential element consists of a diaphragm which moves by reproducing the motion in the time of a signal controlling it (in well-known modes and with the aid of equally wellknown means) and directly transmits this motionto the medium (air) in which it is immersed. The dimension of the diaphragm is related to the range of frequencies to be reproduced, and, more accurately, the lower the inferior range limit, namely the greater is the wave length, the larger is such dimension. The direct irradiation loudspeakers, particularly those having the most appropriate physical and geometrical characteristics, are well suitable to irradiate a frequency range in the air directly.

Regardless of certain peculiar characteristic features, the same results may be attained by horn loudspeakers, using a diaphragm of much smaller dimensions, arranged and operating at the mouthpiece (or throat") of a rigid duct of gradually increasing cross section (or horn) and ending with an opening (or mouth), being sufficiently extended in the air, to which the vibrations are transmitted. The horn is, therefore, an acoustic transformer, transforming a small-area diaphragm into a large air diaphragm by adapting a light medium (air) to a relatively heavy diaphragm.

This definition properly applies to the loudspeakers provided with large throat horn, having a throat being of the same appropriate approximate dimension as the diaphragm.

This condition may be fulfilled, however, without losses due to interference only in respect of frequencies for which the wave length is at least four times as much the dimension of the throat. Therefore, as a rule, the large-throat trumpets are only used for the lower portion of the range.

This limitation does not occur in the case of loudspeakers with a narrow-throat" or compression horn. In this case, there is provided a horn, in which from the diaphragm one passes to the horn surface proper (the duct or more rigid ducts of gradually increasing section) through a compression chamber, wherein the average density of the medium (air) increases. The chamber is formed of the surface of the diaphragm itself and an opposite wall being uniformly very close thereto and broken by openings for the horn passage. This passage consists of channels provided in such number, position and dimensions as to cancel the resonances of a higher order occurring in said compression chamber and due to the radial disturbance reflected by the edge of the chamber and. of suitable development such as to cause the components preceeding from the various'points of the diaphragm to meet in phase at the horn. In this connection, it should be mentioned that the undesired occurrence of the resonances referred to above does not develop in the case of loudspeakers with large-throat horns since there are no cross-sectional reductions in the duct and no sharp direction changes occuring in the passage between compression chambers and horn.

By even overlooking the manufacturing difficulties and the structural complexities occurring to a more or less large extent in the loudspeakers and, more accurarely, in the narrow-throat horns or compression horns in general, the conditions set for the elimination of said inconveniences, among which in the first place those resulting from the presence of higher-order resonances, are incompatible with those necessary to reach the maximum reference efficiency (average frequency).

In practice a diaphragm to throat area ratio of as much as 10 is possible,'corresponding to an efficiency of the order of 10 12%, whereas a ratio of 2 would have an efficiency of 20 22%.

It may, therefore, be assumed that the modern technology of loudspeakers, while still having a complete theoretical analysis of the phenomena occurring in the performance of the above considered loudspeaker type and by taking avail of a widespread experimental and production experience, is still a long way from qualitatively more advanced goals. With the above in mind, it is the object of this invention to provide improved loudspeakers which from certain aspects enjoy the advantageous properties of large-throat loudspeakers and at the same time the advantages of the gradual concentration of the medium (air), proceeding to higher frequencies, namely the desirable properties of the loudspeakers with an air chamber, while avoiding the most serious inconveniences.

More particularly, it is the objectof this invention to provide horn loudspeakers, improved so that therein the propagation takes place, from the very origin, along a duct gradually expanding and with development without sharp direction changes, thereby utilizing radial propagation, characteristically present in the air compression chambers with elimination, of the causes giving rise to the higher modes of resonances of which had to be cancelled in the air chamber loudspeakers within possible limits.

It is a further object of this invention to provide loudspeakers, improved so as to associate with the formerly mentioned advantageous combinations of properties that ensuring such a design as to voluntarily and controllably obtain modifications of the respective response curves.

BRIEF SUMMARY OF THE INVENTION Essentially, the improvement according to the present invention is characterized in that in the horn loudspeakers, including at least an electro-mechanical transducer, comprising at least a diaphragm the usable surface of which is solidly oscillating in an essentially orthogonal direction to the plane defined by the contour of said surface, and opposite to said diaphragm there is arranged an central first member the distance of which is minimum from said diaphragm, at a point internal to its contour, and progressively increases with the distance from said point.

These and still further features of the invention will become more apparent from the following detailed description of possible embodiment examples of the invention, reproduced, together with the graphical representation of some experimental conditions and results on the accompanying drawings, wherein the parts determining prevalently such conditions and results are depicted with a more accentuated graphical evidence as compared to other parts, means, devices and structures which may be individually developed by applying with suitable adaptations the technical'art knowledge.

THE VIEWS OF THE DRAWINGS FIGS. 1 and 2 show schematically a first embodiment example of a horn loudspeaker, improved according to the present invention, said Figures reproducing principally the horn (being in part fragmentarily represented) in its two symmetry planes intersecting orthogonally in the axis (xx) of the diaphragm;

FIG. 3 represents in a larger scale and detail the part being prevalently but not exclusively characteristical of the invention, namely the transducer system, including the diaphragm in a sectional view in any plane whatever containing said axis of the diaphragm itself;

FIGS. 4, 5, 6 and 7 represent in a sufficient graphical approximation the sections of the horn shown in FIGS. I and 2, obtained approximately at the levels designed by IV-IV and V-V, VI-VI and VII-VII, respectively, in said FIGS. 1 and 2;

FIG. 8 is a graph reproducing a series of response curves, obtained by well-known means in the axis of the loudspeaker shown in FIGS. l.-7, said curves being valid, excepting localized unevenesses and of limited width and in regard to the general motion and character for practically each and every loudspeaker which may be performed according to the present invention by applying suitable adaptations;

FIG. 9A represents schematically in a strongly enlarged scale and with limitation to a symmetrical half thereof the initial portions of ducts, complying with certain geometrical characteristics, from which a first series of the curves shown in FIG. 8 are obtained until reaching the flat characteristic of the response throughout the range;

FIG. 98 represents similarly further profiles of said initial portions of the duct, leading to other response curves as shown in FIG. 8 and such as to obtain a desired and controlled accentuation in the response of higher frequencies;

FIGS. 10A and 11A represent fragmentarilymodified embodiments of the improved loudspeaker, particularly arranged in a circular or eventually sectoral manner, and adapted to provide a practically flat response curve over the entire extension of the range, whereas FIGS. 10B and 11B represent similarly modifications of the loudspeaker shown in FIGS. 10A and 11A, respectively, comprising ducts equivalent to those given in FIG. 9B and such as to give rise to voluntary and controlled accentuations of the acoustic response in the highest portion of the range.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The essential feature of the invention will be initially laid out in reference to FIG. 3, reproducing the parts being adjacent the diaphragm. According to this embodiment form, the loudspeaker includes a hollow dome-shaped diaphragm with an active surface 10, being perimetrally supported by a suitable springloaded suspension I2 (see also the detail in FIG. 9A (part G)) and integral with a coil 14, cooperating with a magnetic circuit of traditional type. This circuit may comprise for example the

components

16 and 18, with radial extensions and an annular permanent magnet 20, the cavities of this circuit offering the possibility of being filled with well known sound damping material 22, such as glass wool or others. Opposite the diaphragm 10 there is a rigid wall (which will be called hereafter for simplification reasons opposite wall") designated by 24 and appertaining to a rigid body 26 the outer surface of which defines, together with the inner surface 28 of a hollow flared equally rigid body 30, open so as to accurately delimit (within the limits of the necessary motion freedom) the periphery of the usable portion of said diaphragm 10, the horn proper, generally designated (T). As may be seen also in the detail of FIG. 9A (G), reproducing the geometrical conditions of the loudspeaker shown in FIG. I, the initial narrower portion 32 of said wall 28 tangentially connects to the perimetral portion of the diaphragm 10.

The minimum distance between the diaphragm and the opposite wall 24 coincides with the central point (P) of the diaphragm itself or on the axis (X-X). This distance should be advantageously very small, and in practice the smallest compatible with the maximum width of the axial motion as provided for the diaphragm itself in its service. Starting from this point there is a duct being generally designated by (C) of progressively increasing section which continues without substantial deviation (apart from the bending of wide radius of the opposite surfaces) in the trumpet (T). The current art terms do not suggest any appropriate term to define a duct of this kind, since in its initial fraction extending up to the contour of the diaphragm, it may be considered both a chamber in which the diaphragm operates and the initial portion of the trumpet, in which the propagation takes place.

The geometry of this duct, independently of its large curvature radii relatively to the section of the duct, ,is

adapted for the purpose of the optimum response to different frequencies.

By way of demostration are given below the results and some deductions from experimental tests carried out on loudspeakers improved according-to the present invention, preset to operate in the range extending from 1 to 14.5 Kc. These loudspeakers had been preset to provide a highly faithful yield, retaining however a wide safety margin even in the case of overload. They comprised a magneto-dynamic unit with a flux density of 14,000 Gauss in the air-gap. The diaphragm was made out of light alloy with a thickness of 0.05 mm and the coil formed of a winding consisting of 36 turns of 0.1 mm copper wire over two layers. The spherical dome of the diaphragm had a usable diameter of 34 mm and the overall mass of the movable equipment was of the order of about 0.45 g. The natural resonance of the movable system on the unit was found to be of about 3 Kc. This magneto-dynamic unit, associated with a trumpet of cutoff frequency set at the value of 800 Hz. was associated with opposite walls-of different profile, that is to say, so as to build ducts, complying with the detail representation given in FIGS. 9A and 9B, in which the distance between the diaphragm l0 and the opposite wall changed from case to case to the effect that from the limit geometrical condition given in FIG. 9A (A), in which said distance was constant for almost the totality of the surface of the diaphragm (in practice up to the point (D)), one passed progressively to the geometrical condition shiwn in FIG. 9A (G) (corresponding to that in FIG. 3).

In the graph given in FIG. 8 the curves a, b g correspond to the response curves obtained from these different geometrical conditions A), B) G) in FIG. 3A. It may be seen therein that whilst in the conditions shown in FIG. 9A (A) the response notably drops already starting from about 7 Kc (curve a) the response extended without dropped for higher and higher frequencies until reaching a considerable flatness over its extension in the geometrical condition furnished by FIG. 9A (G) (curve g)). Accordingly, the demonstration was given of the importance attached to the progression of the distance between the opposite walls (fixed and movable walls, but in an equivalent manner also in the case of both movable opposite walls) in the propagation sense and the fact that the duct has no section reductions or sharp direction changes.

It was found that by suitably increasingthe increment of the distance between these walls, commencing from the point (P) and proceeding to the outside, for example by adopting the geometrical conditions in FIG. 9B and by still retaining the condition of the progressive increase of the distance between the walls, it is possible to obtain, to a large extent and practically at will, an increase in the response to higher frequencies. As a matter of fact, to the geometrical conditions shown in FIG. 98 (H and I) correspond the response curves h) and i), respectively of the graph in FIG. 8. The possibility of voluntarily obtaining in the desired measure a higher response to the high frequencies may be most helpful and interesting, for example to obtain outstanding effects in particular musical performances carried out under special environmental conditions.

FIGS. 1-7 represent a particularly interesting solution of the loudspeaker the horn (T) of which evolves from an annular shape in its parts adjacent the diaphragm (at the level VIIVII) to an extended and bent mouth shaped configuration in an orthogonal plane to the axis (XX) (at the level IVIV), passing through sections of progressive evolution, but still retaining a costant path by section increment in each of its sections in the planes containing such axis. On the other hand, other equivalent solutions may be adopted to diffuse under different conditions, e.g. for circular sound diffusion or sectoral sound diffusion.

FIGS. 10A and 11A exemplify loudspeakers complying with the above-described conditions and adapted to furnish response curves of the type as shown in FIG. 8 (g), being considerably flat over the entire range, namely the most desirable motion in all directions comprised between the generatrices of a conical surface and respectively in the axial plane (X-X) of the loudspeaker.

In the example in FIG. 10A, the loudspeaker included a diaphragm l0 and a magneto-dynamic unit being equivalent to those previously described with the difference that the horn (T') is present for a diffusion essentially in a large-opening cone, the rigid walls of the horn connecting under the previously described conditions to the curvature of the diaphragm l0 and that of the opposite wall 24'.

In the embodiment of FIG. 11A, the loudspeaker for circular or sectoral sound diffusion comprises a convexly curved diaphragm operating in opposition to an opposite wall 24", forming with the latter a duct (C") being functionally equivalent to that given in FIG. 9A (G) and therefore adapted to provide a response curve being considerably flat throughout the range (FIG. 8 (g)). Therein, the geometrical progression in the different sections of this duct was obtained from the curvature of the diaphragm 110.

By modifying these curvatures of both the diaphragm and opposite surface, being fixed or possibly movable, it is still possible to controllably obtain the desired acoustic response conditions. For example, the loudspeaker in FIG. 10A may be modified as shown in FIG. 108 by modifying the profile of the opposite wall (24'a), thus obtaining a response curve as given in FIG. 8 (h), the duct (C'a) being equivalent to that in FIG. 313 (H). Similarly, the loudspeaker in FIG. IIA may be for example modified as represented in FIG. 113 by imposing upon the opposite wall (24"a) such a convexity as to make the respective duct (C"a) equivalent to that in FIG. 9B (I), thereby obtaining a response curve being notably of accentuation for the high frequencies, namely the curve in FIG. 8 (i).

The circular diffusion loudspeakers of the type represented in FIGS. 11A and 11B and equivalent ones, are particularly interesting for uniformity in the circular (or a sector ofa circle) diffusion due to the perfect rectilinear sense of the propagation direction. Furthermore, these loudspeakers permit the use of different diaphragms, and in such event the ducts and related horns may be functionally considered split into two parts, separated by the ideal median plane of said ducts and horns. At any rate, since the improved loudspeakers according to this invention have been described and represented by way of nonlimiting example and to provide a demostration of the features and essential principles on the basis of which the aforementioned novel and unforeseeable technical effects may be obtained, it is manifest and apparent that such loudspeakers could be materially performed according to several equivalent technical and manufacturing solutions, without departing from the scope of the present invention.

I claim:

1. An acoustic device, particularly a horn-type loudspeaker, comprising an electro-magnetic transducer comprising a substantially rigid diaphragm having an active surface having a center and being mounted for oscillation in its entirety in a direction substantially normal to the outer contour of said surface; a first stationary member having an outer surface a portion of which is closely adjacent to said active surface; a second stationary member surrounding at least in part said first stationary member and having an inner surface from said outer surface of said first member, the curvature of the surfaces of said members and said diaphragm defining an air passageway extending from the center of said diaphragm in a direction transverse to said direction of oscillation, the cross section of said air passageway being minimal at said center and increasing in said direction.

2. An acoustic device as defined in claim 1, wherein the rate of increase of said cross section of said air passageway is continuous.

3. An acoustic device as defined in claim 1, wherein said members extend Substantially in said direction of oscillation of said diaphragm.

4. An acoustic device as defined in claim 1, wherein said outer surface of said first stationary member is convexly curved toward said active surface and toward said inner surface.

5. An acoustic device as defined in claim 1, wherein said inner surface of said second stationary member flares outwardly in a direction away from said diaphragm.

6. An acoustic device as defined in claim 1, wherein said first member is a rotationally summetric body with curved toward said portion of said first member.

Claims

6. An acoustic device as defined in claim 1, wherein said first member is a rotationally summetric body with respect to an axis extending in the direction of oscillation of said rigid diaphragm.

7. An acoustic device as defined in claim 1, wherein said inner surface of said second member is a surface of revolution.

8. An acoustic device as defined in claim 1, wherein said second member is a sector of a body of revolution having an axis substantially normal to said active surface.

9. An acoustic device as defined in claim 1, wherein said active surface of said diaphragm is concavely curved toward said portion of said first member.