US 3032136 A
Description (OCR text may contain errors)
May 1, 1962 A. s. HEGEMAN, JR
LOUDSPEAKER Filed Sept. 27, 1956 A ORNEYS Uited States Pate I 3,032,136 LOUDSPEAKER Andrew Stewart Hegeman, In 176 Linden Ave., Glen Ridge, NJ. Filed Sept. 27, 1956, Ser. No. 612,415 Claims. (Cl. 181-31) My invention relates to an acoustic radiator construction, as for use as a loudspeaker in a high-fidelity audio system.
It is an object of the invention to provide an improved construction of the character indicated.
Another object is to produce an acoustic radiator having essentially a hemispherical response pattern.
It is another object to provide an improved acoustic radiator construction characterized predominantly by omnidirectional response in essentially a single radiating plane.
A further object is to provide an acoustic radiator inherently capable of strong omnidirectional radiation generally transverse to the predominant axis of the radiator.
Still another object is to provide an efficient means whereby longitudinal mechanical driving oscillations may be caused to produce strong radiation transverse to the oscillatory motion, and avoiding predominant or peaked response at particular frequencies within a broad response band.
It is a general object to meet the above objects with constructions of inherent mechanical simplicity, having broadband omnidirectional response, and utilizing certain commercially available parts, whereby high quality performance may be achieved at relatively low cost.
Other objects and various further features of novelty and invention will be pointed out or will occur to those skilled in the art from a reading of the following specification in conjunction with the accompanying drawings. In said drawings, which show, for illustrative purposes only, preferred forms of the invention:
FIG. 1 is a longitudinal sectional view through a loudspeaker incorporating features of the invention;
FIG. 2 is a similar view to illustrate a modification; FIG. 3 is a fragmentary view in longitudinal section to illustrate alternative features of construction;
FIG. 4 is a longitudinal sectional view of a further construction of the invention;
FIG. 5 is a simplified view in side elevation and on a reduced scale to illustrate an array of my radiators for broader-band response; and
FIGS. 6 and 7 are diagrammatic views illustrating design features of my omnidirectional radiator.
Briefly stated, my invention contemplates the employment of an outwardly flaring or generally conical relatively stiff member as the means whereby driving longitudinal mechanical oscillation is converted into strong omnidirectional radiation generally transverse to the longitudinal axis. The flaring member may be a cone of stiff paper electromechanically driven at the small end or apex of the cone, with oscillation on, or essentially on, the cone axis. Efliciency of transverse radiation is promoted by employment of a fixed acoustic load Within the substantial inner volume of the cone, and in relatively close proximity to the inner surface of the radiating cone; the compression waves generated between the inner side of the cone and the acoustic load are directed along the axis of the cone and provide the axial component used to create an overall hemispherical radiation pattern. The cone may be employed alone as a radiator, or it may function in cooperation with other radiators to achieve particular omnidirectional responses. Various design features are described, with a view to avoiding response peaks at particular frequencies and with a view to controlling the response pattern.
ice I Referring to FIG. 1 of the drawings, my invention is shown in application to a loudspeaker construction employing essentially an outwardly flaring member or cone 10 of relatively stiff material, such as kraft paper, parchment or the like, as the source of radiation. The small end of the radiator 10 is driven in longitudinal mechanical oscillation, and the entire outer surface of radiator 10 cooperates to develop acoustic compressional waves radiating generally transversely of the axis of oscillation, that is, omnidirectionally and substantially radially outwardly from the cone axis. In the form shown, electrodynamic means are employed to drive the small end of radiator 10.
The electrodynamic driver may be of essentially conventional construction and is shown to comprise a magnctic core 11, which may be permanently magnetized. A driver coil 12 is Wound on a tubular member 13 extending into the gap or gaps of the magnetic circuit, and tubular member 13 is, in turn, centrally but axially flexibly supported by means such as a membrane or spider 14. In the form shown, the apex of cone 10 is cut off to define a small end which can be conveniently fitted and secured to the outer projecting end of the coil member 13, as at the location at which spider 14 is connected thereto.
When the coil 12 is excited at frequencies within the response band of radiator 10, the entire radiator 10 is mechanically symmetrically excited with generally longitudinal motion. For each element of area of the outer surface of cone 10, this motion has a generally radially outwardly directed component, and the integration of such components over the entire outer radiating surface is found to produce an omnidirectional radiation response throughout essentially a radial plane and over the full frequency range of the speaker. The radially inward radiation reacts with the air mass inside the cone to provide a generally axial component which, when added to the radial-plane component, may develop hemispherical or other desired Wide-angle response of the unit.
Radiating efficiency is found to be substantially improved by acoustic loading within the interior of radiator iii, as by provision of the solid fixed conical plug 15. The contour of loading member or plug 15 preferably conforms to the inner contour of radiator 10, so as to establish a relatively close annular clearance or pocket 16, thus providing backing pressure for cone 10. The plug 15 may conveniently be a simple casting, as of plastic or plaster; plug 15 may be fixedly suspended from an external framework, but in the form shown, the base of a stud 17 is embedded therein, so as to afford a means of threadedly securing the same to the center pole of the magnetic circuit.
For operation as a tweeter, as for example in the frequency range from 5 or 6 kc./s., on out beyond the end of the audible region, and even to 70 kc./s., the outward flare or slope of the surface of radiator 10 may be in the order of 1:3 to 1:6 (i.e. tangent of half the apex angle of the cone, /3 to /e), and the stiff sheet material may be kraft paper, drafting vellum, or the like, thinly varnished, if desired, for stiffening purposes. The low-frequency cut-off in the response is attributable to the maximum cone diameter (substantially one-half wavelength at low-frequency cut-01f), and to avoid a resonant peak or sharp fall-off at the low-frequency end of the response, I prefer to scallop, undulate, or merely truncate the large end of radiator 10, as at an inclination to the speaker axis, to form the outer edge 18; the outer edge 18 may thus be said to be discontinuous in a plane normal to the transducer axis, that is, to avoid lying in said normal plane. In the form shown, the acoustic load 15 is a right-circular cone, the large end 19 of which is located substantially at the axially inner extreme of the truncated end 18 of cone 10, and the end 13 is reinforced as by cementing a folded hem or seam 2d of the radiator material around the periphery of end 18. The seam or hem 2t acts as a mechanical load to restrict the mode of vibration of the whole structure and to prevent undesirable modes of vibration; alternatively, the element 20 may be viewed as a circumferential ring of mechanical loading material, added to the structure of the cone per se. The acoustic load 15 is found to broad-band the device and to eliminate resonant peaks in the axial component of radiation.
The acoustic-load member 15 may extend further outwardly, as by locating the large end 19 at or beyond the axially outer extreme of the radiator end 18. In FIG. 2, I illustrate such an alternative construction in which the acoustic-load member 15 is longer and is truncated at 19 to conform with the truncated radiator end 18, preferably slightly beyond said end 18. Other contours of the load member 15 with respect to that of the radiating cone may be provided to achieve specific radiation patterns, particularly as far as the axial component is concerned.
For most applications of my speaker, the axis of radiator It} is vertical, so that radiation is omnidirectional, i.e. essentially 360 horizontal and more than 180 vertical. This means that a single axially flexible membrane or spider 14 can sufiice for positioning the movable structure -12-13. For ruggedness, however, I. prefer a two-point suspension, as illustrated in FIG. 3, wherein a second axially flexible membrane or spider 21 is used to support the inner axial end of the coil assembly 13. Whether the suspension is of the single-point variety (FIGS. 1 and 2) or of the two-point" variety, the mass of the moving parts and the compliance of the suspension parts can be and preferably are chosen for mechanical resonance below the response range of the speaker, thereby eliminating the suspension as a factor in the radiation response.
FIG. 3 further illustrates that the acoustic-load memher need not be solid. Such member in FIG. 3 is a hollow cone 22 as of rolled or spun sheet metal, and with an inner-wall coating 23 of sound-absorbing material such as sponge rubber or undercoating compound. A screw 24 in the small-end base of cone 22 secures the cone to the center leg of the magnetic circuit.
FIG. 4 illustrates another variety of speaker embodying the invention, the particular configuration being one which I have found to be well adapted to response in the medium-frequency range, as from 250 c.p.s. to 6 kc./s., or if necessary, to 12 or 13 kc./s. The speaker of FIG. 4 differs from those previously described, in that the free-ended radiating cone 26 coacts with a coxial radiating diaphragm 27, of wider divergence angle. The rim of diaphragm 27 is secured to frame means 28 in the usual manner, and the small ends of both the diaphragm 27 and the free-ended cone 26 are secured to the drivercoil assembly 29. A single-point supporting membrane 30 is shown for the moving parts. The total cross-sectional area at the large end of the diaphragm 27 is preferably about the same as that at the large end 31 of the free cone 26, and the latter is preferably truncated to avoid resonant eifects. A small inverted cone 32 secured within and to the small base of cone 26 provides a means for reducing internal acoustic interference near the driven end of the radiator.
Tests on the structure of FIG. 4 have established not only omnidirectional radial radiation, essentially flat over the frequency range of interest, but have also shown the same uniformity of response over the entire hemisphere above the unit. The forward or upward radiation thus substantially matches and merges with the transverse or radially outward radiation.
The simplified view of KG. 5 illustrates an effective mutiple mounting for speakers of the variety of FIGS. 1 to 3, combined with a speaker of the FIG, 4 variety.
A supporting frame including a base 35 and an upper spider or yoke 36 are spaced by plural legs 57. The frame 28 of the medium-frequency speaker 26 is mounted on the base 35, and the yoke 36 holds the tweeter magnet to position the radiator 16 coaxial with speaker 26. Excellent omnidirectional horizontal response over the entire medium to extreme-high audio-frequency range is achieved by this combination when oriented vertically and coaxially, as shown.
While I know from experience that the described structures do produce high-quality omnidirectional radiation as indicated above, I have thus far been unable to fully analyze and account for'the performance. However, in conjunction with the conical radiators of FIGS. 6 and 7, I offer a simplified explanation which is helpful for design considerations. This analysis relies on the conical flare angle 0:, and the relation between the speed of sound in air and the speed of sound in the material of the radiator. For any given axial excitation at the driven end Stl, as suggested by the double arrow 51, a lateral or transverse radiation component will commence immediately at the small end 51?, as suggested by the lower lateral arrow 52, and proceeding at the speed of sound in air. For the same axial excitation, a compressional wave will travel along the flaring edge 5'3 at the speed of sound transmission in the conical radiator, and each incremental advance of this compressional wave will be accompanied by further axially advanced lateral radiation in air. If the flare angle is substantially equal to the speed of sound transmission in air, divided by the speed of sound transmission in the radiator material, then the first lateral radiation 52 will have reached the location 52. by the time that the said compression wave reaches the large end 5-?- of the radiator, at which time a final lateral radiation component 55 is emitted. For all'interrnediate incremental locations along the edge 53, correspondingly delayed radiation components 56 will have been emitted, to produce a uniform laterally moving wave-front 57, substantially parallel to the radiator axis and having characteristic propagation in the radial direction 58. This will be appreciated as representing omnidirection propagation in the radial plane of the radiator axis, inasmuch as every element of outer radiatingsurface area contributes equally.
FIG. 7 illustrates similar analyses for relationships establishing, on the one hand, downwardly dished omnidirectional conical radiation (designated by arrow 68, inclined at an angle 6 below the horizontal) and, on the other hand, upwardly dished omnidirectional conical radiation (designated by arrow 61, inclined at an angle 6 above the horizontal. If the flare slope exceeds the speed of sound transmission in air divided by the speed of sound transmission in the radiator material, then the eavy dashed lines and arrows suggest development of the downwardly facing wavefront 62, propagated in direction as. if on the other hand the fiare slope is less than this relation between the speeds of sound in air and in the radiator, then the lightly dashed arrows apply, suggesting development of the upwardly facing wavefront 63, propagated in the direction 61.
Although wide-angle response for the axial radiation component has been experimentally confirmed, I have thus far been unable to develop a satisfactory explanation. My present view is that for the case of the truncated radiating cone with correspondingly truncated acoustic load, the radiator in the axial direction is an annulus inclined to the axis. Thus viewed, the radiator is an infinite number of point sources, each out of phase with the other, so that no striking phase coincidences are realized, and the response pattern, instead of being peaked as might be expected from a radiating ring normal to the axis, exhibits a much broader pattern, approaching the spherical radiation which is characteristic of a point source. Actually, something of the same effect is noted for the case of the unloaded truncated cone and for the truncated cone loaded with a right-conical acoustic load (as in FIG. 1) but for best dispersion of axial radiation, I prefer that the profiles of the radiator rim and of the load rim shall be in substantial conformity, as in FIG. 2.
it will be seen that l have described improved acoustic radiator constructions featuring omnidirectional response. Such response may be in essentially a single plane, or it may be conically dished in a desired direction away from said plane. Also, if desired, hemispherical response may be achieved regardless of the spacedistribution of the response pattern. The uniformity of response may be maintained over an extended frequency range, unspoiled by spurious resonances such as any that may be attributable to the suspension mechanism for the moving parts.
While I have described the invention in detail for the preferred forms shown, it Will be understood that modifications may be made within the scope of the invention as defined in the claims which follow.
1. A loudspeaker comprising a hollow relatively stifi outwardly flaring radiator, driving means rigid with and directly mechanically driving the small end of said radiator in longitudinal oscillation, a base fixed relatively to said longitudinal oscillation, axially compliant support means connecting said base to the small end of said radiator and constituting essentially the only support of said radiator, the substantial outer surface of said radiator being freely exposed for radiation generally radially of the flare axis and the enlarged open end of said radiator being free floating, and an outwardly flaring conical acoustic-loading member fixed relatively to the longitudinal oscillation of said radiator and substantially filling the hollow interior of said radiator and in spaced relation therewith, said loading member being of axial length substantially that of said radiator and having an outer surface conforming generally to the inner surface of said radiator.
2. A loudspeaker according to claim 1, in which said member is solid and conical.
3. A loudspeaker according to claim 1, in which said member is a hollow cone of sheet material, the interior surface of which is sound-absorbent.
4. An acoustic transducer having a relatively wide response pattern on the axis thereof, comprising a base, an open-ended outwardly flaring radiator of relatively stiff material, the substantial outer surface of said radiator being freely exposed for radiation generally radially of the flare axis, the open end of said radiator terminating in a radiating edge that deviates from a plane normal to the flare axis, electromechanical driving means rigid with the small end of said radiator for exciting said radiator with axial oscillating movement, and axially compliant support means connecting said base to said driving means, said compliant means constituting essentially the only support for said driving means.
5. A loudspeaker, comprising a hollow radiator of generally conical shape having an axial driving means operatively connected to its small end, the angle of flare of said conical radiator being substantially in the ratio of the speed of sound in air to the speed of sound in the material of said radiator, whereby acoustic vibrations transmitted axially of said radiator will be emitted nor-,
radiator will be characterized by a substantial conical component coaxial with the longitudinal axis of said radiator.
7. A loudspeaker according to claim 5, in which the angle of flare of said conical radiator is less than said ratio, whereby acoustic vibrations transmitted axially of said radiator will be characterized by a substantial conical component coaxial with the longitudinal axis of said radiator.
8. A loudspeaker, comprising a base, an open-ended generally conical radiator of relatively stiff material, the substantial outer surface of said radiator being freely exposed for radiation generally radially outward of the flare axis, the large end of said radiator being truncated at an inclination to a radial plane of said radiator, electromechanical driving means rigid with the small end of said radiator for exciting said radiator with axial oscillating movement, and axially compliant support means connecting said base to said driving means, said compliant means constituting essentially the only support for said driving means.
9. A loudspeaker, comprising a hollow radiator of generally conical Shape having an axial driving means operatively connected to its small end, the angle of flare of said conical radiator being substantially in the ratio of the speed of sound in air to the speed of sound in the material of said radiator, and a fixed acoustically loading member relatively independent of the motion effected by said driving means and substantially filling the interior volume of said radiator and in substantially uniformly spaced relation therewith.
10. A loudspeaker, comprising a base, an open-ended generally conical radiator of relatively stiff material, the substantial outer surface of said radiator being freely exposed for radiation generally radially outward of the flare axis, the large end of said radiator being truncated at an inclination to a radial plane of said radiator, peripherally continuous mechanical loading means carried by the rim of said radiator at the large end thereof, electromechanical driving means rigid with the small end of said radiator for exciting said radiator with axial oscillating movement, and axially compliant support means connecting said base to said driving means, said compliant means constituting essentially the only support for said driving means.
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