US2981359A - Reflex horn loudspeaker - Google Patents

Description

April 25, 1961 R. w. CARLISLE 2,981,359

REFLEX HORN LOUDSPEAKER Filed March 5, 1958 2 Sheets-Sheet 1 INV EN TOR. RICHARD H- C'floQL/SLE April 25, 1961 Filed March 5, 1958 R. W. CARLISLE REFLEX HORN LOUDSPEAKER 2 Sheets-Sheet 2 FIG. 3.

17 FIG. 4.

R CHARD W. CARL/$1.5

cgjiaww United States Patent REFLEX HORN LOUDSPEAKER Richard W. Carlisle, Elmsford, N.Y., assignor, by mesne assignments, to Ling-Temco Electronics, Inc., Dallas, Tex., a corporation of Delaware Filed Mar. 5, 1958, Ser. No. 719,250 6 Claims. (Cl. 181-27) This invention relates to loud-speakers of the folded or reflex type, in which the air column is made effective- .ly as long as that of a straight horn by the provision of internally bent or folded channels generally reversing directions by 180 for each bend or fold. In such horns there is a tendency for cancellation of the high frequencies because of the different path lengths of the sounds traversing the bends. Where these bends have a large radius, as is the case in the final bend of a large horn, phase displacement is more pronounced because of the larger differences in linear distances between opposite walls of the bend. When this distance approaches onequarter wave length of the sound wave being propagated therethrough, acoustic cancellation or destructive interference sets in.

It is the object of this invention to improve the ethciency of operation of reflex horns at high frequencies by improving the transmission of the sound wave around the bends without impairing the transmission of the lower frequencies.

According to the invention, a reflex horn is provided with a desired rate of flare or taper according to conventional designs for all the straight sections (as contrasted with the bent portions) of the horn. In all known designs, the flare rate is such as to increase progressively the cross-sectional area of the horn per unit length along the axis of the sound channel. It is conventional, further, to maintain the flare rate of increase of the crosssection around the bend and to shape the surfaces around the bend smoothly to provide a laminar flow of the sound waves traversing the bend. In order to eliminate high frequency cancellation or destructive interference, the same phase should exist over any plane normal to the axis. This condition has been satisfied to a degree in the past by reducing the radial dimensions at the bend to a fraction, usually A or less, of the wave length of the sound frequencies in the upper portion of the spectrum. Applicant has discovered that a marked improvement in the high frequency response is obtained by providing a surface discontinuity in the form of a reflector in the bend opposite the oncoming path of the sound wave, even if the radial dimensions of the bend are as much as M2 the wave length of the sound waves.

In carrying out the invention a linear or curvilinear discontinuity or insert is provided in the far wall defining the bend to reflect the sound waves with a minimum of high frequency cancellation. The invention will be better understood from the following description in conjunction with the accompanying drawing in which:

Fig. 1 is a side elevation partly in section of a reflex horn and driver incorporating one form of the invention;

Fig. 2 is a front view of the horn of Fig. 1;

Fig. 3 is an enlarged cross-section of a fragment of Fig. 1 showing an inwardly curved insert at the bend; and

Fig. 4 is another enlarged cross-section fragment of the horn of Fig. 1 showing a two-step insert.

The horn is connected to a conventional driver unit 2,981,359 Patented Apr. 25, 1961 11 through any convenient coupling, such as the threaded neck portion 12 having external threads engaging internally tapped threads of a central member 13. The reflex or folded born 10 includes the usual tone arm 14 which may be integrally formed with the base member 13, the reflector 15 which is connected either to the base member 13 or the mouth end of the tone arm 14 in any of the known methods, and a bell 16 which also may be connected to or formed integrally with the base member 13. The bell 16 may be circular or oblong. An oblong bell is shown in Figs. 1 and 2. The cross-sectional area of the horn from the throat 17 of the tone arm to the mouth 18 of the bell increases at a predetermined rate, which may be any of the expansion rates used in horn construction.

The horn 10 as so constructed provides, in effect, a long air column through which the sound wave passes from the driver unit 11 into the throat 17 and through the tone arm 14, expanding as it progresses through the tone 1 arm. At the mouth end of the tone arm the sound wave is deflected by the reflector 15 in the first fold 30 between the increasing annular passageway formed by the outer surface of the tone arm 14 and the inner surface of the reflector 15. The radial dimensions of the bend 30 are usually less than A wave length of the high frequencies thereby incurring little destructive interference. The expanding sound wave is then transmitted around the second fold or bend 19 into the outer expanding annulus defined by the inner surface of the bell 16 and the outer surface of the reflector 15, thereafter emerging into the atmosphere from the mouth 18. Each portion or fold of the horn may have a selected taper or flare rate which may be uniform or vary somewhat from section to section. It is conventional in known devices to continue the expanding area of the successive annuli around each of the bends and to do so with smooth surfaces to minimize the reflections of the sound waves. The shape of the bend of suchdevices is indicated in Fig. l by the dotted line portion 19 and the solid line portion 20. It is seen that the solid portion 20 is smoothly curved with the approximate shape of an arc of a circle, the center of which is usually on the end of the reflector 15. The portion 19 of the bend is also smoothly curved. The bend portions 19 and 20 together form a shape resembling that of a bowl.

According to the present invention, the outer portion of the bowl at the bend 31 is modified into a partial conoid which may take various forms. In Fig. 1 the bowl or arcuate shape portion 19 is replaced by a straight conoid 21.

In Fig. 3 the arcuate portion 19 is modified to an inwardly curved conoid section 22.

In Fig. 4 the arcuate section 19 is replaced by the two cones 23 and 24 with an offset section 25 joining the conical sections. The cone 24 is seen to be closer to the surface of the arcuate section 19 whereas cone 23 is farther away from that surface.

The forms in Figs. 4, 1 and 3, illustrate respectively, successive increases of departure from the original arcuate bowl 19. In general, the less the departure from the original bowl shape, the more nearly the response resembles that of a reflex horn made according to the prior art; whereas the greater the departure from the original bowl shape, the more the high frequency response is improved by a smoothing out of the unwanted dips in the response characteristic.

Applicant has discovered that it is not necessary to modify the inner arcuate portion 20 of the bend 31; and has also discovered that extending the reflector 15 to reduce thereby the opening in the bend 31 has little or no effect in improving the high frequency response.

Experimental tests have shown marked improvements for the frequency response characteristic of folded horns made according to the invention and more particularly for frequencies Wh$ neguarter wave lengths are lessthan the radial dimension at the 'bend 31. It is to be noted that heretofore the high frequency sounds of one-quarter wave lengths less than the radial dimension'of the bend were highly attenuated resulting in sharp dips in the response characteristic adversely affecting the efiiciency and fidelity of the loud-speaker.

.In a preferred embodiment illustrated by the straight conoid 21 in Fig. 1, theradial dimension R between the edge of the reflector 15 and-the conoid portion 21 is 1.33 inches. This corresponds to a wave length at 11,000 cycles per second and a quarter wave length at 2,750 cycles per second. The frequency band of greatest improyement .is experimentally shown to be between 2,750 and 11,000 cycles per second corresponding to onequarter to one wave length. The response characteristic for frequencies below 2,750 cycles per second is substantiallythe same as that of a conventional folded horn without the discontinuity in the bend showing that the improvement of the invention does not impair the low frequency response. 7

It will be observed that the discontinuities 21, 22, 23 and 24 in the outer portion of bend reduce slightly the cross-sectional area along the axis around the bend. This affects the fiare rate of the horn and may be the basis, in part, for'the improved high frequency response. However, it is presently believed thatthe improved high frequency response is due primarily to these discontinuities acting as reflectors of the sound waves thereby inhibiting the expansion of the high frequency sound waves in the radial direction of the bend without causing destructive interference.

Further, it is seen that the improvement of the invention is made in the frequency-space relationship intermediate normal horn wave action and specular or opticaltype reflection. In other words, .the improvement obtains in that region where the dimension of thehorn bend is comparable to the'wave length of the sound. Heretofore, it is noted, .reflex horn loud-speakers were designed on the principle that the dimension of-the bend be much smaller than the wave length of the sound wave to be transmitted therethrough.

.Although the inventionhas been described in atwofold reflexhorn, it is not to be considered to be limited thereto. Thus, for example, a reflex horn having but one fold intermediate the driver and the horn mouth may embody the conoidal reflecting surface of the invention within the single told.

I claim:

1. In a loudspeaker of the reflex type having a continuous sound channel expanding in area at a substantially uniform and predetermined rate and being substantially reversed at least twice to form two or more bends in the sound channel, the outer wall of the inlet and outlet portion of the first bend being smoothly curved, the radius of said first bend being smaller than one-quarter the wavelength of the highest frequency sound desired to be reproduced, and the radius of said second or subsequent bends being larger than one-quarter the wavelength of the highest frequency sound desired to be reproduced, the improvement which consists of interrupting the expansion of the sound channel area only in the outlet portion of the second or subsequent bends by re ducing slightly the cross-sectional area only in the outlet portion of the second or subsequent bends through the provision of a conoidal surface in the outer wall of said outlet portion, the inlet portion of said second or sub,- sequent bends being smoothly curved.

2. A loudspeaker according to claim 1 wherein the radius of said second or subsequent bends is larger than one-half the wavelength of the highest frequency sound desired to be reproduced.

3. A loudspeaker according to claim 1 wherein the radius of said second or subsequent bends is nearly one Wavelength of the highest frequency sound desired to be reproduced.

4. A loudspeaker horn according to claim 1 wherein the conoidal section is straight.

5. A loudspeaker horn according to claim 1 wherein the conoidal section is inwardly curved.

' 6. A loudspeaker horn according to claim 1 .Wherein the conoidal section comprises two straight oifset but not overlapping cones connected to each other by a relatively short wall.

References Cited in the file of this patent UNITED STATES PATENTS 2,494,134

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