US2203875A

US2203875A - Loud-speaker

Info

Publication number: US2203875A
Application number: US139848A
Authority: US
Inventors: Harry F Olson
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1937-04-30
Filing date: 1937-04-30
Publication date: 1940-06-11
Anticipated expiration: 1957-06-11

Description

June 11, 1940. OLSON 2,203,875

LOUD-SPEAKER Filed April 30, 1937 Zhmentor y attorney Patented June 11, 1940 UNITED STATES PATENT OFFICE 2,203,875 noun-SPEAKER Ware Application April 30, 1937, Serial No. 139,848

7 Claims.

This invention relates to loud-speakers, and more particularly to horn structure therefor.

It has been. well recognized that horn type loud-speakers have certain inherent advantages over the direct radiator flat bafile type, and for this reason horn. loud-speakers have been used rather extensively in theaters, public address systems, and the like, the horns usually being of the exponential type. The conventional exponen- 1 tial horn has only a single rate of flare, the crosssectional area thereof along its axis being defined by the equation A AUG where A is the throat area of the horn, m is the flaring constant, and :c is the distance along the axis. The cut-off frequency of such a horn is governed by the constant m.

Where a high acoustic resistance is desired to match, at the higher frequencies, the large acoustic mass reactance of the cone or diaphragm which drives the horn, a horn with a small throat is used in order to obtain high efliciency. On the other hand, if high efficiency is wanted at the low frequency end of the audible range, a horn with a relatively large throat is employed because the acoustic mass reactance of the vibrating system is of practically no consequence at the low frequencies. Now it is obvious that, since a horn with a large acoustic resistance is desirable for the higher frequencies and a horn of small acoustic resistance for the lower frequencies, if a single horn of the conventional type is used to cover the entire audible range, it will be mismatched in the high or the low frequency ranges. To overcome this difficulty, it has been proposed heretofore to employ two separate horns with either a single driver or with two separate drivers, one of the horns being designed particularly for high frequency reproduction. and the other for low frequency reproduction. Under certain conditions, however, it may be undesirable to employ two horns, and it is the primary object of my present invention to provide an improved horn loud-speaker which will meet the requirements of such conditions.

More specifically, it is an object of my present invention to provide an improved single-horn loud-speaker which will adequately cover the entire audible range without being mismatched at either end of the audio spectrum.

Another object of my present invention is to provide an improved single-horn loud-speaker having increased efliciency over similar horn loud-speakers previously employed.

It is also a very important object of my present invention to provide means whereby conventional single-horn systems now in use may readily be converted into systems having the desirable characteristics of multiple-horn systems.

In accordance with my invention, I provide a. horn having several rates of flare so that the acoustic impedance at the throat increases with frequency. The horn may either be formed of several individual sections each having a different rate of flare, or it may be formed as a single horn having but a single rate of flare and provided internally with suitable means to subdivide it into several horn sections, each having a different rate of flare.

The novel features that I consider characteristic of my inventionv are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of several embodiments thereof, when read in connec tion with the accompanying drawing, in which Figure 1 is a sectional view of one formof loudspeaker horn according to my invention,

Figure 2 is a sectional view of another form thereof,

Figure 3 is an enlarged end view of one form of flare rate-changing device employed in connection with the form of my invention shown in Figure 2,

Figure 4 is an enlarged end view of a slightly different form of flare rate-changing device,

Figure 5 is a side elevation of the device shown in Figure 3,

Figure 6 is a diagrammatic view of another form of my invention,

Figure '7 is a curve showing the resistance characteristics of the horns shownv in Figs. 1 and 2 as compared with two conventional horns used in systems of the prior art, and

Figure 8 is a curve showing the resistance characteristics of the horn shown in Fig. 6.

Referring more specifically to the drawing, I have shown, in Fig. 1, a relatively long horn l which is driven by a suitable driver, such as an electrodynamic loud-speaker of which the cone or diaphragm 3 alone is shown. The horn I is formed of two sections la and lb joined into an integral structure, the section Ia being relatively short and having a small throat to provide a large acoustic resistance for efficient high frequency reproduction, and the section lb being much longer and having a large throat to provide a small acoustic resistance for efficient low frequency reproduction. To properly transmit and accentuate the high and low frequencies, respectively, the short horn section la is formed with a large rate of flare and the long horn section lb is formed with a small rate of flare. Thus, if the horn sections la and lb are each made to expand exponentially, the factor m, in the equation set forth above, will be large for the horn section la and small for the horn section lb.

In Fig. 7, there are shown curves representing the resistance characteristics of my improved multi-flare horn as compared with separate horns. For a short horn having. a small throat and a large rate of flare, such as the horn section la, the acoustic resistance is quite high, as shown by the dotted line a, and for a long horn having a large throat and a small rate of flare, like the horn section lb, the acoustic resistance is much lower, as shown by the dash line b. As pointed out previously, this is necessary for proper matching in the high and low frequency regions, respectively, and the same conditions should be met if a single horn is used. By using a double flare horn, such as that described above, the acoustic resistance, at the low frequencies. is the same as that of-a horn having a large throat and a low rate of flare, while at the high frequencies, the impedance is the same as that of a horn having a small throat. and the acoustic resistance characteristic of the born I is shown by the full line 0. Coupling the short horn section la of rapid flare to the horn section lb which flares at a much smaller rate does not change the characteristics of the latter at the low frequencies, but the section I a should not be made too long, as this would deleteriously affect the acoustic resistance in the upper range of the low frequency region by raising it to too great a value. I have found, in practice, that in a horn which is 90 in overall length, excellent results will be obtained if the horn section la is made approximately '7 long with a throat area of about 3 square inches and the horn section lb is made about 83" long with a throat area of about 12 square inches. In such a horn, the low frequency cut-off frequency of the horn section lb is about 45 cycles, and the rate of flare of the horn section la is about five times that of the horn section lb.

The modifications of my invention shown in Figs. 2 to 5 are particularly advantageous in converting conventional single horn systems into multiple-flare horns in order to obtain the advantages of loud-speakers using two or more horns, each designed for greatest efliciency over a certain portion of the audio frequency range. In the modification shown in Fig. 2, I place, within a single relatively long horn 5 of conventional design and having a single rate of flare internally along its entire length, a device I which changes the effective rate of internal flare of the horn 5 in the region where the device I is located. The device I may be formed of a tubular outer member 9 which may be of square cross-section, as in Fig. 3, or of cylindrical form, as in Fig. 4. The tubular member 9 is made thicker at its base end 90. than at its other end 9b and has its wall or walls, as the case may be, of gradually reducing thickness, as clearly shown in Fig. 5, the wall or walls preferably tapering down to an edge at the end 9b. Concentric with the tubular member 9 and spaced therefrom by suitable spacers ll is a central pyramidal member l3 in the device of Fig. 3, or conical member in the device of Fig. 4, which has its base at the same end of the device 1 as the end 9a of the tubular member 9 and which gradually tapers down to a point at the end 9b. Thus, the space l5 between the member 9 and I3 is of gradually increasing cross-section from the base end 9a to the other end 91: of the device 1.

By suitably dimensioning and properly choosing the tapers of the tubular member 9 and the inner member l3, and placing the device I in the horn 5 at its throat end with the device 1 preferably coaxial therewith and its thinnest end So nearest to the mouth of the horn, it is obvious that the device 1 may be made to greatly increase the rate of internal flare of the horn 5. In effect, therefore, the resulting horn would have two sections corresponding to the horn sections la and lb of Fig. 1. For a 90" horn, having a 3 throat, for example, the device "I may have an overall length of about '7", its tubular member 9 may have an internal dimension of about 1 and an external dimension of about 2 at the base, and its central member l3 may have a base dimension of about 1". With these dimensions, the horn 5 behaves, at low frequencies, as if it had a throat area of about twelve square inches, while at the higher frequencies, the throat area presented to the diaphragm 3 is about three square inches.

It will be obvious that a multi-flare horn according to my present invention need not necessarily be limited to only two sections, but may be provided with any number of sections desired. In Fig. 6, I have shown diagrammatically a horn with four sections lw, Ir. ly and la, each succeeding larger section having a successively smaller rate of flare than the preceding section. By employing a horn with several rates of flare, it is possible to obtain a system which will match the acoustic reactance of the vibrating system throughout the whole audible range. For ex ample, in a four section horn such as that shown. the horn section lw may have a flare cut-off at 500 cycles. the section lac at 250 cycles, the section ly at 125 cycles, and the section l2 at 50 cycles. The acoustic resistance characteristic of the horn of Fig. 6 is shown in Fig. 8. the curves w, r, y, a corresponding, respectively, to the horn sections lw, Ix, ly and I2.

Although I have shown several modifications of my invention, it will be obvious to those skilled in the art that many other modifications thereof are possible. For example, the shape and dimensions of the flare rate changing devices shown in Figs. 3 to 5 may be varied at will and need not necessarily be limited to those shown and described. It will also be obvious that a horn having a constant or uniform internal flare throughout its entire length may be subdivided into a multi-flare horn by placing therein a number of devices 1 of suitable shapes and dimensions and at suitably spaced intervals. preferably along its axis, to provide a horn effectively similar to one like that shown in Fig. 6. Other changes will, no doubt, also be apparent to those skilled in the art. I therefore desire that my invention shall not be limited except insofar as is made necessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. A horn having an internal surface defining a space therewithin which has a single rate of flare internally along its entire length, and means disposed within said horn for changing the effective rate of internal flare thereof in the region where said means is located.

2. The invention set forth in claim 1 characterized in that said means is so constructed and arranged that said effective flare is greater than said first named flare.

3. The invention set forth in claim 1 characterized in that said means is disposed adjacent the throat end of said horn, and characterized further in that said means extends axially along said born for only a short distance whereby said region is at the throat end of said horn.

4. The invention set forth in claim 1 characterized in that said means comprises a tubular member the walls of which decrease in thickness from one end thereof to the other, and characterized further in that it is so disposed within said horn that its thickest end is closest to the throat end of the horn.

5. The invention set forth in claim 1 characterized in that said means comprises a tubular member the walls of which decrease in thickness from one end thereof to the other, and characterized further in that said member is disposed within said horn at the throat end thereof with the thinnest end of said member nearest to the mouth of said horn.

6. The invention set forth in claim 1 characterized in that said horn is relatively long, characterized further in that said means comprises a tubular member considerably shorter than said horn and the walls of which decrease in thickness from one end thereof to the other, and characterized still further in that said tubular member is disposed within said horn coaxially therewith and at the throat end thereof with the thinnest end of said member nearest to the mouth of said horn.

7. The invention set forth in claim 1 characterized in that said means comprises a plurality of spaced, concentric members, each of which decreases in thickness from one end there of to the other, and characterized still further in that said members are so disposed within said horn that their thickest ends are closest to the throat end of the horn.

HARRY F. OLSON.