US3027964A

US3027964A - Loudspeaker

Info

Publication number: US3027964A
Application number: US744227A
Authority: US
Inventors: Jr John D Spragins; Leonard T Pockman; George A Brettell
Original assignee: Ampex Corp
Current assignee: Ampex Corp
Priority date: 1958-06-24
Filing date: 1958-06-24
Publication date: 1962-04-03
Anticipated expiration: 1979-04-03

Description

April 1952 J. D. SPRAGINS, JR, ETAL 3,027,964

LOUDSPEAKER Filed June 24, 1958 Q Z W W M.

i E 1 r JOHN D. SPIQAGINS JR. LEONAQDT POCKMAM GEORGE A. BRETTELL lNVE 025 W ATTORNEYS United States Patent 3,027,964 LOUDSPEAKER John D. Spragins, Jr., Palo Alto, and Leonard T. Pockman, East Palo Alto, Calif., and George A. Brettell, Mexico City, Mexico, assignors to Ampex Corporation, Redwood City, Calili, a corporation of California Filed June 24, 1958, Ser. No. 744,227 7 Claims. (Cl. 181-81) This invention relates generally to loudspeakers and more particularly to wide angle horn loudspeakers.

As is well known, horn-loudspeakers are very eflicient for sound reproduction. This is due, in most part, to the ability of the horn to present almost any desired value of acoustical impedance to a generator. As a result, it is possible to achieve maximum overall design of a sound system.

A major problem in designing loudspeakers is that of obtaining a uniform distribution of sound power over a large solid'angle for a relatively wide range of frequencies. At low frequencies, little diificulty is experienced in obtaining a uniform sound pattern. However, at the higher frequencies, speakers become'directional and the directionality is dependent upon the wavelength of the sound. The foregoing is not surprising when it is considered that the audible range includes wavelengths which vary between about 0.6 inch and 50 ft.

In general, loudspeakers are designed for optimum performance at the mid-frequency range. At very low frequencies, the speaker acts as a point source with a uniform radiation pattern. At wavelengths near the largest dimension of the speaker, the radiated sound becomes highly directive and in most cases stays directional as the frequency is further increased. A number of effects combine to cause this directionality. Typical ones are: different path length for sound from different parts of the speaker, phase differences between different parts of the speaker diaphragm, andtransverse vibrations at the mouth.

Attempts to eliminate the directional effect involve creating roughly spherical wave fronts leaving the speaker. Several schemes have been tried in the prior art. A common method is to form a multi-cellular horn in which the horns are arranged in columns and rows with the horns having a common throat and the mouths forming a portion of a spherical surface. However, horns of this type have a wide variation in directivity as the frequency is varied.

The directional characteristics of these horns may be explained fairly successfully as the pattern of one large horn at low frequencies and as the sum of the radiation from several highly directive horns at higher frequencies. Generally, a speaker of this type radiates power uniformly over a wide angle only when the dimensions of its surface are large in comparison to the wavelength being radiated.

It is a general object of the present invention to provide a novel loudspeaker which is relatively smalland which radiates power uniformly over a large solid angle.

It is another object of the present invention to provide a mul-ti-segment loudspeaker in which the sound is caused to arrive in phase at the mouth of all the segments by including velocity determining means in each of said segments.

It is another object of the present invention to provide a horn loudspeaker which employs a plurality of concentric horns forming a plurality of horn segments terminating on a portion of a spherical or arcuate surface with means included in said horns for determining the velocity in each segment.

It is another object of the present invention to provide a horn loudspeaker which employs a plurality of concentric hor'ns forming a plurality of horn segments te minating on a spherical surface with a mixture of gas included in predetermined ones of said horn segments for determining the velocity of the sound within the same whereby the sound arrives in phase at the mouth of the various segments.

These and other objects of the invention will become more clearly apparent from the following description when taken in conjunction with the accompanying drawings.

Referring to the drawing:

FIGURE 1 is a perspective view showing a loudspeaker in accordance with the invention;

FIGURE 2 is a sectional view of the loudspeaker of FIGURE 1; and

FIGURE 3 shows construction lines for forming a loudspeaker in accordance with the invention.

Referring to FIGURE 1, the loudspeaker comprises a plurality of

concentric horns

11, 12, 13, 14, 15 and 16. The space between the horns is a plurality of concentric segments. A driver 18 serves to drive the speakers at the throat 19. Each of the horns 11-16 is a smoothly expanding horn which terminates normal to a substantially spherical surface. The horns expand in such a manner that the ratio of areas of each of the horn seg ments is the same along the axis of the loudspeaker, as for example, at the

planes

21 and 22, in FIGURE 2.

Any suitable driver, for example, the driver shown in FIGURE 2 may be employed for driving the horn. The driver illustrated includes a magnetic assembly 23, a voice coil 24' immersed in the magnetic field at the gap 25, and a diaphragm 26 which feeds into the

horn segments

27 and 28 illustrated. The concentric horn assembly is suitably secured to the driver, as for example, by screws 31.

A gas-tight diaphragm 32 is suitably sealed betweenthe outer horn 16 and the driver and is suitably sealed The corrugations allow for expansion and contraction during atmospheric pressure variations.

Thus, it is seen that the segments form a plurality of enclosures into which mixtures of gases may be introduced for purposes to be presently described. Preferably, the Mylar films are treated whereby the gases enclosed within the segments will not permeate outwardly. For rare gases, the Mylar film may he made impervious by coating it with a relatively thin film metal, as for example by evaporation.

Thus, it is seen that a speaker is provided in which the mouths of the various horns terminate on a spherical surface. If the sound arrives in phase at the mouth of each of the segments, a nearly hemispherical wave front will be generated going out away from the speaker. However, it is seen that unless means are provided for changing the velocity of sound in the various horn segments, it will arrive out of phase at each of the mouths since the path length for the sound is different in each of the horn segments. For example, in the illustration, the path length from the driver to the outer annular mouth is about of the path length from the driver to the mouth of the inner horn. Suitable means are provided for changing the velocity of sound so that it arrives in phase. For example, an acoustical delay line may be introduced. Preferably, the loudspeaker is formed with the gas-tight diaphragms at the two ends and mixtures of different gases are introduced into the segments. As is Well known, the velocity of sound in various gases is different being approximately'inversely proportional to the square root of the molecular weight.

In one particular example, helium was employed in which the velocity of sound is approximately three times that of the velocity in air. By mixing appropriate amounts of helium and air, any intermediate velocity could be realized. Helium being a rare gas did not permeate through the coated Mylar film. By employing helium mixed with air and using as little helium as possible in the inner sections, it was possible to minimize reflections of the sound at the mouths. In the illustrative example, it is noted that the outer horn contains pure air with the inner horns being arranged to contain mixtures of helium and air to give the appropriate velocity, Thus, relatively small amounts of helium were added and the reflections introduced were at a minimum.

It is, of course, to be understood that the Mylar film might enclose all of the horn segments and that other gases might be employed for controlling the velocity. However, because helium has such a large change in velocity relative to air, it is preferable since relatively small amounts need be added.

As previously described, the ratio of areas of the segments for all points along the axis must be substantially equal and the horns preferably should terminate on a spherical surface. True exponential horns have only one parameter which may be varied, while two parameters need be varied to satisfy these conditions. Thus, a more general form of horn equation need be employed to obtain the desired variation. The equation may be used to generate an ordinary exponential horn, a catenoidal horn, a conical born, or any other interediate shapes. The equation involves three parameters: y the radius of the throat, which is virtually fixed by the size of the driver, i the cut-off frequency for the horn which is limited by the cross-over frequency of the speaker, and T which represents the shape factor and can be varied with complete freedom. However, to design concentric horns which comply with this equation would be rather ditlicult with the many limitations which are placed on the parameters. Consequently, a modified type of concentric horn which terminates normally to a spherical surface and which has the proper area ratio was constructed. v Referring to FIGURE 3, a horn similar to that of FIG- URE 2 with the addition of construction lines is illustrated. In construction, circular arcs 41 are drawn with their centers on the axis of symmetry of the horn. The circles are drawn according to calculations from the foregoing equation for the inner horn. The outer horns are constructed so as to intersect all the circles at very nearly right angles. The ends of the horns terminate on a spherical surface and the ratio of areas is preserved throughout.

The horns, as designed in accordance with the foregoing, are only an approximation to the horn defined by the foregoing equation with the outer horns deviating an appreciable amount. However, mathematical expression for horns usually break-down when end effects and other effects are considered. Thus, almost any smoothly expanding horn will perform virtually as well as a true mathematical horn and the above illustration is given only for teaching how a horn might be formed to comply with the invention.

Thus, it is seen that an improved loudspeaker is provided. The horn is relatively small since it includes a plurality of concentric horn segments having their mouths terminating on a spherical surface. The horns are designed whereby they have a substantially equal ratio of areas along the axis. Means are provided for varying the velocity of the sound within the same whereby it arrives in phase at the mouths which lie at different path lengths from the driver. The loudspeaker provides a substantially spherical output wave.

We claim:

1. A loudspeaker comprising:

a plurality of horns arranged with their mouths terminating on a surface to form a common mouth, each of said horns having a difierent path length; and

means comprising different predetermined mixtures of gases included in each of said horns for controlling the sound velocity therein so that each sound wave introduced at the smaller ends of said horns arrives at the common mouth substantially in phase.

2. A loudspeaker comprising:

a plurality of horns arranged with their mouths terminating on a portion of an arcuate surface to form a common mouth, each of said horns having a different path length; and

means comprising different predetermined mixtures of gases included in each of said horns for controlling the sound velocity therein so that each sound wave introduced at the smaller ends of said horns arrives at all parts of the common mouth substantially in phase.

3. A loudspeaker comprising:

a plurality of concentric horns arranged with their mouths terminating on a common surface to form a common mouth;

said horns defining a plurality of concentric horn chambers, each of a different path length; and

means comprising different predetermined mixtures of gases included in each of said horn chambers for controlling the sound velocity therein so that each sound wave introduced at the smaller ends of said horns arrives at all parts of said common surface substantially in phase.

4. A loudspeaker comprising:

a plurality of concentric horns arranged with their mouths terminating on a spherical surface to form a common spherical mouth;

5. A loudspeaker comprising:

a plurality of smoothly expanding concentric horns arranged with their mouths terminating on a spherical surface to form a common spherical month;

said horns defining a plurality of concentric horn chambcrs, each having a throat and being of a different path length; and

different mixtures of gases included in each of sai horn chambers for controlling the sound velocity therein, the mixture of gases in each chamber being selected so that each sound wave introduced at the threats of said horn chambers arrives at all parts of said common mouth substantially in phase;

whereby sound is radiated from said spherical surface with substantially spherical Wave fronts.

6. A loudspeaker comprising:

said horns defining a plurality of concentric horn chambers, each of a different path length;

said chambers having a constant ratio of areas for all points along the axis of said horns; and

different mixtures of gases included in each of said horn chambers for controlling the sound velocity therein, said mixtures being selected so that each sound wave introduced at the throats of said horn chambers ar- 1,840,992 Weitling Jan. 12, 1932 rives at all parts of said common mouth substantially 1,992,268 Wente Feb. 26, 1935 in phase. 2,001,089 Blattner May 14, 1935 7. A loudspeaker as characterized in claim 6, wherein: 2,037,187 Wente Apr, 14, 1936 each of said horns terminates normally on the spherical 5 2,797,766 ullivan July 2, 1957 surface 2,819,771 Kock Jan. 14, 1958 I 5 References Cited in the file of this patent 2820525 Fountam et M Jan 21 19 8 UNITED STATES PATENTS FOREIGN PATENTS 1,770,234 Grant July 8, 19 30 473,046 France Sept. 5, 1914 1,801,521 Milnor Apr, 21, 1931 372,136 Germany Mar. 20, 1923