US 2878887 A
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March 24, 1959 VAN R. POTTER 2,878,887 ACOUSTICAL DEVICE Filed Nov. 29, 1955 1a 7 va lo ls INVENTOR VAN R. POTTER BY a M United States Patent Oiifice 2,878,887 Patented Mar. 24, 1959 ACOUSTICAL DEVICE Van R. Potter, Madison, Wis., amignor to Wisconsin Alumni Research Foundation, Madison, Wis., a corporation of Wisconsin Application November 29, 1955, Serial No. 549,733
1 Claim. (Cl. 181-31) This invention relates to an acoustical device and more particularly to a horn loud speaker for reproducing the entire audible range of frequencies including low frequency sound vibrations with high fidelity.
In order to attain fidelity in sound reproduction, and in particular the inclusion of low audible frequencies, the efficiency of the coupling between the loud speaker and the room or chamber wherein the loud speaker is located is of great importance. Cabinets, baflles or horns should be looked upon as means for coupling the vibrations of the loud speaker diaphragm to the air. Since it is well established that the ear attaches more interpretive importance to directly transmitted sound than to reverberant sound received subsequently, the means by which the loud speaker is coupled to the room preferably should be designed to transmit sound as directly as possible with a minimum of reverberant sound transmission.
It is generally accepted that an exponential horn or modification thereof affords the best sytem for coupling a loud speaker to the surrounding air space. However, it is desirable, and in many instances essential, that the reproducing unit or speaker be of such physical dimensions that the unit may be available for residential use, or for use in other environments where space is relatively limited. In order to obtain results which approach those obtainable with an exponential horn, a number of cabinets or baffles of reasonable dimensions have been designed.
The ordinary and well-known openback cabinet acts somewhat like a plane baffle. However, such a cabinet is found to produce distortion at low frequencies, generally at or below the diaphragm resonant frequency. Besides uneven response, cabinets of this design produce severe loss of bass and frequency doubling. The Acoustic Labyrinth has been designed to provide a loud speaker of small dimensions for reproducing the lower rangeof frequencies of the audio spectrum. In this type of device the transmission line is a pipe, folded to conserve space, fed with energy at one end by the back of the loud speaker diaphragm, and open at the other. However, since the transmission line port and diaphragm are out of phase at higher frequencies, it is common to line the pipe with absorbent material to destroy back radiation of these frequencies. This heavy damping of the pipe, results in a comparative lackof brilliance in reproduction or heavy tone. In addition, devices or cabinets such as the Acoustic Labyrinth and others wherein a transmission tube or battle is so folded, are expensive and difficult to construct for they generally are formed of heavy pieces of plywood of various shapes which must be screwed and glued together to maintain vibration at a minimum. However, even with care in construction such units tend to vibrate because of internal reverberations. These units also lack flexibility in that they are generally designed for optimum results with a particular loud speaker and cannot be easily altered to permit use with other loud speakers.
A horn, which is an acoustical transducer consisting of a tube of varying cross section, can be employed to advantage in many situations. Because a horn can be designed to present practically any value of acoustical impedance to the sound generator, a horn-type acoustical system of high overall efliciency for producing directly transmitted sound can be designed. However, the efficiency of a particular horn is limited to a certain frequency range depending upon its shape, length and diameter.
According to my invention I provide a horn loud speaker of relatively small dimensions and simple and inexpensive construction, which embodies the advantages of the horn and infinite baffle, and is capable of reproducing eificiently and with high fidelity the entire audible range of frequencies and particularly the lower range of frequencies of the audio spectrum. My loud speaker unit produces directly transmitted sound with a substantial absence of reverberant sound. Also my horn loud speaker is capable of large scale sound production, particularly when my unit of small dimensions is so located as to cooperate with proximate surfaces such as the walls of a room.
These and other advantages of my loud speaker unit will be more fully apparent from the following description considered in connection with the accompanying drawing, which is an oblique view, partly in section, of a preferred form of the invention.
The device illustrated in the drawing is adapted to be located in an upright position within a corner comprising side walls 1 and 2, ceiling 3 and floor 4 of a room. The loud speaker unit comprises a cylindrical cabinet or drum 5 formed of non-resonant material, preferably non-resonant fibrous material. One end of cylindrical cabinet 5 is hermetically sealed by an end closure member 6 and the other end is provided with a closure member 7 having a preferably centrally located aperture 8 therein. Thus, the drum and closure members define a cylindrical air chamber 9 having an aperture 8 in one end thereof. Although a single drum 5 is illustrated, the cylindrical cabinet can be formed of one or more drums which are connected to form a cylindrical air chamber. The lower portion of drum 5 serves as supporting means to support the loud speaker unit in a vertical or upright position. A loud speaker driving unit 10 including a conical diaphragm 11 is mounted in obturating relation with aperture 8 thereby forming of chamber 9 a sealed acoustic chamber on one side of the diaphragm. The loud speaker driving unit 10 can function for the entire audio spectrum, or it can be limited to the lower frequencies only, in which case accessory drivers not connected to chamber 9 can be mounted coaxially.
Extending outwardly from chamber 9 in a direction which is longitudinally or axially of chamber 9 is a horn 12 having a throat portion 13 and a mouth portion 14 of greater cross section than the throat portion. Throat portion 13 communicates with the periphery of diaphragm 11. The throat of horn 12 need not be in actual contact with the periphery of diaphragm 11, but there should be a substantially air-tight seal between the throat and the periphery-of the diaphragm. The cross section of the throat should be at least as great as the cross section of the diaphragm and preferably the respective cross sections should be substantially the same in dimensions and configuration.
As is illustrated in the drawing, the mouth of horn 12 preferably cooperates with the trihedron formed of three intersecting surfaces of a room, such as walls 1 and 2 and ceiling 3, to form an expanding air column for transmission of vibrations from diaphragm 11. The corner of the room or trihedron is preferably provided which communicates with each of the three surfaces forming the trihedron, such as walls 1 and 2 and ceiling 3, and cooperates with the horn means to reflect sound vibrations from diaphragm 11.
According to my invention, I provide a loud speaker unit which tends to produce results obtainable with an exponential horn and which comprises in combination horn means and a closed acoustic chamber 9 on one side of diaphragm 11 which acts as an infinite bafiie. If chamber 9 is of a proper size, which is controlled to a large extent by the particular loud speaker with which it is employed, a reduction of peak response at certain frequencies is obtained and very low frequency response is considerably improved because of the eflectiveness with which radiation is prevented from the back of the diaphragm. However, chamber 9 must be of adequate size, otherwise undue pressure is imposed on the diaphragm which raises the resonant frequency of the diaphragm and mars particularly the reproduction of music. Thus, the volume of chamber 9 should be adjusted to obtain substantially optimum results, which volume will depend upon the size of the diaphragm and its resonant frequency. Generally a cylindrical chamber having a diameter of about 16 inches and a length of about 30 to 33 inches will produce excellent results with a loud speaker having a diaphragm about inches in diameter. A chamber of similar cross section and having a length of from 48 to 66 inches produces excellent results with a 12 inch speaker. The optimum size of the chamber for a particular speaker can be readily ascertained by preliminary experiments with chambers of different proportions. As was indicated previously, the drum or cylindrical cabinet is preferably made of a fibrous material. Such fiber drums are inherently strong and Will resist internal pressures of from 600 to 700 psi Because of their configuration and the non-resonant material of which they are constructed, the drums have a very low resonant frequency and do not vibrate or buzz even at the lowest sound frequencies in the audio range. Preferably the cylindrical walls of the drum are lined with a blanket of a substantially non-resonant material such as glass fiber (Fiberglas) blanket 16 to aid in preventing vibration of the drum walls. Likewise end closure member 6 can be advantageously covered with a layer of sand 17, to aid in absorbing sound waves originating from the rear or enclosed side of the diaphragm.
The adaptability of the drums of my device to loud speakers of different sizes and resonant frequencies is readily apparent from the illustration. The size of the acoustic chamber can be readily altered by merely changing the location of end closure member 6 with respect to apertured end closure member 7. Where one or more drums of relatively short length are utilized, an acoustic chamber of desired size can be obtained by combining two ormore drums to form a sealed chamber.
As was stated previously, my loud speaker unit is provided with a horn 12 having a throat portion 13, which communicates with the periphery of diaphragm 11, and a mouth portion 14 of 'greater cross section than the throat portion. The horn means I employ preferably is circular in cross section and takes the form of a frustum of a cone. Of course, the horn can also be of the parabolic, hyperbolic or exponential type; however, a cone type horn is preferred. The principal virtue of a horn resides in the possibility of presenting practically any value of acoustical impedance to the sound generator. This feature is extremely valuable for obtaining overall efficiency in the design of an acoustical system. High efliciency is obtained by designing a system so that the driving force works against resistance instead of inertia of the diaphragm. I have found that by combining the advantages of a horn with those obtained by the employment of a cylindrical acoustical air chamber which acts as an infinite baflle, the diaphragm works against not only the resistance created by the horn but also the resistance created by the enclosed air chamber on one side of the diaphragm, to produce directly transmitted sound including substantially all of the frequencies in the audio spectrum, and particularly. the lower frequencies, with high fidelity. The diameter of the mouth of the horn, rate of flare and length of horn are related to the other elements of my loud speaker unit such as the size of the diaphragm and cylindrical air chamber with which the horn is employed. Also certain other considerations must be made when my loud speaker unit is to be employed so that the mouth of the horn cooperates with a trihedron, formed for example by the walls and ceiling of a room, which considerations will be mentioned hereafter. Generally, a conical horn which is about 18 inches in length and has a mouth having a diameter of about 16 inches will produce excellent results when employed with a diaphragm having a diameter of about 10 inches which is equipped with a cylindrical air chamber having a 16 inch diameter and a length of from about 30 to 33 inches. The horn is formed of a substantially non-resonant material, such as metal, fibre or plastic. Polyethylene has been employed with notable success.
It is often desirable to employ my loud speaker unit so that the mouth of the horn means cooperates with the trihedron formed by three intersecting surfaces, such as the two walls and ceiling or floor forming the corner of the room. The three intersecting surfaces cooperate with the horn to form an expanding air column to transmit sound from the diaphragm. In other words the diverging surfaces act as bounding surfaces of the last stage of expansion of a flaring horn. In the drawing, my loud speaker unit is illustrated as standing in a vertical position in a corner of a room. However, the loud speaker unit need not assume a truly vertical position but may form any angle with the corner of a room provided the expanding intersecting surfaces of the room cooperate with the horn means to obtain good sound production. Preferably, my loud speaker takes a substantially vertical position in a room corner. Although the horn means is illustrated in the drawing as cooperating with two walls and a ceiling, it can be installed in a room to cooperate with twowalls and the floor forming a corner therein. In such instance, the loud speaker unit is provided with suitable supporting means, such as legs, of sufficient height to maintain the horn means in the proper position with respectto the floor and walls. To obtain the most eflicient coupling of my horn loud speaker to a corner of a room, the diameter of the mouth of the horn should not exceed the area of the room corner taken at the junction of the horn with the corner. This area is equal to one-eighth of the area of the surface of a sphere having a radius equal to the distance of the mouth of the horn from the room corner.
The cooperation between the horn of my loud speaker unit and the trihedron formed by three intersecting surfaces can be further enhanced by providing means comprising a surface which communicates with each of the three intersecting surfaces. In accordance therewith, a
triangular shaped reflector 15, whose edges communicate with walls 1 and 2 and ceiling 3 can be placed in a room corner to cooperate with the horn means to reflect sound vibrations from the diaphragm. The reflector makes the smooth connection between the loud speaker unit and the expanding mouth area of the corner of a room. Although a relatively flat, triangular-shaped reflector is illustrated in the drawing, other reflectors, for example curved, concave reflectors can be utilized. However, a flat triangular shaped reflector is preferred, and when such a reflector is used, preferably the angle the reflector makes with the horizontal is about 30 when incident'sound travels from the diaphragm along a vertical axis. The reflector should be madeof a smooth surfaced, hard, dense material to efliciently reflect the higher frequencies. It should also have a natural period of vibration which is very. low and difficult to excite. Advantageously, the rear of the refiector can be weighted to aid in preventing-vibration.
The reflectors I employ can be made of smooth surfaced building materials having the properties previously described. A particularly good reflecting material is tempered r Iasonite. When auxiliary speakers for the middle and highest frequencies are used they are mounted within the horn 12 and mounted coaxially or parallel to the low frequency sound axis.
In this system, either with a single radiator or with two or more driving units, the distance from the sound source to the listener is essentially the same for all frequencies. There is a minimum of reverberant sound within the systern, and there is no cancellation of sound due to phase differences such as are encountered in systems which attempt to utilize both the front and the back of the speaker diaphragm. The reflector picks up the full audible sound spectrum as a direct radiation from the diaphragm or diaphragms and reflects it to all parts of the room within which the loud speaker system is located.
A horn loud speaker comprising means including a drum made of non-resonant fibrous material with the cylindrical walls thereof lined with a glass fiber blanket defining a cylindrical air chamber having an aperture in one end of said chamber and having an end closure member at the other end of said chamber covered with a layer of sand to absorb sound wave energy, a loud speaker driving unit including a diaphragm mounted in obturating relation with said aperture and forming of the chamber a sealed acoustic chamber on one side of said diaphragm, horn means comprising a tube of varying cross-section extending outwardly from and axially of said chamber,
References Cited in the file of this patent UNITED STATES PATENTS 674,210 Loomis May 14, 1901 1,962,300 Ephraim June 12, 1934 1,984,542 Olson Dec. 18, 1934 FOREIGN PATENTS 817,726 France May 31, 1937 496,487 Great Britain Nov. 28, 1938 913,655 France Sept. 17, 1946 54,541 France May 4, 1950 (Addition to No. 913,655) 143,597 Australia Sept. 27, 1951 OTHER REFERENCES Augspurger: Publication Audio Engg., November 1951, pages 24-27 and 67 (page 26 may suffice).
Publication Service, article by W. Martin, December 1953, Fig. 2 is pertinent.