US 3125181 A
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March 17, 1964 R. J. PAWLOWSKI 3,125,181
LOUDSPEAKER SYSTEM Filed June 21, 1961 5 Sheets-Sheet 1 9 Q Q 9' a 9 x L28 22 a V2 (/2 Ii;
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INV EN TOR. Iqbal"? Paul on ska stare, Kerr /a2 ju/Mez'sfe; vmrr/erfaring s United States Patent 3,125,181 LOUDSPEAKER SYSTEM Robert J. Pawlowski, Niles, Mich, assignor to Electro- Voice, Incorporated, Buchanan, Mich., a corporation of Indiana Filed June 21, 1961, Ser. No. 118,673 7 Claims. (Cl. 181-31) The present invention relates generally to loudspeaker systems, and particularly to loudspeaker systems which are particularly adapted for public address applications.
The problem of bringing sound to a large number of people which is both audible and realistic has presented a major acoustical problem. It is more diflicult to maintain the intelligibility of sound in an auditorium than in the ambient surroundings, although less power is required, due to the fact that sound reflects from the walls of the auditorium creating a. plurality of paths between the sound source and the listeners in the audience. This fact adversely affects the image of realism, that is, the listener is conscious of the fact that a public address system has been interposed between him and the speaker or other performer.
If the sound originates in the auditorium, it is necessary to have a microphone which is subjected to the reflected sound from the walls of the auditorium, and therefore, feedback in public address systems limits the power capabilities of the system. It is an object of the present invention to provide a loudspeaker system which will improve the image of realism and reduce the probability of feedback to the greater extent than the presently known and used loudspeaker systems.
Many auditoriums in use today have public address systems which employ a plurality of loudspeakers located in different locations. As a result, the sound impinging upon the ears of the listeners appears to come from a plurality of different directions, thus lessening or destroying the image of realism. A plurality of speakers is generally required in order to provide adequate sound coverage, that is, in order to provide a sound distribution pattern which adequately reaches all of the persons in the auditorium. It is an object of the present invention to provide a loudspeaker system which produces a sound distribution pattern which is uniform over a broad horizontal plane, and which produces a relatively narrow sound distribution in the vertical plane.
It has been known for some time that a column loudspeaker system is capable of achieving a broad sound distribution in a horizontal plane and a relatively narrow sound distribution in the vertical plane over a limited frequency range. Such sound systems may be characterized as line sources of sound, and are described in Acoustical Engineering by Harry F. Olson, D. Van Nostrand Company, Inc., Princeton, New Jersey, 1957, at page 36. A vertically disposed line source of sound radiation will produce a broad horizontal pattern and a narrow vertical distribution beamed normal to the line for line lengths in excess of a wavelength. The frequency range commonly employed for auditorium use is approximately from 100 cycles per second to 10,000 cycles per second, and it is thus clear that a line audio source having a length equal to one Wavelength at the low frequency limit of its frequency range has a length of the order of 100 wavelengths at the high frequency end of its range. A properly designed line audio source for the low frequency portion of 3,125,181 Patented Mar. 17, 1964 the range of a public address system is thus found to produce unwanted lobes at the high frequency portion of its range, leading to non-uniform sound distribution over the auditorium.
One of the advantages of a line source of audio radiation is that the radiation off of the ends of the line is very slight which permits a column speaker to be mounted above a microphone without creating excessive feedback between the speaker and the microphone. However, as the length of the column speaker, or line audio source, becomes more than a few Wavelengths long, spurious radiation lobes appear off the ends of the line and create a feedback condition.
It is one of the objects of the present invention to pro vide a line source of audio radiation which is suitable for use over a wide range of audio frequencies.
It is a further object of the present invention to provide a line source of audio radiation which may be a relatively large number of wavelengths in length without producing significant radiation from the ends of the line.
In accordance with the present invention, a loudspeaker system is provided which has a concave radiating surface and which has a plurality of radiation elements aligned on the concave surface and driven in phase. Curved line sources of audio radiation have also been known prior to the present invention, and are disclosed at pages through 42 of Acoustical Engineering, referred to above, and on page 201 of Volume 2, No. 2, of the Journal of the Acoustical Society of America by Wolff and Malter. However, the curved line audio sources referred to in these publications utilized loudspeaker elements disposed in convex planes, whereas the present invention piescribes the loudspeaker elements to be in a concave plane. As a result, the radiation patterns which are achievable by the curved line sources disclosed in the publications possess spurious lobes from the ends of the lines at the higher frequencies and less uniform radiation patterns measured in theplane of the line source.
Additional objects and advantages of the present invention will become readily apparent upon a further consideration of this specification, particularly when viewed in the light of the drawings, in which:
FIGURE 1 is a front elevational view, partly cut away and in section, of a loudspeaker system constructed according to the teachings of the present invention;
FIGURE 2 is a sectional view of the loudspeaker sys tem illustrated in FIGURE 1 taken along the line 22 of FIGURE 1;
FIGURE 3 is a. schematic electrical circuit diagram of the loudspeaker system shown in FIGURES l and 2;
FIGURE 4 is a graph comparing the energy distribution of the loudspeaker illustrated in FIGURES 1 through 3 in the plane of FIGURE 2 with the radiation distribution of a similar loudspeaker system employing a straight line audio source, the radiation distribution of the loudspeaker system of FIGURES 1 through 3 being illustrated in solid line and the radiation distribution of a comparable straight line source being indicated by dashed lines; and
FIGURE 5 is a graph illustrating the radiation distribution of the loudspeaker of FIGURES 1 through 3 in a plane normal to the plane of the distributions of FIG- URE 4, the distribution for 500 cycles per second being shown in solid line and the distribution for 1500 cycles per second being shown in dashed lines.
The loudspeaker system shown in FIGURES 1 through 3 3. has an enclosure 10which is provided with sides 12 and 1.4-, ends. 16 and .18, a back 24 and a face 22. The sides 12 and .1 4 are parallel flat members, as are the ends 16 and 18. The back 20 is convex, and the front 22 substantially parallel to the back and concave.
The face 22 has an elongated slot 24 which extends parallel to the sides 12 and 14. A pair of parallel recessed ribs 26 extend inwardly from the sides 12 and 14, and a strip 28 of felt, or other soft material, is disposed on the surface of the ribs 26 opposite the back 20 of the enclosure v10. A plurality of loudspeakers 30 are mounted Y on the ribs 26 in abutment with the felt strips 28 and in abutment with each other. Each of these loudspeakers 30-has a cone shaped diaphragm 32 which is coupled to a magnetic driving unit 34. The periphery of the cone shaped diaphragm 32 of each speaker is mounted on a flange 3 6, and it is the flange which is mounted in abutment with the strip 28 on the ribs 26.
A layer 38 of foam plastic material confronts the loudspeakers 30 and extends along the entire face 32 of the enclosure 10. The layer 38 is of porous material and is permeable to sound. The layer 38 is for the purpose of protecting the loudspeakers 30, but permitting passage of the sound therefrom. A perforated sheet 40 abuts the foam layer 38, and is secured at its edges by a plurality of screws 42 which extend through mounting strips 44.
FIGURE 3 illustrates the fact that 12 speakers 36 are employed in the particular embodiment illustrated, and these loudspeakers have been designated 30a, 30b, 30c, 30d, 30c, 30 36g, 3011, 3th, 30 30k, and 301. Loudspeakers 30a, 30b, 30c, and 30d are connected in series between terminals 46 and 43 which are located on the back 20 of the enclosure 10. In like manner, loudspeakers 30e, 387, 30g, and 30h are connected in series between the terminals 46 and 48, and loudspeakers 30L 3'01, 30k and 301 are connected in series between these terminals. Each of the loudspeakers is selected to have an impedance of approximately 6 ohms so that the impedance across the terminals 46 and 43 may be matched to an 8 ohm line.
The enclosure 10 is essentially sealed and acts as a baffle for the loudspeakers 30. A layer 50 of sound absorbent material is disposed on the interior surface of the back 20 of the enclosure 10 for the purpose ofdeadening sound waves within the enclosure 10.
It is to be noted that all of the speakers 30 are mounted with their axes on a common plane and on a concave surface, thereby forming acurved line audio sound source. Each of the loudspeakers has a cone diameter of approximately four inches, so that the arc length of this line is approximately 48 inches. The arc length of the line sound source should be equal to the Wavelength of sound at the lowest frequency to be reproduced in order to achieve the desired energy distribution in the plane of the line source. It is generally not feasible to provide an arc length much in excess of 48 inches, due to mounting requirements and due to degradation of the energy distribution at higher frequencies for longer line sources. For this reason, it has been found most desirable to provide an arc length which is approximately one wavelength at a frequency an octave above the lowest frequency to be reproduced.
In the particular loudspeaker described, the radius of curvature of the line source, or the face 22 of the enclosure, is approximately feet. The radius of curvature of the line source should be between a wavelength of sound at the lowest frequency of the range to be reproduced and a wavelength of sound at the highest frequency of the range to be produced. The loudspeaker system here disclosed is effective over a frequency range from approximately 100 cycles per second to approximately 10,000 cycles per second, and the 5 foot radius of curvature has been found experimentally to produce the desired acoustic distribution pattern in the plane of the line source without creating spurious radiation lobes ofl? the ends of the column or line for the higher frequencies.
The frequency distribution pattern in the plane of the curved line source, that is normal to the face 22 of the enclosure, is illustrated in FIGURE 4 and compared with a similar loudspeaker system, employing a straight line acoustic source. The radiation distribution pattern for both speakers represent the pattern achieved at the same frequency, a frequency at which the length of the line source is greater than four wavelengths. It is to be noted that the straight line source radiator achieves a much narrower radiation pattern. This is of importance in achieving coverage of an audience in an auditorium. A single curved line column speaker, such as here disclosed, can in many applications, replace two or more straight column speakers. The fact that a curved line speaker produces a radiation distribution through a greater angle than a straight line speaker permits the loudspeaker to be mounted higher in an auditorium and still cover the audience, thus reducing the absorption resulting from low level operation of the speaker and increasing the distance between the loudspeaker and the microphones which may be employed. It is always advantageous to increase the distance between the loudspeaker and the microphones since the probability of feedback is reduced and the amplifier system may be operated at a higher level. Additionally, a curved line speaker, such as here disclosed, exhibits fewer spurious lobes in its radiation distribution pattern emanating from the ends of the line at the higher frequencies.
From the foregoing disclosure, those skilled in the art will readily devise many other and additional constructions within the intended scope of this invention. It is therefore intended that the scope of the present invention be not limited by the foregoing disclosure, but rather only by the appended claims.
The invention claimed is:
1. A loudspeaker system for reproducing a predetermined range of frequencies comprising an enclosure having an elongated circular concave face with a radius of curvature between the longest wavelength and shortest wavelength of sound to be reproduced, a plurality ofloudspeakers having members adapted to vibrate along an axis mounted on the face of the enclosure with the axes of the members disposed in a common plane normal to the central axis of face, said loudspeakers being disposed closely adjacent to each other and being responsive to electrical excitation in the same frequency range, thus forming a line source of acoustical radiation when driven in phase, said loudspeakers forming an arc length at least equal to the wavelength of sound at a frequency an octave above the lowest frequency in said range, said face having an opening confronting the vibratile member of each loudspeaker and the vibratile member of each of said loudspeakers having dimensions small relative to the length of the arc of the line source of radiation.
2. A loudspeaker system for reproducing a predetermined range of frequencies comprising the elements of claim 1 wherein the vibratile member of the loudspeakers are in the form of cone shaped diaphragms sealed to the face of the enclosure, said diaphragms being centered on a plane normal to the central axis of the face.
3. A loudspeaker system for reproducing a predetermined range of frequencies comprising the elements of claim 2 wherein a layer of sound absorbing material is. disposed within the enclosure between the face thereof and the surfaces of the enclosure opposite the face.
4. A loudspeaker system for reproducing a range of predetermined frequencies comprising the elements of claim 2 wherein the enclosure has a face with a radius of curvature of approximately five feet, andthe length of the are formed by the line of loudspeakers on the face is approximately four feet long.
5. A loudspeaker system for reproducing a predetermined range of frequencies comprising the elements of claim 4 wherein twelve loudspeakers with cones of approximately four inches in diameter are disposed within the enclosure, said loudspeakers being electrically connected to a pair of input terminals.
6. A loudspeaker system for reproducing a predetermined range of frequencies comprising the elements of claim 5 wherein the loudspeakers have electro-magnetic transducers, the twelve speakers being serially connected in three groups across the input terminals.
7. A loudspeaker system for reproducing a predetermined range of frequencies comprising the elements of claim 4 wherein the face of the enclosure is provided with an elongated slot normal to the central axis of the face, and the loudspeakers are mounted on the face confronting the slot in abutting relationship.
References Cited in the file of this patent UNITED STATES PATENTS Goldsmith Apr. 19, 1938 Waite Jan. 10, 1939 Doubt July 8, 1952 Jordan et al Mar. 8, 1960 FOREIGN PATENTS Germany Apr. 28, 1933 Great Britain Aug. 21, 1957 Germany Feb. 27, 1958