US3080012A - Stereophonic loudspeaker arrays - Google Patents

Description

March 5, 1963 B. B. BAUER STEREOPHONIC 'LOUDSPEAKER ARRAYS Filed Sept. 26, 1958 2 Sheets-Sheet 1 FIG.

INVENTOR. BENJAMIN B. BAUER his ATTORNEYS March 5, 1963 B. B. BAUER 3,080,012

STEREOPHONIC LOUDSPEAKER ARRAYS Filed Sept: 26, 1958 2 Sheets-Sheet 2 INVENTOR. BENJAMIN B. BAUER A T TOFIWE YS 3,tl8tl,012 STEREGPHGNIQ LQUDSPEAKER ARRAYd Beniamin B. Bauer, Stamford, 60nd, assignor to Columhia Broadcasting System, inc, New York, N.Y., a corporation of New York Filed Sept. 26, 1953, Ser. No. 763,499 8 Clmms. (@l. 1813i) This invention is directed to loudspeaker arrays for stereophonic playback systems and, more particularly, to a new and improved loudspeaker arrangement adapted to produce a correct stereophonic impression over a broad listening area.

in conventional two-channel stereophonic systems, two loudspeakers are employed for the reproduction of the individual right and left channels and these are generally placed against one wall of a room at a distance from each other which is equal to about 70% of the length of the wall. Any listeners on the axis of symmetry between the two loudspeakers will hear both channels equally Well and perceive a correct stereophonic impression. Observers oil the axis of symmetry, however, will hear predominantly the sound from the loudspeaker which is closest to them, and this effect provides a distorted perspective of sound.

It the microphones used in recording a two-channel stereophonic performance are highly directional or are placed close to individual performers of a group, there will be considerable separation between the sounds of the two channels and upon reproduction, the sounds will emerge predominantly from one or the other loudspeaker. Since these sounds are basically unlike, they will be perceived as two different sounds, there being no apparent sound coming from the space in between the two loudspeakears creating what is known as a hole. In order to fill out this hole, the two microphones are usually directed to receive equally the center sounds. In addition, a third microphone arranged to receive sound from performers at the center of the group and its output is fed equally into the two channels of the stereophonic system. Even with this arrangement, however, observers on the axis do not perceive the center sounds correctly, but are subject to a shifting center efiect whereby the origin of a sound from a centrally located source appears to be substantially closer to the loudspeaker nearest the observer because of the greater intensity of sound received from that speaker.

Accordingly, it is an object of this invention to provide an improved stereophonic system in which the loudness of the left and right channels are about equal for all observers within the normal listening area.

Another object of the invention is to provide a loudspeaker system capable of conveying an illusion that a centered sound originates from the space between the two loudspeakers, for all the observers in the room.

Still another object of the invention is to provide a stereophonic loudspeaker, system in which substantially equal stereophonic perception is obtained by all of the observers in the listening area.

A further object of the invention is to provide a loudspeaker enclosure capable of producing a variable radiation pattern for use in accordance with the invention.

These and other objects of the invention are attained by directing the two loudspeakers of a stereophonic system so that their axes intersect at a point in front of the listening area. According to the invention, the pattern of sound radiation from these loudspeakers, which decreases in intensity as the angle from the axis of the speaker increases, is utilized to produce substantially equal loudness from both speakers at any point in the listening area when identical signals are applied to the hired h t-sates i dlifiiliii il Patented Mar. 5, 1963 two speakers. inasmuch as only the middle frequencies and the high frequencies of sound are primarily eflfective to produce a stereophonic impression, these frequencies may be selected for reproduction over the two loudspeakers and the remainder of the sound spectrum can be applied to a centrally located loudspeaker in another embodiment of the invention.

In order to improve the stereophonic etiect provided in the above manner, angled loudspeakers having a radiation pattern controlled by selection of the acoustic resistance of the enclosure may be utilized for each of the two stereophonic channels. In this embodiment, the loudspeaker enclosure includes a wall of perforate material covered with fabric behind a speaker, the acoustic resistance of the material and the volume of the enclosure being selected to shift the phase of the sound within the enclosure to obtain any desired pattern of radiation. In addition, a variable shutter allowing for adjustment of the exposed area of the fabric may be used to vary the acoustic resistance and the entire enclosure may be covered with a sound permeable but substantially opaque material to prevent listeners from observing the angular orientation of the loudspeaker.

Further objects and advantages of the invention will be apparent from a reading of the following description in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic plan View of a typical loudspeaker system arranged according to the invention;

FIG. 2 is a plan view in section showing a representative enclosure for a loudspeaker utilized for stereophonic reproduction;

FIG. 3 is a plan view of another form of loudspeaker arranged according to the invention;

FIG. 4- is a schematic view illustrating the radiation pattern generated by one form of loudspeaker arranged according to the invention;

FIG. 5 is a plan view illustrating a stereophonic system employing loudspeakers of the type shown in FIG. 4; and

FIG. 6 is a sectional view illustrating the structure of a form of loudspeaker capable of generating a variable sound radiation pattern.

One form of loudspeaker array according to the in-' vention is shown in FIG. 1 wherein a room It; includes a front portion 11, which may be a stage, for example, whereon two enclosed loudspeakers i2 and 13 receiving the left and right channels, respectively, of a stereophonic signal are positioned. The remainder of the room behind the front line 14- of the stage comprises a listening area 15 and may include the seating arrangement usually found in an auditorium, for example. On the other hand, the room it may be small, such as the living room of a home, in which case the line 14 is merely an imaginary line extending across the width of the room between the speakers 12 and 13 and the nearest listener.

Instead of being placed flat against the end wall 16 of the room in the usual manner, the loudspeaker enclosures .are shifted at a considerable angle so that the axes of the two loudspeakers intersect at a point 17 located generally forward of the line 14, forming an obtuse angle 0 at the point of intersection. Experiment has shown that the optimum value of 0 varies with the size of the loudspeaker and the enclosures. For loudspeakers having a cone diameter of 15 inches and housed in enclosures 2 feet Wide, the optimum angle 0 is approximately l20. For 8 inch diameter loudspeakers housed in enclosures about 1=2 inches wide, the angle 0 should be approximately for the best stereophonic reproduction. Where 5 inch loudspeakers the housed in enclosures 8 inches wide, the optimum angle 0 is almost 180, but as the angle approaches 1180" the stereophonic efiect becomes less evident.

It is apparent, therefore, that the improvement observed in these experiments stems from certain properties of the loudspeakers dependent on their size. More particularly, a loudspeaker mounted in an enclosure exhibits directional properties which are different at different frequencies. At very low frequency, when the wave length of sound is large compared to the dimension of the loudspeaker cone, the wave emanating from the loudspeaker has uniform intensity in all directions, as shown by the dash circles 18 and 13' in FIG. -l. At the higher frequencies for which the wave length of sound is comparable to or smaller than the diameter of the diaphragm, the loudspeaker radiates preferentially along the axis as shown by the outlines l9 and 19', the length of .a radius vector from the center of the loudspeaker cone to the outline portraying the sound pressure radiated in the direction of the radius vector. It is well known that stereophonic perception is more evident at higher frequencies than at lower frequencies and, therefore, it is this sharpening of the radiation pattern at high frequency that is responsible for the improved stereophonic effect, if the loudspeakers are properly oriented according to the invention.

In operation, when an observer is at a position 20 on the longitudinal center line 21 of the room the sound pressure radiated in his direction from the two speakers 12 and 13, which are equidistant from him, is shown by the vectors 22 and 22', respectively. Because of symmetry, these vectors are of equal length if equal signals are applied to the two speakers as by a centrally located source of sound, and the observer will hear both loudspeakers with the same loudness. If the observer moves to a position 23 near one side of the room, the intensity from the loudspeaker 12 is represented by the vector 24 which is longer than the former vector 22 and therefore, corresponds to a greater intensity of sound. In this position, however, the observer is farther from the loudspeaker 12 than before so that the loudness of the sound he hears from this loudspeaker is substantially the same as before. At the same time, the intensity of sound directed toward the position 23 by the loudspeaker 13 is less than that directed toward the position 20, as represented by the shorter vector 24'. However, at the position 23 the observer is closer to the loudspeaker 13 than when he was at the central position 20 so that the sound he hears from this speaker has substantially the same loudness as at the position 20. Therefore, the sounds from the two loudspeakers 12 and 13 arriving at the position 23 will be equally loud, thus giving the observer the required degree of stereophonic perception. By proper inclination of the loudspeakers according to the invention, this satisfactory perception is obtained practically everywhere in the room. At the same time, the center sou'nds, which are transmitted over the two channels with equal intensity, appear to come from an angle intermediate between the two loudspeakers, i.e. on the'line 25, and an apparent central sound source is perceived by the observer regardless of his position in the'listening area 15.

Inasmuch as the inclination of the loudspeaker axes at an angle 6 is important to the production of the desired effect as described above, I prefer to provide a special enclosure for the loudspeakers which automatically positions them at the desired angle. Such an enclosure is shown in FIG. 2 wherein the numeral 26! closures are placed flat against the end wall 16', as shown in FIG. 3, the angle of intersection of the two axes of the speakers will be equal to i=2 (1:. For a loudspeaker with an 8 inch cone the desired angle is found to be approximately 70, giving a total angle of intersection 0 of Both the enclosures 26 and 26' are preferably provided with a foraminous grille or cover 32 which does not allow the observer to see the direction in which the loudspeaker is pointing. This is an important psychological effect which greatly contributes to the effectiveness of the loudspeaker system.

Since it has been shown that stereophonic effect is mainly caused by the middle range of the high frequencies of sound, the stereophonic loudspeaker arrangement may be very effectively utilized in combination with the socalled mixed lows system. According to this system, the outputs of all stereophonic channels at the frequencies below a certain range, say 10 0300 c.p.s., are mixed together and reproduced over one loudspeaker, while the frequencies above this range are kept separate and are reproduced over two other separate loudspeakers. In this manner, the high frequency loudspeakers may be quite small, say 10 inches, and yet capable of providing the improved stereophonic effect described herein.

An example of this arrangement is shown in FIG. 3 in which the loudspeaker 33 is designed to reproduce the mixed frequencies of both channels up to 300 c.p.s., and the loudspeakers 31 and 31 reproduce the high frequency stereophonic information of the left and right channels, respectively, which is above 300 c.p.s. In this case, the enclosures26 and 26 may each be approximately a 10 inch cube. Also, the loudspeaker 33 may be placed anywhere in the room because the origin of the low frequency does not play an important part in producing the stereophonic perception. The enclosures 26 and 26' are placed against the wall so that the axes 34 and 3 4 of the speakers 31 and 31' intersect in the predetermined angle 0 at a point 35 in front of the line 14 to operate in the manner described above. i v

Inasmuch as the radiation pattern of a loudspeaker contributes to the improved stereophonic impression, the use of directional loudspeakers is especially valuable in carrying out the purposes of the invention. A directional loudspeaker may be obtained by taking a conventional moving coil loudspeaker and mounting it in a bafile of suitable design. If an open cone loudspeaker 36 is mounted in a small open baffle 37 as shown in FIG. 4, for example, then the radiation pattern at all frequencies up to and including those at which the wave length equals twice the dimensions of the baflle will follow the cosine law (p==cos 0) shown in the drawing by the two dash circles 38 and 39. I have found this to be a very desirable radiation pattern to carry out the purposes of my invention. Thus, FIG. 5 shows a schematic arrangement which employs two loudspeakers 40 and 41 similar to those in FIG. 4, to reproduce frequencies above 250 c.p.s., for example, the open baffles being represented by the numerals 42 and 43. The loudspeaker 40 receives information from the left stereophonic channel above 250 c.p.s., and the loudspeaker 41 receives the energy from the'right stereophonic channel above the frequency 250 c.p.s., and the two front radiation patterns of these loudspeakers are shown in the drawing by the dash circles 44 and 45. Both of these loudspeakers and their bafiies 42 and 43 are preferably enclosed in foraminous enclosures 50 and 51 to conceal the fact that the loudspeakers are inwardly oriented for the purpose pointed out above. Referring again to FIG. 4, it may be observed that the radiation pattern 38 tends to narrow down at high frequency with speakers of this type. This effect, however, may be diminished by placing a suitable grille or an acoustical lens 52 comprised of appropriate baffles 53 of varying length in front of the loudspeaker, or by using a dual cone loudspeaker, as is well known in the art.

In operation, it will be apparent that an observer at the position 23 receives practically the full amplitude of sound radiated by the loudspeaker 40 as portrayed by the vector 46 while at the same time he receives a much lower amplitude from the loudspeaker 41, as shown by the vector 47. However, since he is so much nearer to the loudspeaker 41, the sounds from both the loudspeakers appear to him to be about equal in loudness. By the same token, the center sounds appear to originate from a central point between the two speakers on the line 48 as described above with reference to FIG. 1. It may be shown by analysis that the similarity of intensity from the two channels employing this system is within less than 2.5 db over practically the entire listening area 15 of the room. As in the system shown in FIG. 3, the frequencies from both channels in the range below 250 c.p.s. are mixed together and reproduced from a single speaker 33.

Inasmuch as the loudspeakers 40 and 41 are mounted in open bafiles 42 and 43 and are not enclosed at the rear, the sound radiated from the back of the loudspeakers is thoroughly dispersed and mixed in the room and it gives an added reverberant effect which creates the illusion of being in a concert hall.

Another useful type of enclosure for use with loudspeakers in accordance with my invention is one which will produce a cardioid radiation pattern (0.5 +0.5 cos 9) or any other pattern in the limacon family (1K+K cos 0), where O K L An example of such an enclosure is shown in cross section in FIG. 6 wherein the enclosure comprises a box 54 of a generally parallelepipedal shape having one wall 55 carrying a loudspeaker 56 and inclined at an angle in the manner described above with respect to FIG. 2. Opposite the wall 55, a rear wall 57 is made of perforated material which may be covered with a layer of fabric 60, and behind this wall a variableshutter 58, slidable in a groove 59, for example, allows for adjustment of the exposed area of the fabric and, therefore, its acoustic resistance. The acoustic resistance of fabric 60 and the volume of the box 54 are selected to shift the phase of sound waves inside the box in such a manner that any desired pattern of sound radiated by the loudspeaker is obtained, such as a cardioid pattern as shown in the drawing by the dash line 61. At high frequency this pattern will sharpen as described before to approximate a circle 62.

In operation, two of the enclosures 54 which are mirror images of each other are arranged against an end wall 16 in the manner shown in FIG. 3. After installation in this manner the shutters 58 are adjusted to achieve the optimum ratio of direct-to-random sound to suit the reverberant conditions of the room. Also, an improved three-channel stereophonic system may be obtained by using two loudspeakers in enclosures 54 as hereinbefore described as the left and right channel loudspeakers respectively along with a conventional loudspeaker for the center channel of the system.

Although the invention has been described herein with reference to specific embodiments, many modifications and variations therein will readily occur to those skilled in the art. Accordingly, all such variations and modifications are included within the intended scope of the invention as defined by the following claims.

I claim:

1. A stereophonic enclosure and loudspeaker arrangement comprising a first area having at least two substantially equal loudness loudspeakers arranged along one side of said first area, said loudspeakers being arranged with their axes at at least 100 but less than 180 with respect to each other, said axes intersecting at a point within said first area, said enclosure having a second area for listeners, the sound from said loudspeakers in said second area being substantially the same for any sounds of equal intensity coming from said loudspeakers.

2. A stereophonic enclosure and loudspeaker arrange- 6 ment according to claim 1 wherein each of the loudspeakers has a frequency range extending above 300 cycles per second.

3. A stereophonic enclosure and loudspeaker arrangement according to claim 2 including a further loudspeaker having a frequency range extending below 300 cycles per second.

4. A stereophonic enclosure and loudspeaker arrangement according to claim 1 wherein each of the loudspeakers includes means for radiating acoustic energy directionally so that the radiation intensity substantially follows a limacon relation as the angle from the axis of the loudspeaker varies.

5. A stereophonic enclosure and loudspeaker arrangement according to claim 4 including housing means for each loudspeaker having an opening behind the loudspeaker so that acoustic energy radiated forwardly and rearwardly from the loudspeaker is combined in the proper magnitude and phase to produce a radiation intensity which substantially follows a limacon relation as the angle from the axis of. the loudspeaker varies.

6. A stereophonic enclosure and loudspeaker arrangement according to claim 5 including acoustic resistance means covering the opening in the housing means behind the loudspeaker to control the intensity of the acoustic energy radiated rearwardly therefrom.

7. A stereophonic enclosure and loudspeaker arrangement according to claim 5 including movable shutter means at the rear of the housing means and adjacent to the opening therein to control the size of the opening and vary the intensity of the rearwardly radiated acoustic energy.

8. A stereophonic enclosure and loudspeaker arrangement according to claim 4 wherein the housing means includes a bafile having an opening in which the loudspeaker is mounted, said bafiie being only slightly larger than the diameter of the loudspeaker.

References Cited in the file of this patent UNITED STATES PATENTS 1,645,231 Dietrich et al Oct. 11, 1927 1,855,147 Jones Apr. 19, 1932 1,902,609 Beers et a1 Mar. 21, 1933 2,065,751 Scheldorf Dec. 29, 1936 2,137,032 Snow NOV. 15, 1938 2,210,477 Benecke et al. Aug. 6, 1940 2,520,798 DeBoer Aug. 29, 1950 2,580,439 Kock Ian. 1, 1952 2,623,606 Corke Dec. 30, 1952 2,710,662 Camras June 14, 1955 2,766,839 Baruch et a1 Oct. 16, 1956 2,858,899 Lopez-Henriquez Nov. 4, 1958 2,904,124 Lyons Sept. 15, 1959 2,924,660 Abrams Feb. 9, 1960 FOREIGN PATENTS 102,888 Australia Dec. 23, 1937 197,911 Switzerland May 31, 1938 229,642 Switzerland Feb. 16, 1944 641,668 Great Britain Aug. 16, 1950 955,960 Germany Jan. 10, 1957 957,044 Germany Jan. 31, 1957 1,142,072 France Mar. 25, 1957 781,637 Great Britain Aug. 21, 1957 OTHER REFERENCES Publication A Stereophonic Magnetic Recorder by Marvin Camras, pages 445 and 446, April 1949.

Textbook, Acoustical Engineering, by H. F. Olson, copyright 1957, by D. Van Nostrand Co., Inc.

Claims (1)

1. A STEREOPHONIC ENCLOSURE AND LOUDSPEAKER ARRANGEMENT COMPRISING A FIRST AREA HAVING AT LEAST TWO SUBSTANTIALLY EQUAL LOUDNESS LOUDSPEAKERS ARRANGED ALONG ONE SIDE OF SAID FIRST AREA, SAID LOUDSPEAKERS BEING ARRANGED WITH THEIR AXES AT AT LEAST 100* BUT LESS THAN 180* WITH RESPECT TO EACH OTHER, SAID AXES INTERSECTING AT A POINT WITHIN SAID FIRST AREA, SAID ENCLOSURE HAVING A SECOND AREA FOR LISTENERS, THE SOUND FROM SAID LOUDSPEAKERS IN SAID SECOND AREA BEING SUBSTANTIALLY THE SAME FOR ANY SOUNDS OF EQUAL INTENSITY COMING FROM SAID LOUDSPEAKERS.

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