A Dipole Speaker System (Sept. 1974)

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EVERY SOUND engineer and hi-fi enthusiast learns early in his career that the selection and placement of loudspeakers has an important bearing upon the enjoyment derived from a stereophonic or quadraphonic system. In considering the desirable attributes of loudspeakers, one customarily thinks in terms of uniformly extended frequency response, freedom from distortion at high and low sound volumes, the ability to handle transient sounds and, finally, sensitivity or efficiency. But these desirable characteristics are of themselves insufficient to ensure superior performance. Another important characteristic, often ignored by the speaker makers and users alike, is the polar pattern, or directional response, of the loudspeaker. Everything else being equal, a loudspeaker with a polar pattern which is especially adapted for the particular application produces the superior sonic performance.

This paper describes a high-fidelity loudspeaker recently designed by CBS Laboratories for the CBS Electro Music Division in which proper attention has been devoted to the directional radiation properties required for optimum stereophonic and quadraphonic performance. The new loudspeaker is called the Leslie DVX series for "Dipole with Variable Axis," for the reasons detailed below.

[*CBS Laboratories, Stamford, Connecticut 06905 ]

Fig. 1-A cosine "polar pattern obtained by mounting a loudspeaker driver on a dipole coupler," or specially shaped baffle. The relative lengths of the arrows represent the radiation strength in various directions in space.

Polar Response Patterns

A few words about the technical significance and practical implication of polar response patterns are in order.

In considering what type of directional response capability one would expect from a loudspeaker, the facile answer is "omnidirectional," i.e., radiating equally in all directions. And, indeed, if one is mainly interested in monophonic sound reproduction and if the listening room has suitable acoustical properties, an omnidirectional loudspeaker probably is a good choice. But when listening to a stereo or a quadraphonic system, an omnidirectional loudspeaker is precisely what one does not want. Directional considerations for multichannel systems have been described in my article in the March, 1973, issue of AUDIO, and they need not be repeated here. But it turns out that some loudspeaker designers who have not studied the directional requirements of multichannel systems have spent much time and effort in developing omnidirectional loud-speakers only to find them mostly suited for the needs of the monophonic listener.

But even for monophony, an omnidirectional system often is not the optimum choice because the majority of rooms in which we listen to recorded music are far from ideal. An omnidirectional loudspeaker radiates sound equally all around, the sonic energy reaching the listener directly is only a small fraction of the total. The balance is heard after reflection from the room boundaries. Such sound becomes unduly colored by the boundary reflections which cause reinforcements and cancellations at low frequency and absorption at high frequency. By contrast, a loudspeaker with a properly controlled radiation pattern can be positioned to cover only the desired listening area in the room, and to provide only sufficient room reflections to contribute beneficially to the ambience without the latter becoming overwhelming. In this connection, the DVX loudspeaker is superior also.

Study of Polar Patterns

Our researches in the directional characteristics of loudspeakers date back to the early days of stereo, when we noticed that the positions of stereo images were significantly affected by the placement of the listeners relative to the loudspeakers. It is a common experience with conventional loudspeaker arrangements that the stereo phonic image is properly reproduced only when the listener is placed on the axis of symmetry. In this position the left signals originate from the left loudspeaker, the right signals from the right one, and any center signals (soloists, etc.) are perceived to originate midway between the loudspeakers.

However, if the listener moves off the center axis, the intermediate sounds rapidly move toward the nearest loudspeaker, leaving a "hole in the middle." Such a stereophonic system is disadvantaged because the area over which a listener can enjoy the program in the manner prescribed by the recording director is very limited. Our study was directed to finding the means for broadening the area of accurate stereophonic perception.

The results of these researches caused us to conclude that for optimum stereo reproduction, the polar pattern of the loudspeakers should approximate the cosine law; in other words, the relative radiation as a function of the angular displacement from the axis, O, should be cos O. Such a pattern has the shape of two circles in contact with the acoustical center of the loudspeaker system, as shown in Fig. 1. Also, we found that this radiation function should be maintained at all frequencies above approximately 250 Hz, which is the range of frequencies most responsible for directional localization. In addition, the loudspeakers should be so oriented with respect to each other that their axes of maximum sensitivity intersect at an angle of approximately 110-120°, i.e., somewhat forward of the principal listening area, as shown in Fig. 2.

The cosine polar pattern with proper orientation allows us to obtain the following result: the listener midway between the loudspeakers, at A, is subjected to equal radiation strengths, as depicted by the arrows 1 and 2. As he moves off to one side, however, as depicted by position B, he becomes favored by the more distant loudspeaker because of the increased radiation vector 3, and correspondingly is "off the beam" of the nearest loudspeaker, as shown by the diminished vector 4. The differential radiation strengths compensate for the differential distances, so that the actual signal perceived from both loudspeakers remains . constant over a wide area of the room. As a result of this compensatory action, correct stereo reproduction can be enjoyed over practically the full listening area.

The dipole coupler used in the upper section of the DVX loudspeaker, which is shown in greater detail in Fig. 3 (with the front grille removed), is seen to consist of a properly-shaped baffle which contains four loudspeakers: the upper one, 8 inches in diameter, reproduces the frequency range 250-1000 Hz; the lower one, 3 inches in diameter, carries the range of 1000-5000 Hz; and the two symmetrically-placed side-domed tweeters provide the desired response characteristics from 5000-20,000 Hz. (The latter ones do not provide the back radiation mode.) The baffle itself is so shaped that it matches the range of wavelengths being reproduced. The full range of frequencies, therefore, is handled with equal efficiency.

Fig. 2-Stereo arrangement of DVX loudspeakers.

Fig. 3-View of the dipole coupler used in the upper section of the DVX loudspeaker.

The baffle is capable of positional adjustment within the cabinet, the angular orientation being shown on a protractor at the base of the baffle.

To obtain the desired performance, the user places the loudspeakers in any appropriate baseline. He then adjusts the dipole coupler with respect to the listening area as shown in Fig. 2.

After satisfactory results have been obtained, the user snaps the front panel back into place, and the system is ready for operation.

DVX In Quadraphonic Applications

The DVX loudspeakers are especially adaptable to quadraphonic arrangements. An example of a favorite placement is shown in Fig. 4. Here the four DVX loudspeakers are placed at the corners of the quadraphonic listening area, the two front loudspeakers, LP and RF, being placed against the far wall, while the two back loudspeakers, LB and RB, are placed against the side walls. For optimum quadraphonic performance, it has been found convenient to angle the dipole couplers in such manner that their axes point toward the center of the listening area. In this case, a centrally located listener, at A, receives the four loudspeaker radiations equally. If he moves to one edge of the listening area, e.g., to positions B or C, he is nearer to one pair of loudspeakers than to the other; but in each case he also continues to receive almost an "on-the-beam" radiation from the distant loudspeakers (as shown by arrows 1 and 2, and 5 and 6, respectively), while he is "off-the-beam" of the nearest loudspeakers as shown by the arrows 3 and 4, and 7 and 8, respectively. The relative radiation strengths again are compensatory of the differences in distance. Therefore, in quadraphony also, all four DVX loudspeakers are heard with equal efficiency over a wide listening area.

Performance Characteristics

In addition to its remarkable directional characteristics, the DVX is among the most carefully designed loudspeakers. The woofer is a 15-inch driver in a 3 1/2 cubic foot enclosure loosely filled with fiberglass terminated by a low velocity (large) vent which forms a fourth-order Butterworth Filter. Counting the woofer and the three-way mid- and upper-end dipole coupler, the system has four separate frequency bands resulting in reproduction of excellent clarity. The dividing network at the upper right of the cabinet uses air-core inductors for the frequency-dividing function resulting in elimination of dividing-network distortion. Three frequency response switches are provided to control the response of the system which in its "flat" position is shown in Fig. 5.

Also see: Speaker Tests--Impedance (by Richard C. Heyser) (Sept 1974)

(Source: Audio magazine, Sept. 1974; Benjamin B. Bauer)

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