dbx Soundfield 100 Speaker System Review

The dbx Soundfield speakers are designed to create a stable sound field in the listening room, so that the stereo imaging and sonic “stage” remain fixed in space over a wide range of listening positions without requiring critical speaker placement in the room. Most conventional speakers produce their best stereo image along a line equidistant from the speakers (down the center of the listening area). Moving any considerable distance to one side of this line usually shifts the apparent location of a sound source. In extreme cases, sitting or standing close to one of the speakers causes its sound to dominate, or masks that of the other speaker, wiping out the stereo image.

The dbx technique is to create a shaped horizontal dispersion pattern from each speaker favoring an inward-angled axis. The speaker’s frequency response and directional properties are also controlled throughout the full 360-degree space surrounding it. Empirical studies showed that such a pattern could create a stereo stage and apparent sound-source location that remain virtually unchanged over a wide range of listening locations. Achieving this result in the first Soundfield speaker, the SF-1, required fourteen drivers and a complex crossover network to supply the necessary phase and amplitude control for each of those drivers. Understandably, it was—and is, in its current version, the SF-1A—a rather expensive speaker.

Improvements and simplifications have been incorporated in subsequent Soundfield systems with no loss of imaging effectiveness. The new dbx Soundfield 100 uses only five drivers, is smaller than the Soundfield 1A, does not require an active equalizer, and costs about one-third as much, yet it has the same essential sound qualities as the larger Soundfield models.

The Soundfield 100 is a floor-standing speaker whose asymmetrical cabinet, crossover, and driver configuration were all computer designed. It is supplied in mirror-image pairs, and except for the top and bottom, the cabinets have no parallel surfaces. The cabinet’s rear panel is conventionally positioned parallel to a wall, which should be about a foot or so behind it. The main speaker panel, which is angled inward about 60 degrees, has a 10-inch woofer at the bottom and a 4-1/2-inch midrange cone driver and a 1/2-inch dome tweeter close together near the top. Another 1/2-inch tweeter is on the narrow front panel, which is angled slightly outward, and the outer panel, which is angled slightly forward, contains a third Vi-inch tweeter and the woofer port. The three identical tweeters, which use plastic diffusers to improve their dispersion, are all at the same height from the floor.

The rated frequency response of the dbx Soundfield 100 is 40 to 20,000 Hz ± 3 dB, and the nominal system impedance is 4 ohms (with a 2.5-ohm minimum). The system’s rated sensitivity is a high 91 dB sound-pressure level (SPL) at 1 meter with an input signal of 2.83 volts. It is recommended for use with amplifiers capable of delivering from 20 to 250 watts per channel into 4-ohm loads.

The walnut-veneered cabinet stands 31-1/2 inches high, 17 inches wide and 13 inches deep, and each speaker weighs about 40 pounds. Insulated binding-post terminals are recessed into the rear, and there are no user-adjustable controls. The removable dark-brown grilles covering the drivers on the three panels are retained by plastic snaps. Price: $899 per pair.

Lab Tests

Although we would expect a speaker with several drivers facing in different directions to have good dispersion, we were unprepared for the excellent room-response curves of the two dbx Soundfield 100’s. When we measured the response on the forward axis of the left speaker and about 30 degrees off the same axis of the right speaker, the two curves were unusually flat and virtually identical from 500 to 20,000 Hz, differing by no more than 2.5 dB (and usually less than 1 dB) over that full range. This sort of measurement result, in a normal listening room, is unique in our experience. Although room boundaries have an unavoidable effect on the response at lower frequencies, the Soundfield 100’s response curves were far more uniform with frequency, and varied less between the two speakers, than we can recall seeing from any other system we have tested.

The close-miked woofer response, including the output from its port, varied only ±4 dB from 20 to 500 Hz. Splicing the woofer-response curve to the averaged room-response curve produced a composite overall frequency-response curve that was flat within ±2.5 dB from 400 to 20,000 Hz and varied only ±3.5 dB from 20 to 400 Hz.

The system’s impedance was about 2.4 ohms from 100 to 400 Hz and from 15,000 to 20,000 Hz. The maximum values were 6 ohms at 50 Hz and 11 ohms at 1,350 Hz. The measured sensitivity depended somewhat on the measurement axis: when we drove the speaker with 2.83 volts of pink noise, the SPL at 1 meter was 91 dB from the main panel, 90 dB from the front panel, and 88 dB from the outside panel. We chose 90 dB as the average sensitivity. With the same drive voltage, the woofer distortion was under 0.5 percent from 100 Hz down to 55 Hz, increasing slightly at lower frequencies. We also measured the distortion at the port and used those readings below 45 Hz, where the port output was dominant. The distortion reached 7.7 percent at 30 Hz, dropped to 3.5 percent at 25 Hz, and rose to 12.3 percent at 20 Hz.

In our peak-power-handling tests, the woofer cone “bottomed,” with audible rasping, at 100 Hz with an input of 2,000 watts into its 2.5-ohm impedance. At higher frequencies, our amplifier clipped before the speaker’s output showed significant distortion (at 1,000 Hz this occurred at 862 watts into 8 ohms, and at 10,000 Hz with 2,060 watts into 2.8 ohms).

Quasi-anechoic FFT response measurements could not be made on the entire speaker system because of its complex directionality. The main panel, however, which plays a major part in establishing its audible frequency balance, had an axial response that was flat within ± 3 dB from 230 to 20,000 Hz. Our close-miked individual measurements of the three tweeters showed that the output of the main tweeter was flat within ± 1 dB from 2,500 to 12,000 Hz, falling at 12 dB per octave at higher frequencies. The outside tweeter’s response was fairly flat from 3,000 to 12,000 Hz but rose sharply to a peak of +7 dB at about 16,000 Hz before dropping off rapidly. The front tweeter had a rising response over most of its range, but its output level was about 10 dB below that of the main tweeter. Apparently it was the combination of the three tweeters, radiating in different directions, that produced the extremely uniform room response we measured.

The dbx Soundfield 100 had excellent phase linearity. On the forward axis of the speaker (with major room reflections removed by our FFT analyzer), the group delay was constant within about 0.1 millisecond from 3,000 to 23,000 Hz. Even the relatively irregular phase response of the complete signal (including numerous reflections) was better than we have measured from many conventional forward-facing speakers, with a 0.7-millisecond variation from 3,000 to 22,000 Hz.

Comments

Our first impression on hearing the dbx Soundfield 100 was of its bright sound, especially compared with other, more conventional, speakers to which we had been listening. We soon realized that this apparent brightness was not due to a peak or other emphasis in the treble range. In fact, as our measurements soon proved, this speaker has one of the flattest middle- and high-frequency response curves we have ever measured.

It seems likely that the brightness we heard was the result of the system’s highly uniform total power response. In contrast, most speakers may have a flat response on their forward axis, but have reduced high-frequency levels in other directions because of driver directivity. Normally we hear the total output of a speaker after reflection (and absorption) of its higher frequencies by the room boundaries and its contents. With the Soundfield 100’s, there is more total high-frequency output to be reflected.

Although we could only guess at its cause, the noticeable brightness of the Soundfield 100’s was quite real. As often happens, a short period of listening to these speakers made their sound quality seem perfectly natural, and switching back to conventional speakers always produced a sense of loss—they sounded so muffled in comparison.

At the other end of the audio spectrum, the bass seemed somewhat thin at first, but that was certainly a by-product of the subjectively bright sound. Actually, the Soundfield 100 is as outstanding in the bass as in the treble. It is one of those rare speakers that does not deliver any bass unless those frequencies are present in the program. Performance like this can be deceptive, especially when it is compared with that of other speakers, most of which add an unnatural emphasis to frequencies in the upper bass. When real bass is present in the music, the dbx Soundfield 100 brings it forth unmistakably.

But the spatial qualities of this speaker are, after all, its major reason for existence. Compared with any of-the conventional forward-radiating speakers we had on hand, its sound was distinctly different. At a moderate distance (12 feet or so), the speakers are not heard as distinct sources; rather, the sound is spread across the wall behind them. Closer to the speakers, the spatial distribution of the sound does not change significantly, but the tonal balance varies slightly (you hear more of the direct sound from their main panels). Under any reasonable listening conditions, you are never aware that the sound is emerging from the speaker cabinets; it always seems to come from the wall behind them (or even from behind that wall).

An interesting experiment is to walk close to the speakers while they are playing and stand directly against one of them. It completely disappears (acoustically), and the entire sound appears to come from the other speaker. Except for this one special case, the subjective loudness and apparent source position remain quite constant no matter where you move relative to the speakers. Having tested two previous Soundfield speakers, with very similar reactions to their sound, we would guess that the Model 100 is very similar to them in all important respects, though without an A/B comparison it is not possible to be more specific on that point.

Although it is difficult to describe the sound of any speaker to someone who has not heard it, our “bottom line” judgment is that the dbx Soundfield 100 is superbly balanced and free from most of the usual speaker colorations. A few minutes of listening to these speakers can make most other speakers seem rather lifeless. And in addition to making the sound largely independent of the listening position, these speakers, according to dbx, are relatively unaffected by their placement in the room. You don’t have to place them—and yourself—at particular locations, with an accuracy of inches or less, to enjoy their full performance. They are a genuine pleasure to listen to. Do so if you possibly can.

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