The Fifth Element #48 Sidebar: 2.5 Cheers For Standmounted Speakers!

Sidebar: 2.5 Cheers For Standmounted Speakers!

Despite the obvious advantage of floorstanding loudspeakers—you don't need stands—as well as the generally held perception that floorstanders fit more easily into a wider range of home decorating styles, many experienced critical listeners aver that, between otherwise similar floorstanders and stand-mounts, the latter usually sound better. Or, to put it another way, many critical listeners believe that below some threshold (such as $5000/pair), floorstanders are often compromised designs, and that a similarly priced stand-mount will usually sound better.

To take one recent example, the lowest-priced speaker in KEF's Reference line, the stand-mounted 201/2, costs $5000/pair. KEF obviously knows how to build floorstanders at varying price points, but for their smallest Reference speaker they opted for a stand-mount. Is there any objective reason why this should be so?

One answer is that different cabinet shapes enclosing the same volume will have different internal resonances. Let's take as examples for analysis the time-honored "two-cubic-foot large BBC monitor" stand-mounted loudspeaker, and a more modern floorstander of the same internal volume. (For ease of calculation, I assume that the take-aways for cabinet-wall thickness and driver intrusions will be the same in both cases.)

The BBC's classic stand-mounted "large monitor" shape is a box 24" H by 12" W by 12" D. This gives a volume of 3456 cubic inches. Let's enclose that same volume in a trendier, floorstanding shape: put the midrange driver on the front, the woofer on a side panel, and crop the sides of the midrange driver to enable as narrow a front baffle as possible—say, 6". To put the tweeter at listening height while dispensing with a stand, the floorstander needs to be at least about 42" tall. Sequentially dividing 3456 by those two numbers leaves us with a depth of 13.71", which, I hope unsurprisingly, is a not-unheard-of depth for a modern floorstander that is tall and narrow.

Why should this make any difference? After all, the two speakers have cabinets of the same volume. The answer lies in the wavelengths of the internal resonances. (I disregard tangential or oblique resonances as too low in energy to be audible.) The two-cubic-foot BBC box has front-to-back and side-to-side internal axial resonances at about 565Hz (the speed of sound, 1130' per second, divided by twice the axial length, because the sound makes a round trip), and a top-to-bottom resonance at 282.5Hz. (It may be the case that the BBC's time-honored but by no means noncontroversial "lossy thin-wall" cabinet design is intended to reduce the amplitude of this resonance.)

The tall, narrow floorstander has a side-to-side resonance at about 1130Hz and a front-to-back resonance at about 494Hz. However, its top-to-bottom resonance is at about 161Hz. This creates trouble, for two reasons.

First, that 161Hz resonance is in the frequency range that engineer and teacher David Moulton calls not the lower midrange or the upper bass, but the "mudrange." This band is particularly crucial for clarity and intelligibility in any element of a reproducing system, whether it be a microphone, a recorder, an amplifier, or a speaker. It's where things are most likely to sound "muddy."

Second, the tall floorstander's 161Hz top-to-bottom axial resonant frequency is too low to be damped effectively with acoustically absorbent materials such as fiberglass or sheep's wool. Its quarter-wavelength is 1.75'; therefore, even 6" of fiberglass will have little effect. Double whammy. Making the speaker wider and less deep won't help, as the problematic mode is caused by its 3.5' height. The combination of a resonance in a critical band plus the difficulty of reducing its amplitude is likely to result in stored energy, overhang, and diminished clarity.

I designed this thought experiment to be as scrupulously fair as possible. In the real world, most stand-mounted speakers are not as tall as the BBC two-cubic-foot monitor, so the contrast in most cases will be greater. For example, the KEF Reference 201/2 measures 16.4" H by 9.8" W by 15.9" D. Its lowest-frequency internal axial resonance is at about 830Hz, within the range where acoustically absorbent materials are sufficiently effective.

Conclusion: If we're going to pick speakers by décor-friendliness, let's all have done with it and buy in-walls. Despite their other virtues, the taller shapes of certain floorstanders may give rise to problematic internal resonances in a band that is critical to clarity, and that acoustical damping materials are less effective in controlling.

From this, one is tempted to conclude that the flowering of the tall, narrow, floorstanding speaker has at least in part been driven by the marketing reality of consumer preference—and, of course, a narrower baffle does contribute to image specificity. This could be a case of people clamoring for what is not best for them, if what they want is the best sound.

As with any individual case, your mileage may vary. A tall, narrow floorstander that has been well designed may sound better than an indifferently designed boxy stand-mount of similar enclosed volume. But if it's the floorstander that sounds better, it's because the designer worked to overcome the design advantage the stand-mount had coming out of the starting gate.

I am indebted to Charles Hansen of Ayre Acoustics, himself a former loudspeaker designer, who patiently answered my questions about this.—John Marks

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