(Photo by Mary Kent)
Of late, Stereophile has written a lot about vibration-isolating footers under loudspeakers. I have been listening to the Magico M-Pods (under the Magico M2 loudspeaker, which won our Loudspeaker of the Year Award for 2020, and which I’ll review, in follow-up fashion, in an upcoming issue of this magazine). Following an impressive demo at a 2019 audio show, I also tried IsoAcoustic footers under the Revel Ultima Salon2 loudspeakers. Michael Fremer tried them under his Wilson Alexxes. We both reported positive results.
The idea of isolating loudspeaker vibrations from floors is controversial. Many (perhaps most) designers believe that dynamic loudspeakers in particular—those with significant moving mass in their cones—should be rigidly connected to the floor as is typically done with spikes. A rigid connection of the speaker to the floor reduces the Newton-3 reactive motion (footnote 1) of the cabinet in response to the motion of the cones, heavy woofers in particular. Cabinet motion could be expected to smear the loudspeaker’s sound.
Against that is another idea: With spikes, vibrations will be transferred from the loudspeaker to the floor, and the transfer of vibrations to a suspended wood floor will be quite different than, say, to a concrete floor, or wood on concrete, and so on. This transfer of energy could affect the sound of a loudspeaker both directly—by altering its output in ways that depend on the floor type—and indirectly, via the subsequent emission of low-level sound from energy transferred to the floor and attached walls. Loudspeaker performance, then, will vary from system to system, not just due to room acoustics—that’s well-known—but also due to complexities of the loudspeaker’s interactions with the room’s surfaces.
In a recent conference paper delivered online to the Audio Engineering Society, mastering engineer and Stereophile contributor Bob Katz investigated in detail the coupling of a loudspeaker to a room with and without isolation, and the effect of vibration isolation on a loudspeaker’s measured response.
The room in question is Katz’s Studio B, which is modestly sized, with a volume of 34.3m3. The floor is well-damped and suspended. Some walls are plaster-and-lath while others are made from gypsum board. The loudspeakers are the Kii Audio Three. The isolators used are EVPs from avroomservice.com; these are essentially low-pass filters with a claimed transition frequency of about 6Hz; above that frequency they are estimated to block about 90% of sound.
When isolation was not desired, a set of blocks were substituted that were identical in form but with absorbing material replaced by kiln-dried white oak, a very hard wood.
Three measurement microphones detected changes in the frequency response (with and without isolators): at the listening position, near the front wall on the right side, and near the back wall on the right side. An accelerometer was used to measure vibrations in various surfaces, on the base of the loudspeakers and at locations around the room.
First the left loudspeaker: At lower frequencies, as expected, the isolated loudspeaker moved more than the loudspeaker mounted on the wood blocks. (Decoupling it from the floor means freeing it up to move.) Measured at the loudspeaker’s base, the difference was most pronounced around 200Hz, where vibrations in the vibration-isolated loudspeaker base were some 20dB higher than when the speaker was mounted on wood. A surprise: At 1kHz, vibration was about 8dB higher with the wood base than with the isolating base.
Throughout most of the audible spectrum, the floor moved more with the speaker on wood blocks than with it on the isolating device; the difference ranged from 5–8dB, rising to 12dB at 1kHz. However, at lower frequencies—below about 60Hz—floor vibrations increased when the isolators were used—another surprise, at least to me.
Results were qualitatively similar for the right loudspeaker, but in detail they were quite different, which reinforces the assumption that the coupling of a loudspeaker to a floor is a complicated business, depending on local acoustics and details of the floor’s construction.
Wall vibration was also complex, the two loudspeakers yielding different results at different frequencies, again demonstrating (or seeming to) that coupling depends on structural details.
Acoustical measurements were taken using Room EQ Wizard, at the microphone positions mentioned previously. Measured at the listening position, there were differences with and without the isolators, mainly in the midrange, but they were modest and inconsistent between the L and R loudspeakers.
Katz believes that the front microphones—the ones closest to a loudspeaker and also to a corner—tell a more interesting story. Starting at about 600Hz and extending through the higher part of the audio band, the SPLs of the loudspeakers with the isolators were lower by an average of as much as about 1dB. (These measurements were reported with one-octave smoothing, to better show trends.) This behavior was common to both speakers. Over such a wide frequency range, the difference is likely to be audible. Between 300Hz and 600Hz, there’s a smaller difference in the other direction, the isolators slightly increasing loudspeaker output in that range.
Response at the rear microphone was once again complex, the two loudspeakers giving divergent results.
Katz made several other acoustical measurements, including total harmonic distortion; he found very similar results with the isolators and the wood blocks, except between about 45Hz and 100Hz, where distortion with the isolators was reduced by as much as 6dB. This suggests (to me) the existence of a floor resonance that is suppressed with the use of isolators; other interpretations are possible. Energy Time Curve analysis revealed many short, negative-going spikes in the wood-block measurements that are significantly reduced with isolators (although a smaller number of short spikes are increased in amplitude with the isolators). Measurements of frequency-dependent reverberation time (T30) were complex.
To me, the most interesting results were the waterfall plots (figs.22 & 23 in Katz’s AES paper), which show a dramatic reduction in delayed energy for the left loudspeaker, with the isolators, compared with the wood blocks, from about 40Hz to above 200Hz, with one strip of delayed energy remaining—even reinforced—between about 95Hz and 110Hz. For the right loudspeaker, the results are ambiguous (figs.24 & 25). Note that the left loudspeaker is proximate to the gypsum-board wall, which could be expected to vibrate more, while the right loudspeaker’s adjacent wall is plaster.
Katz also reports, without providing details, that “some critical listeners have observed” a “subtle increase in clarity . . . when a loudspeaker is isolated from its supporting surface.” Katz concludes that “In the main, isolators placed under floor-standing loudspeakers over a wood floor produce a measurable and often strong improvement in performance.” The exceptions, he writes, “seem to be related to the unique and non-symmetrical shell construction in this room.” In his video presentation, Katz emphasized this effect on subjective sound quality, concluding that “isolation placed under loudspeakers has proved to be an effective tool, resulting in sound quality that is cleaner, tighter, quieter, and deeper.” That’s consistent with Michael Fremer’s experiences (and to a lesser extent my own), but it is merely mentioned, not supported by evidence presented in the paper or the presentation.
I’m inclined to spin the results a little differently. The large differences observed from one loudspeaker to the other show, I’m thinking, how important the fine details of room construction are in affecting the interface between the loudspeaker and the room. In standard construction, such details vary greatly from structure to structure and spot to spot. How closely spaced are the floor joists? How thick is the plywood under a wood floor? How thick is the flooring itself? If the floor is nailed, how closely spaced are the nails? If it is glued, how much and what kind of glue was used? How uniform is all of this? The results also appear to show that coupling via the air is important, which is hardly surprising. I also find myself wondering how the measurements would differ with other commercially available isolators (footnote 2).
Footnote 1: For every action, there’s an equal and opposite reaction.
Footnote 2: For an examination of how the interface between a bookshelf loudspeaker and the stand on which it sits affects the vibrational behavior of its cabinet, see my article here.—John Atkinson
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