The Sound of Surprise (the loudspeaker/stand interface) Page 2

JA returned to this subject in September 1992 (Vol.15 No.9):

I'm proud to be an English audiophile. I say so not in a spirit of misplaced Jingoism, but merely to point out that the English have always placed a high priority on getting the best sound from their systems. This isn't because liking one's beer warm endows the drinker with an excess of virtue, however, or because audiophile hearts are particularly pure across the pond. No, it's because upgrading your system becomes difficult when nothing is folded in your billfold and the term "disposable income" always seems to refer to what someone else has to dispose. Solving sonic problems by tweaking what you already have rather than by throwing money into new components naturally becomes a way of life.

It was the impecunious English, therefore, who first got into loudspeaker tweaks. In particular, they heavily investigated the relationship between the speaker and the stand, and the stand and the floor (though the Mod Squad's Steve McCormack pretty quickly got into the act with his effective Tiptoes, which in turn spawned a flood of imitators). I remember putting on a series of single-blind listening tests at the very first Heathrow Penta Show, where show visitors could audition a pair of Spendor loudspeakers either on stands that rested on the rug, or with the same stands spiked through the rug to the concrete floor below. The difference was not subtle; listeners overwhelmingly chose the spiked speaker stands as sounding superior. Even a couple of engineers from the BBC (an organization not normally regarded as being tweaky) confessed that they heard the spikes improving the sound of the Spendors.

The late Raymond Cooke of KEF, who was a subject in those 1982 listening tests, offered a typical dose of Yorkshire common sense by pointing out that when hi-fi got going in earnest in the 1950s, loudspeakers featured such massive construction as sand-filled cabinet walls. They also often sat directly on uncarpeted floors—the thought that they might not have a secure enough foundation wouldn't have crossed their owners' minds. Since that time, speakers have become flimsier and rugs thicker, allowing the cabinet to move in reaction to the woofer's motion.

This is a practical illustration of Newton's Third Law of Motion: to every action there is an equal but opposite reaction. If a typical 10" woofer cone/voice-coil assembly with a moving mass of 50 grams moves 25mm peak-peak, a cabinet with a mass of 5kg sitting on a friction-free surface will move 250µm: a quarter of a millimeter! Any friction from the support will reduce the cabinet motion, but even if it does by a factor of 100, the resultant 2.5µm cabinet motion is still of the order of that of the tweeter diaphragm's and will frequency-modulate its output. Spiking the speaker or its stand to the floor beneath the rug gives the system a much better mechanical "ground," reducing its reactive motion to the benefit of its sound.

It's been 10 years since those blind UK tests, yet here in the US there is still much skepticism. In the April 1991 issue of Stereo Review (p.62), Julian Hirsch noted that he had "never found [spikes] to make the slightest difference in the sound of a speaker," while a year later, Stereo Review's high-end correspondent, Ian Masters, mentioned in passing that he doesn't feel "spiky feet" to fit his "high-end criteria" (footnote 3). There was also comment earlier this year from an otherwise knowledgeable subscriber to the computer bulletin board The Audiophile Network to the effect that he could not see why the stand should have any effect at all on the sound of a loudspeaker, let alone spiking that stand to the floor.

If listening to the improvement brought about by more effectively securing a speaker to the world on which it sits is not a viable option to these people, then maybe they'd be interested in some more "objective" evidence.

It's an interesting test to sharply rap the top of a loudspeaker cabinet. The resultant "thud" is diagnostic in that it changes character according to the presence of spikes or Tiptoes, the design of the speaker stand, the presence or absence of damping material between the cabinet and the stand top plate, and the nature of the floor. Obviously, then, these factors affect the cabinet's vibrational behavior, which in turn has an effect on a speaker's sound quality (footnote 4). I therefore decided, as a first experiment, to quantify the effect of the speaker/stand interface on the speaker cabinet. A second experiment, which will be reported in a future issue, will quantify the effect of spikes.

To make things easy, I chose two test loudspeakers whose sounds have been found to be very dependent on how they are supported: the ProAc Response Two and the Acoustic Energy AE1. Both have been recently reviewed in Stereophile (in Vol.15 No.7, July 1992, by Corey Greenberg and myself, respectively), and the measurements of cabinet resonances accompanying those reviews indicated that the speakers featured just one or two strong cabinet-wall modes. It would be a straightforward experiment, therefore, to examine whether the amplitudes and frequencies of those resonances changed when the stand and its interface were changed.

Footnote 3: Stereo Review, July 1992, p.96.

Footnote 4: In a paper presented to the 90th AES Convention (Paris, 1991), "An Investigation of Sound Radiation by Loudspeaker Cabinets," Stanley Lipshitz and his coworkers concluded from calculations of the total radiated energy that the sound of the cabinets of the loudspeakers with which they were experimenting would be audible or close to the borderline of audibility. It was noted in the paper that accelerometer measurements of a loudspeaker cabinet's walls varied tremendously according to how the speaker was supported while the measurement was being performed: this comment was the inspiration for this article.