The Sound of Surprise (the loudspeaker/stand interface) Page 6

Though the data in the table are a little hard to unwind, the first thing that pops out (to me at least) is that they reinforce CG's listening impression: that the Target stand designed to be used with this speaker gives the optimal support, resulting in overall lower levels of cabinet vibration across the band when compared with either the solid Celestion stand or the flimsy wooden support. Use of the latter stand, however, does significantly reduce the level of the upper-bass mode apparent when used with upturned German Acoustics cones, while the Celestion stand shifts the mode's frequency from 164Hz to 141Hz, though without any change in level.

In fact, the inability of any of the interfaces to eliminate this mode with the Celestion stand—as they all do with the Target stand—suggests a basic incompatibility between stand and speaker which presumably will have audible consequences. When I auditioned the Response Two on the Celestion stand, I did note a woodiness to its lower midrange which became less audible with the speakers sitting on the Target stands.

Moving higher in frequency, it's interesting to see that all the compliant interfaces raised the overall level of the higher-frequency modes, though these remain at probably harmless levels. That main cabinet resonance at 305Hz, however...? None of the interfaces did much about it, even Blu-Tack. (For those cheap at heart, the ½" layer of polyurethane foam seemed to do about as good a job as any of the expensive toys.)

Fig.8 is the waterfall plot for the ProAc's side panel with the speaker sitting on upturned brass cones on its dedicated stand. Note that there are 55Hz sidebands around the main spectral component at 305Hz, these presumably due to the speaker's fundamental Helmholtz resonance being able to "pump" the cabinet wall with these supports. For comparison, fig.9 shows the effect of replacing the cones with three small pads of Blu-Tack between the speaker and the stand top plate. The entire region below 300Hz cleans up, the lower sideband at 258Hz goes down in level, while the HF hash is also reduced. In his review, CG noted that the Blu-Tack did give the cleanest sound from this speaker/stand combination, a sheet of compliant material recommended by the importer rendering the sound "slower," less dynamic, overall.

Fig.8 ProAc Response Two side panel, Target stand, three German Acoustics brass cones, cumulative spectral-decay plot (8Hz-2kHz, 75ms time window).

Fig.9 ProAc Response Two side panel, Target stand, three flattened pads of Blu-Tack, cumulative spectral-decay plot (8Hz-2kHz, 75ms time window).

Music nonstop
What can the caring audiophile draw from these results? Certainly, I think I've illustrated that the speaker stand and the interface between the stand and the speaker influence the behavior of the loudspeaker's cabinet walls. As a rule of thumb, if you want to suppress cabinet resonances the least, you should support your speaker on upward-pointing spikes or cones. If you want to suppress resonances the most, you should use thin pads of Blu-Tack about the size of a nickel. If you want to arbitrarily boost some modes and suppress others, you should use some sort of reactive interface, of which there are a myriad. These results also imply that the choice of the stand itself will have a major influence on cabinet vibration, an effect independent of that of the interface.

But what differences do the presence or absence of resonances make to the music?

This is where we head into the land of conjecture. It must be a good idea in general to reduce the amplitude of resonant modes, but how audible a high-Q, narrow resonance will be depends not only on its level but also its frequency. If it lies in the upper bass, say between 100 and 200Hz, it will likely coincide with specific musical notes as well as being excited most of the time. An octave or so higher, and it might well fall in the cracks between the discrete frequencies of musical notes. (Table 3 shows the frequencies of the equal-temperament musical notes lying between 100Hz and 523Hz, Middle C, or the note between the bass and treble staves; note the much wider frequency spacing of the notes in the upper octave.) While it would still be excited by wide-band, noise-type signals like drums, the resonance might remain lurking undetected for most of the time, in which case it might as well not exist. And what if you damp such a resonance only a little? This might have the paradoxical effect of making it more audible, as it will now be more likely to be excited more often, due to its lower Q. (A broad, shallow peak covers more frequencies than a narrow, sharp one.)

Table 3
Musical pitches vs frequencies in the upper bass/lower midrange

NoteFrequency (Hz)

* Taken from Music, Physics and Engineering, Harry F. Olsen, Dover 1967, and rounded to one decimal place.

The optimal stand and interface will probably be different for every loudspeaker. I am afraid this sounds like a cop-out, but you will therefore have to audition the effects for yourself, with your own speakers and stands. Listen carefully to recordings you know. Is the clarity in the lower midrange and below disappointing? Do tenor instruments like cello lack clarity or acquire a "woody" sound? Does the bass sound muddy, diffuse, one-note, blurred, or lacking in power, rather than tight, articulate, and extended, as it does in real life? Does the music seem to drag, in rhythmic terms? Do male voices "boom" and female voices "hoot" at some frequencies and not others, with the result that the little inflections of tone characteristic of real voices become diluted? Do central vocal images smear toward the speaker positions at some frequencies? If your answer to some or all of these questions is "Yes," then you should experiment with different stands and different interface materials before you decide to upgrade the speakers themselves.

Don't overlook the obvious. The manual for your loudspeakers often contains valuable advice regarding stands and how to use them. And ask the dealer where you bought your loudspeakers what stands he recommends and how he attaches the speakers to them. If he's a dealer worth supporting, he (or, more particularly, his sales staff) will have done a lot of experimenting in this area. (Of course, if he tells you that the stand doesn't matter, you know that this is a man who knows less than you and should therefore be avoided.)

Happy trails! And don't forget those spikes.