Monitor Audio Studio 6 loudspeaker Measurements part 2
Fig.7 Monitor Audio Studio 6, impulse response on tweeter axis at 45" (5ms time window, 30kHz bandwidth).
Fig.8 Monitor Audio Studio 6, step response on tweeter axis at 45" (5ms time window, 30kHz bandwidth).
The cumulative spectral-decay, or waterfall, plot calculated from the impulse response (fig.9) is dominated by the tweeter resonance, though the woofer mode at 5.1kHz is also noticeable. Replotting these data with a lower -30dB floor (fig.10) chops the top off the tweeter resonance, but more clearly reveals the woofer mode, as well as showing that the persistent notch at 1400Hz is associated with a good deal of delayed energy—presumably this behavior is due to some kind of surround problem.
Fig.9 Monitor Audio Studio 6, cumulative spectral-decay plot at 45".
Fig.10 Monitor Audio Studio 6, cumulative spectral-decay plot at 45" with -30dB floor.
Finally, using a simple PVDF accelerometer, I investigated the speaker's structural resonances. Two modes dominated the vibrational behavior of the side panel (fig.11), these relatively high in frequency at 508Hz and 629Hz. A minor mode at 350Hz can also be seen. The Studio 6's cabinet designer has used bracing to stiffen the walls. While this doesn't damp resonances, it pushes them up in frequency (footnote 1) to a region where they will both be less likely to be excited by music, and will tend to fall in the gaps between the frequencies of musical notes. With modern tuning, 629Hz, for example, lies between the D# and E at the top of the treble staff but is of high enough Q (Quality Factor) that it needs to be hit with exactly that frequency to be fully excited. Any degrading effect it could have on music might, therefore, be minimized.
Fig.11 Monitor Audio Studio 6, cumulative spectral-decay plot of accelerometer output fastened to center of enclosure side panel. (MLS driving voltage to speaker, 7.55V; measurement bandwidth 2kHz.)
Listening to the cabinet behavior with a stethoscope (footnote 2) revealed this to be basically the case, though on some recordings I could hear both of these midrange resonances become excited as a musical line swept through each of their frequencies. And because of their high Q, once excited they tended to ring for a while. Nevertheless, once the stethoscope was put away and I was seated in the listening chair, I could not hear any obvious mid-range congestion on music that I could lay at the feet of the cabinet behavior. On the uncorrelated pink-noise track on Stereophile's Test CD 2, however, the cabinet "talk" could be heard as a slight "color" to the noise. (This signal is very useful, because any resonant coloration, which will be the same in both speakers, tends to pull to the center of the image and become spatially unmasked.)—John Atkinson
Footnote 1: In his January 1993 review for HFN/RR, Martin Colloms conjectured that the hand-rubbed lacquer finish contributes to this effect by adding a very stiff skin to the veneer.—John Atkinson
Footnote 2: For $20, an ordinary medical stethoscope is the most cost-effective investment a speaker designer can make to hear how his favorite cabinet-bracing technique works—or doesn't. It's also useful to investigate speaker-stand behavior. You might expect, for example, the lead-shot'n'sand-filled Celestion stands I used with the Studio 6 to be completely inert, especially considering that the interface between the speaker and stand was four small pads of resistive Blu-Tack. Yet I could clearly hear one of the speaker cabinet's two midrange resonances being echoed by the stand pillar side wall. The relationship between the vibrational behavior of the speaker and the stand is sufficiently complex that predicting the effect of any particular stand on the sound of any particular speaker is impossible. (It would probably make a good subject for a Ph.D. thesis.) A recent report in Stereo Review implying that the speaker stand is of no importance, however, reflects more a poorly designed listening test and the experimenter's bias than it does the truth of the matter.—John Atkinson