Dunlavy Audio Labs SC-IV/A loudspeaker FollowUp
Dunlavy SC-IV/A loudspeaker
When Robert Deutsch reviewed Dunlavy's floorstanding, three-way SC-IV/A loudspeaker in November 1998, he found nothing to criticize: "The SC-IV/A has reached the point where—with the best source materials and associated equipment—it sounds less like a speaker and more like the music that's being reproduced," he summed up. And when I consulted him for his thoughts on where in Stereophile's "Recommended Components" the Dunlavy belonged, Bob was unequivocal: "Class A."
Yet, as regular readers will know, the SC-IV/A appeared in "Class A, Restricted LF." This was because, despite the speaker being specified as having a full-range anechoic response 3dB down at 20Hz, compared to the original SC-IV's 33Hz, the 'IV/A's woofers appeared to be tuned to 37Hz, according to both the impedance and to a nearfield measurement.
This decision of mine produced heated discussion on the Internet newsgroups. John Dunlavy was adamant that the specification was correct. He explained (footnote 1) that the difference was due both to the SC-IV/A's overdamped LF alignment giving a shallower-than-usual rollout for the first octave below the nominal tuning point, and to the fact that the spaced array of the speaker's twin woofers will boost the low bass at the listening position. A loudspeaker featuring "a symmetrical array of drivers, with woofers spaced apart by a significant fraction of a wavelength at or near the crossover frequency," he offered, will have "a directivity gain...typically 3-4dB at frequencies above where the woofers are spaced more than about one-third wavelength."
Now, JD has forgotten more about speaker design than I ever knew, so I was certainly not going to argue with him on this point. Instead, I scheduled a Follow-Up review so I could audition the speakers and hear for myself how the SC-IV/A performed in a real room. In addition, as I have been reviewing minimonitors over the past few issues of the magazine, auditioning a full-range speaker design would come as a welcome change.
Optimizing the placement of the SC-IV/As proved harder than I had anticipated. It's those spaced woofers—one has its center 18" from the floor, the other 58". It's important, therefore, to keep the distances between the woofers and the sidewalls, and between the woofers and the wall behind the speakers, as different as possible from 18" or 58". This is so that the notches and peaks due to destructive and constructive interference between the direct sound from the woofers and the reflections of that sound from the walls do not coincide in frequency. Yet these speakers are physically large, which restricted the placement options in my 16' by 19' room.
The best I could do is illustrated in fig.1, which shows the SC-IV/A's spatially averaged response, centered on the listening position in my room. Yes, the low frequencies are well-extended, the 20Hz and 25Hz 1/3-octave bands being down just 2dB from the 1kHz reference level. It looks as though John Dunlavy is correct. Even though the anechoic tuning of each of the two woofers lies almost an octave higher, at 37Hz, the SC-IV/A's in-room bass extension does reach almost to 20Hz. I believe that this entitles the Dunlavy speaker for full-range membership in Class A.
Fig.1 Dunlavy SC-IV/A, spatially averaged, 1/3-octave, free-field response in JA's listening room.
But look at the broad depression in the lower midrange in this graph. What you can see is the result of the complex interactions among the four widely spaced woofers and the room boundaries. Now, this lack of lower-midrange energy doesn't completely define how the speaker sounded in my room. The graph primarily shows the power response; the perceived balance will also be affected by the speakers' direct sound. But it does go some way toward explaining my main reaction to the SC-IV/A's perceived balance.
Footnote 1: "Manufacturers' Comments," Stereophile, January 1999, Vol.22 No.1, pp.207-208.