To Hans Wetzel,
Hi from Tarragona, Spain. You wrote [in your KEF Q750 review]: “KEF’s Q750 is a superb loudspeaker. It’s one of the most neutral transducers I’ve ever heard, and for the money offers staggeringly transparent sound” [emphasis added]. But [per the measurements from www.hifitest.de‘s review, here and here], I can see a valley at 1.2kHz. I have commented about this issue in GR Research, and in DIYAudio, too. I think it is intentional, [with KEF] looking for a soft “V” response.
The measurements you link to do, indeed, note a roughly -3dB dip around 1.2kHz. Since you seem to be measurement-oriented, it’s worth exploring what we’re looking at. Let’s focus on the frequency-response graph. Are these measurements taken in-room, or in an anechoic chamber? Are the three curves on-axis, 15 degrees off-axis, and 30 degrees off-axis, or something else? Was it measured at 1m, 2m, or some other distance? Without any offered methodology to accompany the graph, you’re extrapolating an awful lot from three curves. That is one very narrow snapshot that is certainly better than nothing, but doesn’t offer anywhere near a complete picture of how the Q750 might sound in your average room.
Take a look at some of our measurements on www.speakermeasurements.com, which we perform in an anechoic chamber at Canada’s National Research Council, where some of the early hi-fi pioneers did their acoustic research, including Dr. Floyd Toole (of Harman International), who quite literally wrote the book on modern loudspeaker design (see below). I’ll point you to the measurements of Revel’s Performa3 F206 loudspeaker. Numerous writers on our staff agree that the Revel (which is a division of the aforementioned Harman International) F206 is the most neutral loudspeaker at its $3500 USD price point, and its measurements are about as “textbook” as you can get for that kind of money, as evidenced by the “Listening Window,” which averages five measurements into one neat plot that is both commendably smooth and flat, with a 2-3dB tilt from left to right. There is nothing amiss between 1kHz-1.5kHz. Chart A, however, which includes on-axis, 15-degree off-axis, and 30-degree off-axis curves, shows a “valley” centered at around 1.2kHz–1.3kHz; surely not neutral!
Hi-fi measurements, like everything else in life, must be placed in context. If you were to only hear direct sound from the Revel, then a deviation like that “valley” would surely be problematic. But we don’t hear like that. What our ears hear is a combination of direct sound from the speaker drivers, along with a variety of reflections that arrive at our ears at slightly different times, which our mind interpolates into something recognizable. The takeaway is that on-axis and near-on-axis measurements like the ones you mentioned in your e-mail only paint part of the picture and have the potential to be misleading if taken in isolation. I’d encourage you to take a look at some of our other measurements, which includes several KEF models. I’ll note that the Q750 is a two-and-a-half-way design that uses passive radiators in lieu of a bass-reflex port (or two). These kind of fundamental design choices may well have contributed to idiosyncrasies in the Q750’s frequency response curve that wouldn’t appear in a two-way or three-way bass-reflex design.
We try to measure as many speakers as we can, but with writers all over North America, shipping large speakers often proves expensive and impractical, so it’s not always possible. Just so you know, I actually wanted the Q750 measured, but we couldn’t make it work logistically -- sorry about that.
To answer your question above in a terribly longwinded fashion, yes, I think KEF purposely designed the Q750 to have the “valley” around 1.2kHz in its on-axis and near-off-axis frequency response curves. I am confident, however, that KEF did not intend for that to be audible, in-room, at the listening position. I suspect that if we were to run our usual battery of measurements on the Q750, the “Listening Window” plot would bear that out, and also confirm my subjective listening impressions.
I recommend Dr. Floyd Toole’s book, Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms. I think you’ll find it highly educational. . . . Hans Wetzel