915Hz Down 6dB

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You would expect that a graphic equalizer wouldn't do as well as a parametric on this test and you'd be correct. A small number of these graphic EQs have parametric notches of some sort or another -- such units should perform the best in this test. The larger pool of devices that have only graphic EQ sliders will show somewhat greater SPL loss after cutting 6dB of 915Hz, but the Behringer DSP, the Rane DEQ, the UREIs and the Presonus were standouts in their field. A further test with four random feedback tones explores the subject in greater depth. Check out the mulitone feedback reduction results.

You can't see it in the table but the Presonus was very resistant to achieving exactly 6dB of cut. None of the other EQs had this problem. The incremental dB steps you get on each Presonus fader are too big for precision cutting of frequencies. Most of the time I had to settle for a cut of 6.5dB or thereabouts at 915Hz. Having clunky level steps is not the end of the world -- the Presonus performance in this trial remains quite good. But having discrete jumps in level as you move a slider is not a feature an owner would be especially proud of. You'll get finer control over level on any EQ if you switch it to ±6dB mode. Presonus owners in search of fine control might think of the ±6dB setting as their default unless they wanted to achieve the better feedback cutting results shown here in the -24 and ±12dB modes. Between a rock and a hard place!

Let's cut to the chase, shall we?

Here are my results, in order of best performance. Less is more. Please note that the dBSPL figures in the left column which track overall SPL reduction are only accurate to a tenth dB or so. Feel free to imagine your favorite 1.3dB EQ beating the competition's 1.2dB unit in a specific application. ;^)

Wish I could go back and test:
dbx 480 (notch filter centers are close but not right on.)
Lake Contour (includes parametric EQ and much more.)
Sabine GRQ 3102 (includes automatic anti-feedback filters.)
TDM 30GE-2 (includes a few analog notch filters.)

Both the transfer and RTA graphs were interesting to examine in this test. I couldn't decide which one showed the action more clearly. Here in a progression of RTA screenshots we see the Yamaha's wide cuts above the Rane DEQ 60L's much tighter pocket compared with a medium-width parametric cut from the Helix and the skinny notch filter you get via AFS on the dbx iEQ31.

In Transfer mode, the phase response becomes visible. Here is the dbx iEQ31 in two different modes. First is the fader-only method. The next two graph shows the same EQ with sliders flat and the AFS engaged for almost 9/10ths of a second. Though the AFS gives us a slimmer frequency notch and retains more of the signal power you can see that the phase response gets a sharp crease at 915Hz. In this case the sharpness was large enough to require a change in SmaartLive coherency settings. I was using 95% coherency (second graph) but this trace disappeared near 915Hz until I reduced the coherency requirement to 83% (third graph.) Some people I've talked to avoid sharp notches so that steep phase changes aren't introduced into the signal. If there's a method by which this kind of crease can be quantified in terms of musicality then I haven't heard of it. I've been using notch filters in sound for my whole career and I'll keep using them until I have seen conclusive test results documenting the adverse effects. I admit I understand frequency response much more than I do phase response. The only thing I'm certain of is that this subject deserves further investigation.