macrae11 says: Quote:
|
I also think it somewhat proves my point. The DSP system can lower the initial output of the trouble frequencies, but still does not affect the longer term effects of the room.
|
Wayne A. Pflughaupt says: Quote:
|
I agree. Some of my esteemed colleges here (and on other Forums) are believers in time-domain equalization, but they haven’t posted or otherwise offered any evidence to back it up.
|
Guys,
Just as a parametric filter operates in the frequency domain, it also has a time response that acts like a modal resonance in a room.
It's easily demonstrated (see below) by doing a frequency response sweep of a Behringer parametric equalizer and adding a single filter.
The time tail is evident, is it not?
From my sticky post on
waterfalls I don't think anyone can argue that this filter is also operating in the time domain?
Waterfall plot plot of a BFD using a single filter of (40Hz, Gain +15dB, BW 10) 
From that thread I noted the following:
But now I look at the resulting waterfall plot of that single filter below.
Look familiar? Sure it does.
It looks like a room mode resonance of any REW measurement at subwoofer frequencies. And it should. It's because the EQ filter, just like the modal resonances of a room, has a time response that acts like a 2nd order biquad. If I apply an EQ filter with the same Q and opposite gain of a room mode, I would completely counteract the effect of the mode. See the time component of the filter (just like a room mode). It rings out, and still isn't in the noise after 300msec. You see, EQ filters don't just affect level. This is why they're so effective at equalizing at modal frequencies below 100Hz. Yes, it is listening position dependant, and only valid at the point where the response was measured, but because of the long wavelengths of low frequencies, the region around that area is fairly large. This is in opposition to higher frequencies where equalization is a bit of a waste of time, since the effective region is so small that eq is impractical.
I remember a post (
from this thread) by JohnM (author of REW) some time ago that was the trigger that really started me thinking about this subject and after lots of playing around with my equipment and investigating, I'm a believer (one of the few, it would seem).
John said:
This is a fundamental misunderstanding of minimum phase systems. Room modes (in particular those below a few hundred Hz) are minimum phase phenomena (substantially behaving as 2nd order biquads) and are effectively countered by 2nd order biquad correction filters such as those implemented in the BFD pro and many similar parametric equalisers. It is, however, unnecessary to engage in lengthy academic discourse on the topic since measurements clearly show the effectiveness of the approach.
brucek