Tuesday, January 6, 2015

Figuring out the Notch

As I posted a few days ago, after the most recent adjustments, my left channel response (uncalibrated in this graph) looks like this:

Generally it follows a nice room curve, showing about 12dB rise at 20 Hz, which is the peak frequency, with useable response down to 10 Hz.  The high frequency shown is 250 Hz, which is in a slight depression about 4dB below the true baseline at mid and high frequencies.  Generally it's relatively smooth compared with the raw response (full of severe room modes, btw).  Right now what bothers me most is the notch at 80 Hz, which got slightly worse as I tilted the response down 4dB at 20Hz (the 20Hz peak was 4dB higher before the adjustment).

The notch looks as though the high pass and low pass got separated somehow.  But they are both set to exactly 80 Hz, and the time delay between subs and panels has been corrected also.  Although, I wondered about the cumulative effect of other time constants in the system response, and so I tried changing the crossover setting on the high side only from 80Hz down to 70Hz, and then down to 50Hz.  At 70 Hz the response was only slightly different.  At 50 Hz, the notch got worse and seemed to  create a new hill around itself (although, even at 80 Hz, you can see a bit of a hill around the 80 Hz notch).
Judging how that went, I began to seriously wonder if separating the crossover frequencies would work better--the opposite of my original visual intuition.  But what I measured instead was the response of the Acoustat left speaker by itself.  That looked like this:

The Acoustat by itself (in current location in my room…which is probably the causative factor) has a series of saw like patterns in the penultimate bottom end.  These may be reflective cancellations, but the back wave cancelation would be the small peak at about 140 Hz…which is strangely a peak instead of a cancellation precisely because the Acoustat is bipolar…the back wave is already 180 degrees out of phase with the front.  But then why are the peaks lower than 140 Hz in increasing size, until we get to just above 70 Hz?  I can't explain the peaks below 140 except to suggest they are also reflections, possibly involving the entire room or the hallway.

The 70 Hz peak in the Acoustat response could be precisely what appears in the combined response (top) which also has a peak at 70 Hz, the left side of the notch.  The notch cut at 80 Hz in the combined response near the notch cut in the Acoust response just below 90 Hz.  The Acoustat response rises oboe that, possibly contributing to the mountain in the combined response.

The notchy Acoustat response doesn't exactly explain the singular pronounced notch in the combined response, but it could be a big contributing factor.  It might be worthwhile to cross the Acoustat over higher with, perhaps 100 Hz (at the peak in Acoustat response) rather than 80 (deep into the notch valley).  From that vantage, the crossover is working with the response error to cut response below 100 Hz, and the falloff below 100 Hz in the Acoustat response will be slightly filled in, but beyond that slightly removed, leading to a smoother overall curve.

It could be I also need to reduce the bass level rather than apply an overall tilt in opposite direction to room curve (reducing room curve boost by 20% or so across the board) as I am doing since Saturday.  Reducing the level would lead to less undesirable reinforcement (the unwanted bloom) in the 70-150 Hz area.  Reducing the level 4dB would have the same effect as the tilt on 20 Hz, but additional reduction for higher frequencies up to the crossover point.  (It's hard not to see this suggested level change as "less bass" even compared with the tilt.)

Additional note: the deepest notch occurs at 280 Hz.  That has a wavelength of about 5 feet.  At the wall reflection polarity is retained.  The back wave starts out-of-polarity, so as it goes through a full wavelength in reflection, it cancels maximally.  That suggests a wall reflection is occurring at 2.5 feet, and indeed that's about how far my Acoustats measure from the wall perpendicularly.  On the listening axis, it's more than that, and that could explain the dip at 180 Hz.

Thanks to angling and other factors, these reflection notches are not as bad as they could be.  I think at minimum about 3db Eq with bandwidth of about 1-1.5 octaves could be used here, with center frequency of 230 Hz.   Reflection and/or aborption treatments?  I don't know, maybe they could help…but I'd be afraid of them hurting.  Moving the speaker much is basically impossible.  I chose this 2.5 feet from wall (it seems more like 3 feet) as the best possible in a multipurpose room.   As it is, the room is already squarely in man cave territory.  I have friends and parties, but I live by myself now, and my friend has accepted the speaker portion of the living room (but wants changes elsewhere).  Speakers 4 feet from the wall would not work with my schedule of monthly parties.  All that speaker moving…which needs to be done to the nearest 0.1 inch or better, I'd go nuts.

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