Sunday, January 11, 2015

Current Crossover Settings

As of January 10, 2015

Behringer DEQ (used for Acoustat high pass and EQ)
  Parametric EQ's
    80.5 Hz, LC, -15.0dB; 1/2 of LR24
    80.5 Hz, LC, -15.0dB; 1/2 of LR24
    227 Hz, 1Octave,+3dB; sweetener Eq added 1/4/2015
  Gain Offset 0dB
  Delay 4.14 msec Left and Right

Behringer DCX (Used for subs and super tweeters)
  SW Left
    A -14.1dB
    LR24, 80 Hz
      1. 20 Hz,  +1.0dB, Q3.5; room curve boost
      2. 45 Hz,  -7.0dB, Q4.0; #1 room mode
      3. 39 Hz,  -3.6dB, Q5.6; #2 room mode
      4. 32 Hz, +4.0dB, Q4.0; room curve boost
      5. 68 Hz,  -3.0dB, Q3.2; #3 room mode
      6. 20 Hz,  -3.0dB, Q1.0; Tilt down to cancel excess up tilt
    Delay 0.1msec

  SW Right (Gain pot straight up)
    B -14.1dB
    LR24, 80 Hz
      1. 27 Hz,     0dB, Q1.6; old boost discontinued
      2. 45 Hz, -5.0dB, Q5.0; #1 room mode
      3. 66 Hz, -3.0dB, Q3.2; #3 room mode
      4. 32 Hz, +3.0dB,Q2.0; room curve boost
      5. 25 Hz,      0dB,Q1.0; old boost discontinued
    Delay 1.15msec+0.29msec

  ST Left
     A -2.0dB
     Delay 2.18ms+1.31ms

  ST Right
    B -2.0dB
    Delay 2.56ms+1.02ms

All Polarities are NORMAL.

Both Subs fed through XLR 10dB attenuator.

N.B.  Cutting Subwoofer level is difficult; SVS's are high sensitivity compared with Behringer DCX which has high output, even with 10dB attenuator.  "gain pot straight up" is at least -3dB, which may be what is dialed in other side but not sure.  Experimental cut to SW Left will require adjusting SVS input level, fortunately it's a digital level control.  With level adjust, EQ 6 may go away.

Currently listening to my altMusic folder random shuffle through Sonos, taking a break from Pandora and William Orbit.  Beatles playing right now.

Bedroom System

    -13.7dB Sum
    80 Hz LR24 LP
    Delay 0
    EQ 50 Hz -4.0dB Q4.0; Room Mode
    -1.9dB A or B
    80 Hz LR24 HP
    Delay 3.49 ms
    EQ 107 Hz +4 Q3.5; sweetener added 1/4/2015 re floor/ceiling cancellation

Friday, January 9, 2015

Music is experienced constructively

One of the things I was taught in Cognitive Psychology is that all perception is constructed.  We directly sense various nuances…and construct those into complete experiences.

This is true for visual images…and it is even more true for Music, which doesn't happen at once but over time, requiring memory.  And the unique feelings that Music evokes…are of course constructed from the harmonies, rhythms, and more complex relationships.  Most of the ultimate experience comes from the mind-as-it-exists-at-the-time, which itself changes from one listening to a piece more than once.  Your memory has already grown because of the first listening by the time you start the second.  But still, most comes from many previous experiences.

So a little bit of information, taken from a very information rich source such as 16 bit PCM, is sampled, and that little bit of information is turned into far more virtual information in the complete experience…that would swamp the largest computer clusters to simulate.

The variability in experience from the same CD comes from many levels…how we choose to sample the original, and how we choose to expand on it to be come an experience.

We sample far less information than is on a CD (making sampling differences possible) but that doesn't necessarily mean that a higher resolution source, having more information still, wouldn't sample sufficiently differently to be detectable as such.

In every set of samples there are common elements.  The larger the ultimate information space, the more diffuse those common elements, and therefore the richer the possible experiences.

If information is perceivable, more information ultimately makes for noticeably richer experience the second and all subsequent times.  (This might not argue for higher sampling rates…but it would argue against all lossy compression systems.)

At the same time, much of the experience comes from state of mind, attention, need.  This is more stuff that is not really part of the recorded information.

State of mind is highly affected by belief, such as belief in strategically placed crystal resonators.  If you have the belief, you will of course experience that you need those crystals, and be sure that you need them when you don't.  And you wouldn't be lying if you said your experience was affected by them, even if they had negligible audio effect, if you had another way of knowing if they were in use.

Belief may also cause you to seek out confirmatory examples, and find alternative explanations for examples that don't confirm.

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.

Sunday, January 4, 2015

R.E.W. The Revolution is Here!

Right Channel before changes today

Left Channel before changes today
I finally dusted off Room Eq Wizard this weekend.  This is a marvelous audio measuring tool!  Much nicer than anything I've used before.  Over the first night with R.E.W., I could not help but tune both living room and bedroom audio systems to the highest standard ever.  I love the quick and easy and accurate measurements with R.E.W., but I've still been setting parametric EQ's manually with cut-and-try (and learning more about how to set Q's, as described in previous post) rather than using R.E.W.'s namesake EQ setting features (which I haven't figured out yet, but probably wouldn't use completely anyway, I always prefer manual control to automatic).

When I previously looked at R.E.W. last year, it complained that I didn't have a calibrated mike.  I did some online research and chose to buy a Dayton EMM-6 calibrated by Cross-Spectrum Labs.  With calibration curve, this becomes something like a laboratory quality mike, for just $99.  Calibration data files are copied from the USB memory included with the microphone, and then the relevant file is loaded after starting R.E.W, and then it doesn't complain about uncalibrated mike when you press the "Measure" button.  So you then have a state-of-the-art audio measuring tool--for $99 instead of the $99,000 it might have cost in 1970.  In cases like this, computers are truly wonderful, when powered with incredible free software.

I got the microphone a week or two later, but then the whole project was sitting there for six months until I had to give a long promised and already previously delayed demonstration for a computer club meeting.  Unpacking the microphone for the first time, I struggled to get it to work with R.E.W. and my (!@#$%) Windows PC laptop and Emu Tracker Pre.  I quickly figured out how to select the Tracker Pre as my audio input device, but still wasn't getting any signal.  I was worried that the Tracker Pre might need an AC power adapter, but checking online (on my smartphone at the meeting) it seemed as though it did not need more than USB power except when used standalone without a USB connection.  Finally I figured out that this microphone needs phantom power, which a switch on the Tracker Pre turns on.

I measured the living room system with all my existing parametric EQ's operating.  The room modes are essentially eliminated already.  However there were noticeable defects, and I was immediately motivated to try and fix them.  The first was an apparent notch at 80Hz, the crossover frequency, in both channels.  I thought to myself, this might be better if I overlap the crossover a bit, perhaps by lowering the Acoustat high pass filter to a lower frequency.  I tried 70 Hz, seemed to have no measurable effect except lowering the entire region a tad.  So I tried 50Hz, and the notch actually got deeper.  So things are not necessarily as they appear in a frequency response graph, and worse, trying to make any correction to a complex speaker/room system is like pushing on string, you never know exactly what is going to happen, and this is especially (though not uniquely) true when you are trying to remove notches.

A picture of the Acoustat response alone (I measured right channel, which also without any crossover shows a series of saw shaped curves below 200 Hz, getting bigger down to a deep notch at 80Hz, followed by a final response peak beginning at 60 Hz and fading below baseline by 40Hz.

So obviously I cannot fix this problem by extending Acoustat response lower, because the problem is in the Acoustat/Room response itself.  I didn't try raising the subs higher…that thought just occurred to me writing this.  In general I am opposed to using subs higher than necessary.  I'm not used to having such a powerful tool give me guidance on such issues.

Leaving that unsolved problem behind, I next looked at a notch around 32 Hz, also in both channels.  I had already added a 3dB boost to the left channel, which has a stronger corner loaded position and doesn't seem to have to work as hard to generate high spl's.  I made the Q higher and raised the boost to 5db, which seemed to help a little.

Finally I noticed that the response of the left channel was generally rising compared with the right channel, probably helped by that corner position (and also a few eq's selectively added only to the right because of its stronger position).  I suppose I could have chosen to lower the sub level.  Instead I decided to try a new kind of low frequency tilt filter.  I set the center frequency to 20 Hz, a cut of -4dB there, and Q=0.7.  That lowers the bass below 100 Hz with a tilt reaching maximum at 20 Hz, where it is 4dB down.  (However, by design, only in the subs).

This tilt change measured exactly as expected, and brought right and left channels into much better match, looking reasonably good IMO and still with generous room curve (slightly less OT) below 100 Hz reaching fullness at 20Hz, and fantastic response remaining to 15 Hz and below.  (Note for all measurements in this post, the microphone calibration had not been correctly set.  This means about 5dB flatter response below 20 Hz, little else of consequence to low frequencies.  So the actual response below 20 Hz is better than what is shown here…which is already pretty impressive.)

Left Channel after changes today
The rise going downwards from 100 Hz to 20 Hz is significantly attenuated, but bass below 20 Hz is not attenuated much.  The notch at 32 Hz is significantly reduced.  However the notch at 80 Hz is worse, but it may that it was just as bad before--or just about--but didn't look as bad for technical reasons.  I didn't do much to make 80 Hz worse, just a tiny bit of string (about 1dB of attenuation resulting from the now -3dB at 20Hz tilt, in sub channel only.)  It needs attention as a separate problem now.

(Note that both changes were made only to the left sub, so the right sub was same as before.)

Tested bass with Bass Erotica and Grouse We Want to Be Loved.  Fantastic bass transparency!  However, I decided to roll back the tilt to -3dB to add more casual impressiveness to the bass, a tiny change based more on caution (don't eq too much) than listening test result.

I moved on to the bedroom system, where the Behringer DEQ 2496 had recently died, taking with it the hand tuned room EQ I started developing in about 2005 which reached untouched perfection, or close enough, around 2009.  I don't remember where I might have written down the adjustments, it was before this blog started.  Things have changed a little anyway, I have new padded hard floor instead of shag carpet, taller bed with head position farther from wall, and other changes.  I now know I can program parametric EQ's into the still-working DCX-2496…I just needed to know what adjustments.

Measurement result of full range system with subs was so good I wondered if it even needed any eq.

Left Bedroom w Sub, No EQ
Right Bedroom w Sub, No EQ

Closer look revealed two defects, which I made some headway correcting.  For one, there was a peak around 50 Hz.  A similar (and desirable, IMO) peak at 20 Hz (good for room curve) made the 50 Hz peak look innocuous, but it was not--a peak at 50 Hz is far more audible and less desireable.  Strangely, I had already dialed in a -5dB cut at 50 Hz (I think a few months ago, I tried doing a quick EQ with pink noise but decided going full range with satellites worked better.  My EQ'd version sounded too bass-shy, while the flat out version sounded boomy.)

That 50Hz was in fact the precise place to effect a cut, but it needed a far sharper Q as I had been discovering.  So I upped the Q to 4.0, reduced the cut to -4dB, and measured and it looked about as good as possible.

The other defect looks tricky, a response valley centered just above 100 Hz.  It may have something to do with the slightly vaulted ceiling and the bed on the floor, a floor-ceiling bounce which results in cancellation around 100 Hz.

Never one to trust stern warnings not to try potentially useful things that might not work, I tried a bit of EQ on this and it seemed to help.  I threw a full +4dB at 107 Hz with Q of 3.5.  I could possibly throw another +4dB at this without making a bulge, so a "mere" +4dB is a good compromise between reality and ideality.

Bass measurement is now almost fantastic, with a trace of the 100 Hz cancellation remaining, but otherwise reasonably flat, reaching ultimate low peak at about 18 Hz with usable response down to 10 Hz.

Just one day with R.E.W. and everything has changed for the better.

Parametric EQ Simulator

This simulator was very helpful to me in fine tuning my parametric EQ's this weekend.

Generally, I needed higher Q's than I was thinking.  Previously I was too (?) worried about high Q's meaning resonant systems.  I'm still a bit worried about that, but if I use 1/3 octave equalizers, as I often have, that means I've been freely using Q=4.3 already.

Here's a Q vs Bandwidth calculator.  Some useful numbers:

Bandwidth = 1/6 octave    Q = 8.7
Bandwidth = 1/3 octave    Q = 4.3
Bandwidth = 1/2 octave    Q = 2.9
Bandwidth = 2/3 octave    Q = 2.1
Bandwidth = 1 octave       Q = 1.4
Bandwidth = 2 octaves     Q = 0.6

Thinking about this, I also created a low frequency tilt control with a Q=0.5.  Setting a center frequency at 20 Hz, a 4dB cut with Q of 0.5 yields a downward tilt starting around 100 Hz with maximum reduction of 4dB at 20 Hz, which is exactly what my living room left sub needed to have flatter bass (there was previously a frightening rise at low frequencies…room curve too extreme).  After some listening, I decided to dial back this ultimate tilt control to -3dB to give bass a bit more life, but without sacrificing a new sense of low to midband transparency.