Wednesday, April 12, 2017

Loudness Compensation

Loudness compensation dialed into 2 bottom EQ's

Even my most purist audiophile friend agrees: loudness compensation is a necessary feature, not a frill.  Playing at realistic levels (which are actually much higher than commonly expected) in a home is not something one can do always  (if ever).  Whenever the reproduced level is substantially lower than the realistic level, one of the biggest deficits from reproduction is the loss because of spectrally imbalanced hearing insensitivity at lower levels.

My friend doesn't think very highly of all my digital eq, with all crossovers, room correction, and linkwitz/grundy dip realized through digital signal processing.  But as flexible as this signal processing is, it doesn't actually include any loudness compensation per se.

The biggest loss, and the only one usually corrected by most loudness compensation circuits, is in the bass.  I would argue that this is the part most needing correction, and I'm not even sure if the high frequency loss can be corrected for very well or is worth bothering with.  The basic issue is that as the volume is reduced, the sound becomes "thin" because it lacks proportionate bass (the bass is still there, actually, but we hear it less).  This thinness makes listening much less pleasurable.  Boosting the bass up a bit, even if not as a perfect compensation, reduces the thinness and makes listening pleasureable again.  Compensating the highs might complicate the picture by bringing some of the thin sound back.  Unless it could be done very well, compensating the highs is probably better not done at all.

For a long time I've had no clear idea how to do loudness compensation in my living room.  It would be very convenient, or so I thought, to use the parametric EQ's in my Tact 2.0 RCS which was upgraded to the Tact version that includes parametric EQ's.  But since I'm not actually using the Room Correction feature (I didn't like the way Tact did this--full spectrum) I can't use the parametric EQ's either.  Bummer.  I've long thought about really hacking the software of the Tact (nobody seems to do this) to permit parametric EQ's even in Room Correction Bypass mode, which I am always using.

Or, if I were even more clever, I could program the Room Correction curves as specific loudness compensation curves for different relative volume settings.  So Correction1 might serve for -10db, and Correction2 might serve for -20dB.

Well even using some tricks I've learned (one can connect the Tact output to the microphone input with an impedance/attenuation network, and then the Tact is correcting a straight wire--no correction at all essentially--and then set the target curve to the desired loudness correction curve, voila!) this would be a lot of trouble and needless to say I've never done it.

Then I've had some other ideas about sticking in an analog preamp somewhere in the system, such as between the disc machines and the Lavry AD10 digital converter, or between the Audio G_D Dac and the Krell.  (Neither idea is very appealing...I like the transparency of these connections which are now just short pieces of wire.  Nevertheless, I've spent hours looking at Quad 34 Preamps and the like on eBay just wondering if something like that would do it.)

Finally, last weekend, I found a quick, easy, and zero degradation way of doing the loudness compensation.  I can use the Graphic EQ function on the digital equalizers used as crossovers and room correction.  Currently I only use the Parametric EQ features to achieve crossover and room correction, and the Graphic EQ is set to flat.  That makes it easy to just lay in the desired loudness compensation via Graphic EQ.

The problem with that, as I realized some time ago, is that if I just dial in loudness compensation for the subwoofer, the effects are limited and in fact the loudness compensation would be messing up the integration between the subs and the panels.

Well, then the obvious solution would be to dial in the required graphic EQ correction into all 3 digital equlizers, though probably no need to bother (yet) with dialing in bass boost to the super tweeter.

What I finally did last weekend was to dial in bass boosting correct, the identical bass boost correction curves, into the equalizers for both the subs and the panels.  Because the boost curves are identical, the integration between subs and bass is not affected in any way.  (I did also consider and temporarily try dialing in correction to the subs only, or dialing in a different correction to the subs and panels, but decided I liked the way that doesn't affect the integration the best, so far, at least for peace of mind).

And to make this even easier, I didn't have to set levels for each 1/3 octave because the Behringer DEQ 2496 graphic equalizer function has a control for "bandwidth" so I selected a low frequency of 63 Hz as the "center" of boosting, with a 4 octave (actually 11/3) bandwidth.  Once I have selected that bandwidth, I can just turn the level control up and down in both EQ's to vary the loudness compensation (as shown in picture above).

Simple, free, and it works!

More sophisticated solutions might be possible, such as fine tuning each 1/3 octave of compensation, but this solution seems to work well enough while being very easy to adjust or turn on and off.

A more typical (or "correct") loudness compensation would boost the deepest bass most of all.  There are issues with that for a system like mine that attempts to reproduce bass down to 15 Hz.  Headroom in the deepest bass is limited and I'm loath to apply very much EQ there, though I suppose at low volume it could be boosted nearly back to 'realistic' level and be OK, but I was thinking this would be worthy of some serious investigation first.

My limited looking boost curve is pretty much what someone might add with a 'bass' tone control.

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