Room matters

Agreed - the room is the elephant in the room - possibly why many of the regular contributing experts here will not even share a few pix. Nevertheless there are differences in speakers that are significant too.

However, every one of these systems that I have auditioned works wonders, particularly in the bass. To reconcile the conflict, I use DRC only for subwoofers - where it makes the biggest improvement - and run a (mostly) "purist" main signal path.

I agree with Kal - in theory there is no "purity" reason that DRC cannot be used throughout, however, from an acoustics perspective the ultra LF frequencies (below about 100 Hz) is the only area where DRC can effectively correct specific frequency related room modal effects. A waveform at 1000 Hz is about a foot long so any specific corrections are not going to apply to a large enough area of the room to be worth doing, and, above the ultra LF freqencies, DRC is really just an EQ adjustment, which can help correct a bright room, for example. In this case, it is always best to get appropriate speakers and acoustic room treatments first rather than make EQ adjustments. The same can be said for the ultra LF - bass traps and an ideal room that needs no correction is likely to sound much better than a system with heavy DRC. However, it is nearly impossible to achieve good acoustics down to 20 Hz so DRC becomes the only "practical" solution.

So in a sense "purity" applies in an acoustic sense - get the room as good as we can (as pure as possible) before doing anything else.

The impure part of DRC is that we are artificially adjusting BOTH the level of the primary AND that reflected signal in order to adjust the COMBINED level to be flat (our ears hear the combined signal but we also have a sense of primary versus reflected as well - although this sense is very poor or weak as you get to low bass frequencies). In reality, in an ideal world, your speakers would have a flat response and therefore you should only want to adjust the level of room reflections/modes (too strong or too weak).

Perhaps, the ultimate solution requires active canceling such as is used on noise-canceling headphones - imagine an array of active woofers with built in microphones that compare the audio in room delayed response and acoustic decay to the original signal (feedback) and which are able to cancel modal peaks and adjust your room acoustics to perform as desired. Designs such as this probably exist already (in labs) - for example Meyer speakers have a microphone in front of the woofer to reduce primary signal distortion already. The only issue is cost - in theory you could re-create the acoustics of any auditorium if you had such a system.

Meridian's use of correction in the sub-300Hz range on all channels (optional and modifiable) is a great solution and one that should be an option on all other room EQ systems.

My choice of 100 Hz may be a bit low and as you correctly point out - a lot of problems occur between 100 Hz and 300 Hz and, I might add, even further on up as far as about 600 Hz, as the sound goes from omnidirectional (bass) to directional (Lower midrange) and during this transistion the sound is affected at various frequencies by floor and ceiling and side walls until the sound becomes mostly of a forward direction (and the room becomes much less of a problem).

Roy Allison and many others are well aware of this problem with virtualy every free-standing speaker. However, this well known fact is hardly mentioned by the majority of speaker manufacturers these days...despite the fact that professional acousticans continue to take into account these very real acoustical problems in pro studio designs.

I decided to find out what was going on with loudspeakers and room interaction. I'd had a hint of it while doing some papers at AR. There was an unexplained phenomenon—nobody could tell me why it happened: a suckout in the middle bass range in almost every loudspeaker, almost every room transmission curve that we measured. That got my curiosity aroused. I wanted to find out what was causing it.

This transition zone from 100 to 600 Hz is often the most problematic. This is why studios tend to either

1) Use small monitors in near field close to the listener and away from walls/boundaries.
2) Use large main monitors that are built into a wall - a soffit mount

Kal,

I may not have been clear enough or I oversimplified things too much but I can confirm we are in complete agreement.

It is only below the critical frequency that spatial issues dominate.

I very much agree with that and for the sake of a "critical frequency" Meridian's choice of 300 Hz is fair enough as a ball park number.

Here is an article which is based on Olsen's work that shows how a mere "baffle" can have some interesting effects. From this is follows that the effect of baffles and therefore room boundary surfaces can actually affect response over a "range" of critical frequencies - depending on the specific situation - leading to suckouts or peaks in what are sometimes broad frequency ranges.