Why do so many automatic configuration systems set centers/rears to "large"? (1 Viewer)

[Brian L]

Not so, unless things have changed and the quoted article is wrong.

My Marantz SR96 THX AVR is 24/12 (as is my non-THX NAD 762), which is consistent with the quoted article. Used with a main channel speaker that also has a -3dB point at 80 hertz and a 12db roll off sums to 24/24. But again, the crux of the matter is to get an effective 24/24 with a crossover that is 24/12 requires a main channel that has a natural 12dB roll off and a -3dB point of 80 hertz.

That was the crux of the Secrets article. Most AVR's use a 24/12 arrangement, but for that to function in an optimum fashion requires a main channel that matches the THX roll-off characteristics. Going with a main channel that goes an octave below 80 hertz gives you a mid-bass hump in the crossover region.

....I think!

Brian

 

[LanceJ]

Whew.

All I will say about this is that concerning the M&K page, I'll bet they list all their THX speakers with the 80Hz spec with nothing else simply to avoid confusion for the people not totally into this stuff: "O.K. here we go - a whole set of speakers that only get down to 80Hz, so they must all be good matches for my THX receiver" (and obviously, the THX logo is right there too so that helps).

And THX's gentle cut-off slope of -3dB cannot really be attained by bass reflex designs because of that design's relatively *sharp* low-end cutoff (because of the woofer/port interaction issue, that is, if the speaker's official spec rates it flat to 80Hz in the first place). I'm not ragging on bass-reflex speakers - that's just the way they operate. FYI: notice how with the supplied port plug being used that come with several of those M&Ks, their bass response changes directly to 80Hz.

The following is from the .pdf manual for their B-1500:

While the above doesn't say anything about THX specifically, IMO it sure sounds like it is implying acceptable THX usage.

Anybody know what a "BMC-MINI" filter is?

 

[Kevin C Brown]

Yes, I guess that first quote is completely wrong!

But yet still:

12 dB/40 Hz = 24 dB/80 Hz

I guess it depends on whether you look at it per octave or by Hz. I say by Hz works because it's a small increment, (vs 20 Hz to 20 kHz for example). By octave means that for the common 12/24 to work, you'd have to buy all speakers that were down 3 dB at the crossover you choose. Which seems artifically limiting to me, especially for most gear where you only get a global choice for 1 single xover for all speakers.

But there's the practical effect too. With a 12/24 combo, I've always held to the "1/2 to 1 full octave away" setting for crossovers, and freq response plots with discrete test signal values or even with ETF5, I've always managed to get a flat freq response through the crossovers. But I also adjust sub phase to compensate too.

Maybe the bottom line is, no matter how you do it, if you actually measure, you can see what you get and if it's working or not.

 

[LanceJ]

I asked the people at Audyssey, the firm that designed the auto setting system for Denon's receivers, about this issue. I received a reply a day later from Chris Kyriakakis, Chief Technical Officer:


No wonder so many people report that the auto distance settings for their sub seemed weird! FYI: IIRC when the "distance" is set, actually what a distance compensation system is doing (whether its manual or automatic) is altering the phase of each channel's soundwave so that when all the waves from the various speakers reach your head, the crests & troughs of all those waves match i.e. they are in phase with each other. This helps explain why so many people that set their front mains - or all channels (like me) - to large with a subwoofer present have reported that the bass quality is excellent: whether it's an automatic system doing the setting or the user doing a careful manual set-up* by ear, any shared soundwaves are either 100% in-phase or very close to it by the time they get to their ears. Hehe!!!

Conventional speakers contain a crossover - I wonder if even though it is a *fixed* crossover, could they also have an audible effect? I'll ask Chris about this.

* a continuously variable phase control on the subwoofer & distance settings for each individual channel (in very small increments) can sure help with this.

 

[Kevin C Brown]

This is not how most systems do it. For most systems, if you set the distance to each speaker correctly (or when it is automatically set correctly), then each speaker is now in phase with each other speaker at the listening position. There are tests in Avia to check for this. But when auto systems adjust the distance to the sub, that channel and that channel alone is being set up to be in phase with all the other speakers. So that's why that distance can be way off, but the phase is correct.

So ... it is relatively easy for auto systems (or people themselves for that matter) to get the phase correct between each main speaker. Then, it is a simple matter to set the phase (i.e., "distance") to the sub with respect to the main speakers. Not that easy manually (unless you know what you are doing), but very easy automatically.

And, to complicate things even further, in most cases you do not even want perfect phase in between the sub and mains, but you do want the flattest, smoothest freq response. That's what a lot of us who use ETF5, TrueRTA, etc, strive for.

 

[LanceJ]

Yesterday I sent what I wrote to Chris & this is what he had to say today:


************************************************** *******

For people that don't get into this stuff, time alignment between woofers/midranges/tweeters can also be aided by their physical positioning on the speaker itself. This is why there are speakers that have stepped or tilted front baffles like from Thiel, so the physical centers of each driver is lined up. But usually time-alignment is mostly done in the crossover because building cabinets like that is expensive. Or, some companies use coaxial speakers i.e. tweeters mounted inside the midrange or woofer, like Thiel, KEF and Tannoy . But these can have some (unrelated) drawbacks of their own. Designing speakers within a specific price range always involves a certain number of compromises - how they eventually sound reflects what sonic aspects their designer considered more important.


* I'm just guessing here but maybe the in-phase aspect can be accomplished with just tiny shifts in delay; but for distance compensation that tiny shift as a whole can be "moved" back & forth in a macro manner as needed to maintain the imaging. I can see what I'm trying to explain but lack the technical vocabulary to explain it properly. So here's an analogy, sort of: picture a van moving at 60mph. But inside the van when you toss a can of beer to your buddy in the back seat, the can - relatively speaking - is only moving a few feet a second. Back in HT land.....so when you adjust the distance setting on a receiver (auto or manually), its DSP system automatically maintains that *separate* in-phase setting, the in-phase setting being that beer can. But whatever the case, if it sounds good, it sounds good.