Dialing in main/sub crossover point using measurements -- what is the best approach? - Page 2 - AVS Forum
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Audio Theory, Setup, and Chat > Dialing in main/sub crossover point using measurements -- what is the best approach?
craig john's Avatar craig john 03:43 PM 07-18-2012
Quote:
Originally Posted by arnyk View Post

That no two different speakers work the identical same way is another one of those unhelpful truisms. Every speaker works a little different, even different THX speakers.
The above is an example of cherry picking the details of an example to make a point as if it were a useful generality.
You seem to be trying to make it out that if a speaker doesn't work the identical same way as a THX speaker that there is an cataclysmic catastrophe that can't be altered, adapted or managed in any reasonable way.
Of course. That is another one of those truisms. But is the THX way the only reasonable way? Is it the best way in every case? Is it the most economical way in every case?
I actually got you to say that without payola or threat of violence? ;-)
Its a crap shoot anyway, because of room acoustics. And who says that THX knows better what will blend best in a given situation, than someone else who happens to have full access to that situation?
You know, you've apporached this from the start as if you think I'm married to THX. I am not. My current speakers and subs are not THX certified, (you can see them in the link in my signature.) I am simply trying to explain how the THX crossover works with THX speakers, and how it works differently with other speakers. You've jumped to the erroneous conclusion that I think the THX way is the only way. Clearly that's not true.

THX is just a set of specifications. If a product meets those specifications, it can get certified to meet those specifications, and the user can be assured that the product will perform to the minimum THX standard for the certification level, (it could actually work better than that, but it must meet the minimum to get certified.) You can use a THX certified product with other THX certified product and be assured they will work together to meet the minimum THX standard. This allows users to put together systems that they can be assured will work properly without having the technical knowledge to test everything and ensure satisfactory performance.

You can also use THX certified products with non-THX-certified products. However, you can't expect that they will work the same way they would work with other certified products. Actually they could work *better* or worse together, depending on the design. The THX crossover used with non-THX products is but one example of that. 80 Hz with a 12/24 set of filters could work perfectly for some speakers; not so well for others. It's a crap shoot, and it's up to the end user to optimize the results. Lots of people just throw a system together without considering the implications. The OP said: "I realize that THX suggests an 80Hz crossover point, but my ears suggest that it sounds better around 60Hz." I was merely trying to explain what the THX crossover is, what it does, and how it may or may not work for his speakers and subs, which are not THX certified. Somehow, from that you decided I was on the THX marketing staff. eek.gif

Craig

AJinFLA's Avatar AJinFLA 05:03 PM 07-18-2012
Quote:
Originally Posted by arnyk View Post

It makes plenty of sense if your response shows some evidence of trying to understand it in context, and doesn't immediately jump to the conclusion that the writer is a total idiot.
And do tell us oh great seer Arnie, as Earls official interpreter/spokesperson, what "context" did he have in mind that makes a 3D acoustic soundfield an "accurate facsimile" of an electrical input?
Quote:
Originally Posted by arnyk View Post

Obviously, the "accurate facsimile" "at the listener" is acoustical. What else can it be?
Right, we gathered that oh great one. Now WTF does that have to do with representing the recorded electrical input "accurately"?
Measurably "accurate facsimile at the listener"? Perceptually "accurate facsimile at the listener"? _____ " accurate facsimile at the listener"?
And this is verifiable how?
rock_bottom's Avatar rock_bottom 05:16 PM 07-18-2012
Quote:
Originally Posted by craig john View Post

What you missed in your "research" is that speakers and subs designed to THX specifications are designed to work in conjunction with the slopes of a THX crossover:

A speaker designed to THX specification has a -3 dB point of 80 Hz and a acoustic roll off of 12 dB/octave. When COMBINED with the 12 dB/octave roll off of the crossover, you get a COMBINED 4th order, 24 dB/octave roll off.

Exactly. This was originally proposed in an AES paper by Marshall Leach called Electroacoustic System Realizations for the Linkwitz-Riley Crossover Networks. I think Holman picked up the idea from that paper.

So if you know the (ideal) -3dB frequency of the mains, and they are sealed and have QTC near 0.707, and you have the ability to set the crossover frequency to the -3dB frequency of the mains, you can generalize the THX approach.

One thing I like about the Onkyo receivers is that they have the biggest choice of crossover frequencies, making this approach more practical.

But 40 Hz seems rather low to try this, because the electroacoustic phase shift of the sub probably hasn't gotten close to zero at this frequency yet.
craig john's Avatar craig john 07:59 PM 07-18-2012
How can you possibly infer from this:
Quote:
Originally Posted by craig john View Post

A sub designed to THX specification is flat to 160 Hz, or 1 octave above the crossover. Apply the THX crossover to that sub and you get another 4th order, 24 dB/octave roll off

... that I said this?:
Quote:
Originally Posted by arnyk View Post

BTW many subwoofers are reasonably flat to 160 Hz and above, not just those with the THX label on them. So right up front there was an obfuscated truth - an apparent claim that THX subwoofers are the only subwoofers that are reasonably flat to 160 Hz and above. The truth is that there are many subwoofers that are reasonably flat to 160 Hz and above. Sounds like a sales pitch for THX subwoofers.

Are you developing reading comprehension issues in your graceful old age? Obfuscated truth??? Are you kidding me? I never said anything like what you posted. Obviously many other subs are flat to 160 Hz. My Seaton Submersives are fully capable and they're not THX certified, (although I'm sure they could be if Mark wanted to bother with it):

316

I'm sure there are many others that are similarly capable of response to 160 Hz. You can go to the Data Bass website and see several of them: http://www.data-bass.com/home

Now, having said that, there certainly are also some subs that are not capable to 160 Hz. The Data Bass website lists a number of subs that don't do 160 Hz so well. The Rythmic F15 comes to mind:

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(They make a version with a different amp that is flat to 200 Hz, but this version rolls off at 90 Hz.) Obviously, it can still be used with the THX crossover. It will have a different response above 80 Hz than a THX subwoofer, but it will "work". Or it could be used with any other "appropriate" crossover. rolleyes.gif I wouldn't use anything higher than 90 Hz, but anything else below that should "work".

Craig
craig john's Avatar craig john 08:07 PM 07-18-2012
Quote:
Originally Posted by arnyk View Post

You seem to be trying to make it out that if a speaker doesn't work the identical same way as a THX speaker that there is an cataclysmic catastrophe that can't be altered, adapted or managed in any reasonable way.
Wait... What??? I don't know how you interpreted anything I said to mean that.^^^ You're losin' it Arny. eek.gif

Craig
arnyk's Avatar arnyk 06:02 AM 07-19-2012
Quote:
Originally Posted by rock_bottom View Post

Exactly. This was originally proposed in an AES paper by Marshall Leach called Electroacoustic System Realizations for the Linkwitz-Riley Crossover Networks. I think Holman picked up the idea from that paper.

I'm trying to defend Holman here - the Leach paper is from 1987, almost a decade after Linkwitz's original paper. The ideas that Leach shared in that AES engineering reports was one of those things that probably popped into the heads of many people who read Linkwitz's original paper right then and there. If you're an EE with a background in filter design it might be pretty obvious. I know that we had implemented our first L-R crossovers that way a year or two after Linkwitz's original paper, not thinking that it was rocket science.

If memory serves we were looking at the effects of driver roll-offs on measured response and asked ourselves the question "Would this be smoother if we bypassed one of the two stages of Butterworth filtering". It was. If we could figure it out, a designer of Holman's stature would probably get it all by himself even sooner.
Quote:
So if you know the (ideal) -3dB frequency of the mains, and they are sealed and have QTC near 0.707, and you have the ability to set the crossover frequency to the -3dB frequency of the mains, you can generalize the THX approach.

Exactly. Caveat - the mains should be sealed boxes, not vented. Venting gets you 4th order Butterworth which is a shade different from the cascaded second order Butterworth filters that Linkwitz specified. Also not everybody tunes their sealed boxes for maximally flat Butterworth response. Many speakers are a little underdamped for warmth.

Let's be real - the acoustical response of speakers is not nearly as exact as electrical networks. Everything we do is observed through the blurred lenses of room acoustics and human perception.
Quote:
One thing I like about the Onkyo receivers is that they have the biggest choice of crossover frequencies, making this approach more practical.
But 40 Hz seems rather low to try this, because the electroacoustic phase shift of the sub probably hasn't gotten close to zero at this frequency yet.

When you're doing an acoustical/electrical Linkwitz Riley, the phase shift of the sub is part of the design and you actually hope that the electroacoustic phase shift is there to match the design specification. That's part of the charm of the approach - from the standpoint of the design the normal electroacoustic phase shift of the speakers can actually work for you to achieve a higher goal.

Also, lets not forget that the final response of a properly-implemented Linkwitz Riley crossover is a big fat all-pass filter with a quick 360 degree phase shift at the crossover frequency. What it does to square waves isn't pretty at all. It even takes an observable toll on tone bursts.

However, have no illusions you are choosing the lesser of two evils. Changes in speaker directiivity though the crossover frequency can be very audible because it can cause frequency response variations that might not be expected if you just look at the magnitude response of the two speakers being blended, and even if you factor in the magnitude response of the electrical crossovers.

I can't think of any musical instrument that puts out clean tone bursts or even far less probable clean square waves. A little ringing and some group delay is the lesser evil. We observe these effects through the very fuzzy lens of room acoustics and the somewhat fuzzy lens of our hearing. Anybody with illusions about the sensitivity and reliability of the ears as test equipment take notice!
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