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Old 16th May 2012, 11:53 PM   #31
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by LafeEric View Post
Please tell, what did you mean when you said it in post#24?
Hi, I said in post #26 i may have overstepped the mark, rgds, sreten.
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Old 17th May 2012, 01:49 PM   #32
KSTR is offline KSTR  Germany
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PRTG,

I would think you tend to illustrate things more complicated than they are.

Basically you have the raw in-situ driver's IR's, impulse responses, (that can be displayed as magnitude + phase response vs frequency), preferably with frequency dependant windowing applied, combined/merged with close field measurements for the LF stuff. These IR's also must contain time of flight differences (use HolmpImpulse to get there without fiddling).

Then you simply must find the correcting filter function (and then implement it -- which is the section you put emphasis on) to acheive your choosen total and complete driver response, which not only does contain the XO but also the XO of adjacent sections (or the final LF rolloff). These correcting filter functions may contain additional low/high passes and/or allpass sections (things get tricky when the tweeter is ahead of the woofer, though). See this post for an (analytical) example of time-of-flight compensation.

The core question is now if 0deg phase-matched responses (of final acoustical response) of all driver sections throughout their whole operating (down to -30dB or so) is acoustically to be preferred or not. I personally do prefer it (and I think this is generally agreed on), for technical reasons like lobing and distortion, except for D'Appolito type of arrangements were 0deg tracking phases doesn't work well (you need constant 90deg phase offset there).


For example I recently designed (the electrical part of) an active sub+sat system which was planned to work along the THX-guideline of a "textbook" LR4 XO at 80Hz. The sats were ported and highpassed (8th order total), so resulting IR was obviously only able to partially follow the LR4 reponse. But of course the sub has its own LF-rolloff (6th order, at ~35Hz) and I managed to tailor the additional phase from the 8th order sat alignment to complement the additional phase from the woofer's LF-rolloff for a perfect 0deg phase tracking (+-5deg down to -30dB points) with almost equally perfect flat summing. The subwoofer's HF roll-off, when viewed in isolation, is perfect LR4, which required the woofer passband being EQ'd flat up to 500Hz and then the textbook LR4 applied (which had to be variable from 50...120Hz as a design requirement)

Note that raw reponses were measured only once, all the rest was done with simulators (LTspice mostly, and some LSPcad), then built. I happened to land spot on when finally measuring the thing.

Last edited by KSTR; 17th May 2012 at 01:52 PM.
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Old 17th May 2012, 02:11 PM   #33
PRTG is offline PRTG  Latvia
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KSTR, thanks for suggestions, I see your point. While these things are relatively easy to do with I'm aiming for minimalistic as possible passive filtering here for usage with single SET amp (quite a a niche, still many people enjoy it). Thanks for pointing to HolmImpulse and time-of-flight measurements. I was going to analyse mostly with group delay available in ARTA.
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Old 17th May 2012, 02:46 PM   #34
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PRTG what drivers are you thinking for your project ?
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Old 17th May 2012, 04:28 PM   #35
PRTG is offline PRTG  Latvia
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Quote:
Originally Posted by lduarte1973 View Post
PRTG what drivers are you thinking for your project ?
For the "easy part" two paper cone 12"-inchers with ~95dB sensitivity in bipole DTQWT baffle arrangement + 2 matching waveguided tweeters arranged in dipole configuration. More details will follow!
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Old 17th May 2012, 04:29 PM   #36
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Quote:
Originally Posted by PRTG View Post
...I'm aiming for minimalistic as possible passive filtering...
PRTG: I noticed this before, and now that you mention it again I thought I would comment. You stated that you want to use "first order" filtersin your first post. Please keep in mind that first order filters, especially passive ones, are not very good for loudspeaker crossovers because they do not sufficiently attenuate the signal to the drivers. If you do not attenuate the signal to a woofer enough at higher frequencies the breakup region will not be sufficiently suppressed. If you do not sufficiently suppress the signal to the tweeter at low frequencies enough, the cone will still be driven to high excursions (for a tweeter). Both of these will increase distortion, and for the tweeter, more power than necessary will be sent through the voice coil, which will cause it to heat up more than needed. In general, first order filters lead to a rash of bad thing, and this is why they are not typically used in high performance speakers. Occasionally you see it, but the designer must really know how to address (and measure) the problems that can occur in order to make sure that they are controlled and minimized.

You might want to keep that in mind. If you are planning on using ARTA, use STEPS to measure distortion. Do some modeling of the tweeter excursion to see how power you can apply before reaching Xmax. Using a tweeter with a resonance frequency closer to where you will cross it over will help, because the tweeter's own rolloff will decrease excursion. On the other hand, this region of the tweeter's response typically has higher harmonic distortion... and the tweeter's phase response will still be changing in its passband, making the phase matching/tracking harder to achieve... also, flattening the impedance peak at resonance does not change the phase behavior of the driver output, so you will still have that to contend with... there are only difficult tradeoffs to choose from I am afraid.

Speaking of phase, in your first post you mention aligning the tweeter and woofer using a waveguide on the tweeter to "align acoustic phase". Unfortunately this does not align the phase, only minimizes the acoustic delay part of the phase response. Remember, phase = minimum_phase + excess_phase, the excess phase coming from the delay of the signal that results when the distance from the drivers' acoustic centers to the listening position is different for each driver. So, when excess phase is zero, there is still the minimum phase response to consider, which is solely a function of the frequency response of the drivers. Since the woofer and tweeter have different frequency responses, their minimum phase responses MUST be different. This is why, in general, the relative phase cannot be the same everywhere, unless you use some DSP tricks.

-Charlie
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Old 17th May 2012, 09:41 PM   #37
PRTG is offline PRTG  Latvia
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Thanks, CharlieLaub, I knew someone would touch this sooner or later.

Quote:
Originally Posted by CharlieLaub View Post
Using a tweeter with a resonance frequency closer to where you will cross it over will help, because the tweeter's own rolloff will decrease excursion. On the other hand, this region of the tweeter's response typically has higher harmonic distortion... and the tweeter's phase response will still be changing in its passband, making the phase matching/tracking harder to achieve... also, flattening the impedance peak at resonance does not change the phase behavior of the driver output, so you will still have that to contend with... there are only difficult tradeoffs to choose from I am afraid.
This isn't quite so. My observations are that when you flatten driver's resonance peak electrically (or mechanically) excursion is minimized so is THD in this region as THD comes from resonant character and not the opposite way. Same can be said about impedance. Electrical correction of impedance peak is matching LRC values to electrical equivalent circuit of driver's mechanical properties to counter-measure its resonance. When it is done f response roll-off gets less steep as Q changes its value, impedance is flattened, THD is lowered and phase curve gets straightened. Actually that's the first and most important point of the method. I believe directivity smoothness must also be improved as my observations are that HF driver's resonance also mean extremely wide dispersion at resonance therefore crippling directivity smoothness. We'll see.

Regarding "high performance speakers" I believe you actually mean capable of playing "loud and clean". First, we don't have defined criteria here (still you're welcome to suggest), second the goal isn't set for such, rather for pleasant listening at moderate levels. With target speaker sensitivity at least ~93dB/2.83V and cleverly designed baffle I don't see much of the trouble here as 1-2W power will make it "quite loud" for in-room listening with minimum of excursion and audible distortion.

For any conditions requiring more power crossing actively with high filter orders and damping tweeter resonance (for reasons explained above) would give the best result. Many would disagree for second part to be reasonable at all, so its yet to be proved unless "this was all already done before" (still waiting for some referrals to related articles). Still active filtering/DSP-based correction/multi-amping is approach that is far from being minimalistic, simple and easy to build, SET amp friendly.

Thanks for suggestions anyway!

Last edited by PRTG; 17th May 2012 at 10:08 PM.
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Old 17th May 2012, 09:52 PM   #38
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Quote:
Originally Posted by PRTG View Post
This isn't quite so. My observations are that when you flatten driver's resonance peak electrically (or mechanically) excursion is minimized so is THD in this region as THD comes from resonance-like character not the opposite way. Electrical correction of impedance peak generally gives counter-measuring equivalent circuit of driver's mechanical properties. When it is done f response roll-off gets less steep as Q changes its value, THD is lowered (and output as well), and phase gets straightened. Actually that's the whole point of the method. Plain physics.
I don't have much experience with this, though I have been in electronics for years.

It seems to me it might be possible by carefully manipulating the Q of the resonance compensation network to minimize power losses by balancing it's effect. Do you know if this has been tried?
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Old 17th May 2012, 09:58 PM   #39
DavidL is offline DavidL  United States
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Well how about the OP builds his design and lets see what he ends up with. Otherwise it will never get off the ground with all this arguing. I'm betting he doesn't end up meeting his goal, not by a long shot.
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Old 17th May 2012, 10:22 PM   #40
PRTG is offline PRTG  Latvia
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Quote:
Originally Posted by LafeEric View Post
I don't have much experience with this, though I have been in electronics for years.

It seems to me it might be possible by carefully manipulating the Q of the resonance compensation network to minimize power losses by balancing it's effect. Do you know if this has been tried?
Have a look, I added a bit to original reply to CharlieLaub, still you were very quick

I see that power loss is in place only in R in parallel LRC filter. R value is somewhat 2-4 times Ro for result close to total damping. I'd say energy loss would be about 25-50% in this particular region (a very rough estimate). Also part of energy gets shifted to lower frequencies due changes in slope steepness. So we'll be expecting dip in combined output here if LF driver isn't very capable or if we have tweeter waveguide diameter too small to compensate for it. Still it will be "clean" dip without artifacts that usually accompany resonances.

Regarding balancing it is surely possible to compensate for a lesser degree as a compromise, depends on whether you have to do it at all.
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