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Old 7th June 2012, 12:05 AM   #101
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Originally Posted by turbodawg View Post

- Shift the time delay of the sub that is corner loaded, crossed at 80 hz / 1st order, from what is an audibly and distance calculated correct setting, by say, .5 ms ahead of the mains. The sub goes from being audibly invisible, to sounding out of sync, sped up. Or laggy, if you were to delay it the same amount. That's equivalent to a physical shift of about 6". Wavelengths at 80 hz are 14ft, so phase shift is minimal. I do understand that there is higher frequency distortion from the sub that may blend better with good time alignment. But why is a .5ms shift so audible here?
Expectation effect ? Seriously. Have you done proper controlled blind testing or only sighted testing ?

Whilst I tend to agree that group delay at high frequencies may affect imaging and coherency, perhaps more than generally accepted, I really don't see how 0.5ms of delay below 80Hz could be audible when our sensitivity threshold for group delay is much much higher than this at low frequencies. (by at least a factor of 10)

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- Woofer to tweeter crossover is 3.5k, 1st order. If I shift on tweeter channel by .3ms, this results in a roughly 360 degree full phase shift at the crossover frequency. The positioning of the stereo image will be relatively unchanged, but it will sound "off", and not be as clear. Thoughts?
Yep, if you had measured the resulting frequency response when doing this you would see that it introduces a lot of (audible) ripple in the amplitude response and a lot more group delay than neccessary due to more rapid phase shift.

0.3ms will only be 360 degrees at one frequency, it will be less at lower frequencies and more at higher frequencies, thus the relative phase shift introduced will be varying with frequency throughout the overlap region. In a 1st order crossover the overlap region is large, so the ripples in response will extend across a wide frequency range, including the presence region where the ear is most sensitive.

If you had good driver phase tracking before you've just completely destroyed it by adding the delay. Just because the phase wrap is 360 degrees at your nominal crossover frequency doesn't mean the result should be equivalent. Far from it.
Quote:
- Why does 1st order sound and image much much better than 4th order?
Your opinion only. I find properly implemented 3rd/4th order phase tracking crossovers image better than 1st order, probably due to the decreased overlap region where the drivers can interfere off axis.
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Old 7th June 2012, 02:27 AM   #102
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Originally Posted by DBMandrake View Post
Expectation effect ? Seriously. Have you done proper controlled blind testing or only sighted testing ?

Whilst I tend to agree that group delay at high frequencies may affect imaging and coherency, perhaps more than generally accepted, I really don't see how 0.5ms of delay below 80Hz could be audible when our sensitivity threshold for group delay is much much higher than this at low frequencies. (by at least a factor of 10)
Sorry, no ABX'ing. But I've certainly tried the sub at many different points from no time correction all the way past optimal. Changes are very noticeable. If it's not audible, then why is subwoofer time correction very common.

Overall though, I can't imagine doing audio in a car, period, without time correction. The difference with and without is amazing.

Quote:
Yep, if you had measured the resulting frequency response when doing this you would see that it introduces a lot of (audible) ripple in the amplitude response and a lot more group delay than neccessary due to more rapid phase shift.

0.3ms will only be 360 degrees at one frequency, it will be less at lower frequencies and more at higher frequencies, thus the relative phase shift introduced will be varying with frequency throughout the overlap region. In a 1st order crossover the overlap region is large, so the ripples in response will extend across a wide frequency range, including the presence region where the ear is most sensitive.

If you had good driver phase tracking before you've just completely destroyed it by adding the delay. Just because the phase wrap is 360 degrees at your nominal crossover frequency doesn't mean the result should be equivalent. Far from it.
Fair enough, that's probably what I'm hearing. Thank you for the detailed insight.

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Your opinion only. I find properly implemented 3rd/4th order phase tracking crossovers image better than 1st order, probably due to the decreased overlap region where the drivers can interfere off axis.
I'm only basing this off my experiences with the CD8053, which is an exceptionally well regarded DSP preamp head unit; there is not even an amplifier section, only a 8v or 16v balanced pre out. All things otherwise equal, the 1st order sounds much more coherent, cleaner, lifelike, much better imaging. I doubt there's that much more processing involved in the 4th-order.....?

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Old 7th June 2012, 05:45 AM   #103
AllenB is offline AllenB  Australia
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Quote:
Originally Posted by turbodawg View Post
All things otherwise equal, the 1st order sounds much more coherent, cleaner, lifelike, much better imaging.
OK, so are all things otherwise equal? Has the wider overlap region caused the reduction of an annoying peak, or the filling in of a hole, or has the extra 90 degrees of phase difference that the first order has over the fourth order improved your response?
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Old 7th June 2012, 07:41 AM   #104
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Quote:
Originally Posted by turbodawg View Post
Sorry, no ABX'ing. But I've certainly tried the sub at many different points from no time correction all the way past optimal. Changes are very noticeable. If it's not audible, then why is subwoofer time correction very common.
If you've got a physical separation of many feet from main speakers to subwoofer it may be worth doing time correction, but you're talking about 0.5ms which is a delay equivalent to half a foot, which is extremely small for frequencies below 80Hz.

It's not just how far it is in terms of wavelength to consider, there's also the fact that our threshold for detecting group delay skyrockets at bass frequencies - we become very insensitive to timing at low frequencies. A good part of the reason why is time/frequency uncertainty. At 100Hz it takes 10ms for a single cycle of a bass note to be completed, we aren't even aware that a 100Hz bass note has begun until at least one cycle has been completed, eg 10ms later. As frequency goes down it gets even worse, 20ms for 50Hz etc. This sets fundamental limits on our ability to resolve small timing differences at low frequencies.

As for expectation effect, don't underestimate just how powerful it is. I've been doing critical listening to small changes on speakers for years, I'm fully aware of expectation effect and the ways that it can catch you out, and try my hardest to be skeptical of what I hear, and yet if I get lazy and perform sighted tests of subtle changes I still sometimes get caught out where I can hear an expected change when in fact there is no audible change at all. No matter how good a listener you think you are, everyone is susceptible to it.

Hearing isn't just a simple process of turning the sound waveform into impulses and sending them to the brain like some digital recording, it's a complex cognitive process that is constantly trying to interpret and make sense of the acoustic environment around you. A lot of what we "hear" is actually the brain interpolating and interpreting multiple senses including sight, and "guessing" to fill in missing pieces of information based upon past and expected scenarios. There is surprisingly little information (in terms of bitrate) being sent from the ear to the brain, so there's a lot of "reading between the lines" being done by the brain that we are not even aware of consciously.

That's why proper scientific listening studies, (such as those designed to measure the threshold of audibility of certain parameters) have to be done double blind and have the right procedures in place to prevent expectation effect and information from other senses from giving false results. Placebo effect in audio sadly is stronger than it is in medicine
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Old 7th June 2012, 11:33 AM   #105
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Originally Posted by turbodawg View Post
Thanks for clearing up the stereo vs. mono point. I mainly observe soundstage shifts with the mains, as you say. Questions...

- Shift the time delay of the sub that is corner loaded, crossed at 80 hz / 1st order, from what is an audibly and distance calculated correct setting, by say, .5 ms ahead of the mains. The sub goes from being audibly invisible, to sounding out of sync, sped up. Or laggy, if you were to delay it the same amount. That's equivalent to a physical shift of about 6". Wavelengths at 80 hz are 14ft, so phase shift is minimal. I do understand that there is higher frequency distortion from the sub that may blend better with good time alignment. But why is a .5ms shift so audible here?

- Woofer to tweeter crossover is 3.5k, 1st order. If I shift on tweeter channel by .3ms, this results in a roughly 360 degree full phase shift at the crossover frequency. The positioning of the stereo image will be relatively unchanged, but it will sound "off", and not be as clear. Thoughts?

LOL.
As Simon said, the summed response will be different even if the 3.5k point is in phase. You will have added a lot of phase rotation by that delay (at a relatively high frequency). The effective crossover is a span of frequencies until one side or the other is down 20 to 30 dB and can't mess up the sum.

With exactly one phase rotation 3.5 k would be in phase but the phase curve is tilting strongly downwards so frequencies above and below would diverge in phase between woofer and tweeter giving rough response.

You see this frequently with a passive crossover between a long mid horn and short tweeter. Always a few rotations of phase between one unit and the other, so a narrow band of comb filtered response with neither connection polarity looking very good.

Not sure what to say about the woofer crossover as that hasn't been my experience.

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Old 7th June 2012, 02:52 PM   #106
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Originally Posted by AllenB View Post
OK, so are all things otherwise equal? Has the wider overlap region caused the reduction of an annoying peak, or the filling in of a hole, or has the extra 90 degrees of phase difference that the first order has over the fourth order improved your response?
What I'm hearing is a full spectrum improvement in imaging and what I believe is better transient response.

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Old 7th June 2012, 03:16 PM   #107
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My 'unofficial' interpretation of those comments is firstly that you've balanced your frequency response somehow as this can cause our focus to relax and widen. With regards to the transient response I believe that proper transient response doesn't sound like anything, it's just natural. In the past I may have felt the need to use the term when I've caused my system to make material jump out and sound fast.
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Old 7th June 2012, 03:37 PM   #108
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Originally Posted by AllenB View Post
My 'unofficial' interpretation of those comments is firstly that you've balanced your frequency response somehow as this can cause our focus to relax and widen. With regards to the transient response I believe that proper transient response doesn't sound like anything, it's just natural. In the past I may have felt the need to use the term when I've caused my system to make material jump out and sound fast.
Clearly, I'm getting better transient response with the 1st-order. If anything, frequency response is worse with the 1st-order, and there is distortion from the tweeter if the crossover point is too low.
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Old 7th June 2012, 03:42 PM   #109
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Hi,

Since the beginning of the thread number of considerations for the way to multi-way phase coherent speaker has widened, also measurement techniques. A quick summary (please add if I have forgotten something):

a) Lobing effects in vertical plane due acoustical phase shift from physical distance between drivers (CharlieLaub) with 4-th order active filters (reference). Would be interesting to see how similar tests with 1-st order active compare with 4-th order active. I believe 1-st order filter lobing will be less pronounced but present within wider frequency range;
b) Measurement of excess group delay using middle of bandpass region of each driver as relative acoustical phase reference point (DBMandrake). I'll probably need some of your help for interpretation of results when I get to this point and also about the method itself.
c) Clear controversy is still present about smoothness of frequency response (direct and very importantly also indirect radiation) in conjunction with time-delay correction as being the most important objection we must deal about, or not just that. An opposed view is that electrical phase alignment must be dealt additionally as being orthogonal to acoustic phase but with similar importance. Btw, I'm one-fifth into Toole's book, thanks to SpeakerDave for pointing to it.
d) There are still not enough clues about possible positive/adverse effects of tweeter resonance damping except that it could be favourable for current-source amps. My listening experience is that with passive filtering close to resonance frequency damping of resonance with RLC favors rather for use with voltage source amp (reduced amp IMD from reduced back-EDS? Else?). Quite contrary SET amp seems to provide something like "power compression damping" of HF driver's resonance. Generally raise of load impedance will shift valve operation mode into more linear region (less THD) but lessens some of its output power. Effect may vary significantly amp by amp, though in my opinion so it gives the pleasant "resonance softening" effect (along with other SET amp related bonuses/tradeoffs). Some reference here. Clearly, measurements must be done to prove this.
e) Regarding 1-st order filters and tweeter damping I found interesting ad-like article from Thiel, could be plain marketing, or could be something more in it indeed.

Many thanks to all of you for shared insight and experience. I have revised my measurement process, so lets re-evaluate it.

Sreten was right about the lack of depth for grounding of my initially proposed building method, so instead of comparing two filter building methods I'll take more elaborate approach by performing different measurements and gathering enough measurement data of parameters that are considered crucial for sound reproduction accuracy. And that is different filter orders, crossover frequency, amp type, physical driver placement, etc. So instead of comparing a couple of filtering design approaches I'd rather go for various measurements around two most critical points of the system - crossover region and tweeter resonance frequency and at least three octaves below and above.

After finishing prototype speaker I'm going to compare different filters for fixed, low crossover frequency (around tweeter Fr x 1.5) of 1-st order passive filter with current source SET amp (JJ ECC83+Valve Art 300B), 1-st order passive with voltage source hybrid amp w. solid-state output stage (6N11+LM3886), same repeated with passive 1-st and 3-rd order combo, then same with active analog 4-th order with Linkwitz/Riley alignment with adjustable time delay (Rane 23S).

Measurement set for each type will include:

- summed impedance along with electrical phase response;
- on- and off-axis frequency response with 10 degree steps for off-axis within 90 degrees one-side horizontal pointed at median of driver placement;
- on- and off-axis frequency response repeated at ear level, lets say 30 cm above tweeter placement;
- THD;
- IMD;
- excess group delay;
- anything else?

Then shifting the tweeter along Z axis to get the best excess group delay alignment and remeasuring f-response again.

Sorry, no blind listening tests, just occasional WAF

I see no quick wins here as it will take time, but by omitting something from the big picture we leave space for another guesswork wrestling. Table of results of this measurement tournament would give at least something valuable.

Please feel free to add more suggestions for the measurements.
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Old 7th June 2012, 03:44 PM   #110
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Meanwhile I have also played with Monacor DT-300 tweeters and find them cleverly designed and having inexpensive complementary waveguide. Unfortunately four drivers that I've received had low consistency of assembly quality: two of them were far out of specs. I found an easy way how to correct this and informed Monacor about the flaws providing Z and f-response curves. They responded immediately with promise to investigate possible root cause of these problems. Until their response or grace period of two months (whichever comes first) I'll won't be giving any more details in public.

While it would be easier to get better THD/IMD results with, lets say, Scan-Speak Illuminator series I don't feel that many of fellow DIYers would afford buying them without sacrificing other important qualities of their life. I thing that for filter comparison reasons absolute quality values aren't so important, plus we'll find out what moderately priced driver is capable of.
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