Physical time alignment

[MechEngVic]

14 hours ago, Chris A said:

And the alignment requirements due to the very short wavelengths at the crossover frequency are so small that even if you do manage to time align the super tweeter for one listener position, if the listener simply stands up, the super tweeters are out of alignment to next the lower frequency drivers/horns by more than 90 degrees (i.e., about 1/4 inch alignment requirements or less are typical).  It's a "head in a vise" problem that cannot be fixed.  I recommend using dual-diaphragm coaxial drivers instead to eliminate this problem.

 

Chris

Once again, Thank you for the valuable comment.

 

What you describe here is exactly what I am finding. I'm thinking that instead of physically aligning the super tweeter, it may be more advantageous to use it as a sort of equalizer, so much is the effect on the overall sound as its position changes. I can make the KLF's sound much brighter with the super tweeter in certain spots, but the amazing thing is I can make the KLF's sound even darker and warmer with the super tweeter in certain other spots than the KLF's can be on their own with no super tweeter! Certainly if you have bright horns this might help tame them. Now I will see the effect of the super tweeters in these darker spots on imaging and detail.

[MechEngVic]

On 12/21/2019 at 1:32 AM, Chris A said:

  I recommend using dual-diaphragm coaxial drivers instead to eliminate this problem.

 

Chris

Chris, 

Your comment started sinking in and I started looking at dual coaxial drivers and the effects of time aligning... While searching I ran into a video where a guy talks about super tweeter placement and how if there are duplicated frequency ranges (which I have, using a super tweeter with a two way speaker), that you'll have comb filtering no matter where you place the tweeter. He recommends turning the tweeter around to face backwards or placing the tweeter on top of the speaker facing upwards. The idea being that the timing will be far off enough to not cause comb filtering.

 

I put my tweeters in a combination of both positions, pointing backwards and upwards... No more comb filtering, and a big increase in soundstage! I feel like this achieves my goal of filling in the high frequency roll-off of the KLF's.

 

Thanks again for inspiring me to look deeper!

 

This video is worth a look:  

 

 

[WMcD]

FWIW, the Linear X LEAP system manual IIRC has a suggestion on where to assume the source of the sound from a driver.  It is the front plate of the magnetic structure which defines the hole in which the voice coil sits.  But I think that is a reference, only.  The diaphragm is going to be a bit in front or could be a bit behind physically.  Or curved to make it both physically behind and ahead.  Ugg.

- - -

 

FWIW. The driver and the rest are what is called a "causal" system.  Whatever we use as an input, the output occurs later in time.  That is the real world.  We can't make a system which which looks at lottery results on Tuesday and sends it to us on Monday so we can place bets.  The sci-fi of time travel paradox is built on this fiction.

 

An impulse input contains all frequencies with the same zero crossing.  When we look at the output even at the diaphragm, the output is later in time.  The input impulse is smeared out when we measure at the output.  We see graphs of that.  Similar to a bell ringing though that is a narrow freq.  It means that the various frequencies are delayed by different amounts of time.  It arises because of the bucket brigade nature of interacting components which store and release energy.

 

Anyway, we can look at the frequency components of the smeared out impulse.  We see sinusoids (e.g. sine and cosines are the very same shapes and generally called sinusoids).   These are always delayed frequencies.  There is no "leading" when compared to the input frequencies in the input pulse.   Nothing happens before it is caused in the real world. At best, if output sinusoid B is delayed less than sinusoid A, then it might be said to be leading A, but it is not a matter of the system moving it ahead in time.  There is no Delorian here.

 

- - - 

End of rant.

 

 

[MechEngVic]

36 minutes ago, WMcD said:

An impulse input contains all frequencies with the same zero crossing.  When we look at the output even at the diaphragm, the output is later in time. 

 

 

I can't claim to know much about impulse response other than some modeling we did in college using Matlab. 

[WMcD]

Well, I got into the rant which was basically my retelling of how I figured out some of mysterious explanations.  Leading and lagging imply time advance and delay but that is really not the story.

----

I recall in college walking up the hill from the engineering building to the main campus with a good professor of physics.   Seems like a charming mentor and student situation, don't ya think?

 

I asked, "How does a bicycle  work?"  He said,  "That is a very complicated question."  Then silence.  I thought, "He doesn't know either but he doesn't want to admit to it."

 

- - - 

On impulse response of a device:

 

I just re-listened to Days of Future Past  on the Internet.  At the end there is a gong sound.  So we  hear the impulse response of the big cymbal.  Not a bell-like response.  Sort of multifreq shimmering  with decreasing amplitude.

 

What is interesting is that in the opening seconds of the album there is a shimmering sound of increasing magnitude.   I think it is a reverse playback of the same.  Therefore we get to hear the impulse response reversed in time.  

 

End of Rant 2 and Good Night.

[MechEngVic]

5 hours ago, WMcD said:

Well, I got into the rant which was basically my retelling of how I figured out some of mysterious explanations.  Leading and lagging imply time advance and delay but that is really not the story.

----

I recall in college walking up the hill from the engineering building to the main campus with a good professor of physics.   Seems like a charming mentor and student situation, don't ya think?

 

I asked, "How does a bicycle  work?"  He said,  "That is a very complicated question."  Then silence.  I thought, "He doesn't know either but he doesn't want to admit to it."

 

- - - 

On impulse response of a device:

 

I just re-listened to Days of Future Past  on the Internet.  At the end there is a gong sound.  So we  hear the impulse response of the big cymbal.  Not a bell-like response.  Sort of multifreq shimmering  with decreasing amplitude.

 

What is interesting is that in the opening seconds of the album there is a shimmering sound of increasing magnitude.   I think it is a reverse playback of the same.  Therefore we get to hear the impulse response reversed in time.  

 

End of Rant 2 and Good Night.

Well I'd be happy to read any and all of your rants on the subject. 

[Chris A]

11 hours ago, MechEngVic said:

I put my tweeters in a combination of both positions, pointing backwards and upwards... No more comb filtering, and a big increase in soundstage! I feel like this achieves my goal of filling in the high frequency roll-off of the KLF's.

 

Thanks again for inspiring me to look deeper!

 

This video is worth a look:  

 

Well...I think that there is more than a little smoke and mirrors in that video, even though Mr. Richie (a fellow Texan from Iowa Park just west of Wichita Falls) is taking the time to explain why super-tweeters really don't work very well. 

 

Basically he's saying that there is no way to keep the lower frequency driver ("midrange-treble") from forming strong output cancellations with the super-tweeter due to the vertical distance separating the two drivers more than about 1/4 wavelength (Richie didn't say this in the video, but Tom Danley has discussed this in the Synergy horn series) such that you're going to get strong cancellations at certain frequencies in the crossover frequency band between the drivers depending on the exact position of the microphone vertically and horizontally.

 

So the consolation prize is to aim the super-tweeter upwards (noting that it still needs to be time aligned with the crossing midrange-treble driver mounted on the front panel).  You get an ambience effect from the super-tweeter and you avoid much of the strong out-of-phase cancellations in the crossover interference band.  And if you place a microphone near ear height while sitting, you will see less cancellations than a front-baffle mounted super-tweeter. 

 

But notice that he didn't show you a SPL/frequency plot of the combined response in-room at multiple microphone heights and lateral off-axis positions, then show you each of those.  It is wise that he didn't show you these, I think, because it would show you the complexity of the situation that has been created.

 

To understand what occurs when you aim a super-tweeter upward, think of having multiple loudspeakers placed together, each with very narrow coverage angles (polar coverage) vertically and horizontally, splayed out from the center loudspeaker by the angle of coverage (like a multi-cell horn of old).  Some of those loudspeakers have good SPL response on-axis of each loudspeaker, others have not-so-good response, but they're all playing the same music at the same time in-room.  Depending on where you sit or stand, you're going to hear a different timbre of sound depending on your location.  The spectrograms (instead of SPL vs. frequency plots) will also show you a totally different picture.

 

The above situation is exactly what can't happen if you go to a movie theater and sit somewhere in the audience.  You want the same sound throughout the theater, no matter where you sit.  This is the difference between a "head in a vise" setup that so many audiophiles tend to accept without acknowledging, and a real cinema-style setup that puts every place that you choose to listen as a good place to listen from. 

 

Which system sounds much better in-room?  I think it's obvious.

 

Chris

[MechEngVic]

On 12/28/2019 at 5:21 AM, Chris A said:

 

Well...I think that there is more than a little smoke and mirrors in that video, even though Mr. Richie (a fellow Texan from Iowa Park just west of Wichita Falls) is taking the time to explain why super-tweeters really don't work very well. 

 

Basically he's saying that there is no way to keep the lower frequency driver ("midrange-treble") from forming strong output cancellations with the super-tweeter due to the vertical distance separating the two drivers more than about 1/4 wavelength (Richie didn't say this in the video, but Tom Danley has discussed this in the Synergy horn series) such that you're going to get strong cancellations at certain frequencies in the crossover frequency band between the drivers depending on the exact position of the microphone vertically and horizontally.

 

So the consolation prize is to aim the super-tweeter upwards (noting that it still needs to be time aligned with the crossing midrange-treble driver mounted on the front panel).  You get an ambience effect from the super-tweeter and you avoid much of the strong out-of-phase cancellations in the crossover interference band.  And if you place a microphone near ear height while sitting, you will see less cancellations than a front-baffle mounted super-tweeter. 

 

But notice that he didn't show you a SPL/frequency plot of the combined response in-room at multiple microphone heights and lateral off-axis positions, then show you each of those.  It is wise that he didn't show you these, I think, because it would show you the complexity of the situation that has been created.

 

To understand what occurs when you aim a super-tweeter upward, think of having multiple loudspeakers placed together, each with very narrow coverage angles (polar coverage) vertically and horizontally, splayed out from the center loudspeaker by the angle of coverage (like a multi-cell horn of old).  Some of those loudspeakers have good SPL response on-axis of each loudspeaker, others have not-so-good response, but they're all playing the same music at the same time in-room.  Depending on where you sit or stand, you're going to hear a different timbre of sound depending on your location.  The spectrograms (instead of SPL vs. frequency plots) will also show you a totally different picture.

 

The above situation is exactly what can't happen if you go to a movie theater and sit somewhere in the audience.  You want the same sound throughout the theater, no matter where you sit.  This is the difference between a "head in a vise" setup that so many audiophiles tend to accept without acknowledging, and a real cinema-style setup that puts every place that you choose to listen as a good place to listen from. 

 

Which system sounds much better in-room?  I think it's obvious.

 

Chris

I hear what you're saying and I agree with it, as I think most would. But many will turn right around after agreeing and tout the virtues of their "head in a vise" arraignments. Well I only grudgingly accept and grudgingly tout as an improvement to what I had before, due to limited resources and trying to do the most with the equipment I have on hand. But know that I am paying close attention as I slowly but surely make improvements.

[david.purton@ntlworld.com]

Please let's not confuse time alignment with phase, as so many still do. If you are using analogue active or passive crossovers time alignment is virtually impossible, a doddle with dsp and a programme such as ARTA and an impulse response measurement.

 

I am using horns and a horn super tweeter on an active analogue system, crossing around 8Khz. The mid horn has a passive 1st order LP and the tweeter an adapted passive 3rd order. I established putting a 10 (8-12) ohm resister in parallel with the first capacitor of the HP filter took the knee out of the 3rd order roll off and now created a gentler

- 6dB at the point the 1st order LP was -6dB, and voila, as they say...reverse tweeter polarity, absolutely flat response through the crossover point and still sufficient attenuation at lower frequencies to protect the tweeter. Xsim was invaluable at simulating the response from the circuit designs and proved wholly accurate when compared to the ARTA plots (calibrated mic) taken in real time. Both free programmes so no excuse for subjectivism and comment with no scientific imperative.

 

The impulse responses clearly showed that the time arrivals were different (as expected) by around .5ms, but the response was ruler flat and minimum phase plot showed the pair of drivers were in phase.

 

The wavelength at 8000Hz is around 40mm..at a listening distance of say 3m or 3000mm it represents a difference in distance of .013% . Given these frequencies are also easily reflected and absorbed and you are allowed to move your listening position a bit, the chance of you hearing the difference in arrival time of .0005 seconds of direct sound ( particularly where the ear is not so sensitive as say around 1kHz) is probably unlikely. The chance of you hearing the effects of a suck out of around 6 dB at crossover is entirely likely particularly as the response anomolies will vary on and off axis.

 

It's a choice you have to make because everything is a compromise. You may like the analogue route (I have gone back to it from 8 channel of dsp) and bear in mind once chosen everything in the chain, bar a dac to convert digital sources (RME), should stay in the analogue domain all the way through. I opted for Rane 23S active crossover which enabled me to use the "delay" function between bass and mid and mid and mid horn..it phase shifts, and is not physical time delay as in dsp, but you can tune for minimum phase and flat response through the crossover with the 24dB/octave LR filters. The mid horn to super tweeter is a passive network as outlined above.

 

Or you can go digital with a super clean 96KHz+ ,  24 bit processing (and upwards) distortionless LMS and make all you adjustments, eq, crossover, and time alignment in a simple 1U controller. Should be a no brainer really, and for professional application its difficult to argue the benefits of any other system. But we are humans and personally I like the "sound" of analogue, class "A" valve EQ,s, etc, so having to tolerate an error of .0005 second time difference between super tweeter and mid horn at 8kHz is a compromise I can make. In truth, I lose little sleep over it!