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Old 13th February 2010, 09:39 PM   #11
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Quote:
Originally Posted by Bill13 View Post
time-aligning the acoustic-centers of the drivers?
Do keep in mind that the acoustic centre of a driver changes as soon as the driver starts moving so aligning acoustic centres can only ever be approximate.

dave
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Old 14th February 2010, 05:03 AM   #12
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with a 4th order x-over, about 1 octave on either side of the crossover point is off. Say at a 1khz crossover, the phase shoots toward positive infinity after about 500hz. Then just after the 1khz, the tweeter phase comes up from negative infinity towards zero.

An 18db crossover at 80hz causes the sub about 20ms delay then more as it gets closer to the crossover point.

Thats why a deep 2 way seems quicker, it has better phase.

And a passive or active crossover, the components or opamps shift the phase. So if you have a 6db filter on 2 full range drivers, 1 driver will be closer to you than the other for alignment (passing closest to a square wave) because the inductor shoves the driver back and the cap moves it forward (or vise verse, I forget).

And a driver may be rolling off, turning an 18db crossover to a 24db acoustic.

Crossovers are messy, and the driver's changing impedence doesn't make it any easier.
You pay for a well designed crossover, not just the price of the drivers.

Norman
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Old 14th February 2010, 05:58 AM   #13
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Norman,

Every pole in the filter introduces 90 degrees of phase shift. 4th order will introduce 360 degrees of shift.

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Old 14th February 2010, 01:57 PM   #14
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from Roy Hall (Green Mountain Audio)
"If a speaker uses a 4th-order crossover at 3,000Hz (common for a 1" tweeter), what will we actually hear from this system?

*

Down at 1,500Hz, the tweeter isn't really contributing. Thus, the image is formed by the woofer. However, the woofer moves back by one inch because the circuit imposes 75 degrees of shift at 1,500Hz. But the dimensional shift is always relative to that 3,000Hz wavelength reference point, where (75 degrees/360) x (13,500ips/3000Hz) = 1 inch.
*

At 2,000Hz, the tweeter output is now audible (12dB softer then the woofer). We hear the image from the woofer move back by a little more, to 1.1" (90-degrees of 3,000Hz equals ¼ wavelength). Yet the tweeter is actually forward by 3/4ths of a wavelength of 3,000Hz, or 3.4". As a soprano's harmonics move through this region, we'll hear the image begin to split by 4.5" total. The image will begin to diffuse from front-to-rear, and harmonics structures will begin to fuzz over -- the sound will become grainy.
*

At 3,000Hz, the woofer and tweeter contribute equally. However, the woofer image has moved backwards by 2.25", for the same total split of 4.5". At this point, the image is very confusing and textures are at maximum graininess. The voice is split into two parts -- 'esses' and 'tees' from the tweeter arrive first, stretched out from the sound of the throat as delivered by the woofer -- an unnatural occurrence. The voice hisses and spits, and strings are edgy*. There'll be comments such as "the transients are etched..." "...detail thrust at the listener..." "...this speaker seems very fast..." "...analytical..." and "...very sensitive to electronics...." Of course, these comments would be expected because the tweeter arrived first!

* Not all higher-order designs sound like this, because there are ways to disguise the problem. Again, if the designer tunes the "Q" of the circuit (computer-optimized, or by ear), or otherwise misaligns the actual crossover point, the graininess will be reduced, along with transient response. The spatial distortion remains, however, and even a solo flute can be heard to wander.
*

At 5,100Hz, the woofer is now 12dB softer than the tweeter -- just audible. It's now 3.4" behind the tweeter's highest frequencies, and the tweeter is still 1.1" ahead -- a 4.5" total split.
*

At 12,000Hz, two octaves above the crossover point, the woofer output is non-existent, but the tweeter output will still arrive 30 degrees ahead (0.4 inches) of its very top end at 24,000Hz, where phase error returns to zero. This will disturb the timing of a wooden stick striking a bell, blurring its image. A musician striking the bell .4" late is definitely behind the beat as well."
Green Mountain Audio - Speaker-Time-Phase-Coherence - Loudspeaker Phase Accuracy and MusicalTiming

"Higher-order circuits, from 2nd on up, cannot be made time-coherent, because that Phase Difference each one exhibits at the crossover point does not remain a Constant Phase Difference when the tones move away from the crossover point. In other words, the drivers' Relative Phase Difference is always changing."
AudiogoN Forums: Phase Coherence or Time Alignment: Which More Imp?

Here on page 2 and 8 show the phase of a 24db crossover from eaw.
http://www.aalt.com.au/FAQ/DSP_setti...over_point.pdf
here is a picture
Click the image to open in full size.

Norman

Last edited by norman bates; 14th February 2010 at 02:04 PM.
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Old 14th February 2010, 05:56 PM   #15
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That looks like a good dissertation... but no mention of phase heading off to infinity.

Sure makes you think that the choice of a FR for at least mid-top is a really good one.

dave
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Old 14th February 2010, 07:59 PM   #16
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Quote:
Originally Posted by norman bates View Post

"Higher-order circuits, from 2nd on up, cannot be made time-coherent, because that Phase Difference each one exhibits at the crossover point does not remain a Constant Phase Difference when the tones move away from the crossover point."
Here is something "timely" with regard to this discussion:

Siri's Killer Note

Note that you can have a 1st order high pass with a 4th order *bessel* low pass and maintain a minimum phase design. It's the technique that Spica used.

Stereophile: Spica TC-60 loudspeaker

You can also use Duelunds approach in a 1st order filter and maintain minimum phase with a net "higher order" filter:

http://www.htguide.com/forum/showthread.php4?t=34839


..btw, this should probably all be "split-off" into it's own thread and placed in the Multi-way forum.
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Old 14th February 2010, 09:50 PM   #17
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planet10, yup you are right, it shoots off to -180 and pos 180 degrees.

And of coarse the physical delay, a 24db moves an acoustic center back and the other forward, but you can slide the drivers back and forth till the match up.

Yes, the fast way (fullrange augmented sub technology, is that right ?) should be the new wave.

My old thiels ran 6db time aligned, but the midrange was clearer on my dual 4", but the thiels had some sparkly highs and more bass. But the cabinet (stuffed big time) had some resonances that added well to male voices. I've given them to my kid to get him hooked into stereo equipment like me. All good drug dealers give the first sample of crack for free. It's nice when he say "yea, I'm hearing all sorts of stuff in games and music I hadn't b4."

To me the crossover, even a 6db, adds some haze to the drivers. And the 2 different materials can act quite differently. But I'm time aligned.

Click the image to open in full size.

Norman

Last edited by norman bates; 14th February 2010 at 09:52 PM.
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Old 14th February 2010, 10:28 PM   #18
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Default Phase plots for 4th order filters

Norman,

You said about a 4th order crossover: "Say at a 1khz crossover, the phase shoots toward positive infinity after about 500hz. Then just after the 1khz, the tweeter phase comes up from negative infinity towards zero."

The way phase shift graphs are plotted for crossover networks, and by speaker measurement software like SoundEasy, can be misleading - phase plot is actually continuous as the plotted phase wraps depending on the particular phase plot's scale. The reference you cite with the phase plot picture says:
"Attention needs to be given to the “wrapping” of the phase plot. The y-axis of this plot ranges from –270 degrees to +90 degrees. Phase plots usually only represent 360 degrees of phase; since 0 degrees and 360 degrees are equal, as are 90 and –270, 180 and –180, etc. It may not be intuitive that the phase responses shown consistently decrease."

In the "LoudSpeaker Design Cookbook" by Vance Dickson, for typical 4th-order crossovers, the high & low section summed resposes are plotted (summed crossover phase, magnitude, and group-delay). The phase plots resemble the picture you posted. For example the 4th-order Linkwitz-Riley is an all-pass network which sums to a flat magnitude, and the group delay is a smooth curve with a only slight peak just below the nominal 1 kHz crossover frequency. Dickson's 4th-order "Linear-phase" crossover filter has a summed group delay which is nearly flat.

From Wikipedia: "Group delay is a measure of the transit time of a signal through a device under test (DUT), versus frequency. Group delay is a useful measure of phase distortion, and is calculated by differentiating the insertion phase response of the DUT versus frequency. Another way to say this is that group delay is a measure of the slope of the transmission phase response."

So if a 4th order crossover filter's group delay is a continous, fairly smooth function, then the phase shift doesn't have discontinuities.

The 4th order LR filter (1 kHz nominal crossover frequency) has roughly 0.5 millisecond group delay error when plotted from 20 Hz to 20 kHz (Dickson's plot). This implies that phase is continous, although not linear Vs. frequency.

A controversial question is: How audible is a time offset of 0.5 millisecond?

Bill
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Old 14th February 2010, 11:03 PM   #19
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Scott,

The PDF of the 2008 YG Acoustics advertisement appears somewhat similar to the 2010 YG Acoustics ad in TAS, but the current 2010 ad version is more extensive with more plots and even more spectacular performance claims -- my impression.

The current TAS ad seems to put more emphasis on claimed phase-linearity & coherent superiority to even a competitor's very-well-designed first-order crossover speaker (e.g Thiel ?).

As I recall, YG Acoustics cliams that their optimized 4th-order crossover beats the example 1st-order design in linear-phase error (+-5 degrees) over a wider speaker banwidth of 200 Hz to 20 kHz.

I got the impression that YG Acoustics 'KIPOD' speaker seems to offer improvement in the 2010 advertisement measured response plots - I understand that some (recent) changes may have been made to the wide-band woofer and tweeter, and perhaps the matched crossover as well - again, from memory.

Anyway, it might be worthwhile to look at TAS current issue.

Bill
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Old 15th February 2010, 02:01 AM   #20
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Interesting topic. One thing that comes to mind about any speakers mentioned, is that at some point of course, they must also be connected to an amplifier.

From Stereophile: (note highlighted reference)
"The Anat Reference II's voltage sensitivity was significantly higher than average, at an estimated 90.1dB(B)/2.83V/m, though it is a little lower than the specified 92dB. The impedance is specified as 4 ohms, with a 2.7 ohm minimum. However, while our sample was basically a 4 ohm design, the magnitude did drop to almost 2 ohms in the mid-treble—which is why WP felt the tubed VTL amplifiers lacked sparkle—and below 2 ohms above 25kHz (fig.1). The phase angle also varies widely, and the combinations of 4 ohms magnitude and –53° capacitive angle at 110Hz, and 4.8 ohms and –46° at 2.4kHz, mandate the use of a power amplifier that can deliver unrestricted amounts of current. As the Main Module is not fed from a crossover when used with the woofer modules, the former magnitude/phase combination will be unaffected."

This makes me think that at some point, adding complexity to simplicity may not yield all the results being searched for. It wouldn't seem sensible to take a full range driver, designed to stand alone (or almost), in a low power application, tubed or otherwise, and make things even more difficult on the amp which affects performance negatively.

Also, since you are not re-engineering the drivers themselves to fit a criteria, then you could be trying to fit 10lbs of sand in a 1lb bag, or so to speak.
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