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Old 13th August 2007, 08:59 PM   #1701
jlo is offline jlo  France
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back to group delay :
Quote:
Also, SL has progressively changed his position on the audibility of group delay in low frequency crossovers. He progressed from 2nd order between mids and woofer to 4th by "convincing himself" that the additional GD order wasn't audible. Then with the ++ and the addition of (sealed) subwoofers to the Orion I guess he further convinced himself that he couldn't hear the GD with the woofer to sub crossover either. This is in contrast to the building evidence that GD associated with higher order crossover at low frequency is audible.
I had the same questions about audibility of GD in low frequencies so I did a software to change GD without changing the FR (an allpass filter).
I found that with some signals (sawtooth), low freq GD is quite audible : curiously, longer GD gives "more" bass. I am wondering if some of you had same findings.
You can also try it with my soft here
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Old 13th August 2007, 09:33 PM   #1702
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Hi jlo,

Amplitude, phase and group delay are inextricably linked.
EQing and crossover modification of amplitude response affect the other two.

The sounds upon which I found sub/LF phase change most noticeable were an outdoor recording of a distant canon firing, and the hall ambience background of recorded music. Both changed considerably with LF phase change.

There are those who claim we are incapable of hearing LF phase change, however, I just wish they would speak for themselves only !

With regard to group delay, I found this has a separate effect.
Take the kick drum. Where there is EQ boost induced group delay, the initial thud sounds as if it has less amplitude than the following membraneous vibration, and again sounds quite different, though if in phase can be reasonably tolerable.

Lynn is proposing to increase LF output by increasing driver area with reducing frequency, and if carefully done this should not develop phase and group delay induced deviation or EQ amplitude modification of either.

You mention adjusting group delay, but you must simultaneously have also been altering either the amplitude or phase response.
Is this what provides 'more' bass.


Cheers .......... Graham.
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Old 14th August 2007, 12:19 AM   #1703
mige0 is offline mige0  Austria
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Default How to measure a driver for use as a dipole

Hi

Quote:
... I'd still like to see a 1M measurments with 3 to 5 msec FFT window. I'm sure there will be differences but my spread sheet includes an imperical model of the efffect of asymmetry between front and rear radiation which I based on a variety of my own measurments for a number of different drivers. This is, for example, why the 90 degree off axis response doesn't go to identically zero at all frequencies. Anyway, if you have the time to do a windowed 1 M measurement to get reflections out of the picture I'd be interested to see it.
JohnK, here it is and as it is only a few clicks more I include the CSD plot as well:
( As you know for sure and indicated by the yellow bar THIS measurement is only valid above 200 Hz due to the 5ms window )



Click the image to open in full size.
Click the image to open in full size.



What can be seen is that this driver in this baffle refuses to show the first baffle peak, instead almost exactly - there is a 4 dB dip at 600 - 700 Hz where the 4 dB peak should be. Hope it is of any help though.

Anyway I wouldn't recommend that driver. Not because of that FR which always can be EQed out ( no resonance can be seen in the CSD plot between 300 - 1000 Hz ). This driver also has a good motor, extended FR, low moving mass and low distortion (3rd HD up) BUT one thing that bothers me the longer the more is that tupperware sound coloration of the plastic ( magnesium filled ) cone material. This may have increased since I replaced the original foam by rubber surroundings.

I am also exploring a high sensitive WESTRA paper cone chassis around 10 years older then the almost ancient Dynaudio above. This was kind of el cheapo speaker at that days but in contrast to the high priced Dynaudio after roughly 35 years they are still in perfect working condition having a flawless linen surrounding and NO broken voice coil braid either as is very common with Dynaudio chassis.

Sound is fast and transparent with a nice singing in the bass department. I really have to measure it more extensively.
A interesting detail is the acoustically transparent dust cap made of black gauze a early version of what now is done with phase plugs.

I am curious with which speaker Lynn will come up for the mids. It is easy to find PRO speakers with paper ( maybe even hemp ) cones but it isn't easy to find PRO speakers with phase plugs avoiding the rear coughing through the pole piece ventilation hole - essential for OB's IMO.


Greetings
Michael
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Old 14th August 2007, 12:32 AM   #1704
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Thanks guys. I have now changed my mind and ditched the WWMTMWW 4 way and LR4 at low XOs. I would follow Lynn's OB, however, my plan had been drawn up well before this thread started and I have all the drivers on hand already so I will press on. I have now opted for a MTMWW 3 way + sub, passive acoustic LR4 at 2k, active acoustic B3 at 120Hz. Sealed box for 120Hz- with a natural LR2 roll off at 50Hz (no electrical high pass), Sub with LR2 low pass at 50Hz. MTM is still necessary to get the sensitivity to 93dB with a maximum SPL within linear excursion 118dB from 120Hz up (from effective wide baffle). Technically, I would go with TM or MTM but not TMM, given the M is a 6.5". I don't know if using a sealed box for 120Hz- would degrade the sound more than the GD introduced in LR4 in a U-frame. Power response is less even, but perhaps some power increase at the low end is better since our ears are less sensitive at low frequencies. However, I have to work with the drivers I have on hands, which would give up the SPL capability of 115dB at 80Hz in a U-frame so U-frame does not seem to be an option unless I use a LR4 for driver protection. So at the end, for reduced GD while maintaining high SPL capability I have to go with a sealed woofer box from 120Hz-.
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Old 14th August 2007, 03:13 AM   #1705
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Good choice, backing down on the complexity.

Voicing MTM's can take a very long time in my experience. Even JohnK alludes to it not being a quick or trivial process, and he's using a lot more sophisticated simulators than when I voiced the Ariels in 1991.

The discrepancy between measurements, simulations, and actual audition is a big warning sign you're in over your head. To some extent, all of us experience this on a new design, but the odds go way up with complex radiation patterns, which have a way of defeating expectations. In the end, the ear is the final guide to neutrality, but that assessment can only reached with a lot of tedious listening to pink-noise and music with a wide selection of quite dissimilar amplifiers.

By "dissimilar amplifiers" I mean radically different technologies - a variety of different price-point Class AB transistor amplifiers, Class AB pentode amplifiers with feedback (vintage style), and non-feedback Class A triode amplifiers. A good speaker will be even-handed, reveal the sonic differences inherent to the technologies, but not fail completely with one topology or the other.
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Old 14th August 2007, 06:46 AM   #1706
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Lynn,

Many thanks for your posts. Your posts are always very enlightening and very delightful to read.

I have tried to tune the XO of my previous WWMTMWW (designed by somebody, I have taken out the drivers for the new OB speakers) and understood how hard it was. It took me a very long time (many nights and long weekends over one year) and numerous trials of different XOs to get it right. I only got the MTM right, but not necessarily the WW -MTM- WW right. But that is a different story because the woofer boxes were not well designed.

I read an article by somebody who wanted to investigate why MTM gave a particular sound and he found that at 45 degree off axis there was a 20dB dip while for a TM the dip was much less. My guess is that although that experiment certainly showed something, it can be far more complicated. On that particular axis with the given drivers, baffle and XO the MTM would produce a 20dB dip but under different parameters the TM may work out worse. The mathematics involved would be too complicated to comprehend and it is hard to theoretically prove one way or another. Since a number of people mentioned the same thing I guess from a DIYer (like myself) perspective one should approach MTM with cautions, and I do.

There are many factors to consider. If I need to have two Ms to make up the sensitivity and SPL, I can only do it either MTM or TMM. For TMM with a 6.5" M, I consider that the polar response would be unacceptable because the distance between the T and the second M is too long comparing to the wave length of the XO frequency. Secondly, TMM would certainly place the midwoofers 200mm closer to the floor if we maintain the ear level of the tweeter (floor reflection is increased). At last, even if we can prove that the off axis behaviour of TM is generally better than MTM, we can not deny one advantage of MTM - the superiority of its vertical response over that of the TM. So, I chose MTM over TMM, and prefer to deal with the complexity of the XO.

Of course, I would choose a 8" or 10" pro driver to make a TM instead of two 6.5" to make a MTM if I could. But I am not ready to give up my existing 4 x 6.5" drivers yet because they are some of the best of the HiFi drivers and cost a fortune. If I did not have them, for sure, I would try a TM with a pro driver.

I would be very interested in how your design goes. I can see a lot of goodies in it already. The only thing I still have a question is how the dipole peaks and nulls are dealt with. Using dipole simulation software, I have never got it right with a large baffle unless, I imagine, the back waves are damped. A large baffle makes the first dipole peak and null low, which is hard to be dealt with in a XO. John K's approach is to make a narrow baffle to shift the first dipole null to the XO region to be dealt with by the XO. I would like to have a lot more driver excursion headroom so I would not want a narrow baffle. What I will try is to use a narrow front rounded panel to get the diffraction very smooth, with extended, angled side wings to make it effectively a large baffle to preserve sensitivity, SPL / headroom (with dipole=monopole at 100Hz). In addition, the side wings will allow me to place 100mm to 150mm thick stuffing material. If the back waves, which I don't necessarily want in a small room, can be reduced by 20dB, for example, the dipole peaks and nulls will no longer be a problem. Why not a monopole then? because I don't want the "box" sound. One may argue that the stuffing materials are not linear. My thought is that it is for damping the back waves, and if one observes the measurement it is still far more linear than the reflections from the front wall. It is fairly linear and effective from 250Hz to 4k, in the range we want it to be. Below 250Hz, the stuffing materials gradually become ineffective, making the radiation pattern closer to a dipole. At higher frequencies, it is closer to a monopole, giving an even power response when blending with a monopole tweeter (I have tried back firing tweeter and did not really like it).

By the way, how close stuffing material to the driver magnet before it creates the "mass loading" effect?

Regards,
Bill
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Old 14th August 2007, 06:58 AM   #1707
jlo is offline jlo  France
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Quote:
You mention adjusting group delay, but you must simultaneously have also been altering either the amplitude or phase response.
with an allpass filter, you only modify phase (and group delay, group delay being only another view of the phase/frequency curve) but you do not modify frequency response (it' not anymore a minimal phase system). So you can really hear if the phase itself is audible or not.
Now those digital filters (FIR and IIR) and real-time softwares, you can easily simulate lot of psychoacoustics effects and really decide if something is audible or not without modifying other parameters (I played with phase and GD, crossover types, box diffraction, early reflections, room modes,...have a try with the softs, it's free)
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Old 14th August 2007, 07:23 AM   #1708
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jlo,

Thanks for letting us know your software which I am sure is very interesting to many of us. I love to play with them but rarely have the time (I use internet only at work). I promise that someday I will. Before that happens, do you have a summary page in which you tell us your findings in your numerous software (even though it represents your personal opinions)? it would be extremely helpful.

Regards,
Bill
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Old 14th August 2007, 12:16 PM   #1709
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Hi jlo,

I think it might be illuminating to study superimposed 'scope sims of input/output for first 2 cyles of sine at LF.

Group delay in RLC circuits is related to steady sine, not initial start of wave, as with kick-drum, bass guitar picking.....

The same in/out relationships need to be observed with digital processing, for the reproduction becomes altered where fundamental tones are shifted in time wrt their own unaltered harmonics.

Cheers ....... Graham.
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Old 15th August 2007, 07:51 AM   #1710
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Quote:
Originally posted by HiFiNutNut


There are many factors to consider. If I need to have two Ms to make up the sensitivity and SPL, I can only do it either MTM or TMM. For TMM with a 6.5" M, I consider that the polar response would be unacceptable because the distance between the T and the second M is too long comparing to the wave length of the XO frequency. Secondly, TMM would certainly place the midwoofers 200mm closer to the floor if we maintain the ear level of the tweeter (floor reflection is increased). At last, even if we can prove that the off axis behaviour of TM is generally better than MTM, we can not deny one advantage of MTM - the superiority of its vertical response over that of the TM. So, I chose MTM over TMM, and prefer to deal with the complexity of the XO.

Of course, I would choose a 8" or 10" pro driver to make a TM instead of two 6.5" to make a MTM if I could. But I am not ready to give up my existing 4 x 6.5" drivers yet because they are some of the best of the HiFi drivers and cost a fortune. If I did not have them, for sure, I would try a TM with a pro driver.

I would be very interested in how your design goes. I can see a lot of goodies in it already. The only thing I still have a question is how the dipole peaks and nulls are dealt with. Using dipole simulation software, I have never got it right with a large baffle unless, I imagine, the back waves are damped. A large baffle makes the first dipole peak and null low, which is hard to be dealt with in a XO. John K's approach is to make a narrow baffle to shift the first dipole null to the XO region to be dealt with by the XO. I would like to have a lot more driver excursion headroom so I would not want a narrow baffle. What I will try is to use a narrow front rounded panel to get the diffraction very smooth, with extended, angled side wings to make it effectively a large baffle to preserve sensitivity, SPL / headroom (with dipole=monopole at 100Hz). In addition, the side wings will allow me to place 100mm to 150mm thick stuffing material. If the back waves, which I don't necessarily want in a small room, can be reduced by 20dB, for example, the dipole peaks and nulls will no longer be a problem. Why not a monopole then? because I don't want the "box" sound. One may argue that the stuffing materials are not linear. My thought is that it is for damping the back waves, and if one observes the measurement it is still far more linear than the reflections from the front wall. It is fairly linear and effective from 250Hz to 4k, in the range we want it to be. Below 250Hz, the stuffing materials gradually become ineffective, making the radiation pattern closer to a dipole. At higher frequencies, it is closer to a monopole, giving an even power response when blending with a monopole tweeter (I have tried back firing tweeter and did not really like it).

By the way, how close stuffing material to the driver magnet before it creates the "mass loading" effect?

Regards,
Bill
In practice, mass loading can be mostly avoided if the (wool/cotton) felt is kept at least 2~3" away from the cone surface. (Some kind of simple frame or spacer will be needed.) These are minimums - bigger cones probably need a little more distance.

Much of the "low-efficiency" reputation of transmission-line enclosures is due to the filling getting too close to the drivers, thus mass-coupling, which results in substantially degraded midrange and HF and lower efficiency. If the MF/HF response of the driver gets congested, "closed-in" sounding, or starts to measure different, the filling is too close, and interacting with the cone in an unfavorable way.

You should seriously re-evaluate your position on the drivers. Either they're good enough, or they're not, and that decision should be rigorously isolated from sunk costs or arbitrary decisions about the system topology. You really must be ruthless here, otherwise you could literally spend years trying to force-fit XYZ drivers into a design where things are never going to be optimal. This is part of the reason why companies with in-house drivers are frequently less successful than companies that are not tied to any one driver vendor - if your hands are tied to XYZ drivers and ABC design goals, frankly, the amount of work goes way up, and the chances of success go way down.

Don't mean to be harsh, but the only speakers I ever designed that I liked were ones where I started with a clean piece of paper - a process that inherently means throwing out a lot of drivers and also throwing out topologies that just don't work. Plenty of stuff looks good on paper, or in a computer simulation, and sounds terrible in the real world. Even though I'm kinda-sorta retired, I value my time, and I have a lot better things to do than force drivers to do things they don't want to do. That is the biggest time-waster of all, with the least to show for it.

I would do the simplest, most basic thing of all - listen to XYZ drivers with no crossover, full range, on a baffle. Do you like them or not? Do they put a smile on your face, or are you trying to convince yourself to like them, because they're "supposed" to sound or measure good. Pay attention to your emotional reaction; are you tensing up, or relaxing? Are you mentally justifying the sound, making excuses for the system? These initial reactions in the first minutes of listening are highly significant and relate directly to long-term satisfaction. As the Buddhists like to say, "First Thought, Best Thought."

For me, drivers are a go/no-go decision. If they are unlikable, that's it, out they go, no second chance, no 100-hour "break-in", none of that audiophile BS. If the vendor can't make a driver that sounds musical on a flat baffle right out of the box, forget it. Just about any driver out of a dusty 1955 console TV set can pass this test, why not an overpriced techno-wonder driver?

The absence of gross and obvious break-up on the FR curve is only the first pass; after that, it has to sound good - as in, similar to music, with a sense of realism and a "breath of life" that good drivers have. These qualities come down to subtle manufacturing decisions about types of glues between VC former and cone, speed-of-sound mismatches between the VC former material and cone material, and small asymmetries in the spider assembly. These manufacturing decisions only slightly show up in the waterfall/CSD curves - they're right down in the noise - but make all the difference between a musical and unmusical (hifi) sounding driver.

As a preview of what I'm going to do, I'm going to measuring and auditioning a lot of 8~12 inch drivers, listening to them full-range on an OB, no crossover, just as-is. The leading candidates are the 18Sound 8NMB420 and 12NDA520 mentioned earlier, an EnABL'ed Alnico Lowther, and an assortment of hemp-cone drivers from Hemp Acoustics and Tone Tubby. Not a Fostex fan, no plans to audition those. The Fertin has an exotic charm, but I've never heard one, and they're way too expensive to buy and then re-sell if I don't like 'em.

Fooling around with the "variable-geometry" concept and the assorted LF shaping networks follow the choice of wideband driver, and will be complementary to the overall sonic character of the primary driver, extending the dynamic range and sense of LF "presence" and tactility. I plan to do a lot of A/B switching between the LF array + primary driver and the primary driver alone to make sure the basic character remains the same.
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