Ukko Kari

Faital 15PR400, BMS 4594 MTM

50 posts in this topic

If it's working, you should also see zero crossings at each of the two peaks.  For example:

post-1549-0-82867400-1464241433_thumb.png

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The numbers are cut off on the right, but I assume that there is a zero crossing where you put the cursor?  So it looks like you have a zero crossing coinciding with the right peak, the other two are shifted for some reason.  Weird.

Maybe someone who understands the measuring apparatus better can comment?

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The closest I could place the cursor was -0.9 degrees, yes it coincides with the peak on the right, but the others are shifted some. I get 0.5 degrees at 0.7 hz above the left peak, and centered it's already at 20 degrees.

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Looks like the sound card has the dreaded Behringer crosstalk issue. There's a strong rising response below 20 Hz. That may mess with phase.

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1 hour ago, lilmike said:

Looks like the sound card has the dreaded Behringer crosstalk issue. There's a strong rising response below 20 Hz. That may mess with phase.

Oh yeah, I see that now.  Bummer.  At one point, I considered making I considered making an impedance measurement jig, but then I decided to just buy a Woofer Tester 2 instead.

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Yeah, the Woofer Tester 2 is pretty nice. I am pleased with mine, I have used it for years. I shared my version of the REW impedance jig that the OP's gear is based on for cases like this where that sort of an investment may not be warranted.

It might not be perfectly exact, but it is a measurement of the impedance of the drivers in the cabinet, which has a LOT of value, even with the crosstalk issue. It's pretty clear that the tune is closer to 30 than to 40, which was his target. Odds are, when he takes the tune up closer to 40, the crosstalk will be less severe and the results might look a little better. I have no direct experience with the specific gear he is using, I do have its little brother though (Xenyx 302USB). It's not used in my measurement rig due to the crosstalk. 

EDIT - my bad - OP is using a UCA202 for the USB input and source. Those definitely have the crosstalk issue we're seeing here. I have plenty of those and know the crosstalk issue very well. 

 

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Had a bit of time today to pull the lower woofer, wiggle the ports out of the baffle, shorten to 8" long, installed they are 8 3/8" total.

Net interior volume went up by 0.11 cu ft, to a total of 9.63 cu ft. I had not saved the impedance .mdat prior, but looking at the screen shot, the upper impedance peak fell a small amount, as it would when the box volume increased.

Removing 2 inches on each port only raised the tuning ~ 1 hz, observed excursion minima was at 34-34.5 hz. I also found a leak around a screw during testing, the foam seal had compressed some since assembly.

8 inch ports impedance.JPG

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As an experiment ( per Lilmike ), I hooked up a pair of UCA-202's, output on one, and measure on the second to get rid of the cross talk issues. Apparently, the absolute value for impedance is off, but the <20hz cross talk is gone.

 

 

twin UCA 202's.JPG

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4 hours ago, Ukko Kari said:

As an experiment ( per Lilmike ), I hooked up a pair of UCA-202's, output on one, and measure on the second to get rid of the cross talk issues. Apparently, the absolute value for impedance is off, but the <20hz cross talk is gone.

Some of the phase zero crossings still aren't correct either.

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2 hours ago, SME said:

Some of the phase zero crossings still aren't correct either.

I also checked for excursion minima with sine waves at high levels. With the shorter ports, excursion minima was 34-34.5 hz. We can debate minutia all day long, but that doesn't change much. When the voice coils have some heat in them, the impedance will shift as well.

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Well, I was just pointing out another remaining issue with the impedance measurement.  The excursion minimum check does the trick too.  As long as you are satisfied with the result.  :)

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I had hoped that the tuning would shift up enough by shortening the ports 2 inches, but their proximity to the side walls and floor of the enclosure means that they are acting like they are much longer than they physically are.

It is a bit of a pain to finesse the port length, but I would like to get the tuning point up just a wee bit higher. I can live if they aren't 40 hz.

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Mic 1/4" from top woofer dust cap:

Edit: I don't recall if eq and crossover were enabled, it's been a while since this was measured. Might have to disable the DSP and re-measure this.

close mic.JPG

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27 minutes ago, Ukko Kari said:

I had hoped that the tuning would shift up enough by shortening the ports 2 inches, but their proximity to the side walls and floor of the enclosure means that they are acting like they are much longer than they physically are.

It is a bit of a pain to finesse the port length, but I would like to get the tuning point up just a wee bit higher. I can live if they aren't 40 hz.

I had the same problem with my mains, which are at 45 Hz vs. closer to 55 Hz (to be used with subs) to limit excursion related distortion.  My ports have the two flared ends connected directly together so can't really go any shorter.  It's not a big deal though.  These speakers handle movies at reference levels without flinching.  I'm thinking of enhancing them with low-mid/MBM units to create a kind of shadowed pseudo-line array.  With those in use, the mains woofers won't be pushed nearly as much.

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Perhaps juggling chainsaws would be easier than picking trade-offs. Baffle width, roundover size, cabinet depth, height, volume, adequate port area, bracing, ability to get cordless drill inside enclosure for installing the pocket screws on the last side... portability, ease of assembly, etc.
 
 
 

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9 hours ago, SME said:

I had the same problem with my mains, which are at 45 Hz vs. closer to 55 Hz (to be used with subs) to limit excursion related distortion.  My ports have the two flared ends connected directly together so can't really go any shorter.  It's not a big deal though.  These speakers handle movies at reference levels without flinching.  I'm thinking of enhancing them with low-mid/MBM units to create a kind of shadowed pseudo-line array.  With those in use, the mains woofers won't be pushed nearly as much.

At the end of the day, you have to ask yourself is the juice worth the squeeze? Additional amplifier channels, more complexity, an additional surface for diffraction from your main woofer.

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1 hour ago, Ukko Kari said:

At the end of the day, you have to ask yourself is the juice worth the squeeze? Additional amplifier channels, more complexity, an additional surface for diffraction from your main woofer.

So far it absolutely has been.  And I'm already way far gone on the complexity thing.  I'm running 14 output channels (but only 5.1 input), completely custom DSP with practically unlimited filtering capability, matrix processing, etc.  I find that there is a narrow region around "perfect" within which the great are made.  That's  true despite the variability in the source content.

As for diffraction, that's something that happens at edges and not surfaces.  And often times, more edge is a good thing if the sound wave passes around it over a greater distance / time period.  Diffraction is an unavoidable fact in audio, so the right approach is to design for and take advantage of it.  That's essentially how horns/waveguides work to steer the sound into a desired pattern.

But I'm maybe a special case because I love sound so much and actually have the skills to do what I do.  That doesn't mean I have the knowledge of what is right to do.  It's been a very fun learning experience, and it's great to go back and listen to stuff that had been harsh, muddy, or congested before and to hear clear, sweet, smooth sound.  You know, the kind where it doesn't even sound loud until you try speak and can't hear your own voice or you turn it off and realize that your ears need time to recover.

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One way to cure diffraction is to flush mount the cabinets in a baffle wall with a layer of absorption, like 1" thick.

Realistically, larger radius corners should have been used on my boxes, but 1 1/4" is getting a bit hairy to handle in a hand held router even with a large add on baseplate, and 1 1/2 is about the limit without going to MDF substrate and using machined corners.

http://www.tapeease.com/cabinet1.htm

I could have gone the skeletal route with multiple layers of thin plywood instead, ending up with curved sides, large radius on the baffle. Did I mention the dust was *incredible* when routing all of the corners to 1 1/4" radius? Doing this outdoors with a slight wind, it was still problematic, it was like it was snowing wood dust out. :D

 

 

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Small radius corners are pointless if you are using a good horn.  The horn should keep most sound away from the edges, so it can't diffract.  For low frequencies from the woofer, a small radius doesn't do anything because it needs to be on the order of the wavelength to matter.

My cabinets have sharp edges, but I may some day build multi-segmented pieces to attach to their sides, essentially a rough round-overs from front to back, to effectively smooth out the diffraction of the lower frequencies.  The low frequencies won't care about the individual segments, and it'll be much easier and cheaper to build than anything curved or multi-layered.

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1 hour ago, SME said:

Small radius corners are pointless if you are using a good horn.  The horn should keep most sound away from the edges, so it can't diffract.  For low frequencies from the woofer, a small radius doesn't do anything because it needs to be on the order of the wavelength to matter.

My cabinets have sharp edges, but I may some day build multi-segmented pieces to attach to their sides, essentially a rough round-overs from front to back, to effectively smooth out the diffraction of the lower frequencies.  The low frequencies won't care about the individual segments, and it'll be much easier and cheaper to build than anything curved or multi-layered.

What is a good horn in your experience? For a common 90h x 45v horn of 12 inches width and 6 inch height, the vertical polar pattern will widen below 3.7khz, exceeding 90 degrees at just below 2khz, according to this article:

https://www.prosoundtraining.com/2010/05/24/understanding-horn-directivity-control/

Even the 18 Sound XT1464 is 'small'. Used with other elements in a vertical 'array' of 3 elements [MTM] leads to increased directivity in the vertical dimension, even when the pattern control is falling from the horn.

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I'm a fan of the SEOS horns, and they do lose vertical pattern earlier like you describe.  I'm not sure it's that big of a deal though as far as diffraction is concerned.  My SEOS-15 measurements (in a box with approximately 1" of border) suggests that it loses its vertical pattern starting at around 2.5 kHz.  The wavelength at that frequency is about 5", so the roundovers are going to have to be pretty big to control that.  However, the primary effect of diffraction is to basically hold a vertical pattern (a somewhat wider one) to a bit lower than frequency than the horn alone would probably hold.

If I do the pseudo-line, the horn will be sandwiched between the mid and the mid/bass-woofer sections, so it will probably allow more sound to travel up and down the baffle front.  Again, I rather doubt this is a big deal.

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A baffle of infinite size can be considered a 180 degree horn. The acoustic impedance will change rapidly from the mouth of the horn to the baffle. giving rise to diffraction, unless there is absorption, or a very large radius at the mouth where it transitions to a baffle, but this also depends on the frequencies of interest from the horn, the dispersion and the physical sizes of the device in question.

In other words, diffraction and re-radiation is still a real problem for horns of finite dimensions.

 

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The SEOS horn is rounded along the vertical edges, although not as much as the horizontal edges, which I believe are much more important for the same reasons that we typically arrange drivers in a vertical pattern as opposed to a horizontal pattern.

I guess you're right though that a lot of horns have little to no rounding at all.  The trade-off on the SEOS is that it doesn't hold pattern as low for its size.  But if one was to go through the trouble of creating large round-overs to reduce diffraction, more space would be needed anyway.  It's rather more convenient to relying on diffraction control within the horn, IMO.

Every application is different though.  The SEOS horns are not known to load as much as many other horns, so for pro application also, they may not be best.

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