Best 8" to 12" driver for 130hz tractrix horn

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Get WT3 or similar

Yes, well I've measured as shown and with voltmeter across the driver. The dip becomes a peak when you measure across the driver, and is more obvious, as long as you feed enough volts into the combination. The high output impedance just means the voltage across the driver tracks the impedance - I understand this .
As I said earlier, I was able to easily measure the driver+horn resonance without the back-chamber - this was 80Hz . It was obvious and quite sharp . I can measure the driver alone at some stage just for completeness if you like . I can measure the other ( not run-in ) driver this evening .

Now the problem is that I lose any form of resonant peak in the expected area once a chamber of ANY size is attached over the driver. I've scanned widely and I've scanned slowly . It's not as if there's a 'bit of a ripple' or a small peak or dip or anything - there is NOTHING ...other than a broad peak lower down, around 50 or 60, which must be something else.

JLH - have you ever measured the 8PE21 on anything other than a full (90deg) mouth tractrix ? I wonder if there is some kind of non-theory deviation from the hornresp model in this case ?

Hopefully I will have the Fanes in a week or two and I can get more data .

Just get the Dayton WT3 or the WooferTester thing and forget about f'ing around. You already spent too much time on this not very complex issue and a computer based measuring system will take your design skills to another level altogether.

Save yourself some aggravation and pay the money - I did a long time ago and I haven't looked back since.

Best
Kris
 
Hey Kris
You're a very aggressive young man, aren't you ?
I've never spoken to you before and you decide you can tell me to "stop f'ing around" .
The measurement method is quite capable of showing the peak if it's there .
I do actually work as a scientist (optical designer) in my job and am quite familiar with both the theory of this and the practice of operating test equipment & taking measurements . So don't speak down to me as if I'm some kind of numpty .
It I use WT3, as far as I can see, it's just going to automate the test and draw me a curve.
It doesn't help to solve this specific problem which is one of the not being able to pick up a peak .
(Also I will be £150 worse-off )

John, the volume I have currently with the foam in place is at least as small or smaller than the pic you enclose.
What I think I'll do next is try borrowing a Fluke DMM from work, as I don't know the limitations of the AVO-meter in measuring AC volts when the frequency is not 50Hz for example. Perhaps there some oddity there or an input impedance problem or something .

MJ
 
Not True!

No, I am not a member. I do not feel qualified to join. I'm a mechanical engineer, not a PhD in EE. I'm just pointing out that there is an opportunity here for JLH to make a positive difference if in fact he's found an error in the rear chamber calculations.

My off hand, top of the head, back of the envelope idea is that the equations only apply to straight horns that are radiating into precisely the space for which they were designed. The equations may also only be appropriate to full size horns as well, I don't know. There could be many reasons why the equations do not "work" in the real world.


For about the cost of 4'x8' sheet of plywood anyone can join. Quote
"Your AES membership payment expires in August 2011. A one year renewal is $99 for an online membership or $149 if you also wish to receive the printed version of the AES Journal." As a graduate ME you are more than qualified.
Regards,
WHG
 
The old-school method of finding a resonance (or the lowest point of an impedance curve) is use a scope on XY display mode, measuring current on one axis (with a 0.1 or 0.22 ohm sampling resistor) and voltage on the other axis (via direct connection to amplifier output). Use a transistor amp that is known to be flat to a few Hz - tube amps as a source will only confuse things.

If there's any reactance (phase angle) at all, the display opens up to an ellipse, becoming a perfect circle at pure reactance, or 90 degrees. When this happens, all power is reflected back to the power amplifier, and none is used at the load. 45 degrees of reactance isn't that unusual in vented-box cabinets (with woofers that have low values of mechanical loss in the suspension), which makes a challenging load for the amplifier (since so much of the power re-appears at the plates or emitters of the power devices).

To determine the frequency of a peak or minimum on the impedance curve, look for the ellipse to become a straight line, and zoom in as much as necessary - this is either the exact top of a peak or one of the low points of the impedance curve. Very useful for determining the exact center between the two peaks of a vented-box alignment, or Fb. It's also useful for finding Zmin, typically around 200~300 Hz for direct-radiator drivers. The exact impedance of Zmin, compared to the DC resistance of the voice coil, tells you interesting things about the air-load on the driver - this is one of the measurements where you get a different value if the driver is measured in a vacuum.

I should add you also need a frequency counter that is accurate to a fraction of Hz - a few percentage points of counting error grossly throw off the T/S calculations. That's why computer measurements are so convenient - no hassle with finding precision frequency counters, no twiddling with XY scopes, and less opportunity to slip a digit on the hand calculator (this is how I did it back in 1975, when I was drafted into speaker design). It was even more tedious and error-prone on a slide-rule, which is how I justified the purchase of a HP-15C calculator (still works perfectly, which is more than I can say for the HP 2025dn color printer sitting a few feet away).
 
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Quick check here: 1.) Can you find your resonance with the woofer mounted on the horn with the back chamber open?

2.)With everything emptied out of the back chamber but the woofer, can you find the resonance with the back chamber closed?

3.) Slowly start filling the back chamber to reduce its volume. Take your time and work your way to your goal resonance. It's possible that you might have over dampened the rear chamber and squished the resonance some how. You got me scratching my head.
 
When I built some 78 hertz conical horns, I was getting some large numbers for the back chamber. I talked to Eric Stevens (formerly) at Image Dynamics (car horn company who then shared a building with Dr. Edgar), and he recommended on making it small...almost too small to fit the 15. Turned out he was right, the tiny back chamber was the way to go after doing impedance sweeps.

I wouldn't be discounting what JLH is/has been telling you.
 
Hey Mark,

About the WT3 it actually works great even if you prefer to do it the other way. I have measured a JBL 24440 either with or without backchamber and Altec 414 in free air. Works great. But actually there is no need for this gadget.

Was thinking about your foam and if that could be the problem. Maybe you should remove it and do like Edgar and use small waterfilled polyethene-bottles(glass?) to see if it works better.
 
Hey Kris
You're a very aggressive young man, aren't you ?
I've never spoken to you before and you decide you can tell me to "stop f'ing around" .
The measurement method is quite capable of showing the peak if it's there .
I do actually work as a scientist (optical designer) in my job and am quite familiar with both the theory of this and the practice of operating test equipment & taking measurements . So don't speak down to me as if I'm some kind of numpty .
It I use WT3, as far as I can see, it's just going to automate the test and draw me a curve.
It doesn't help to solve this specific problem which is one of the not being able to pick up a peak .
(Also I will be £150 worse-off )


MJ

It does a whole lot more than just draw you a curve. But, to each their own method...
 
For about the cost of 4'x8' sheet of plywood anyone can join. Quote
"Your AES membership payment expires in August 2011. A one year renewal is $99 for an online membership or $149 if you also wish to receive the printed version of the AES Journal."

If downloading the classical papers for $5 instead of $20 you soon have gotten the membership fee for free. There are lots and lots of papers about horns and other loudspeakers.
 
I’ll renege on my non-reply statement this one time only. There is no opportunity for some new miraculous formula for back chamber volumes. While Leach’s formula is mathematically correct for its intended purpose, it cannot address all the other variables that would otherwise alter the real world results. In addition, it does not apply to the tractrix, but more suitably to the hyperbolic-exponential horn. I have a fifth order polynomial that when solved simultaneously with 6 horn variables predicted the tractrix back chamber volume within a +/- 12% error. However, it was very limited because it fell apart once the horn was outside a 105Hz to 160Hz flare rate. It also fell apart if the room volume or boundaries changed. I derived it from the multiple tractrix horns I built and tested in this size range. While it was somewhat interesting, for all practical purposes it was useless and a tremendous waste of time. The type of detail and the number of variables required to address all horns would be so complex it would be too monotonous to use.

That in itself would, in my opinion, make for an interesting article. Showing that Leach's equations do not work in the real world, and why they don't work, is in itself a valuable contribution.
 
Quick check here: 1.) Can you find your resonance with the woofer mounted on the horn with the back chamber open?

2.)With everything emptied out of the back chamber but the woofer, can you find the resonance with the back chamber closed?

3.) Slowly start filling the back chamber to reduce its volume. Take your time and work your way to your goal resonance. It's possible that you might have over dampened the rear chamber and squished the resonance some how. You got me scratching my head.

Step No.1 is fine, I've measured a good peak at 80Hz.
Step No.2 is where it falls down - cannot get a fix on any sort of peak once a back-chamber is fitted. Started with a back-chamber of 5.2L ( 4L with the driver inside ) & couldn't measure anything . Went down to around 1.5L ( maybe ) from there & found nothing. Added another 2 litres with an extension , to give me about 6L free ; no peak found ; then filled it with foam down to ? 1L , 1.25L maybe - nothing .
Hence I can't move on to step 3.

Lars's advice is good re. the type of stuffing, but I need to understand first where the peak went , when I fitted the empty 4L chamber .

I will say that I've listened to it with some of these different volumes on there, and I'm not getting anything I'd call HiFi yet !

I'm taking a long weekend from Friday and will try to go through some of this again slowly & double-check everything . There must be an answer .....
 
Your Step No.1 result tells me you have a good seal between the woofer and horn throat. I can tell this because all the 8PE21 I've measure have a free air resonance between 94Hz and 98Hz. You get 80Hz because the air mass in the horn body is loading the front cone and lowering resonance.

Your Step No.2 completely perplexes me. I've never seen this ever. Something has to be wrong. Not to insult your intelligence in any way, but do you have a local friend that can give your setup a second look. I have missed things before, so it can happen to anyone. Sometimes a second pair of eyes can uncover something we've missed.

Step No.3 - Last resort - Plane tickets for JLH :D
 
SWAG

Your Step No.1 result tells me you have a good seal between the woofer and horn throat. I can tell this because all the 8PE21 I've measure have a free air resonance between 94Hz and 98Hz. You get 80Hz because the air mass in the horn body is loading the front cone and lowering resonance.

Your Step No.2 completely perplexes me. I've never seen this ever. Something has to be wrong. Not to insult your intelligence in any way, but do you have a local friend that can give your setup a second look. I have missed things before, so it can happen to anyone. Sometimes a second pair of eyes can uncover something we've missed.

Step No.3 - Last resort - Plane tickets for JLH :D

When the back is closed, air column resonance shifts to be out of phase with that of the driver. This condition would kill the resonance peak you are looking for in step 2 and 3 as well.

Regards,
WHG
 
Hi Lynn
Thanks for that idea . I'll see if I can coax some life into the oscilloscope , it's an old one I got cheap from work, needs a good warm-up! I'll need to borrow a second scope lead too .
So it's just a small series resistor then - eg. 1 ohm ?
Cheers
Mark

If it's an old scope, make sure both channels match, and you're using DC coupling for both channels. You can test this by feeding both channels the same signal directly from the amplifier (in XY mode) and seeing if the trace is a single very narrow line with no opening at any frequency. If the line opens up into an ellipse, the scope is no good, sorry. Repairing old scopes is not worth the effort, considering how inexpensive modern Asian scopes are. If the line is nice and straight at all frequencies, then you're good to go.

A 1-ohm series resistor is fine (put it in series with the "ground" side of the speaker cable), attach the scope's probes across the resistor, attach the other channel directly to the amplifier output, and adjust channel gains so you get a line at a 45-degree slope. Sweep from a low frequency up through a few hundred Hz and write down the exact frequencies where the ellipse closes up into a straight line. Those are the tops of the peaks and bottoms of the troughs of the impedance curve.

After you measure a speaker - multiway or full-range, doesn't matter - it's a little sobering to realize that the number of frequencies where the speaker is truly resistive can be counted with the fingers on one hand. Everywhere else, it's a mixture of resistive and reactive, reflecting some of the power back to the amplifier.
 
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When the back is closed, air column resonance shifts to be out of phase with that of the driver. This condition would kill the resonance peak you are looking for in step 2 and 3 as well.

Regards,
WHG

That's an interesting idea. Is that applicable to the wavelengths we are trying to measure at? Would it have a wide enough Q to mask the entire range from 80Hz to 120Hz? While it has not been stated, I'm guessing the back chamber heigth can't be much more than 6 inches. I've never ran into this phenomena before. It just seems very strange to me.
 
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If I remember rightly, Dinsdale said something about the throat reactance being essentially capacitive, and a closed box inductive, and therefore a fortunate match. Not that I've studied this.

JLH, you've mentioned the rear chamber length before. Are you thinking of anything other than simply making sure it is not a significant portion of a wavelength over the horn's bandwidth?
 
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