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Multi-Way Conventional loudspeakers with crossovers

Are these drivers defective?
Are these drivers defective?
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Old 3rd July 2017, 10:04 AM   #1
tcpip is offline tcpip  India
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Default Are these drivers defective?

Hi all,

I got a pair of 6.5" midbass drivers made locally, from a manufacturer which is not exactly famous for high quality. I am trying to build a pair of small speakers with budget drivers, to see what sound quality I can get.

When I do the impedance measurement in free air (I'm using ARTA LIMP), I get the following curves:
Are these drivers defective?

Are these drivers defective?


The impedance below 50Hz is strange, to say the least. I don't remember seeing this shape in any other driver. As you can see, the two drivers are quite close to each other -- piece-to-piece consistency is pretty good. The Fs in the graph is about 54Hz, which is close to the published 49Hz figure.

ARTA LIMP goes bonkers when I ask it to calculate T/S parameters -- it seems to go into an infinite loop. I'm talking to Ivo, the creator of ARTA about it. If I reduce the frequency range from 20-20K to 35-20K, LIMP works well and I get the T/S parameters (sans Vas). But this may not be an adequate solution, since I'll need to calculate the Vas too, and for that, I need to get two graphs, and I don't know how LIMP will behave when I have two graphs both looking weird like this.

My question is not about ARTA. My question is: are these drivers "bad" or defective in design in any way? It's unlikely that they're defective in construction, since both have almost identical curves. It must be a defective design if at all.

Any ideas? Should I scrap these drivers and go for other models?

(I've just started using LIMP from the ARTA suite, but already I like it a lot, compared to Speaker Workshop. )

Last edited by tcpip; 3rd July 2017 at 10:10 AM.
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Old 3rd July 2017, 11:49 AM   #2
goldyrathore is offline goldyrathore  India
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It looks like you have trouble measuring below 30hz. I doubt the measurement setup. Probably there is some high pass filtering in the setup that is limiting the lows.

I have measured drivers using LIMP and Xonar DX/D2X and never ran into 'infinite loop' kind of problems. I ve often had trouble measuring the lows because the impedance curve kept fluctuating near 20hz probably due to acoustic noise.
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Old 3rd July 2017, 12:44 PM   #3
vacuphile is offline vacuphile  Netherlands
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Impulse time alignment? Just guessing, but I can't think of any mechanism in a driver that would create this effect. It must be in the measurements. Another indication is that phase does not behave as one would expect from an LTI-system in the sub-50Hz region.
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Old 3rd July 2017, 12:45 PM   #4
tcpip is offline tcpip  India
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Quote:
Originally Posted by goldyrathore View Post
It looks like you have trouble measuring below 30hz. I doubt the measurement setup. Probably there is some high pass filtering in the setup that is limiting the lows.
Thanks a lot. I'll try this now.
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Old 3rd July 2017, 01:38 PM   #5
system7 is offline system7  United Kingdom
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Measuring the resistance with a multimeter will give a more sensible 4 ohm reading, I'm quite sure. There must be a capacitor on the input of the ARTA software measurement.
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Old 3rd July 2017, 02:53 PM   #6
DUG is offline DUG  Canada
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Try a sweep with a 20 ohm resistor.
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Old 3rd July 2017, 03:05 PM   #7
tcpip is offline tcpip  India
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Quote:
Originally Posted by goldyrathore View Post
It looks like you have trouble measuring below 30hz. I doubt the measurement setup. Probably there is some high pass filtering in the setup that is limiting the lows.
I've never set the measurement range to start below 20Hz. I'll try that.

Quote:
I have measured drivers using LIMP and Xonar DX/D2X and never ran into 'infinite loop' kind of problems.
Just to point out a matter of detail: I don't get an infinite loop when I measure. I get it when I ask the software to calculate T/S parameters. And this is reproduced by Ivo too, using my .LIM file.
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Old 3rd July 2017, 03:13 PM   #8
tcpip is offline tcpip  India
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Quote:
Originally Posted by system7 View Post
Measuring the resistance with a multimeter will give a more sensible 4 ohm reading, I'm quite sure. There must be a capacitor on the input of the ARTA software measurement.
See this measurement, of a different driver:
Are these drivers defective?

As you can see, the portion below the resonant peak goes down to "normal" levels, using the same measurement rig. The one below is that of a tweeter:
Are these drivers defective?


I've measured quite a few other drivers, and their impedance curves all look "normal". I've also measured a resistor and capacitor, and gotten exactly the curves I'd expect to see. I can do it again if you want.

The only thing I've not done till now is extend the low end of the frequency range below 20Hz. I'll do that too.
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Old 3rd July 2017, 05:43 PM   #9
tcpip is offline tcpip  India
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Default PROBLEM SOLVED!

Thanks to all of your inputs, and to Ivo's patient guidance over email, the problem has been solved. It was the USB audio interface.

I first did a measurement of a 27 Ohm power resistor using the hardware and software I was using till now, just increasing the frequency range to 5Hz. This is what I got.
Are these drivers defective?


From the graph, it appeared that there was a capacitor in series with the load somewhere and at low frequencies, it was increasing the impedance. At this point, I guessed that this was not my laptop or software, it had to be the audio interface. So I kept aside the Behringer UCA202 I was using earlier, and fished out my earlier USB audio interface, a venerable 12-year-old Creative SoundBlaster Digital Music, bought in perhaps 2004 or so. I had retired it after buying the UCA202. After searching for and downloading modern device drivers for it, and after fiddling with controls of various kinds, and rebooting a few times, I got the impedance jig to work with the same 27 Ohm resistor:
Are these drivers defective?

Here, if anything, the series capacitor effect has been replaced with a series inductor -- the impedance actually drops below 20Hz. I was thrilled -- I could now try the drivers with the new interface.

So, here's the 5.25" driver I had used earlier as a test, now with the Creative SB interface:
Are these drivers defective?


And finally, the driver whose graph had started it all: the 6.5" driver:
Are these drivers defective?


Thanks to all of you, I think the problem is licked. I am getting fairly consistent readings when switching between FFT and stepped-sine methods. All is now well.

Last edited by tcpip; 3rd July 2017 at 05:49 PM.
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Old 4th July 2017, 07:30 AM   #10
TBTL is offline TBTL  Germany
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Great to have it solved, though it looks like there still is an error below ~40 Hz. If you calibrate your setup (if I remember correctly that means measuring a resistor with a known value) you correct for this error.
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