Impedance Measurements in ARTA

Feb 20, 2013
1,058
21
38
Richmond, VA
#1
Hi, all. I finally made a little impedance measurement setup in LIMP today, and I'm getting some surprising results. First, when I measured one of the B&C 12NDL76 drivers I had from the DIY Mid Hi project, it showed an Fs of 58Hz, which is markedly higher than spec. I went on to double check the Re value, and that's significantly off, too - 4.8 Ohms instead of the specified 5.3 Ohms. Are these measurements typical deviations from spec, or should I look at my setup for potential issues?

Thanks.
 
Jan 11, 2011
803
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Florida
#2
I have a spec sheet for the B&C 12NDL76 that shows a Re of 5.1, 4.8 still would be off by quite a bit, have you checked your ohm meter for accuracy against others?

Fs of 58 vs 50 is way off, is the driver broken in, and supported vertically away from surfaces?
 
Feb 20, 2013
1,058
21
38
Richmond, VA
#5
I in fact have two of these drivers, both of which measure within an ohm of each other. I went into my university’s lab and finally got a reasonable seeming result by using a voltage divider to determine the resistance of the circuit. In all honesty, even the test bench DMM did a poor job of measuring driver and resistor resistances. Not really sure as to why. At any rate, I found them both to be hovering around 5.1 ohms, which is closer to reality, I’d hope. This was done at about half a volt for the entire circuit. Similar results were found at 1 volt.

I still need to figure out what is wrong with my free air impedance measurements.
 
Jan 11, 2011
803
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Florida
#7
In all honesty, even the test bench DMM did a poor job of measuring driver and resistor resistances. Not really sure as to why. At any rate, I found them both to be hovering around 5.1 ohms, which is closer to reality, I’d hope. This was done at about half a volt for the entire circuit. Similar results were found at 1 volt.

I still need to figure out what is wrong with my free air impedance measurements.
Max,

Driver build specifications can "evolve", as I mentioned, the spec sheet for the B&C 12NDL76 shows a Re of 5.1, exactly what you measured. Small changes in resistance in test leads and connections can make testing low resistance values frustrating- you'd not think much about an ohm or three measuring 600 ohms, but even a quarter ohm seems like a lot measuring around a 5.1 Re.

The 2 Hz or so difference you measured in the resonant frequency between the two drivers may be within typical tolerances. However, being around 8 Hz high seems fishy- try a simple "reality check", and measure the Fs the "old fashioned way".

To measure a driver’s resonant frequency, connect the “+” output of a frequency generator/amplifier output directly to the driver “+”, place a 10 ohm (or so) resistor in series with the “-“ to the driver “-“ . Connect an AC voltmeter across the resistor leads. Adjust the frequency up and down until the voltage across the resistor reaches it’s minimum. The voltage minima is the resonant frequency.

I just performed that test on a brand new B&C 8NDL64-8 and measured the Fs at 87 Hz, 7 Hz higher than the spec sheet rating of 80 Hz.
Hmm, sound familiar?

I then “broke in” the speaker by pushing the cone in and out by hand to around Xlim (as far as the suspension would allow) several times, then repeated the test, the Fs dropped to 78 Hz !

I had used around 4V for the two initial tests, after a few minutes decided to try 1V (”small signal”) and read around 82 Hz, 4 Hz higher..
Bringing the voltage back up to 4V, the Fs dropped to 80Hz. The suspension probably is not fully broken in, and these stout little cones need a bit of juice to move.

At any rate, your 12NDL76 have an Xmax of 6.5MM, Xlim is likely around 13MM.
To “break in” the speaker without days of ridiculously loud use requires the suspension to be pushed a bit past Xmax, over 1/2” peak to peak. At a very low frequency, like 20 Hz, very little voltage is required to reach that excursion, far less than the speaker can handle safely.
By hand, you should be able to safely push those cones to around 1” peak to peak, if you have not, I suspect your speakers will read a lower Fs after.

If after "really" breaking in the speakers your Fs readings the "old fashioned way" don't agree with your new test gear, something's wrong with the new tester.

If that's the case, other than checking it's series resistor is within spec, no more suggestions ;^) ...

Art
 
Last edited:
Feb 20, 2013
1,058
21
38
Richmond, VA
#8
After “exercising” the suspension a bit, I coerced the Fs to drop to a more reasonable 55hz, and then 53Hz after some more reps. Good to know my setup is reasonable and I’m not going absolutely insane. I also checked it with a ported cabinet of known tuning, and that came out right where it should have.

Still reading as 4.5-4.8 ohms on nearly every ohm meter I try. I’m thinking what might be happening is that the spec sheet has some VC heating involved, but to be honest that seems disingenuous of B&C not to state clearly, so I’m still looking for answers there. Either way, the impedance trace is more important for everything, so I won’t be too worried about it.
 
Jan 11, 2011
803
7
18
Florida
#9
Voice coil heating would raise impedance, not lower it.
At any rate, we now know that "break in" makes a large difference in Fs on suspensions designed for high power use.
At 53 Hz vs 50 Hz, still have a ways to go, but a lot closer than 58!
 
#10
Still reading as 4.5-4.8 ohms on nearly every ohm meter I try. I’m thinking what might be happening is that the spec sheet has some VC heating involved, but to be honest that seems disingenuous of B&C not to state clearly, so I’m still looking for answers there. Either way, the impedance trace is more important for everything, so I won’t be too worried about it.
Well, the B&C website (& the pdf specsheet) do clearly state "Thiele-Small parameters are measured after a high level 20 Hz sine wave preconditioning test", so I'm not sure how disingenuous that could be?

Voice coil heating would raise impedance, not lower it.
At any rate, we now know that "break in" makes a large difference in Fs on suspensions designed for high power use.
At 53 Hz vs 50 Hz, still have a ways to go, but a lot closer than 58!
That's exactly Max's point - Max is measuring a lower Re than the spec sheet, so Max's power voicecoil heating theory may not be that far off the mark at all.

HTH,
David.
 
Jan 11, 2011
803
7
18
Florida
#11
David,

Both good points.
Of interest, the 12NDL76 spec sheet I downloaded 1/27/13 does not state "Thiele-Small parameters are measured after a high level 20 Hz sine wave preconditioning test", though many others do, including the B&C 8NDL64-8 I recently tested.
For what it's worth, that driver tests cold at 5.6 ohms, spec sheet says Re of 5.4 ohms.

Art
 

Bennett Prescott

Just This Guy, You Know?
Staff member
Jan 11, 2011
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Wallingford, CT
www.bennettprescott.com
#13
Hey Max,

Your Rdc values are within production tolerances, as is Fs. Our tolerances for the 12NDL76 are Fs: +/- 15% and Re: +/-10%. As you’ve seen, exercising the suspension is important as our spec sheets are designed to be “in use” numbers rather than factory fresh.

I’ll ask what the excitation voltage is for these, my brain says it’s 10V RMS but I just landed at LAX so can’t double check.
 

Bennett Prescott

Just This Guy, You Know?
Staff member
Jan 11, 2011
10,842
7
38
34
Wallingford, CT
www.bennettprescott.com
#14
I’ll ask what the excitation voltage is for these, my brain says it’s 10V RMS but I just landed at LAX so can’t double check.
Here's the news straight from the horse's mouth:
The preconditioning used before the measurement of the woofer T/S parameters is actually done by the Klippel LSI analysis which is performed to determine the Xvar Value. The voltage used is variable and depends on the excursion characteristics of the woofer under test, so it's not possible to use a fixed voltage good for everything. The LSI analysis lasts 1 hour.

So our suggestion is set up a voltage that allow to reach the Xvar excursion at a frequency equal to Fs, then leave the speaker working this way for an hour.

Please note that T/S parameters must be measured after an adequate rest of the speaker, at least 24 hours.