# Checking MDAT (impedance files)



## grokaudio (Feb 11, 2013)

Whats the process for validating Files generated with REW - just took some impedance measurements (ignoring Rleads) as I couldnt get REW to measure them and want to make sure the system is reading accurately


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## EarlK (Jan 1, 2010)

grokaudio said:


> Whats the process for validating Files generated with REW - just took some impedance measurements (ignoring Rleads) as I couldnt get REW to measure them and want to make sure the system is reading accurately



- Other systems use ( & include ) a resistor of known impedance ( to calibrate the process before making measurements ) .

- One could buy a 1% precision resistor ( say 10 ohms from *Parts Express* ) & then measure it ( obviously, one looks for a resulting trace of @ 10 ohms, + or - a couple of percent ) . 

:sn:


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## grokaudio (Feb 11, 2013)

Thanks 
- I've already got the impedance rig all set up
- I'm using 3x344 Ohm 0.1% 1W resistors in series - yielding 1032 Ohm sense resistor

Using front headphone out and front line in off computer system:
- calibrated sound card with loop-back using same ports
- ignored the SPL message when starting the impedance calibration (will be using a UMIK)
- calibrated Impedance Rig
- DID NOT MEASURE RLeads
- attached Resistor
- attached Leads to driver
- took impedance sweeps of Driver(s) (two woofer two tweeter)

Since I used a larger value resistor, and ignored Rleads - wondering how accurate I can expect measurements to be. 

compared to manufacturer site for drivers they're close, but enough variation to question my process or the manufacturer specs...

Any ideas on steps I should have taken, or validating above was ok?

I've saved the measurement data or can post img if needed.


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## EarlK (Jan 1, 2010)

grokaudio said:


> Thanks
> - I've already got the impedance rig all set up
> - I'm using 3x344 Ohm 0.1% 1W resistors in series - yielding 1032 Ohm sense resistor
> 
> ...


I can't really help you here since I use Woofer Test II for these sort of measurements .

Hopefully someone else will chime in that does this all the time ( along with their tips for success ) .

:sn:


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## JohnM (Apr 11, 2006)

May as well attach the mdat for us to take a look. That is a very high sense resistor, which will result in the soundcard measuring quite low levels (assuming your driver impedances are not in the hundreds of ohms range) and so degrade signal to noise a little. From anecdotal comments it seems it is more common for drivers not to match the manufacturers specs than to match them...


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## grokaudio (Feb 11, 2013)

Thanks John - I'll upload them tonight when at home, as well as the driver links.

I agree with manufacturer specs, these particular drivers were measured with "broken in drivers" (per the site anyway)

While it's a high R value, would leads play less of a role or same role as I didn't get that measured properly

Just trying to figure what I need to tweak in setup before going through break - in and T/S measurements.

I appreciate yours and the forum feedback here as well!!


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## JohnM (Apr 11, 2006)

Generally don't need to worry about the impedance of the leads, assuming you're not using string soaked in brine


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## grokaudio (Feb 11, 2013)

:rofl:

Not as of yet, though it did cross my mind the use them to boost voltage during break in process...


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## grokaudio (Feb 11, 2013)

I've attached the MDAT files - not yet broken in drivers - first pass at impedance measurement:

the a & b in filenames denote separate drivers, and as i said - the manufacture states they did measurements on broken in drivers.

woofers are SB15RNX uncoated versions from madisound - essentailly these: http://www.sbacoustics.com/index.php/products/midwoofers/5-sb15nrxc30-8/ and more specifically here: http://www.madisoundspeakerstore.com/approx-5-woofers/sb-acoustics-sb15nrxc30-8-uc-5-paper-cone-woofer/

tweeters are http://www.sbacoustics.com/index.php/products/tweeters/sb29rdc-c000-4/

Woofer
1032_good_impedance_A: Frequency of 58.2 Hz at 180.7 Ohm
1032_good_impedance_B: Frequency of 56.8 Hz at 185.2 Ohm

Tweeter
1032_good_impedance_A_tweeter: Frequency of 619.3 Hz at 10.8 Ohm
1032_good_impedance_B_tweeter: Frequency 663.5 Hz at 11.3 Ohm

MFR: Woofer: 38 Hz at 77 Ohm-ish
MFR: Tweeter: 600 Hz at 10 Ohm-ish


Thanks in advance for all your support


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## JohnM (Apr 11, 2006)

Woofer measurements look quite a bit different to the specs, don't they! Could try using a lower sense resistor (e.g. just one of the 344R resistors) to see if that alters the impedance levels measured, though it won't make any difference to the resonant frequency. The measurements look pretty clean though.


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## grokaudio (Feb 11, 2013)

They do - which is kind of why I've been scratching my head:

1. Tweeters - They seem relatively consistent to each other - and fairly close to the spec 
2. Woofers - They seem relatively consistent to each other - but not the spec 

Which leads me to (probably false observations) :
a. Manufacturer really does break in the woofers before measuring (though +/-20Hz is hard to imagine)
b. I goofed up by not doing the SPL meter setup (though all measurements would be off in this case)
c. Too high an R value - (this would only be a factor in the impedance peak at FS and Tweeter seems ok)
c. The published specs may not be as accurate within this production build, air pressure, Temp, phase of moon...
d. The published specs may use some sort of smoothing, etc...

I'll see what the woofers do with 344 Ohm Sense - and if different - post observations.

Outside of that - Would anyone have difficulties using these "AS IS" as a first pass for TS measurements?


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## grokaudio (Feb 11, 2013)

Ok - so I have an update - remeasured with the following - (one driver woofer only Driver B)

Sense Resistor Impedance peak Frequency
1032 Ohm 185.2 Ohm 56.8 Hz
344 Ohm 165 Ohm 56.6 Hz
111 Ohm 136.2 Ohm 56.0 Hz
56.2 Ohm 122.5 Ohm 55.3 Hz
34.8 Ohm 115.5 Ohm 54.8 Hz
24.0 Ohm 111.8 Ohm 54.7 Hz
15.0 Ohm 108.6 Ohm 54.0 Hz
3.0 Ohm 103.6 Ohm 54.0 Hz

MFR: 
150 Ohm 77 Ohm-ish 38 Hz (with broken in driver)

All Sense Resistors are Vishay/Dale 1 Watt resistors with 0.1% Tolerances and measurements done one after the other to speed things up...

So from above - and noting my previous concerns about the differences in (impedance peak and resonant Frequency) between my measurements and Manufacturers posted...

Can anyone shed some light on explaining it away and the approach I should take as the impedance data and resulting T/S measurements will be used for crossover development???

FYI - I can post MDAT as requested

onder:


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## JohnM (Apr 11, 2006)

Might want to enquire on the DIY speakers/subs forum. If you go back and use 344 ohms again do you get the same results as when you first did it?

Could set things up to use an amp to drive the unit you are testing in case the headphone output is struggling, but would need resistive dividers for the feedback to the line in (and ideally some zener clamps and care with levels).


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## grokaudio (Feb 11, 2013)

Thanks John,

I have a little 8 watt amp kit (volume pot) - which I figured I would need for the driver break-in anyways.

Can you point me towards the help setup for that?

and If I'm only using for breakin would I still need the resistive dividers for the feedback and some zener clamps as you recommended?

UPDATE:
took two more measurements of same driver as suggested:

ORIGINAL:
Sense Resistor Impedance peak Frequency
344 Ohm 165 Ohm 56.6 Hz

NEW MEASUREMENTS
344 Ohm 163.5 Ohm 56.4 Hz
344 Ohm 172.1 Ohm 56.4 Hz


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## AudiocRaver (Jun 6, 2012)

A question, and beg pardon if I missed this in reading through the thread. Are you still using the headphone output as your driving source, and if so are you taking its source resistance into account? Most headphone outputs have a source resistance of 50 ohms or so.


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## grokaudio (Feb 11, 2013)

AudiocRaver,

Thanks for input - I was not aware or accounting for that - outside of the sound card calibration already done using the headphone output.

Are you indicating that would be responsible for the ZMax (peak impedance) differences I am getting...

And this only affects my 8 Ohm woofer measurements - my 4 Ohm tweeter measurements are close enough to manufacturer specification.

Can you expand on how I could further test....


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## fbov (Aug 28, 2008)

My apologies, replied in wrong thread!
---------------------
There has always been debate over driver break-in, and you're smack dap in the middle of the debate. New LF drivers are the one time when a clear mechanical break-in of cone suspension components will affect driver performance and measured characteristics. May I suggest:
- palpating the cone through it's range of motion, at a minimum. 
- if possible, run at 1/2 rated power at the impedence peak frequency for 24 hours. 

This latter is normally done at rated power, since current draw is minimized at the max impedence, while cone motion is maximized. I suggest 1/2 rated power because there's reason to expect your peak will drop in frequency. 

Conversely, I've seen some drivers that just don't meet their published T/S specs. 
http://www.parts-express.com/pe/showdetl.cfm?partnumber=295-344

Compare the datasheet with the Klippel results under Manuals/Resources. T/S are small signal parameters, Klippel is a large excursion analysis that also gives small signal parameters. You'll see some big differences, but when input to simulators, these differences are minor at best. Since I built with these, I can compare predictions with reality.

Note the bass extension the simulations promise but the speaker fails to deliver... That's all due to a driver that doesn't meet it's specs. (I have a woofer tester III-based T/S measurements, but can't look at the files on this PC.)

Just so you know... it could be driver break-in, it could be a measurement issue, or it could be the driver's off.

HAve fun,
Frank


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## AudiocRaver (Jun 6, 2012)

grokaudio said:


> Are you indicating that would be responsible for the ZMax (peak impedance) differences I am getting...
> 
> Can you expand on how I could further test....


Not sure how much difference it will make, it could account for some of your measurement questions.


```
--Rinternal--> <--
          |                        |
   headphone                  V1
     output                      |
          |                        |
           ------------> <--

           --Rinternal--> <--------
          |                         |       |
   headphone               Rload     V2
     output                       |       |
          |                         |       |
           ------------> <---------
```

Do an open circuit (no load) voltage test with a 1 KHz sine wave, that value is V1.
Do a test with resistive load "Rload" - one of your 344 Ohm resistors would be perfect - with the same 1 KHz sine wave, that value is V2, it will be slightly smaller than V1.
Formula: Rinternal = Rload x ((V1 / V2) - 1)

Once you know Rinternal, you should add it to your Rsense value for your calculations (consider it part of Rsense).

*EDIT: As JohnM points out in post #21, this measurement and calculation, while a valid way to determine output impedance, is not needed for the driver impedance measurements being discussed in this thread.*


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## AudiocRaver (Jun 6, 2012)

There could also be a series coupling capacitor further increasing the source impedance and affecting your woofer measurements. Thinking about how to measure it or negate it with a calibration file.....

*EDIT: As JohnM points out in post #21, this statement has no bearing on the driver impedance measurements being discussed in this thread.*


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## grokaudio (Feb 11, 2013)

AudiocRaver

Excellent - thanks for the clarity in the explanation - I'll try later tonight and post any observations.


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## JohnM (Apr 11, 2006)

In principle the output impedance of the drive stage should have no effect on the measurement as its influence is removed by the loopback connection to the left channel, the measurement process calculates the current flowing through the sense resistor (and the load) by measuring the drop across it, and the right channel gives the voltage across the load so allowing the impedance to be determined. Thermal effects within the output driver tend to be more of a problem.


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## AudiocRaver (Jun 6, 2012)

JohnM said:


> In principle the output impedance of the drive stage should have no effect on the measurement as its influence is removed by the loopback connection to the left channel, the measurement process calculates the current flowing through the sense resistor (and the load) by measuring the drop across it, and the right channel gives the voltage across the load so allowing the impedance to be determined. Thermal effects within the output driver tend to be more of a problem.


Aha! I had not thought through the total setup. I see now precisely what you mean. Thank you for the clarification, John.

The method for determining output impedance in post #18 is still correct, but it has no bearing in the current discussion. I have added a note there so no one will be led astray.

Thanks again, John.


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## grokaudio (Feb 11, 2013)

All,

Thanks for feedback on this....

I've been thinking about this over weekend (ah what a few days in Newport Beach will do) and have some observations and questions:

1. I've observed that as the Sense resistor changes - the ZMax and FS are changing: In otherwords as I increase sense resistor - the measured FS and ZMax goes up

2. I've compared to Manufacturer - the tweeters (4 ohm) are in line - yet the woofers (8 Ohm) are way off.

3. I've also noticed that increasing the output (even with lower sense resistors (344 Ohm) the woofer measurements are much closer to MFR specification.


Questions:
1. Is this more a factor of independent variables when manufacturer measures and i'm not using the same current to driver when measuring?

2. I thought electrical driver parameters were fairly constant - why does FS and ZMax keep changing as I change sense resistors?

3. Am I making a mountain out of mole hill cause when taking T/S measurements - what I really care about is the driver itself its measurements and not the delta I see to the current published spec???

THANKS AGAIN ALL FOR CHECKING THIS THREAD AND PROVIDING FEEDBACK !!!!!!

grok


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## AudiocRaver (Jun 6, 2012)

grokaudio said:


> Is this more a factor of independent variables when manufacturer measures and i'm not using the same current to driver when measuring?


Possibly. Speakers are not perfectly linear devices.



grokaudio said:


> I thought electrical driver parameters were fairly constant - why does FS and ZMax keep changing as I change sense resistors?


See below.



grokaudio said:


> Am I making a mountain out of mole hill


Maybe.



grokaudio said:


> cause when taking T/S measurements - what I really care about is the driver itself its measurements and not the delta I see to the current published spec???


Yes, but you should get repeatable results over SOME range of Rsense values. Try using a lower Rsense value that gets the measurement current closer to actual operating range, like 50 ohms or below.


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## grokaudio (Feb 11, 2013)

Thanks again.

Maybe John could answer on this:

Does REW use Current Source or Voltage Source to measure impedance?


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## JohnM (Apr 11, 2006)

grokaudio said:


> Does REW use Current Source or Voltage Source to measure impedance?


No 

The sound card is a voltage source, behind its output impedance and the sense resistor. Those impedances are typically large compared to the load, so the load is driven in a more current source like than voltage source like way, but neither is 'used' to measure the impedance as such. The soundcard connections allow REW to measure the voltage across the load (right channel signal) and the voltage across the sense resistor (left channel signal minus right channel signal). The voltage across the sense resistor divided by the value entered for the sense resistor gives the current through the sense resistor. Assuming the soundcard input impedance (typ 10k to 100k) is much, much higher than the load impedance, the current through the sense resistor can be treated as flowing entirely through the load, so the load impedance is found from the voltage across it divided by the current flowing through the sense resistor.


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## grokaudio (Feb 11, 2013)

Latest update...

got myself a new sense resistor - 1/8 Watt 75 Ohm 0.1% tolerance

New measurements:

Fs = 50.82
Zmax = 86.0
6.28 Ohm at 292.4 Hz

(tons closer to published spec - and reasonable enough to believe that with some breakin on drivers - I'll be in the ballpark.

MDAT attached


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## JohnM (Apr 11, 2006)

That's encouraging. One thing did occur to me, if the 1W resistors you were using previously were wirewound they would need to be non-inductively wound, otherwise the basic assumption underpinning the measurement (that the sense resistor is purely resistive) would be invalid.


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## grokaudio (Feb 11, 2013)

All,

Just a quick update - I've finally gotten around to it.... Broke in the woofers with a small little 8 Watt amp I built over the weekend (currently using a 12 V power supply with it - can increase to 18V)


I've posted some initial results:


Broke in gradually
Increased the voltage hitting driver about every 15 minutes to avoid clipping
Figure with an 8 Ohm load it didn't get much past 3.5 to 4 Watts or so
Ran it with no crossover or low/high pass filters
used a 40 HZ wav file off the net...

When I first used the sine wave generator - it kept giving clicks at the start/end of looping. Running the 40 Hz Wav through a standard player (rhythmbox) on continual loop avoided it.

so I'm at about 43 HZ - with published spec at 38... I'm going to hit it a bit more through the next week or so, but by the end of this - the last few measurements showed little change.

Any idea whether more time breaking in/more watts while breaking in/ or sense resistor could be throwing this off a bit.

Generally I'm pleased - just want to ping others for feedback.


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