Impedance Plots (POST THEM HERE FOR DISCUSSION)

[WalledSonic]

The purpose of this thread is to post your measured impedance plots (or values) for discussion.  There are competitors that may not be inclined to share their data, but I still hope some experienced people will chime in.

Its not called box rise, its called impedance rise, and there's a reason for it.  Impedance rise is affected by the final impedance you're wired to, the characteristics of your enclosure, and perhaps some other factors.  So it cannot be completely contributed to the enclosure alone.

This is a really busy subforum so I was apprehensive to start this thread here, but since the primary constituents are subs and enclosures, this is where the discussion belongs.  Hopefully we get enough traffic to get it stickied, since there's a lot to learn from even a single impedance plot.  

[WalledSonic]

I have (4) 15's in a center slot ported flat wall.  12.5 cubes volume, 152 sq in of port area, tuned to 26 Hz.

I used a IM-SG to take the following data points.  Then I threw them in Excel and made it look pretty.

The dark green line is the impedance rise while the subs are wired down to 0.8 ohm (DCR).  The faint green line is the predicted amplifier power output based on the impedance curve.  The red line is the impedance plot when I wired down to 0.2 ohm.

I thought perhaps my 'rise' would stay the same when I wired down.  For instance, at tuning frequency, if I measure 1.1ohm wired down to 0.8ohm, then I had 0.3ohm rise.  If I wired down to 0.2ohm, would that give me 0.5ohm at tuning frequency?  Nope.

It turns out that the characteristic shape of the curve remains when wiring to different final loads, but the amplitude difference changed.  

An interesting thing to point out, the factor by which the measured impedance changed from the DC resistance remained the same.  Example: 

Green line shows 5.1 ohm at 21ish Hz when wired to 0.8ohm.  Thats a factor ( 5.1/0.8 ) of about 6.4.  On the red line at the same frequency, when wired to 0.2ohm, I measured around 1.3ohm.  Thats a factor of 6.5, which is about the same factor as in the 0.8ohm wiring configuration.  That 'rule' holds true throughout the graph.  Whatever the 'rise' is at some frequency at some wiring load, when you wire your final load differently, you can multiply the rise factor at that frequency to your final wired load to get your new rise point at that frequency.  

EDIT: I have no clue what that peak is at 36Hz.  Im guessing some function of the enclosure, such as sharp corners instead of rounds, an affect of bracing, etc. but I am not actually sure.

EDIT EDIT: All values taken with the front two windows down, demo style.  This is my daily so thats how I drive it.

TRIPLE EDIT: I have a Sundown SCV-6000D, which has been dyno'd at 6600W at 1ohm, but I used the assumption that its only putting out 6000W at 1ohm for this power curve.

[Triticum Agricolam]

Thanks a lot for making this thread WalledSonic!  I think impedance plots are a VERY underutilized tool in the car audio world.  There is a LOT of information you can get out them and I think it would be great to see them used more often.  

Your graphs and analysis do a good job of showing that impedance rise is multiplicative and not additive.  

The spike in the impedance you are showing around 36 Hz is most likely caused by a resonance somewhere in your system.  These can be all sorts of different things, it could be a standing wave in your vehicle (which I consider to be the most likely cause in your case due to the low frequency it occurs at).  Spikes in impedance like that are often seen at higher frequencies too and can be caused by a pipe resonance in the port, a loose joint on a panel somewhere, a standing wave in the box, a panel vibrating due to insufficient bracing, etc.  Using an impedance plot to track down these resonances and fixed them can be very useful. 

The next step in your analysis is to take impedance measurement at high(er) power.  This will show things such as port compression and power compression.  Unfortunately you can't take those measurements with your IM-SG, but you can with an AMM-1.  I think there is a way to do it with a resistor and a DMM too, I'll have to look into it.  

 

[Kyblack76]

How or what changes most when more power is applied? Using the imsg we plotted everrrrythang. Even looked at shit venting the cabin. (Door(s) open, each window down just to look at changes for fun and some knowledge. When stepping up power, what are a few changes you would see ? Asking for a friend  lol. 

[Triticum Agricolam]

16 minutes ago, Kyblack76 said:

How or what changes most when more power is applied? Using the imsg we plotted everrrrythang. Even looked at shit venting the cabin. (Door(s) open, each window down just to look at changes for fun and some knowledge. When stepping up power, what are a few changes you would see ? Asking for a friend  lol. 

Power compression (caused by the voice coil/s heating up) will increase the impedance across the board. Port compression will raise the impedance around the tuning frequency. If it's severe enough the impedance plot will stop looking like a typical ported/4th order one with two peaks and begin to look like a sealed box plot with one big peak. 

[Kyblack76]

Interesting... and actually makes sense. Dig it. 

[Kyblack76]

And again, what is "higher power ? Like, 75 percent of what you have on tap? Or plot at full each time?  Is doing several full tilt burps nessacary or is 75-80% cool beans?  Just seems like at full, so many variables come into play. Door jam breaking opening, heat, damage lol. That shit. If ya feel me  

[ShadeTreeMechanic]

Very interesting. It would seem that plotting curves at a few different power levels would be good to learn when different things would start to become a problem. It would maybe help pinpoint what is becoming a problem at each level? To tell if the fix helps at one level and hurts at another?

[Triticum Agricolam]

22 minutes ago, Kyblack76 said:

And again, what is "higher power ? Like, 75 percent of what you have on tap? Or plot at full each time?  Is doing several full tilt burps nessacary or is 75-80% cool beans?  Just seems like at full, so many variables come into play. Door jam breaking opening, heat, damage lol. That shit. If ya feel me  

From an analysis perspective testing at full power would be best. However since playing sine wave tones at max volume has been known to let the magic smoke out of things, some discretion is advised.   You want to be at a high enough power level that whatever bad things are going to happen are happening (compression, vibrations, etc), but not so high you break stuff.  Use your best judgement. 

[Kyblack76]

Ugh. My best judgement... hmm.. 

 

Full tilt it is ...