Impedance Plots (POST THEM HERE FOR DISCUSSION)

[Triticum Agricolam]

42 minutes ago, Kyblack76 said:

Ugh. My best judgement... hmm.. 

 

Full tilt it is ... 

Lol, I'd expect nothing less :-)

[WalledSonic]

Sure would be great to see some 4th or 6th order impedance plots.  I'm building a small 4th with a buddy, but wont have it together and tested for another 2 weekends.  

[Triticum Agricolam]

So I may have a less expensive way for people to gather high-power impedance plots.  I just picked up a Uni-T Mini Clamp meter from Amazon, this is it: https://www.amazon.com/Uni-T-B4Q094-UT210E-Current-Capacitance/dp/B00O1Q2HOQ

For $32 its pretty tough to beat.  It will measure DC & AC Amperage.  By measuring the A/C amperage we can find the impedance pretty easily.  I just got done comparing it to my AMM-1 and my Fluke DMM.  So far it seems pretty accurate.

To take a high power impedance measurement, you begin by playing a sine wave tone, frequency isn't important but something in the middle of the range like 50 or 60 Hz would be good.  Then you set your volume at your desired output level and then measure the A/C output voltage.  Write that number down.  You then start taking amperage measurement at different frequencies and write the results down.  For my test I started at 20 hz and worked my way up in 2 hz increments.  It actually goes pretty fast.  Once you have all the measurements its easiest to drop them in a spread sheet.    To calculate impedance you divide the voltage by the amperage.  So if you tested at 20 volts and recorded 10 amps at 32 hz, the impedance at that frequency is 2 ohms.  

The AMM-1 is faster since it shows the impedance directly (no math required) and it can do several things the Uni-T clamp meter cannot, but for $32 the Uni-T should let people get pretty good impedance measurements.  

[WalledSonic]

So I woke up this morning and was thinking about my impedance rise and how my amp just cant put down enough power in the 30-50Hz range (see original post).  I could wire down to quarter ohm, but thats probably a bad idea.  I dont really want to recone the subs, but I do want to run more power to put those subs to work.  It dawned on me that I could just run 2 amps at half ohm.  I was thinking about how there was a very solid trend between my previous 0.2ohm and 0.8ohm plots, however, I dont think I can use the IM-SG to test 2 subs at a time in my center-slot-port enclosure.  Perhaps I can estimate what each amp would see in terms of impedance from my previous data.

At any frequency on the plot, the value at 0.8 ohm was 4 times the value at 0.2 ohm.  So I took all the impedance values at 0.2ohmDC and multiplied them by 2.  Then I took all the impedance values at 0.8ohmDC and divided them by 2.  Then I plotted all the impedance curves:

I believe if I run (2) amps at 0.4ohmDC each, that my impedance rise will be low enough to have more power output throughout my playing range, but be high enough to not pop the amp.  

Thoughts on the estimated plots?

[rocking.that.eclipse]

Subs don't hold to the laws of physics right? JK

This is interesting to see. I would like to see a plot of the spl score at those different frequencies too. I'll try to find the one I did for my friend a while back and post it up

 

 

[rocking.that.eclipse]

So here is what we got with my buddies 6th order for his 2 18s.

I'd like to see this over-laid with the impedance curves to see what they show

 

[WalledSonic]

@rocking.that.eclipse , can you guys measure impedance of that setup?  That would be VERY cool and would really tie some interest into this thread.

[Triticum Agricolam]

I would think your idea of running each amp at .4 ohms should work fine.  

It will be interesting to see how your impedance plots are different now that you can measure them at higher power.  That will be the real test. 

[the_garynator]

I love this thread! I did something similar on some of my old build threads but unfortunately the pictures were on photobucket.  I'll have to dig them up and post them.

 

Anything that increases pressure on the softparts (loading) will increasethe impedance rise.  For example, if you have the subwoofer firing at a window that's really close to it, this will increase pressure on the cone, raise impedance, and decrease excursion.  I have tested this by swapping a sub, box and amp from a hatch vehicle into a trunk and the rise was quite a bit less firing at an undeadened trunk lid instead of a large chunk of glass attached to a heavy hatch frame.

A hypothesis I've come up with based on experiences, but have not fully tested to validate is that it can change at higher power / spl levels. For example, if the object you are loading off of no longer can maintain adequate resistance to the pressure, it can cause the woofers to unload (not completely, just the loss of additional loading) and therefore the rise no longer stays constant and rise should drop as power increases past this point.

I don't recall if I have testing to back it up but I believe loading the port off of an object such as the back of a trunk (having the object closer to the opening of the port than the ports diameter) will increase this.

Increasing the volume of your enclosure will increase rise. A ported box has more rise than a sealed box since the pressures inside a ported enclosure are higher than a sealed box.  Impedance rise will peak at the tuning frequency, as pressure is the highest at that point, and as you all probably know, excursion will be at the lowest.

Something I have not had a chance to do a controlled test with yet but have noticed with different builds is the effects of the flow from the cone into the port and out of it on impedance rise.  I believe the easier time the sound waves are able to enter and exit the port, the lower the pressure the driver sees. For example, if my theory is correct, an up firing, port back enclosure should have more impedance rise than the same net volume enclosure with sub front, port back and a large bell mouth on the port.

These factors are why opening doors on big builds have detrimental consequences in some builds, as well as why bandpass enclosures generally have higher impedance rise, parallel tuned 6th orders having more than the equivalent series tuned and that having more than a 4th and so on.

[Triticum Agricolam]

1 hour ago, the_garynator said:

....A ported box has more rise than a sealed box since the pressures inside a ported enclosure are higher than a sealed box.  Impedance rise will peak at the tuning frequency, as pressure is the highest at that point, and as you all probably know, excursion will be at the lowest....

You have that part a little backwards.  Impedance is at a low point (along with excursion) at the tuning frequency in a ported box.  With a sealed box impedance peaks at the sealed resonance frequency.  

When looking at impedance plots for sealed and ported boxes, sealed boxes have one impedance peak (at the resonant frequency) and ported boxes have two, one above and below tuning.