Impedance testing

[Bill Fitzmaurice]

I don't think I have ever made anything simpler than this. A bit of wire, a few connectors and a resistor. I threw it together in a rather haphazzardish way, so I'm not going to make any pictures of my sloppy solder work and wires hanging loose. Instead, here's the schematic.

Schematic.gif

The program that I used is called LIMP. It comes with Arta (clickable).
Personal license is €79, commercial license €149. Screenshots were made from the evaluation version.

I think the same thing can be done with another program called "Speaker workshop" which is free. I haven't tried that one.

Basically it more or less does the same thing that WT3 does, WT3 plugs into the USB port on one end, the driver/speaker on the other. WT3 also measures driver parameters, inductors, and box Q.

[Greg Plouvier]

The program that I used is called LIMP. It comes with Arta (clickable).
Personal license is €79, commercial license €149. Screenshots were made from the evaluation version.

I just bought ARTA and it's great feature set for the money. $119.27 USD. I hope to be able to do some polar plots of the tops soon.

[AntonZ]

I'm being curious here Greg. Did you happen to buy Arta just before this thread, or did this thread inspire your purchase?

[AntonZ]

I would think there is something not quite right with the number 4 cab. Could this be a leak of some sorts? Can something like a sloppy driver gasket show up like this in the impedance plots?

Yes. The reduced and shifted lower peak indicates the cab is tuned too high, and either a leak or an over-sized port will do that. But as the upper peak is also shifted higher a driver gasket leak is the most likely source.

Today I have removed the driver on the faulty #4 Omnitop 12, filled the holes, evened out some more irregularities on the surface for the driver gasket and sanded it smooth. I have also added some caulking to where the baffle meets the baffle braces. Just one other location that could theoretically have a leak. A pain to get to that location, now that I had it open it was a 5 min job to do the caulking. Then reinstalled the driver with fresh screws. Slightly larger screws, as the originals felt a bit tiny for a 12" driver. The effect on the impedance plot was not what I had expected: hardly any change. The cab construction is identical to its' three siblings, no bigger ports.

Other things I could do? I may try a line of caulk around the driver perimeter, if that doesn't help then play subsonic tones and listen for leaks

[DJPhatman]

play subsonic tones and listen for leaks

+1 This should have been the first order of business. Diagnose, then fix, not the other way around.

[AntonZ]

You may be right there, although from the first impedance plot the gasket leak was diagnosed as very likely (either that or ports too large - and the latter can be ruled out: identical ports on all 4 cabs).

Oh well, back to the man cave it is
Not before next weekend, maybe later.

[el_ingeniero]

It might be the driver. Swap drivers with a known good cab.

[Dave Non-Zero]

It might be the driver. Swap drivers with a known good cab.

It might be the driver. Swap drivers with a known good cab.

+1

and/or put dodgy driver in different cab.

[aackthpt]

You can do this with REW (free) also using the same sound card jig mentioned. I just built a similar jig, though I used a 50 ohm sense resistor as it sounded to me like it's more sensitive that way per the article (linked later). The basic schematic is in REW help, but there's a bit more extensive writeup here. Haven't run a test with it but will very shortly.

[Dave Non-Zero]

Even cooler!

[AntonZ]

The first incarnation of my impedance testing jig was ugly. Just some loose wires, connectors, a resistor, all hanging from the back of a pc's audio jacks. Something dropped off my desk and tore the whole thing apart. By now I am convinced by the usefulness of these measurements. So I made a better version that will hopefully last me a long time.

Front drilled, connectors and switch installed.

JigFront.JPG

All wired up. This is all there is to it, even I can do this

JigWired.JPG

Here is the finished jig.

JigFinished.JPG

I intend to take plots of all my cabs over time. If somethings goes wrong with any of them, I can use the existing plots as a reference of a good cab.

[Dave Non-Zero]

That looks great.

Did you swap drivers and find the weak link?

[AntonZ]

(hang on there, Dave - I'm still typing )

With the new jig ready, I decided to take plots of a few drivers. No cabs, just drivers free air. Here are some popular drivers:

TangBand W8 740E (fs = 32Hz)

Tangband W8 740E free air.GIF

HiVi B3S (fs = 80Hz)
(this is a nice and good looking 3" driver that can be used for small versions of SLA and SLA-curved (see pics on Leland's website http://www.speakerhardware.com ) )

HiVi B3S free air.GIF

Eminence Alpha 8A (fs = 73Hz)

Eminence Alpha 8A free air.GIF

Eminence DeltaLite II 2510 (fs = 53Hz)

Eminence DeltaLite II 2510 free air.GIF

Eminence KappaLite 3012LF (fs = 37Hz)

Eminence KappaLite 3012LF free air.GIF

[AntonZ]

From most of these drivers, impedance plots can be found online. My measurements pretty much match the online plots. Good

Note from the plots above that the peak in the impedance plots happens more or less at fs (resonance frequency) from the driver specs. The little HiVi driver is a bit off, but my guess from the above is that the impedance peak should match fs. Can anyone confirm this?

[AntonZ]

Now it gets interesting. I did not get around to swap drivers on the problematic OmniTop 12 cab just yet. But a few weeks ago I opened the cab. I had the driver out to inspect the gasket, the mounting surface and renew screws. While the driver was out, I took a free air impedance plot, hoping I would be able to compare to plots online. Alas, no impedance plot to be found online of the P-Audio SN12MB driver. Here is my free air plot:

PAudio_SN12MB_Omnitop12_nr4 free air.GIF

Then I accidentally destroyed the first impedance jig. Fast forward to this weekend and the new jig. Note the location of the impedance peak, at roughly 130Hz. The P-Audio specs state fs at 62Hz. I think the issue with my #4 OmniTop12 cab is not tied to a leak in the cab or gasket, but rather this particular driver is not OK.