Impedance testing

[Dave Non-Zero]

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

Interesting! Still think you need to swap drivers. And I hope someone with more knowledge confirms your hypothesis, but you may well be right about a dodgy driver.

[Gregory East]

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.

P-Audio huh, no surprise after Bill giving them the raspberry.

If one wanted to sit with meter and beer could one plot it manually?

[AntonZ]

Dave, you are probably right. But I'm a bit reluctant to take out drivers, since they are mounted with wood screws. I prefer to not touch the mounting screws in the good cabs. We will see what the more knowledgable have to say on the assumed relation between impedance peaks and fs. If the driver is the cause, then I'm in some trouble. Neo prices have risen sharply, and this particular P-Audio driver does not seem to be widely available anymore either. And I do not know if I should mix 3 P-Audio loaded cabs and one Deltalite.

P-Audio huh, no surprise after Bill giving them the raspberry.

If one wanted to sit with meter and beer could one plot it manually?

Yes, I know about the P-Audio reputation. The other cabs are running fine though, and I'm not sure if this particular driver was a lemon or has suffered too much power somewhere during it's lifetime.

It can be done by hand, if you have patience and enough beer that is. It takes a signal generator and at least one but preferably two multimeters. Take measurements of voltage and current at different frequencies, use these to calculate impedance at each of those frequencies. It is how I originally intended to do this. Then Bill said:

Getting a useful impedance plot requires at least 100 separate points on the chart, otherwise it's very easy to miss something. Taking a sweep with a WT3 takes about 5 seconds.

I decided to do some web surfing and found ARTA, which comes with a separate program LIMP that can do this for you with nothing but a computer with a sound card and a handful of dirt cheap components. At that point:

So tonight I took a few scrap components, threw together a Q&D jig, and took impedance plots of 6 cabs all in one evening and it cost me nothing but a little time.

[Dave Non-Zero]

Dave, you are probably right. But I'm a bit reluctant to take out drivers, since they are mounted with wood screws.

I know tops are a bit different, but I just can't bring myself to trust screws. Plus I have resigned myself to the fact that one day, my neo drivers may blow, and I'll have to replace with something heavier. :/

I borrowed 4 t48 from a friend the other week and 2 of them blew during the night, prob from airleaks. I suspect the screws and/or gaskets.
Tnuts for me...

[Dave Non-Zero]

Ps, I mix deltalites and delta pro cabs all the time. I'm sure it wouldn't be much of an issue.

[aackthpt]

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?

It appears you are correct. It turns out there's a wiki entry on this very subject. In determining the resonance frequency, it's probably more accurate to look at the point the phase plot crosses 0 rather than the impedance peak... not that high accuracy is probably needed for the task at hand. The +/- phase swing is actually the feature of this that I thought to look for first, though I don't really know why; the answer is probably buried in the back of my brain somewhere from that controls course 15 years ago.

This frequency is known as the "free-space resonance" of the speaker and is designated by Fs. At this frequency, since the voice coil is vibrating with the maximum peak-to-peak amplitude and velocity, the back-emf generated by coil motion in a magnetic field is also at its maximum. This causes the effective electrical impedance of the speaker to be at its maximum at Fs, shown as Zmax in the graph. For frequencies just below resonance, the impedance rises rapidly as the frequency approaches Fs and is inductive in nature.

At resonance, the impedance is purely resistive and beyond it—as the impedance drops—it behaves capacitively. The impedance reaches a minimum value (Zmin) at some frequency where the behaviour is fairly (but not perfectly) resistive over some range. A speaker's rated or nominal impedance (Znom) is derived from this Zmin value (see below).

Since you said your other plots look close-ish to ones you saw from elsewhere, your conclusion that you have a bad driver seems quite plausible.

[AntonZ]

I may be able to purchase some Deltalite 12 drivers, supposedly from an Avatar cab. Can these be used for OT12, or are there significant differences with the off-the-shelf version?

[Bill Fitzmaurice]

are there significant differences with the off-the-shelf version?

They are different, appropriate for use in a 2-way cab with a midrange driver, even though Avatar doesn't use a midrange. OK in an OTop 12 only with the 1.2kHz compression driver option.

[AntonZ]

The price was very good, but going compression drivers is not an option for me. I'd have to change 4 cabs. And it seems these Avatars are 4 ohm. No good for a larger number of cabs (would be for an 8 ohm OT212). Thanks Bill.