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Discussion Starter #1
I'd just like to share my impedance measurement results from my Maelstrom that I will be putting in a box soon. I've given it about 24 hours of break-in. Measurements were performed with speaker workshop, a Wallin Jig v2, and EMU 0404 usb.

It is one of the recent single-coil 4ohm versions

Fs: 17.8 Hz
Re: 3.43 Ohms
Le: 3.92 mH
Qms: 5.786
Qes: 0.454
Qts: 0.42

When these parameters were modeled (using the spec'd Vas and Bl values) the results were practically identical to the model with standard values. This speaks a lot about the quality and consistency over different runs and coil configurations, bravo Kevin.

My unknowns are still Bl, Vas, Cms, and Mms. If I can get just one of them the others can be calculated. I wonder which should be the closest to the specified value - perhaps Mms?

I will try to measure Vas using the added mass method tomorrow but for something with so low of a tuning this probably wont work as well as the sealed box method - which I can't do.
 

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Kevin will probably comment here to clarify but in my experience the parameters that are the most solid are the RE, BL, and MMS. The rest float around a bit ,but usually end up modeling pretty close anyway. Accurately measuring VAS is tough. I've not had much success. The added mass method is not that great as ideally you want the driver to be horizontal.

Everything looks good except for the LE looks high.

Like I said Kevin can better explain.
 

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Discussion Starter #3
I measured the Vas as 379 l using the added mass method. Oddly, to get a clean peak in the impedance measurement I had to measure it vertically. I averaged a few measurement though.

I had also thought that Bl and Mms should have very little variation.

It was difficult to measure Re, I had to move to a very quiet environment. I guess maybe my number includes the binding posts and leads as they go to the VC and this extra series resistance should be removed from the parameter calculations. I guess if I can figure a way to get a better Re measurement I can recalculate Qes, Qts so that they fit better with the given Bl and Mms values.

The Le value was what speaker workshop fit it to over the range 5-300Hz. This was to match the impedance curve closely in the 50-100 Hz range. If I extend the fitting range up to 1 or 2KHz the Le parameter ranges from 2.5-3.5mH. Remember that this is a single 4-ohm coil so it should be higher because the posted number are 2 coils in parallel.
 

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The biggest fundamental value that changes is Cms. Mms shouldn't vary significantly across a production run nor should BL.

The inductance of a woofer is a complex subject. To get a meaningful picture of what it is doing you have to plot inductance with frequency, displacement, and power. The single Le spec that is usually given is simply a measurement of the voice coil @ 1K with virtually no power so it is of limited use.

A Klippel is about the only way to look at it from all angles. When we design a woofer we try to optimize with stroke (using the shorting rings), and with power (the thickness and type of material determines shorting efficiency with power) and the goal is to have stable inductance as you add power.

One of the things to realize about measured T/S is that they are all derived from the impedance sweep. It is pretty amazing that you can get so much information from such a simple measurement.

Kevin Haskins
Exodus Audio
 

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Will the WT3 from Parts Express do everything you are looking for in sub driver measurements. I have been thinking about getting it.
 

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Sonnie,

the WT3 is what I use and it's well worth it IMO. It is not perfect however. The best thing that it does is just get you an accurate impedence sweep of the driver and also of the driver/s in an enclosure which is possibly even more handy. The VAS and LE measures are subject to a lot of variables even sweep to sweep and I wouldn't put too much weight on those from the WT3.
 

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If you want one Sonnie I have an extra that I'd be willing to unload. I've used it once to see how the measurement compares to my standard Praxis measurements.

In my opinion, it is a better tool for small woofers. Large subs need a larger signal level to get the best parameter readings. A couple mV doesn't cut it.

Kevin Haskins
Exodus Audio
 

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If you want one Sonnie I have an extra that I'd be willing to unload. I've used it once to see how the measurement compares to my standard Praxis measurements.

In my opinion, it is a better tool for small woofers. Large subs need a larger signal level to get the best parameter readings. A couple mV doesn't cut it.

Kevin Haskins
Exodus Audio
I'd agree with that statement about the big subs.

As long as you don't put too much faith in it as some sort of concrete measure it's a great little tool. I still think it really shines as a straight up impedance measurement device, cables, drivers, driver/ enclosure combos, finding your port or PR tune, etc.

How did it compare to your Praxis measurements? Was it in the ballpark?
 

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I'd agree with that statement about the big subs.

As long as you don't put too much faith in it as some sort of concrete measure it's a great little tool. I still think it really shines as a straight up impedance measurement device, cables, drivers, driver/ enclosure combos, finding your port or PR tune, etc.

How did it compare to your Praxis measurements? Was it in the ballpark?
It is accurate and a good tool. The electrical side of the measurement is simple and it should be as accurate as any of the more sophisticated measurement systems. The fact that it only uses a very low-level signal and doesn't get the suspension moving is the only reason that the Cms might be off on larger woofers.

On smaller woofers and midwoofers it should be right on. I used the older model more than this one and it gave good results, pretty much the same as Praxis but you cannot control the signal level like you can with Praxis. If you use a real low signal level both give pretty much the same readings though.

Kevin Haskins
Exodus Audio
 
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