More impedance measurements

[Alex M]

Now I am on a roll with my impedance measurements of the speakers I built more than ten years ago!

I have just made some measurements of the Audax H100Z0 midrange driver in its cabinet (a substantially stuffed closed box with internal volume of 20-25 litres), and I am surprised by how different the results are from the curves in the datasheet:



The dashed red curve is the uncompensated driver, while the solid red line is the same, but with what I previously calculated to be an optimal Zobel network for the HM100Z0: 6.4 ohms in series with 4.7 uF.

The raw impedance curve agrees well with the manufacturer's data above 2kHz, but is much lower below 1kHz, even though the resonance frequency of 280-290 Hz is spot-on. Is this likely to be the influence of the box? I wasn't expecting this, since this is a stiff little cone with Vas=0.84 litres, much less than the box volume. I am reasonably confident in my measurements, as when I measured the 8" woofer I got much better agreement with the data sheet right down to 100 Hz (below which the cabinet loading look over).

Anyway, the humpy curve for the compensated driver would certainly explain the non-ideal frequency response I have measured for my crossover filter...

Alex

[AllenB]

I'm not sure what you're question is, but I'd start by increasing the compensation capacitor value until the midrange is workable, and if the top end fell too far I'd increase the resistance.

[Alex M]

Hi Allen,

Thanks for your response. My next step is certainly to tweak the Zobel values to get a reasonably flat midrange.

My main question, though, was whether the difference in the size of the main resonance between my measurements and the dataset was plausible. I wasn't expecting that putting a driver with a Vas of less than a litre in a box bigger than twenty litres would have such a large effect on the low-frequency response.

Alex

[jerome69]

Hi Alex,

The frequency resonance has not changed but the amplitude and the width have changed. A problem with the magnet, ie magnetic field better ?
A set of TS parameters will say more things.

Regards.

[Alex M]

Originally Posted by jerome69

Hi Alex,

The frequency resonance has not changed but the amplitude and the width have changed. A problem with the magnet, ie magnetic field better ?
A set of TS parameters will say more things.

Regards.

Hi Jerome,

I don't have the facilities to measure T/S parameters, but here are the published data for the HM100Z0:

http://https://www.madisound.com/sto...oducts_id=8928

To verify my measurements I would have to measure the midrange driver from the other channel, and also measure one of the drivers outside the box.

Alex