Spider and surround creep

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Tried to measure compliance and motor strength by putting the driver on its magnet (the magnet sitting on 0.3 m² particle board to avoid compression effects from the carpet), taping a paper pointer to the cone somewhere near the suspension, putting various weights on the dustcap and observing the pointer move on a meterstick that I had dangling from a table. The partition is in mm, but it is possible to read to position to about 0.2 mm.

One thing I observed that when a weight was added or removed or a current switched on and off, the pointer would move to its new position and then change by up to 3/4 mm over the next half minute.

The worst was a Peerless CC 260 (creep 0.75 mm, time constant > 2 minutes, would not return to original position even when cone was moved manually) with flat light-yellow spider and rubber surround. I got it on ebay and the guy packed it into carton without a standoff on the basket, so the carton actually pressend the surround down by a few mm. When I received the driver, the spider was bent inward by about 4 mm. Storing the driver suspended upside down for a day didn't do the trick (isn't it amazing that a couple of days with mechanical pressure at temperatures somewhere between room temperature and freezing, given current weather conditions, leave such a lasting impression?), but blowing with a moderate-heat hairdryer at both surround and spider for a few minutes while the driver is upside down and letting it cool almost did the trick after two rounds. I have the visual impression I am still missing 1 mm, and while the measured compliance agrees with the data sheet, the motor force measurement tells me the same as my eye. Will have to do the hairdryer spiel once more and remeasure...

Then came an Audax HT300G0 with rubber surround and brown cup spider, maybe 0.5 mm and 30 s.

Next was a Mivoc 10" with rubber surround and brown cup spider, 0.3 mm and 10 s.

The best actually was an old 8" Vifa with flat spider (same shape and color as on the Peerless) and foam surround that had almost no obervable effect. This one turned out to have the softest suspension, too, so I cannot say whether it is the rubber or the spider causing this.

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What is the best way to measure these parameters? Wait for creep, don't wait, measure dynamically? The latter would require an electronic displacment meter. Any good recommendations what to use for a transducer?

I also wonder about the sonic implications. If things are observable on a 1 - 10 s time scale, things most likely also happen on shorter timescales. This sounds a bit like memory distortion...
 
I'm away from the lab right now, but one way to measure compliance is to find fc, then put the driver in a sealed enclosure of known volume, run the driver up to temperature, and then find fc again. The speaer compliance and the compliance of the volume of air are subtractive. There's a simple equation for calculating the compliance of the driver in units of volume. Damn equation just escaped my memory.

I suggest you check when the driver is at operating tempurature to see if the creep changes either in displacement or time constant.

I'd also flip the driver upside down to see if the creep is the same in both directions.

If you decide to use a creepy driver as a woofer, it may not be a good idea to mount it horizontally. Gravity is relentless.

Isn't it great not knowing if the VC is centered in the gap?
 
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