We have a lot of experience measuring driver parameters. Unfortunately we had to delegate hours and hours into getting a system setup to accurately measure parameters on all of our woofers. We finally have the system functioning the way it needs to for repeatability. We needed to measure the same driver on multiple passes and get the same results. A tolerance of 10% doing the same test was not an option. In reality, with running multiple passes, we need to be within no more than 1% tolerance. Nothing is changed, so the measurement shouldn't change.
The first thing we want to note is that you CANNOT fire a driver upwards. No matter what the woofer tester or any other system says, you just can't if you want accurate measurements for several reasons. Your Bl measurement will be low. The driver instantly sags to some extent, putting it away from it's magnetic center, where Bl is low. Depending on the driver this can be up to a 10% decrease in Bl right away as you are away from the magnetic center. Firing upwards means not only is the motor working against the suspension but against gravity. Extra force to fight against makes the Bl seem even lower. Then when you add mass you are fighting even more against gravity and further sagged from center. The further you sag from center, the stiffer the compliance seems also. Now this begins to effect your measurements of Fs, Vas, Mms, etc.
The next thing is do not use the delta mass method. Use instead delta compliance. Adding mass to the cone means you probably have to fire it upwards, or have something sticky on the cone so you can fire vertically. With the cone firing up with added mass, you further stress the suspension into a place it isn't supposed to be and futher put the coil away from center. Bl is even more decreases, suspension seems stiffer that it is, and you can't get an accurate measurement of the shift with the added mass anyway which means your electro mechanical parameters are all off.
Then, ANY movement of the driver, mass added, or baffle holding the driver will throw off the impedance curve drastically. A door slamming, a fan blowing at the driver, truck driving by, etc can all throw it off. This doesn't mean you need a perfectly silent room, but make sure you minimize the noise. It also won't throw it off the same every time. This will show up as an irregularity in the impedance curve. Blips, bumps, dips, etc that you will see in both the magnitude and phase at and around the driver Fs will drastically throw off every possible parameter. The following are a series of measurements done by a customer. When measuring the driver he got Mms measurements that varried from 130grams up to 250grams. The mass was physically verified on a postage scale of 210grams.
Here are the images:
You can see all the bumps in both the magnitude and phase around the peak at Fs and in the one driver a double peak even. When calculating parameters a curve fit is done of the measured impedance curve. With these irregularities, the entire curve fit is thrown off.
So how do we measure accurately? First start out with the Delta Compliance method. This means you need a free air baffle, and an enclosure of known volume. We use 50L for 15" and 25L for 12" right now. Make sure to fire the driver horizontally, not up or down. You want both the coil to be centered. Make sure nothing moves. This is very critical as you have seen in the measurements above. The woofer creates and equal and opposite force on the enclosure, table, etc. We recently built our "shark cage" which is a steel structure over 250lbs bolted with 6 threaded rods into the concrete floor to be the bench for clamping our measurement enclosures down to. While you may not need to go this extreme, make sure it is well mounted. Make sure you can repeat the same impedance curve nearly identically. Praxis allows you to hold a curve, then remeasure. If the two are nearly perfectly overlaid, we realize the test is valid. If it looks different, something is wrong. If there are weird peaks, bumps, a flat top on the impedance curve, rounded top, etc, something is most likely moving.
When measuring the second step in the sealed enclosure, make sure it is perfectly sealed. If it is not, the enclosure will seem larger than it actually is, showing less shift in the given volume than it should. This throws off all your compliance related measurements. Your in box peak should be taller and narrower than the original. If it isn't, something is wrong.
The following shows two good impedance curves that you can then withdraw accurate parameters from:
The first red peak shows the free air and the yellow shows the in box measurement. The small blips at 600hz and above are due to surround resonance, etc and are far out of the range we are concerned about for T/S parameters.
Also one final note, all the equations for Vas, Mms, Bl, etc are based on using 1/3 of the surround diameter and not 1/2. I see many software packages incorrectly stating to use 1/2 for your Vas measurements. This just doesn't work as it inflates Vas and Bl to accommodate for it. Praxis is the same way. We tried doing it, but we would get an Mms and Bl value both inflated by about 20% when doing so. Properly measuring 1/3 of the surround gave correct values. We always check our measurements based on known Mms values physically weighed on a postage scale. We then fine tune the diameter measurement until it shows the correct Mms value being measured. Then we know the rest will all fall in line as well.
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