For Vas measurements, I haven't yet gotten around to sticking the drivers in a sealed box, so I wanted to try the quicker added-mass method. Is this method much less accurate than the sealed-box method? I have official data sheets for the driver, and the Sd given there matches with the Sd I'm measuring from middle-of-surround to middle-of-surround. (Everyone tells me that added-mass Vas depends heavily on an accurate Sd.)

I'm placing two five-rupee coins (exactly 10g each) on the cone, and taking the added-mass impedance measurements. They give me nice clean graphs, sometimes with twin-peaks. And they have given me Vas figures of 24L, 27L, 29L and 30L. The published figures in the datasheet are higher: 36.8L. The data sheet is here:

Peerless India 6.5" Kevlar cone woofers

I repeated the tests with three coins instead of two (ie. 30g weight), and the Vas measured was just 5-7% away from the 20g measurements. I believe this is how it should be, within a reasonable range of weights?

The other parameters I've measured tally quite closely with the data sheets. I'm getting 0.39 as measured Qts for three out of the four pieces, and Fs in the range of 42-44Hz (published is 39Hz).

My question basically is: can I trust my added-mass Vas, or should I necessarily do the sealed box measurements too?