I recently did some measurements on some B&W 8" drivers from one of their subwoofers. Using the added mass method I kept getting the message stating I hadn't added enough mass to lower Fs enough. I eventually plonked on a tape measure on top of the blue tac that I had run out of. I think this gave me an added mass of about 360g.
Voila, VAS measured and came out at 9.2 with an MMS of 101g.
I forgot to save this so I redid it a day or so later. Didn't add as much mass this time, can't remember exactly how much, but it have me a VAS of approx 12 and Mms in the 70g region.
Quite a change from my initial attemp and makes for different results in WinIsd obviously.
What do I need to for repeatable or more accurate results?
Voila, VAS measured and came out at 9.2 with an MMS of 101g.
I forgot to save this so I redid it a day or so later. Didn't add as much mass this time, can't remember exactly how much, but it have me a VAS of approx 12 and Mms in the 70g region.
Quite a change from my initial attemp and makes for different results in WinIsd obviously.
What do I need to for repeatable or more accurate results?
No clue what these machines are designed for, but the pioneers concluded that lowering Fs 1.56x was the goal and worked fine for me.
The added mass should be roughly the same as the driver's mass, that would lower Fs to ~0.707*Fs. With GMs figure, it would lower Fs to ~0.641*Fs.
If you add too much mass, the other parameters like Rms and such can change a lot vs frequency. When you add tons of mass, the excursion changes a lot as well, and parameters are nonlinear so Cms will change between measurements.
Parameter measurements are a lot more approximate using these simple measurements than most people think. When you see 3 of 4 sig figs on parameters, you should laugh.
If you add too much mass, the other parameters like Rms and such can change a lot vs frequency. When you add tons of mass, the excursion changes a lot as well, and parameters are nonlinear so Cms will change between measurements.
Parameter measurements are a lot more approximate using these simple measurements than most people think. When you see 3 of 4 sig figs on parameters, you should laugh.
