Test boxes for VAS?

[REDSKIN]

Just about to make a test box for measuring the VAS of a 12" driver. All the info I've read suggests making a 1cuft box for this size of driver with the goal of making fb 1.5-2 times fs.

However the driver to be tested is designed for car audio and I'm assuming that the VAS will be lower than a comprable home driver, therefore would it be wiser to build the test box slightly smaller to be sure of getting fb to rise to the correct level?

[richie00boy]

Build it at 1 cu ft then you can always pad it out later with MDF blocks or something if you don't get enough resonance shift.

[John_E_Janowitz]

We use a 50L sealed enclosure for all of our 15" driver testing and a 22.5L enclosure for all of our 12" testing. Also you'll need to make sure that when you do the measurements you account for the volume displaced by the driver. This will make a big difference in getting accurate Vas measurements. Making sure the enclosure is completely sealed is also critical as a lossy enclosure will appear larger and throw off Vas as well. As a result Mms and Bl are thrown off also. As Mms is a known value based on the physical mass of the parts used, you can use this value to verify your results. If your value for Mms isn't coming out right, the others are off too. Also it is important to use 1/3 the surround when measuring Sd, not 1/2 the surround. The Theil and Small formulas used to calculate the rest of the parameters are only valid for 1/3 surround. Many companies try to use 1/2 surround to boost sensitivity numbers, but it doesn't work that way. If you do this, you inflate the Mms value as well. Some other notes on measuring parameters are here:

http://www.aespeakers.com/phpbb2/vie....php?f=4&t=927

John

[REDSKIN]

Thanks for the quick replies. Off to make some sawdust. Going with the 22.5L box.