Help, Determining T/S VAS

[gjeff80]

Hi everyone, I'm looking for an accurate way I can determine the T/S specs for a woofer I have. I have repeatedly tried using Speaker Workshop and just gave up on it. So if possible, I would like to try and manually measure the T/S specs of the woofer, does anyone have any good measurements/calculations that have been tested and known to work, and also I'll need some tips on determining the VAS of the woofer. Any help would be greatly appreciated! Ideally I want to determine the T/S specs and the VAS for the woofer, so I can then plug them into a box building program and figure what size port I need in the cabinet to tune it to 40Hz.
Thanks
Glenn

 

[Vivek]

Try the method described at www.diysubwoofers.org/measure.htm. It works well.

Vivek

 

[roddyama]

Here's a site that doesn't have the best visuals but does have a decent explanation on how to manually measure the T/S parameters. Although, to get Vas, you will need a sealed test box of a known volume. There is another method for getting Vas in which you add mass to the cone but I haven't seen those calculations on-line.

 

[gjeff80]

Cool, thanks guys. I'm going to try then when I get a chance. Yeah, my biggest concern is going to be figuring out the VAS of the woofer, I've seen the added mass method somewhere, but was wondering how to do it. And to use the seal volume method, it says that you need to put the woofer on the outside of the enclosure. If possible I think the added mass method would be easier to use if anyone has any experience using this method??

Thanks
Glenn

 

[roddyama]

And to use the seal volume method, it says that you need to put the woofer on the outside of the enclosure.

Actually, all you need is a box with solid sides of a known volume (measure it), and one side open. Then you only need to make a test baffle (a piece of 3/4" ply with a whole in it) and seal it over the open side. It doesn't have to be pretty, just functional. If you have access to a table saw and a saber saw, you could make the whole box in a few hours.

 

[wintermute]

Hi,

Added mass method isn't too hard with smaller woofers, but can get tricky adding enough weight on larger ones. David Weems recommends 5g for small woofers (under 6.5") 10 grams for 6-8" and 20g for larger woofers. Dickason says the mass should be sufficient to change the free air resonance by 20-25%.

For the weight use modelling clay or blue tack (modeling clay can leave marks if it has oil in it).... and make a ring which you place evenly around the union between the dust cap and cone.

do your impeadance measurements again with this added mass and work out the new Fs

when you have that plug it into Circlotrons Best ever T/S parameter calculator spread sheet below (which he is now pretty sure is correct) and you should get your VAS. edit: just realised you might not have excell........ I think the formulas are somewhere in that thread....

http://www.diyaudio.com/forums/showthread.php?postid=300333#post300333

You need to measure the weight accurately and also the Fs, Dickason says to .1g and .1Hz respectively. I bought a highly accurate scale with .1g resolution which comes with a 50g +- .05g calibration weight (would have been a lot cheaper to build a box, but at least the scale is easy to store!)

pic of scale attached. Note it is showing a weight because it was upside down on my flatbed scanner (no digicam)......

Tony.

 

[sreten]

Hi everyone, I'm looking for an accurate way I can determine the T/S specs for a woofer I have. I have repeatedly tried using Speaker Workshop and just gave up on it. So if possible, I would like to try and manually measure the T/S specs of the woofer, does anyone have any good measurements/calculations that have been tested and known to work, and also I'll need some tips on determining the VAS of the woofer. Any help would be greatly appreciated! Ideally I want to determine the T/S specs and the VAS for the woofer, so I can then plug them into a box building program and figure what size port I need in the cabinet to tune it to 40Hz.
Thanks
Glenn

Vas is easy if you can measure the frequency of the impedance peak.

With a known box volume Fb will be higher than Fs.

Fb = Fs x square root ( Vas/Vb + 1 )

Vas = Vb x [ (Fb/Fs)squared - 1 ]

You can use just about anything for the box volume.

sreten.

 

[gjeff80]

Could I use a box that I already have build, BUT the woofer mounts inside the enclosure? It's a W-Bin cabinet that isn't insulated, and I haven't installed an ports in it yet. But I think mounting the woofer on the inside of it, would mess up the measurements. I might go check the wood I have outside, might be easier to just build a square box where I can measure the enclosure volume.

-Glenn

 

[roddyama]

Could I use a box that I already have build, BUT the woofer mounts inside the enclosure? It's a W-Bin cabinet that isn't insulated, and I haven't installed an ports in it yet. But I think mounting the woofer on the inside of it, would mess up the measurements.
-Glenn

Just subtract an approximation of the volume of the woofer, or make a baffle that fits with the driver on the outside. Than you have to add the volume of the air in the cone and hole in the baffle.

 

[Pjotr]

Added mass method isn't too hard with smaller woofers, but can get tricky adding enough weight on larger ones. David Weems recommends 5g for small woofers (under 6.5") 10 grams for 6-8" and 20g for larger woofers. Dickason says the mass should be sufficient to change the free air resonance by 20-25%.

For the weight use modelling clay or blue tack (modeling clay can leave marks if it has oil in it).... and make a ring which you place evenly around the union between the dust cap and cone.

Found synthetic clay and blue-tack rubbish as a weight. It will leave some hard to remove residue on the cone. And what with speakers with a phase plug?

I favour the added mass method. For the weight I used key-holding rings of several sizes and weights. I attached them to the cone with a few small strokes of double sided adhesive tape of the week sticking kind. But it stays tricky with paper cones. For paper cones the volume method seems better to avoid cone damage. And never use added mass to metal cones! Metal cones are very easy damaged! My own experience

Cheers

 

[gjeff80]

Ok, can you guys help me. I know I'm doing the steps correctly because I measured two seperate woofers, and one comes out fine, but the other I am having a problem getting the Fl value to check. What should I do for this one woofer? Here is what I end up measuring. I determine Fs to be 37Hz, and then when I find the upper frequency that has the same -3dB voltage, it is 63Hz, and the I find the lower frequency that has the same voltage and I get 15Hz, BUT when I use those two numbers, the Fs (sanity) Check value comes out as 30Hz, what should I end up doing here? I know I'm doing the procedures correctly, because I used another woofer, and the FsCheck value came out withing 1Hz of what I measured. Should I just ignore the Fs insanity check value?

Does anyone know if these values would sound about right for a Cerwin Vega 189 E - 18 Inch woofer?

Qts- .28
Vas- 165.0l
Fs- 37.00
Re 4.56
Qms- 1.25
Qes- .37


Thanks
Glenn