Calculating a negative Vas?!?

[SoundmanDave]

I have been trying to measure the Vas of a couple drivers I have (one 8" and one 6.5") and for both I get a negative result. One note: I didn't try the closed box method of finding the Vas because I just don't have a box available; I used a method where changing the mass of the cone would result in a different Fs, thus allowing me to calculate the Vas. I could have done it wrong.

I was finally able to get a Vas of 58.8L for a 6.5" driver. Does that sound reasonable?

Thanks.

[kelticwizard]

The Vas of 58.8L, (nearly two cubic feet), for a 6.5" speaker sounds high unless the SPL @ 1W/1M is over 90 dB or higher. The more efficient the speaker is, the greater the Vas, all other things being equal.

[SoundmanDave]

Yes. The SPL rating for these drivers (both of them) is 90dB. They were both originally taken from an infinite-baffle ceiling speaker that had a passive xover and a 1" tweeter.

My plan is to use two 8", a 6.5" and a 1" tweeter in a tower with a new xover network. So here is another question: given that the two 8" drivers will handle all the low end, is it critical to have a tuned/ported enclosure for the 6.5" driver?

Obviously, to get the most out of the two 8's I'll need to tune (and likely port) the enclosure for them. Can I put them both in the same enclosure and just double the required volume or is there some other "double-driver" consideration needed?

Thanks.

[kelticwizard]

If I understand you correctly, you plan to use the two 8 inchers for the deep bass, the 6.5" for midbass and a tweeter.

Making a crossover for these is going to be very difficult, won't be able to help you with that.

But on your other question, if you plan to use the 6.5 incher for midbass, I would not port it, I would seal it. Much as I like ported enclosures for their extra bass, the sound is not as clean as a well designed sealed system, so for midbass, (above 90 Hz or so), I would definitely want a sealed 6.5 inch.

[SoundmanDave]

Yeah, I'm coming to see that the crossover will not be easy. I'm not sure if they should both be in the same enclosure or separate and I'm also not sure if I should put the two drivers together electronically, either series or parallel, in one crossover circuit or if they should each have their own crossover branch... any tips or ideas would be greatly appreciated!

I was leaning toward no port for the 6.5" mid but I just wanted to get some outside opinion. My concern now is that I'm going to end up with a much larger cabinet than I originally planned. Will porting the enclosure for the 8" drivers allow me to make it smaller?

[kelticwizard]

For the same woofer, within the most common band of Qts ratings generally a ported enclosure results in an enclosure about twice the size of sealed but with much, much deeper bass.

Yes, for two woofers, whether hooked up in series or parallel, you double the volume that you would use for one woofer.

Just for verification on that Vas of the 6.5 incher, here you see a Vas rating of 1.38 cu ft, (abut 40L), for a 6.5 rated at 90 dB. Not too far from yours. Only thing is, it is a 4 ohm, so you cut the sensitivity down 3 dB as compared to a true 8 ohm, so it's really an 87 dB sensitive speaker @ 2.83 V/1W. And it's Vas is not far off from your measurement.

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[AndrewT]

if the 8inch drivers came from vented boxes then it is likely that they have a lower Qts than otherwise.
Fitting them to a sealed box raises the Qbox to substantially above Qts.
You want Qbox ~=0.7 to avoid a bass or midrange boom. You might even like a Qbox<<0.7 for the different bass sound. You can adjust the Q as well as the Fbox by choosing a different volume for the sealed box. But Fbox and Qbox cannot be changed independantly of each other. Vbox going bigger, lowers Qbox and lowers Fbox. Vbox going smaller, raises Fbox and raises Qbox. This is how manufacturers tune their little boxes to give artificial bass boost a high Qbox exaggerates the upper bass to kid us on that the little box is reproducing real bass. It is also why little boxes can be quite intolerant of room position and need tuning to suit the room. Just listen to a variety of male spoken voice to hear the boom effect of high Qbox.

If the 6inch are similarly mounted in a sealed box, again aim for Qbox~=0.7
If this has a 2pole Q=1 filter in front of the Mid amplifier, then the overall filter on the mid unit will be a 4pole Q=0.7 and make it very predictable and ideal for a Linkwitz Reilly crossover to the bass driver.

[SoundmanDave]

Thanks everyone for all your input. I recalculated some of my numbers this morning and I am happier with the results. (It seems doing speaker measurements at midnight is not always the best time for clear thinking)

I am comfortable with the data I have collected for the two drivers. Now I just need to figure out how to work out the enclosure dimensions needed for the drivers. None of the online box calculators can do the compound box I am thinking is needed. (Would it make sense to calculate two different enclosures and then 'join' them together?)

Basically, I'm looking at an overall height of ~40"; 22" for the 2x8" drivers (possibly ported), 19" for the 6.5"; the overall width will be ~17" and overall depth of ~14". That's what I have found through online calculators. I would like to reduce the width to ~12" and increase the depth to ~18". Space limitations require less width and more depth.

Any thoughts?

[AndrewT]

Hi,
use the box volume and driver data for each driver completely separately.
I like WINisd pro, but it does not give Qbox, just the shape of the roll-off. Instead you choose Qbox (closed/sealed) and see the resulting shape.

Find the Vbox that each driver needs. Do that in whichever volume units you are comfortable with, eg. cub inch, cub ft, litres, cub metres.

Then find what internal sizes are required to give you those volumes.

If you prefer a certain width and know the material thickness then that fixes the box width for all the drivers.
Is the tweeter sealed or does it need it's own sealed box?

[SoundmanDave]

Hi Andrew,

I have played with WinISD a bit but it didn't seem to work right. Maybe I need to adjust my data a bit.

So I can calculate each driver enclosure then just combine them into one construction with the appropriate dividers built-in. Seems simple enough. I'm afraid the double 8" drivers will require a far too big enclosure. I suppose I can just use one instead of 2...

The tweeter is all enclosed. I plan to make a small 'enclosure' just to mount it and it will be part of the 6.5" enclosure.