Midrange Driver QTC in 3-way system: Important?

[RalphL]

I'm an educated newbie (dangerous and annoying, I know), so thank you in advance for your patience.

I'm building a 3-way system. Using BassBox 6 Pro and X-over Pro, I designed the towers with a QTC of .707 based on the bass driver. If I just mount the mid on the same baffle as the bass driver, the mid driver has a very low QTC due to the volume of the bass section of the tower. Should I mount it in its own enclosure within the tower to get a QTC of .707?
In this situation, is the QTC of the mid driver important?

I am sure I am missing some theory here, so please speak up and give me the skinny! I'm a polite and fast learner!

[speaker dave]

You absolutely need to give the mid its own enclosure, simply so it won't be pushed around by the woofer.

As to what Q to give it, that is harder to say. In the end you will be adding crossover to it and shooting for some kind of acoustical rolloff that is the combination of the mids rolloff and the crossover highpass. No need to shoot for .707 and probably best to keep the resonance somewhat low to keep it away from the crossover point.

Regards,
David S.

[RalphL]

I should have added that this has only become an issue during xover modeling. The mid enclosure is part of the xover calculations. The xover point on the low end of the mid is 300hz.

A .04 cu.ft. enclosure for the mid driver gives a QTS of .707. A .5 cu.ft. enclosure (much easier to build with my cabinet design) gives a QTS of approximately .45. Is this a meaningful figure in my situation? What issues might I run into?

Thanks again!

[Moondog55]

If your midrange driver isn't acting pistonically the size (volume) really isn't a problem.
Have you determined the resonant frequency in each sized enclosure? if you have and the resonance is more than to or three octaves above the drivers Fs then the size isn't that important.

300 Hz means that the mids Fs should be lower than 300/4 or 75Hz in the box

[RalphL]

Pistonically? Is that a real word? :-) That's wonderific!!
Please define, thanks.

The mid driver is a 4" cone style. Its Fs is 68hz.

A .0468 cu. ft. enclosure gives a Q of .707, an Fb of 132.5hz, and an F3 of 130hz.

A .5 cu.ft. enclosure gives a QTC of .428, an Fb of 132.5hz and an F3 of 139hz.

When switching between these choices, the xover curve is effected between 100 and 200hz. Oddly, the .5 cu.ft enclosure gives a flatter response with my xover modeling, but my fear is I am missing some essential knowledge concerning this issue. I know that I'll inevitably have to tweak the xover once I get everything built, but rebuilding the cab is just not in the budget.

Should I be concerned about going with the .5 cu.ft. enclosure?

[AllenB]

Something's up here. Your larger enclosure should show a lower fb. The lower fb will be the good thing here.

my fear is I am missing some essential knowledge concerning this issue

Simply, if you can get your box resonance out of the way, the crossover will remove it from play and become the new rolloff controlling the response, the excursion, etc. so your larger box may be OK. If it is too large the response may droop before you cross it, as the modelling should show.

[RalphL]

I assume you mean Fb. Isn't Fs the resonance of the unenclosed driver?

And yes, I thought it should go down as well. Checked everything and I cannot account for it.

[Moondog55]

Pistonically When the driver is moving back and forth in a piston like manner
Happens around the resonant frequency and is why we need to do box size calculations.

[RalphL]

Thank you Moondog55. I get your point now.

AllenB, can you please give an example of getting the, "...box resonance out of the way?" What might that look like in my situation? Thanks!

[AllenB]

I did mean fb, sorry I fixed it later.

What modelling software are you using?