Acousta-stuff and box size

[Chris40]

I am currently designing a sealed box for home and had questions about the change in Qtc that occurs with stuffing. I searched for a while, but did not find anything that answered my question. The driver I purchased is a car audio sub that is recommended to use a 1.65 cu. ft. box in the car. Using the loudspeaker cookbook and T/S parameters, I found that a 2.4 cu. ft. enclosure would give a Qtc of .7 in the home. On my last project, I didn't use stuffing, but I am trying to make a smaller box, so I had some questions about it. Using an equation from the book, it estimated that a sealed box of around 1.3 cu ft. would give a Qtc of .7 with 1 lb/ cu ft. of stuffing. This seems like a huge change to me. My number one priority is sound quality, so does stuffing degrade the sound quality, and does it really have this drastic of an effect on Qtc. I have read that it actually helps, but is it more predicatable to go with the larger box? Also, I have also read that you can decrease your base extension when your Qtc drops significantly below .7. If I would make a 2cu ft. box and stuff it, would I risk loosing low output by dropping the Qtc too low, or is this really not an issue. Finally, how do you keep the stuffing off of the speaker, and is this imporant.
I really appreciate any help, and just in case numbers would be helpful- Qts = .434, vas = 111.6l, fs = 24.3, xmax = 21.6mm, sd = 100.4sqinch Thanks- Chris

 

[Volenti]

I personally prefer the sound of a stuffed sealed enclosure, your predicted drop in Qtc is in the ball park, this is mostly caused by the resistive losses the fill creates in the enclosure. The thermal effects of fill (making the enclosure appear larger) are quite minor in comparason, a 2-3hz change in resonant frequency verses halving the electrical impedance at resonance. Indeed in some suitations you can have a lower Qts with fill in an enclosure than the driver has free air.

I've used polyester based fill for over decade with no particular attention paid to keeping the fibers away from the driver and have had no issues to speak of, though a layer of polyester batting (instead of the loose fill) directly behind the driver would prevent any possible problems in this regard I would think.

In the end I would opt for the larger enclosure if possible since it's easier to take up space in a box than to create more.

 

[Chris40]

Thanks for the reply. I really appreciate the response. So I know I can always go in and remove space on the larger box, but do boxes with a Qtc around .6 to .65 still sound good. The transients should be great, but I heard that you start to loose deep extension. Also, is there any risk to the speaker if it was designed for a 1.65 cu ft. enclosure and it is in a 2.4 enclosure. At some point, one would think it would loose some of the "air" suspension. Is this that important.
Thanks again.