subwoofer TL with leftover wood

[krikus050]

Hello. i was looking for somebody more capable than I to run a simulation for me. I am building some fullrange speakers, half chang bofu's, and will have some left over wood to work with.

I want to make a subwoofer box for my car with a really crappy 15" sub that I have. It will work okay for my car audio needs. The wood and space available limit me to about a 4 cubic foot box. The box will be an angle back.

The side profile with show a height of 18", top width of 8", bottom width of 16". The length of the box will be 40". all external dims

I was going to do a ported box but was thinking i might be able to work out a TL. The line length would be a little under 80" which would put the tuning around 40hz i think. i guess a little lower if the line is stuffed.

If anybody could help me out with a simulation. I would appreciate it. here are the T/s that i could find on it.
QTS: .67
QMS: 3.96
QES: .81
Fs: 29 Hz
Vas: 116l
Sd: .086 sq m
SPL: 91dB
xmax: 16mm
z: 8 ohms

Also i had a question about doing iso-barik, i know it doubles the apparent box size and allows for lower tuning but how would a TL be affected by this?

[GM]

Greets!

Its specs says it doesn't want to 'feel' a box volume (Vb) that small.

Anyway.........

L = 74.564"
w x d = 68.03"^2
zdriver = 24.75" down from closed end

All dims i.d. and approximate. Sim uses 1.0 lb/ft^3 polyfil stuffing density throughout the TL.

BTW, for my records, what brand/model # driver is this?

GM

[GM]

Isobaric reduces the box size due to an effective halving of Vas. In a TL this means that the cross sectional area (CSA) is halved, i.e. the w x d dims are each reduced 0.707x, so if the w x d is 10" x 10" for a single driver, then for an isbaric pair they need only be 7.07" x 7.07".

GM

[Hezz]

Krikus,

There is so much cabin gain in a car. Why not go for an aperiodic in that 4 cubic foot enclosure. It will sound better than a bass reflex and be easier to built than a TL which might be hard to do in those confines.

[krikus050]

Originally posted by Hezz
Krikus,

Why not go for an aperiodic in that 4 cubic foot enclosure. It will sound better than a bass reflex and be easier to built than a TL which might be hard to do in those confines.

I think you are spot on. I think that was the last generic box design i was not aware of. So thank you for that.

Quickly looking at the vents from scan speak and dynaudio. I would need about three of them. So I guess i will make a cutout with about the same surface area as three of those and stuff it and tune by ear.

I can't put the vent on back like suggested. I am limited to the top 8"x40" or front 18x40". Where would you suggest it be? I am mounting the sub as far to one side as possible so i don't know if the vent should stay near the sub or be pushed to the other side.

Are there any programs that can model these or any guidelines that could be followed?

[krikus050]

Originally posted by GM


BTW, for my records, what brand/model # driver is this?

GM

you must have a huge record. It is a Volfenhag ZX4715. Nothing great at all but for around 60 bucks for a 15 inch that is decently efficient I can't ask much more from it. It's sound quality is ok for my car but it's Qts makes for a huge box.

[GM]

Thanks, I do.

FWIW, the TL I simmed is probably much closer to being truly aperiodic than a BR using OTC vents and has more gain BW (lower F3, Fp) for the extra woodworking effort to boot. Note how flat the impedance peak is in the sim and it can be completely flattened (aperiodic) with a bit more stuffing, but MJK designed his software to only be accurate up to 1.0 lb/ft^3.

Anyway, any box program that allows you to adjust the vent's Ql can in theory sim it, but unless you know the OTC vent's acoustic properties you'll have to either rely on the manufacturer's general guidelines or do impedance traces while altering box and/or vent damping. I've never tried it, but I imagine you'd have to really heavily stuff a cab with fiberglass insulation to get similar results with only one or two OTC vents.

GM

[Hezz]

Originally posted by krikus050


Quickly looking at the vents from scan speak and dynaudio. I would need about three of them. So I guess i will make a cutout with about the same surface area as three of those and stuff it and tune by ear.

I can't put the vent on back like suggested. I am limited to the top 8"x40" or front 18x40". Where would you suggest it be? I am mounting the sub as far to one side as possible so i don't know if the vent should stay near the sub or be pushed to the other side.

Are there any programs that can model these or any guidelines that could be followed?

Some of the better software programs can model the aperiodic enclosure as a leaky sealed enclosure. I have always tuned by ear. I'm not sure how much the placement matters but I would try to put it as far away from the driver as you can at any convenient place.

If you decide to go with an aperiodic instead of a TL you might try using a six or eight inch perforated metal snap on speaker cover. The kind that has a metal ring that the cover snaps on. THis could make for a larger vent area that could be easier to adjust and be less expensive than three Scan Speak variovents.

This is the idea that I have for a larger cabinet that I am going to make. Mount a perforated piece of aluminum plate to the inside of the cabinet over the hole where the aperiodic vent will be. Or mount it recessed so that there will be a gap between this perforated plate and the outer speaker cover of between 1/2 to 1/4 of an inch. Then mount the speaker cover on the outside of the hole. THe cover can then be removed and replaced easily to experiment with different stuffing densities and materials.

[krikus050]

Originally posted by GM
Thanks, I do.

FWIW, the TL I simmed is probably much closer to being truly aperiodic than a BR using OTC vents and has more gain BW (lower F3, Fp) for the extra woodworking effort to boot. Note how flat the impedance peak is in the sim and it can be completely flattened (aperiodic) with a bit more stuffing, but MJK designed his software to only be accurate up to 1.0 lb/ft^3.

Anyway, any box program that allows you to adjust the vent's Ql can in theory sim it

GM

I realize now that TL's are also aperiodic. I have not expanded my definition of aperiodic yet since i just learned about it. It makes complete sense though. The simulation looks pretty good too. Looks like i have some options. Thank you very much for helping me out.

I can adjust Ql in WinISD. What exactly is it?

Originally posted by Hezz


Some of the better software programs can model the aperiodic enclosure as a leaky sealed enclosure.

If you decide to go with an aperiodic instead of a TL you might try using a six or eight inch perforated metal snap on speaker cover. The kind that has a metal ring that the cover snaps on. THis could make for a larger vent area that could be easier to adjust and be less expensive than three Scan Speak variovents.

I can see now that i did not clearly state that i was going to make my own vent. Sorry for that. I would definitely just make the vent with some heavy gauge metal mesh or a speaker grill like said. If I put the vent on top it would have to be the metal because the top is only 8 inches wide external. If i put it in the front i could use a grill though. I was just wondering if the surface area of three ~4" vents would be enough. I really need to learn MathCad so I can really get into my hobby. Thank you too for your suggestions.

[GM]

You're welcome!

Q = quality, l = loss, so it relates to the Q of the cab at Fb resulting from all the leakage losses, ergo how you define Hezz's leaky sealed cab. Some programs such as LspCad also has Qp, the port's quality factor, which allows a more accurate way to model an 'aperiodic' port. MJK's allows you to damp the vent also, though its properties are again limited to 1 lb/ft^3 stuffing density of polyfil, which isn't enough to sim it accurately.

Well, (3) 4" diameter vents = ~37.7"^2, so comparing it to the TL dimensions I simmed, I believe I can safely say that to match its damping performance will take a seriously resistive material, so you'll have to start with heavily stuffing the cab as the pioneers of audio recommended, leaving an open tunnel between the back of the driver and the vent, creating an acoustic tunnel or labyrinth as Harry Olson called it. Obviously, the further away the two are the better, so we're right back to the stuffed TL in essence.

GM