ever design/build a speaker?

gmorris

Sorry I left you out jcaut. You also know your ****.

Good idea on the polyfill stuffing. About how much should be used? I saw that stuff in the Parts Express catalog, pretty cheap.

If you feel like running the specs through the software you refer to, the sub is a JL Audio 12W1-8. I went to the website, clicked on subwoofers, then discontinued models, then was able to download a .PDF of the spec sheet that had all the T/S parameters on it. (i'm going to try to attach that .PDF to this post, I'll .zip it)


Also, I am going to design a box with a port just to compare. What is meant by the tuning freq. of the box? What would I want to select for the tuning freq?

Thanks again,
Greg

jcaut

Don't worry about leaving me out of that list. Sometimes I don't know mine as well as I think I do, meaning: Sometimes my fingers get ahead of my brain..:)

Tuning frequency is the frequency you tune to box to, using a port. It depends on the size of the box and the length and diameter of the port. For flattest response, you usually tune to something near the free air resonance frequency (Fs) of the woofer. That's not the only consideration, though, as I'll get to below.

I took a look at the specs for that subwoofer, and I ran some numbers on that spreadsheet I mentioned. I don't pretend to be a subwoofer expert--- as I mentioned before, you might want to try to consult a forum member like Dr. Spec before you get too involved in this design--- but here's the deal with the JL Audio sub, as I see it: The problem you run into in trying to squeeze a low F3 from this sub is cone excursion. That might not be a huge problem if your output goals aren't too high, but this is not the woofer to try to get 115db output @25Hz out of.

In the spec sheet you attached you'll find JL Audio's recommended box sizes for sealed and ported enclosures. And they're substantially smaller than what you were coming up with on WinISD. That's because those program's default, "ideal" enclosure calculations generally give you the box volume for the flattest response without regard for things like power capacity or woofer cone excursion capability. JL Audio's box recommendations sacrifice a little low end to keep you from bottoming the woofer (running out of excursion capability). Plus, since they're really designed as a car subwoofer, and the interior of a car is a small space compared to most any room in a house, the low frequencies get a boost anyway.

There's a specification called Xmax that stands for the peak linear cone excursion that the woofer is designed for. Usually it's given as a one-way peak, but some manufacturers "cheat" and give a peak-to-peak figure. The woofer cone won't necessarily "bottom-out" at this distance, but that depends strictly on the design of the driver. What it actually means is that beyond the Xmax figure, the voice coil is traveling further out of the gap than it's designed to, which has all kinds of undesirable effects including distortion, bottoming, etc. Your JL Audio sub's 8.0mm Xmax is not bad, but it's not really a lot either compared to most modern subwoofer drivers. For instance, a Dayton Titanic MKII's Xmax is 18mm.

Your woofer actually looks better, to me, in a vented box because of the excursion limitation. At the frequency the port is tuned to, the motion of the cone is severely reduced and the sound comes primarily from the port. I think you could get away with a 100 Liter box tuned to about 30 Hz, for an F3 of 28Hz. Feed it a lot of power below about 25Hz, though, and it will most likely bottom out.

If you can receive e-mail through the forum, I'll try to send you some more information.

Jason