Replacing a HT subwoofer with a car sub.
Jun 30, 2004 at 2:43 AM Post #16 of 89
What do you think of the phase angle as it approaches 30hz and under, gerG? Gets kinda steep down there, but maybe it's not that much an issue.

I get fluttering down below 20hz, but I don't consider that real output
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Actually, my box isn't strong enough to handle bumping up the gain too much. If the frequencies under 35hz get too strong, my box and all sorts of things start to shake up and rattle. One day I'll get rid of the alligator clips I'm using (my soldering iron was strong enough to heat up the clips -_-) and brace the box up and epoxy seal the rear connector plate. It's currently just caulked up
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Luckily, at my normal levels of listening, my sub doesn't get much strain.
 
Jun 30, 2004 at 2:52 AM Post #17 of 89
ooheadsoo, it does roll off too soon for my taste. I am going to look for a woofer that can do a better job in a cabinet of that size. I knew of one, but they changed the parameters on me. Usually when I am working with a 12" woofer I end up around 4 ft^3. Just the nature of physics.

Just for fun, measured response of my system with the Shoguns when I had it in the garage. It is back in the house now, but I have a lot of work to do on the room. Mostly structural reinforcement and a full ceiling bass trap.



Garage2b.jpg



gerG
 
Jun 30, 2004 at 3:09 AM Post #19 of 89
The triangles are 8x8.

Is there anyway you could help me out on reading the graph. Im kind of new to this stuff so I'm not to sure on how to take it.

Will the amp be ok where I have it mounted?
 
Jun 30, 2004 at 6:41 PM Post #20 of 89
Reading the graph: first you probably need a target response. Ideal is usually considered flat down to 20 hz. There is some hand waving out there about room gain helping out at the low end, so some rolloff is acceptable. In practice I have never had a room help as much as it hinders. I get the best results with flat response.

The red trace shows the predicted response for the driver and alignment that you have chosen. For subwoofers the response is judged relative to the level at the crossover point. For a first cut I usually just call that 100 hz, which gives me a consistent reference. From the graph you are -3 db at 50 hz. That means that output is at half the amplitude of the 100 hz point. -3 db is considered the rolloff point. That is rather lean for a subwoofer because it rolls off early and it rolls fast. I tried tuning the cabinet lower, but it did not help much. More cabinet volume did, and the 4 cubic foot example moved the f3 down by about 10 hz, and added significant output in the 25 to 40 hz range. The tradeoff is a bigger cabinet, and reduced max output levels (I can plot the latter as well). I would suggest either going to a larger cabinet, or to a different driver. I will fiddle with the design some more tonight. I didn't have that driver in the database, so I got off to a slow start.

Amp mount: I know that these "subwoofer amps" are made to mount to the cabinet, so in theory you should be ok, at least for the warranty period. I have failed a lot of connections due to vibrations, so I personally would not mount any amp to the sub cabinet. Just me being overly cautious of my gear. I have not tried any of the latest generation of cabinet mount amps.


gerG
 
Jun 30, 2004 at 10:23 PM Post #21 of 89
Pic one

Pic Two

Thats what I have done so far. I have already messed up though, because I think the brace is going to hit the port, so Im probably going to have to cut one of the braces.

Thanks for you information gerG, I really appreciate it. However, I have already purchased the driver and made half the box. Is there anything I can do now to help it out? Do I need to put fiberglass inside the box? Anything at all?


Edit: Was just reading some stuff on the sub and saw this. Is it just a big sales pitch BS or is it possibly true?

Quote:
"In the home theatre environment, a vented alignment will generally yield more impressive results. The vented enclosures reduce the cone excursion enabling higher output at low frequencies. While the Quatro™ Series subwoofers may not calculate out to particularly low F3's, this is somewhat deceiving.

The high efficiency, low Q design causes output levels that peak at a frequency above 100 Hz. This high reference sensitivity makes the F3 also appear to be high. However, since most home theatre subwoofers are crossed at 80 Hz or below, the reference level is reduced drastically whereby the F3 will typically be below 30 Hz even for the 10" model. The tuning frequencies will typically be in the mid-to-high 20's, which will minimize cone excursion at critical home theatre frequencies. With a typical 250 watt subwoofer plate amplifier, the Quatro™ subwoofers will produce room shaking bass, well above THX® certification levels. It is now possible to build a high-performance ported home theatre subwoofer for less than $200 in parts! "
 
Jul 1, 2004 at 2:19 AM Post #22 of 89
Dang Chris, you are cranking right along! I wish I could get things done that fast. It is looking good.

That quote is about one part truth to one part marketing hype. They are right about vented alignments having more output for a given driver size, especially in the low reaches. The tradeoff is mostly larger cabinet size.

I can think of 2 ways to extend lf response in this design. One is to equalize (carefully). This should also be combined with a lf cutoff filter to reduce the risk of excessive cone excursion. The other trick would be to make it a compound loading, where you place a second driver in front of the existing one. You just mount it backward on the front and wire it in parallel (but reverse the polarity). This will have the effect of doubling the apparent cabinet size to the driver. It is an easy mod that you can do later if you think you need it. Your amp needs to be able to handle 2 ohms to do this trick. You can also wire in series, but the power will drop in half (8 ohms).

hmmm, I just thought of a third way, but I will have to do some numbers. Adding an active cuttoff close to the tuning frequency can extend response, but requires some special electronics.


gerG
 
Jul 1, 2004 at 2:37 AM Post #23 of 89
Quote:

Originally Posted by gerG
One is to equalize (carefully). This should also be combined with a lf cutoff filter to reduce the risk of excessive cone excursion.

gerG


This one seems intresting. How exactly could I do this? Is it cheap? My funds are running low on this project very quickly.
 
Jul 1, 2004 at 3:46 AM Post #24 of 89
Wow I wish I had your box skills and speed.

Wouldn't EQ need a lot of power? I'm not sure your amp could handle it.
 
Jul 1, 2004 at 3:50 AM Post #25 of 89
What amp did you order? I can see what the built in controls can do.

Oh, I missed one. You need some acoustic material on the back wall and 2 adjacent side walls. A couple of inches of fiberglass is plenty. Don't block the port.

gerG
 
Jul 1, 2004 at 2:39 PM Post #27 of 89
That's the stuff. Plain old itchy pink fiberglass. Polyester batting works as well (fabric shop). The reason for the insulation is to absorb any standing waves that might occur. Although the sub will be working below the fundamental frequencies of the cabinet, noises can be generated by the amp or crossover that can excite resonances.

gerG
 
Jul 1, 2004 at 6:23 PM Post #30 of 89
Oof, good question. Outside, by all means. Inside mount means you no longer have access to the driver or cables, unless you do a removable panel, and I would not do that. Removable panels always vibrate.

If you want the recessed driver look, you can always add an extra panel to the front with a hole cut to the OD of the woofer flange. Much easier than doing stepped cuts with a router. This trick can be done later, once you decide how you want to finish the cabinet.


gerG
 
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