View Full Version : Recommend box size for single Alpine SWR-1241D

12-16-2002, 08:29 PM
Hey folks...

I'm working on adding a subwoofer to my system and have decided on an Alpine SWR-1241D to be powered by an MTX 6500D amp. My goal is to have tight, accurate, deep bass with an emphasis on sound quality. SPL is a nice secondary benefit, of course... but it is not my primary goal. The car in question is a 1998 Ford Mustang GT.

My preliminary box design is a sealed "irregular rectangle" (wedge, shorter top, longer bottom) that will fit almost flush against the rear seat area from inside the trunk. I will most likely mount the box in such a way that about 2" of space exists between the actual back of the seat and the face of the box / subwoofer so that there's little chance of the sub ever bottoming out against the seats.

The airspace that I have been using for my box calculations thus far is 1.0 ft^3 -- taking into consideration the approximate 0.128 ft^3 displacement of the sub basket and motor assembly. Total volume, therefore, is 1.128 ft^3.

What do you all think; is this enough airspace or should I go for more or less?

Alpine's specs recommend a range of .7 - 1.0 ft^3 for the sub but when I ran the T/S parameters through BlauBox (yeah... I know) it showed that 1.0 would actually provide a flatter response curve.

I'm completely open to suggestions, so fire away. :D

12-16-2002, 09:13 PM
Thanks for the reply, Jmac...

No, it will be just a single SWR-1241D sub wired 2-ohm mono. Alpine rates them at 300W RMS (150W per coil) and the MTX amp should be feeding it a little over 200W per coil at +12Vdc.

I intend to keep the gain on the 6500D set around 80% of maximum and rarely crank the volume on the HU past 75% of maximum so I figure this will be well within the 1241D's ability to handle and certainly below the 6500D's clipping threshold. Good clean power.

Granted, maxing out the 6500D would definitely be more than the sub can handle... and if I ever get to that point I will likely upgrade the sub to something a little more robust. But SPL isn't really a concern of mine at this moment. Just accurate bass.


12-16-2002, 09:15 PM
As a side note... I can go with two 1241D's and wire the coils in series on each and then parallel the two subs so that they each see around 125-150W per coil if that sounds like a better approach for safety's sake. I'm just concerned that two 12's will be a little more bass than I really want for the system.

12-16-2002, 09:27 PM
Hmmm... MTX is really under-rating their amps these days, I guess. I'm not doubting you -- I'm just looking at a spec sheet from www.mtx.com that claims 500Wx1ch @ 2ohms and a peak output of 775Wx1ch @ 2ohms.

Are you thinking of the 8500D perhaps? That's my amp's bigger brother. :)

BTW: I am all for getting more power than I expected out of the amp; it just changes things dramatically if it's the case. :D

12-16-2002, 09:37 PM
Originally posted by Jmac
Nope ... that's how much the 6500Ds do ... The 81000Ds put out outrageous power (1700-1800 RMS into 2 ohms) ...

MUAHAHAH... sweet. :D Thanks for the info, man. It sounds like I may need to have a second set of plans drawn up for a dual sub box just in case. I can order another 1241D and do the series/parallel wiring scheme and keep it safe that way.

So anything upwards of 1.0 ft^3 per sub for a sealed enclosure, then?

12-16-2002, 09:44 PM
Thanks again! I think I will revise the design for 1.25 ft^3 and then draw up another box using the same per chamber on a dual-sub setup just in case I decide to order another 1241D before actually beginning construction.

Can't wait to get this put together and see how it actually sounds in the car. :D

12-16-2002, 11:44 PM
Okay... I designed a sub box that provides 1.242 ft^3 airspace per sub taking into consideration a 0.128 ft^3 speaker displacement.

I guess the 0.008 ft^3 difference between the actual volume and the ideal volume of 1.25 ft^3 per sub really isn't going to make that much of a difference in terms of Qtc. Besides, this is the closest I could get without really splitting hairs and making some really unusual board cut lengths. :D

12-17-2002, 11:09 AM
Originally posted by Jmac
Sounds good :) Let us know how it turns out :D

Will do. Actually... I am looking at the idea of returning my 1241D and using two 1021D subs instead. I can wire them series/parallel and actually present a 2-ohm load to the amp while limiting the subs themselves to about 125-150W per coil, well within their rated limit.

You've been most helpful, Jmac! :)

12-17-2002, 01:40 PM
Originally posted by Jmac
1.25 ft^3 per sub gives a Qtc of .707 (Good SQ and SPL mix)
1.5 ft^3 per sub gives a Qtc of .67 (Less SPL, more low-end, more accurate response)
2 ft^3 per sub gives a Qtc of .625 (Even less SPL, even more low-end, even more accurate)


Do you have any figures (or can you calculate them or point me toward a program or formula that will) for box volume -to- Qtc comparison on the Alpine SWR-1021D sub?

I may end up going with two of them instead, as described in my previous post.

Thanks! :)

12-17-2002, 03:37 PM
I started playing around with WinISD and a few others again and came up with a Vb of 0.416 cubic-feet for the Alpine SWR-1021D subwoofer to achieve a Qtc of .71

That sounds small. Is my math right? Thiele/Small parameters for this sub are as follows:

Alpine SWR-1021D
Coil Height (Hvc) : 36.7mm
Cone Area (Sd) : 320.47 sq. cm
D.C.Coil Resistance (Re) : 1.8 ohm + 1.8 ohm
Electrical Q (Qes) : 0.40
Equivalent Suspension Stiffness (Vas) : 29 liters(1.02cu.ft.)
Free Air Resonance (Fs) : 29Hz
Frequency Response : 28Hz - 1kHz
Gap Height (Hag) : 10mm
Impedance (Nominal) : 2 ohm + 2 ohm
Inductance (Le) : 2.67mH at 1kHz (0.88mH at 20kHz)
Linear Excursion (X linear) : 13.35mm
Maximum Excursion (X peak) : 26.7mm
Mechanical Excursion (Peak-to-Peak) : 55mm
Mechanical Q (Qms) : 8.93
Sensitivity : 85 dB/W/M
Total Loudspeaker Q (Qts) : 0.38

12-17-2002, 08:24 PM
Originally posted by Jmac
I would go with 0.8 ft^3 per chamber :)

Is that before or after you consider the displacement of the speaker itself?

12-17-2002, 09:04 PM
Originally posted by Jmac
After speaker and brace displacement :)

Speaker displacement is 0.094 ft^3 per sub ...

Excellent. I was able to modify one of my blueprints so that it yields .92 ft^3 total internal volume without the subwoofer displacement / .84 ft^3 with the subwoofer. Very close to your recommendations.

While we're on the subject, what software are you using (if any) to do your calculations?

12-18-2002, 07:57 PM
If I use two of the Alpine SWR-1021D subs (2ohm + 2ohm coils):

Each subwoofer has it's voice coils wired in series to present a 4ohm load (per sub).

The two subs are then wired in parallel to the MTX 6500D amp to present a 2ohm load.

The amp yields 900W (RMS) at 2ohms, so this means...

Each sub "feels" 450W and each coil gets 225W or pretty close to it. Right?

12-18-2002, 08:06 PM
Originally posted by Jmac
Yup :) Although I would watch the gains and not use bass boost .... The 10"/12" Type-R motor is only rated to handle 300 RMS or 150 RMS per coil (Although mine did find with 365 RMS) ...

Yeah, I will keep the gains on the amp turned to about 75% - 80% of maximum. I like to have a WIIIIIIDE safety margin on stuff like that. And I think the bass boost circuitry adds unnatural artifacts to the sound, so I'll leave it off anyway. :)

BTW, I redesigned the box from scratch and came up with .82 ft^3 per subwoofer (considering displacement of basket and motor structure) which is within .02 ft^3 of the ideal target range you came up with. Hopefully it'll sound tight and clean! :D

12-18-2002, 08:53 PM
Originally posted by Jmac
Yup :) Although I would watch the gains and not use bass boost .... The 10"/12" Type-R motor is only rated to handle 300 RMS or 150 RMS per coil (Although mine did find with 365 RMS) ...

My 2 12's in .9 each never whimperd with 1200 watts on them ! And well I pushed em hard for a good while !

12-18-2002, 10:08 PM
First and foremost...


Thanks, I feel better now. :D

I decided to get some large pieces of cardboard and cut out a template for what an end piece of the box I had designed would be like when cut from MDF, and went to test fit that into the trunk of my car. No dice. The angle of the face (wedge shaped box) was too steep and didn't mate up right with the firewall seperating the trunk from the cabin.

So I took another piece of cardboard and a felt tip pen and made a tracing of the firewall's angle so that I could start revising my design. I came up with a 66.5 degree slope, cut another template and went back to test fit it again. Success!

Now I notice that the way the rear wheel wells cut into the trunk, there's going to be less width to play with than previously thought. This means the box will either need to be deeper or taller. Taller is okay, but deeper is definitely out as there's only 11" worth of depth on the trunk opening to slide a box in from the rear of the car.

Have I mentioned ARGH!!!! lately? Heh.

Okay, so I'm sitting here now with a template that fits so long as I am willing to remove the speakers from the rear package tray. That's not a big sacrifice as I don't care to have a rear sound-stage anyway. They're just a cheap pair of Kenwood 6x8's and the gains are turned way the heck down on the rear channel... so done deal. Now I've got the height that I needed.

But wait! There's an unusual hump of some sort (about 5"w x 2"d x 0.5"h) right in the middle of the trunk floor. Another ARGH moment.

So I'm not real sure what I am going to do yet. :crazy:

12-19-2002, 12:20 AM
Okay, I think I have it now. This box can be slid into the trunk through the cabin of the car with the seats folded down and provides 0.808 ft^3 per chamber after the speaker displacement is deducted from the actual Vb.

Depth 1 (top) = 10.5 in
Depth 2 (bot) = 15.0 in
Height = 11.0 in
Width = 32.0 in
Divider @ 1/2 width (16 in center)

Material used = 3/4 in MDF
Speakers = (2) Alpine SWR-1021D

WHEW!!! :banghead:

The good news is that the depth of the top panel is sufficient to allow me to mount my amps horizontally and have nearly 2" of clearance between the top of the heatsinks and the bottom of the rear package tray. Plenty of room for airflow around the amps.

12-19-2002, 07:45 AM
I'll be sure to take pictures of the finished box and post them here. It's turned into something of an adventure. :D

12-19-2002, 05:36 PM
Just sent the SWR-1241D subwoofer back and will be ordering the two SWR-1021D subs as replacements next week (Thursday or Friday). Hope to have them in my hands before the end of the following week.

This is all good anyway as it gives me time to tinker around with cardboard mockups of the box design and make sure that I have all of my bases covered before I actually start installation. Heck, I may even build the box between now and then. :)

12-19-2002, 05:40 PM
Originally posted by Jmac
Where did you get the sub from ?

Good luck with the box :)

I ordered it from Ike Sound (www.ikesound.com). They didn't have the 1021D's listed so I called them up and found out that they had them anyway. :)

12-19-2002, 05:52 PM
Originally posted by Jmac
Did you have to pay for the shipping back ?

Sounds like pretty good customer service :)

Yeah, but we get a pretty good discount on FedEx service here at work so it didn't hurt too bad to ship it back to them. They should have it Monday and then I can set the ball into motion on those other subs.

I think they'd rather give me credit toward the new speakers rather than lose the sale all together... but yes, they do have good service from what I have seen so far. :)