Small-ish simple subwoofer idea - diyAudio
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Old 5th July 2013, 10:15 AM   #1
Seb C is offline Seb C  United Kingdom
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Default Small-ish simple subwoofer idea

Hi All,

I've been planning my new (and first ever) DIY HT/music setup for a little while, and have been playing around modelling a few subwoofers. The constraints are all too familiar: can't be expensive, must be small, can't be ugly (WAF). An agreement has been struck: I can build a subwoofer, as long as it can serve dual duty as a side table!

Size constraints are (more or less) no more than 30cm wide, about 45 long (max) and 45cm tall (max).

I was almost set on going down the Peerless XLS 10" route (830452), but that really doesn't seem to model too nicely without using a PR, which I'm trying to avoid on cost grounds as much as anything.

One design I knocked up (apologies, can't post a picture from work) was a dual JBL GTO 804 setup. In brief: Two 8" subs, in about 40-45 litres, port tuned to mid-late 20's Hz, it looks pretty darn flat down to 25Hz, and with two 120w RMS amps (which I already have) specs to about 105dB at 25Hz (within excursion limits) ignoring any room gain. Group delay looks reasonable, XMax isn't reached until ~120w at under 20Hz. I know I'm not going to get any real 20Hz and under authority out of two low-powered 8s, but it should have some authority at 30Hz which will beat anything I've had before. Being a simple box I can knock it up fairly easily, veneer it with iron-on oak(!) and put a real oak top on it = instant side table! The JBL 8s are also very cheap in the UK - a pair can be had for about £70...

Any thoughts? Sound like a good plan? Is 105dB at 25Hz a reasonable goal for this sort of thing (no demands of 120dB @ 20Hz in 20 litre box with a £100 budget or anything!)

Thanks,

Seb
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Old 5th July 2013, 10:29 AM   #2
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I have a pair of those drivers and they are good. Your plan is OK, but you may find it difficult to keep port noise down, i.e. you need a lot of mouth area which means a long port. I looked at a slot port that ran along the bottom, up the rear and folded back down, almost like a transmission line.
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Old 5th July 2013, 10:37 AM   #3
Seb C is offline Seb C  United Kingdom
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Exactly what I was thinking. I was planning on using the floor as the bottom face of the port, running under the enclosure, then turning the corner. So three sides of the box would sit on the floor, the fourth cut short to reveal the mouth. I radius on that edge could help port noise too, I guess.
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Old 10th July 2013, 09:36 AM   #4
Seb C is offline Seb C  United Kingdom
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Okay, I did a little more WinISD modeling on this last night. I came to a point where I wasn't sure which way to go in the design.

I can get flat to 26Hz, then pretty steep drop-off below that (F10 maybe 23Hz - really steep!) or a slightly falling response to about 26Hz - not flat- but then a more gradual fall off below that. What would be the preferred option?
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Old 10th July 2013, 10:37 AM   #5
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You should keep an eye on group delay and cone excursion too.
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Old 10th July 2013, 11:04 AM   #6
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I'll be watching this thread closely, looks like a good small sub! Good luck!
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Old 10th July 2013, 02:41 PM   #7
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Yes, look at your group delay and excursion curves. What is your tuning frequency in either case? I suspect the second option will be better in practical use.
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Old 10th July 2013, 03:45 PM   #8
tuo is offline tuo  Finland
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You are building pretty similar what I'm doing at the moment. The result should be a subwoofer with a TangBand W8-670 element with a plate amp. The enclosure will be 23 liters with a tuning frequency of 30 Hz, the 3 dB roll-off at 25 Hz. The size will be 40 cm * 32 cm * 38 cm (H * W * D) and the port will be flat on the bottom.

I already started a new thread with a title: Subwoofer with Postal Money.
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Old 10th July 2013, 07:02 PM   #9
Seb C is offline Seb C  United Kingdom
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Having a quick look at this again now. XMax at around 20Hz depending on tuning with 120w RMS. What's my target for group delay?

I saw your thread tuo. I'll be keeping an eye on it
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Old 11th July 2013, 03:19 AM   #10
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How about using a nozzle vent or bowtie-shaped shelf vent? It should reduce chuffing by quite a bit. You'll need to use something like HornResp to model the results though. I've been seriously tempted to try something like that since reading a research paper on the subject of nozzle vents.
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