Help with building an ultimate small subwoofer

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soundcrave

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#1
Hi everyone,

I'd like to try my hands on building my own subwoofer for fun. As much as I would like to build a big mega sub, I just can't

I am quite limited in size. I'd really prefer something no bigger than a 15'' or 16'' cube. However, if I can do a lot better, I can forgo the 16'' limit on the height dimension (but not the width or depth).

I want as much deep bass output as possible with good sound quality. While this will be used for 90% music, but HT performance is my priority in this build (otherwise I would have just bought a JL subwoofer). However, I tend to watch movies louder, and I want as much room shaking deep bass as I can get. The room is about 3000 cubic feet.

The budget is quite large, up to $2000 including the amplifier since the alternative is buying something like the JL E112. I think I’m gonna get a Behringer iNUKE 3000 or 6000 for power.

Any recommendations on the design? What would give me the best HT performance? Should I do 12'' with passive radiators or dual opposed 12'' or a 15'' sealed? What drivers should I be looking at?

Thanks guys
 
#2
You can get excellent low bass with going "mega" if you position the sub so that you are in the near field. This would be right next to or behind your seat, essentially. Since the sub is near you, it does not need to generate as much raw SPL as if it was across the room, and the frequency response will be much less effected by the room modes, e.g. smoother.
 
#3
If you go 16x16x22 tall and using 1" thick sides, you end up 14x14x20 internal for volume of about 2 cu ft (taking off some for the driver, bracing and amp). Lots of good 12" drivers will work in that volume sealed and can be equalized for deep bass. You should be able to do this for about 1/4 of your budget if you build the cabinet yourself.
 
#4
I use a pair of JBL GTO1214s in a box about that big. Bridged QSC USA850 with drivers in series.
Amp clips as the drivers run out of excursion. Lots of EQ to get anywhere, but a cheap way of getting pretty low, pretty loud out of a small box.

Chris
 
#5
Thanks for the suggestions, but I'm not looking to save money here. I want the absolute best performance I can build within the dimensions constraint.

What is the disadvantage of having very low internal volume for the drivers besides a loss of deep bass efficiency (which I can overcome by using a really powerful amp)?
 
#6
Remember that as you throw more power into the coil, you're going to get a number of effects:

- driver heating: with small enclosure volumes, you get little excursion for lots of power. Cooling is compromised and you should derate the drivers accordingly.
- deformation of the magnetic field: these days, the high-end drivers have copper plated pole pieces. The physics is more than I want to go into right now, so here's the quick version - you've got a permanent magnetic field (from the magnets bolted to the speaker), and then you're putting another magnetic field in the same space (by putting current through the coil). Ideally, the permanent field would remain perfectly still, propelling the cone forwards. In reality, the permanent field is deformed. This gives rise to (IIRC) primarily 2nd and 3rd order distortion. The copper in the magnetic circuit allows eddy currents to flow, so the permanent field better resists deformation.


Look at Dayton Ultimax, and, given that you want loud HT, set the dynamic EQ on the DSP amps to avoid excursion issues.

Chris
 
#7
The funny thing with speakers is that the best driver for a given situation may not be the most expensive. As Chris stated you run into power issues pushing hundreds of watts in a small cabinet size. You need to read up on Hoffman's iron law. In a nutshell it says that if you want big low bass, you need big cabinets. You can't buy your way out of a physical limitation.

I have two 6 cu/ft ported cabs tuned at 24 Hz with 15" drivers. Connected in series for 16 ohm load, I connected a tiny chip amp powered by a 9 volt battery. At 35 Hz, it pushes 3.2 volts rms or 640 milliwatts total power into the speakers (neglecting the actual impedance of them at 35Hz). I have a sealed 1.7 Cu ft. sub with singe 12" driver. To match the SPL of the big subs at 640 mw, I had to drive the smaller sub with 20 watts or 31.25 times the power. Now scale that by putting twenty watts into the big subs (10w each) and I'd need 625w to match them with the small sub. I'd have to figure in compression and I'd be over 1000 watts. Won't happen with that small sub.
 
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#8
 
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#9
soundcrave said:
Thanks for the suggestions, but I'm not looking to save money here. I want the absolute best performance I can build within the dimensions constraint.

What is the disadvantage of having very low internal volume for the drivers besides a loss of deep bass efficiency (which I can overcome by using a really powerful amp)?
Click to expand...

The disadvantage of a low volume (low ratio of internal volume to maximum swept volume of the cone, equal to Sd*Xmax) is increased DISTORTION! This is why putting a driver in a small box and then using 200W of power to "extend and equalize" the response is not the best for sound quality. "Muddy" or "flabby" are some of the words thrown around for a high distortion sub.

If you truly want small size as well as low end output that is extended and clean, you should consider Rythmic audio products. They have a proprietary feedback between driver and amp that reduces distortion quite a bit. You can buy driver(s)+amp combos from them and then build the enclosure yourself if you want to DIY it. This will be much better than anything you can come up with yourself from scratch. Link:
Rythmik Audio • Servo subwoofer products
 
#10
johnr66 said:
You need to read up on Hoffman's iron law. In a nutshell it says that if you want big low bass, you need big cabinets. You can't buy your way out of a physical limitation.
Click to expand...

You get the general idea about Hoffman's Iron Law, but are missing a critical word: efficiency.

Hoffman's iron law describes the tradeoff between efficiency, corner frequency (related to Fb, F3, and bass extension) and enclosure volume.

You can still get "low bass" in small cabinets (albeit with somewhat increased distortion) if you compensate for the resulting low system efficiency with a lot of input power. This is why you see on the order of many hundreds to thousands of Watts of power for subwoofers used today. That is at least 10x more than the nominal power level for midband/midrange drivers. If instead the cabinet volume were increased 10x you could use 10x less power and get similar extension. But wifey no likey.

Some good info here:
GlassWolf's Page on Hoffman's Iron Law
 
#11
CharlieLaub said:
You get the general idea about Hoffman's Iron Law, but are missing a critical word: efficiency.

Hoffman's iron law describes the tradeoff between efficiency, corner frequency (related to Fb, F3, and bass extension) and enclosure volume.

You can still get "low bass" in small cabinets (albeit with somewhat increased distortion) if you compensate for the resulting low system efficiency with a lot of input power. This is why you see on the order of many hundreds to thousands of Watts of power for subwoofers used today. That is at least 10x more than the nominal power level for midband/midrange drivers. If instead the cabinet volume were increased 10x you could use 10x less power and get similar extension. But wifey no likey.

Some good info here:
GlassWolf's Page on Hoffman's Iron Law
Click to expand...

That's why I said big low bass, not just low bass.
 
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#13
Cal Weldon said:
If you need a small box, perhaps consider a Linkwitz Transform.

Linkwitz Transform Subwoofer Equaliser
Click to expand...

An LT just shapes the frequency response by applying more power the the lower frequencies to boost them up. It's essentially a special type of shelf network. That won't help with distortion or power, those problems remain and are related to the relative sizes of enclosure and driver.
 
#17
Small box with Linkwitz transform and fill the inside with a bladder containing activated charcoal and xenon gas. That should give you perhaps 50% more effective volume in the chamber. Xenon is pricey stuff though...

The activated charcoal is porous and acts like ultra-stuffing, the xenon has a heavy molecular weight and slows the speed of sound down, making the box, effectively larger. Buy a nice Tedlar or similar gas sampling bag from a chemical lab supply company - it won't leak like a latex balloon.
 
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#19
It's no joke - activated charcoal was used in Celestion commercial product and measurement of xenon or other heavy gas filled speaker indeed shows larger effective volume due to slower speed of sound. Airtight sealed alignments only - use bladder for has full as most drivers and enclosures are not gas tight.
 
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