I've been running an SVS SB-1000 for a while and I've finally hit the end of what it can do. It's a good little sub and I have no complaints about it for what it is — sealed 12" in about a cubic foot, 300 W, published at 24-260 Hz ±3 dB. But I keep noticing the bottom octave isn't there, and no amount of EQ fixes a box that small. So rather than buy something bigger I'm building one.
Going with a sonosub because I want the volume without the footprint, and because a 15.5" ID form tube is the cheapest 5 cubic feet you can buy. Everything is on hand now, so here's the design before I start cutting, partly to write it down and partly because if I've got something wrong I'd rather hear it from you lot than discover it after the glue sets.
The parts
net volume 122.9 L / 4.34 cu ft
tuning 15.0 Hz
port 4" ID, 30.99" assembled, flared both ends
alpha (Vas/Vb) 0.402
h (Fb/Fs) 0.556
cavity 45.75" between caps
overall height 55.4" including legs
estimated mass 84 lb, CoG 21.1" off the floor
That's an extended-bass-shelf alignment rather than a maximally flat one, which the tube volume more or less chose for me. Modelled half space at 1 m it comes out flat within ±0.4 dB from 14 to 28 Hz, then tilting up about 5 dB to 100 Hz, and −3 dB at 13.4 Hz relative to its own bottom-end plateau. The tilt gets a shelf filter in DSP and the low pass sits at 80 anyway, so I'm not bothered by it.
For context against what it replaces — SB-1000 is published at 24 Hz, this models to 13.4 Hz, though I'll be the first to say those are different measurements under different conditions and I'm not pretending it's a clean comparison. It's about four and a half times the internal volume and three times the power, so it ought to be a decent step regardless.
The part I'm less happy about
The 4" port is the limiting element in this design, not the driver, and I want to be upfront about it rather than have somebody point it out on page three.
Small's minimum vent diameter for Vd = 1.21 L at 15 Hz works out to 4.48". I've got 4.00", which is 12.57 in² — 15% of Sd. Near tuning that reaches 17 m/s peak, the usual ceiling for a flared port, at about 84 watts. So the bottom octave is port-limited, not amp-limited:
12 Hz 93.1 dB port-limited 117 W
15 Hz 98.9 dB port-limited 84 W
18 Hz 103.6 dB port-limited 253 W
20 Hz 106.3 dB port-limited 498 W
25 Hz 109.0 dB amp-limited 900 W
40 Hz 110.9 dB amp-limited 900 W
63 Hz 112.8 dB amp-limited 900 W
Half space, 1 m, no room gain — add 6-12 dB below 25 Hz in a real room.
I kept the 4" anyway, and here's why. Port length scales with area, so with diameter squared. A 5" port for 15 Hz in this box needs 52" and the cavity is only 45.75". A 4.5" needs 40", which fits, but leaves the inner mouth 5.75" off the bottom cap — less than one and a half diameters, close enough that the mouth starts loading against the cap. A 6" is off the table entirely at 76". So 4" is what physically fits at this tuning, and if I wanted more clean output down there the honest move would be to tune higher, not to fight the port. I may end up doing exactly that — the port length is the one thing that's trivial to change later.
The amp is going inside
The 1200AS1 is going on a shelf in the box rather than in the wall. The datasheet is specific about cooling: 0.8 K/W reference heatsink, 40°C ambient limit passively, PSU shutdown at 95°C, and 27 W dissipated at idle with 80% efficiency at rated power.
Sealed inside the tube, the whole cabinet becomes the heatsink. External surface is about 1.84 m², and through the cardboard wall with natural convection on both sides that's roughly 3.4 W/K, which gives:
27 W idle internal air 30°C baseplate 52°C fine
60 W sustained internal air 40°C baseplate 88°C at both limits
120 W sustained internal air 57°C baseplate 153°C shuts down
Bolted straight to MDF with no heatsink it doesn't even survive idle — the module's own baseplate into still air is about 4 K/W, which is 143°C on 27 W. So a real heatsink is going in there regardless.
Where it lands is that idle is comfortable and sustained high output isn't. Bass has enough crest factor that programme material rarely sustains 60 W of dissipation, but that is genuinely the argument I'm making to myself and I'd be interested if anyone has run a 1200AS-series module sealed in a sub long term. The clean fix is an aluminium plate let into the tube wall with the fins in room air, and I don't want to break the cylinder. Compromise is the biggest heatsink that fits, sited under the port's inner mouth but offset from the centreline — that's the only place in the box with real air movement, and it moves hardest exactly when the amp is working hardest.
Construction
Three internal braces: an amp shelf at 16", a brace at the port's free end at 27.4" that grips the tube above the flare rather than at it, and one at the port's mid point at 38.4". Port runs all the way up to the top baffle and exits through it, driver fires down through a 2.25" laminated bottom cap.
The end caps are a flush outer disc at tube OD + 2 mm so the vinyl wrap finishes level with no lip, plus inner discs that slip inside for glue area. I want it reading as one continuous cylinder rather than the usual chunky wood overhang — the legs overhang, nothing else does.
What's next
Cutting discs for this soon. Once it's together I'll run an impedance sweep to find where it actually tuned — the two inputs I'm least sure of are the driver's displacement, which Dayton doesn't publish, and the flares' end correction, which moves Fb by a couple of tenths either way. Worth noting the box air runs 8-18°C above room with the amp inside, and that alone lifts Fb another 0.2-0.45 Hz, so I'll be measuring it warm.
Going with a sonosub because I want the volume without the footprint, and because a 15.5" ID form tube is the cheapest 5 cubic feet you can buy. Everything is on hand now, so here's the design before I start cutting, partly to write it down and partly because if I've got something wrong I'd rather hear it from you lot than discover it after the glue sets.
The parts
- Dayton Audio SS12-22, 12" DVC, ±23 mm Xmax, Sd 528 cm², Fs 27 Hz, Qts 0.35, Vas 49.4 L
- 15.5" ID form tube, 48" long, staying uncut
- ICEpower 1200AS1, mounted internally
- DSP, connectors
- Five 24×48 3/4" MDF hobby panels
- 4" Schedule 40 PVC port with flares on both ends
- 1 mm vinyl wrap, 6" metal legs, three of them in a tripod
net volume 122.9 L / 4.34 cu ft
tuning 15.0 Hz
port 4" ID, 30.99" assembled, flared both ends
alpha (Vas/Vb) 0.402
h (Fb/Fs) 0.556
cavity 45.75" between caps
overall height 55.4" including legs
estimated mass 84 lb, CoG 21.1" off the floor
That's an extended-bass-shelf alignment rather than a maximally flat one, which the tube volume more or less chose for me. Modelled half space at 1 m it comes out flat within ±0.4 dB from 14 to 28 Hz, then tilting up about 5 dB to 100 Hz, and −3 dB at 13.4 Hz relative to its own bottom-end plateau. The tilt gets a shelf filter in DSP and the low pass sits at 80 anyway, so I'm not bothered by it.
For context against what it replaces — SB-1000 is published at 24 Hz, this models to 13.4 Hz, though I'll be the first to say those are different measurements under different conditions and I'm not pretending it's a clean comparison. It's about four and a half times the internal volume and three times the power, so it ought to be a decent step regardless.
The part I'm less happy about
The 4" port is the limiting element in this design, not the driver, and I want to be upfront about it rather than have somebody point it out on page three.
Small's minimum vent diameter for Vd = 1.21 L at 15 Hz works out to 4.48". I've got 4.00", which is 12.57 in² — 15% of Sd. Near tuning that reaches 17 m/s peak, the usual ceiling for a flared port, at about 84 watts. So the bottom octave is port-limited, not amp-limited:
12 Hz 93.1 dB port-limited 117 W
15 Hz 98.9 dB port-limited 84 W
18 Hz 103.6 dB port-limited 253 W
20 Hz 106.3 dB port-limited 498 W
25 Hz 109.0 dB amp-limited 900 W
40 Hz 110.9 dB amp-limited 900 W
63 Hz 112.8 dB amp-limited 900 W
Half space, 1 m, no room gain — add 6-12 dB below 25 Hz in a real room.
I kept the 4" anyway, and here's why. Port length scales with area, so with diameter squared. A 5" port for 15 Hz in this box needs 52" and the cavity is only 45.75". A 4.5" needs 40", which fits, but leaves the inner mouth 5.75" off the bottom cap — less than one and a half diameters, close enough that the mouth starts loading against the cap. A 6" is off the table entirely at 76". So 4" is what physically fits at this tuning, and if I wanted more clean output down there the honest move would be to tune higher, not to fight the port. I may end up doing exactly that — the port length is the one thing that's trivial to change later.
The amp is going inside
The 1200AS1 is going on a shelf in the box rather than in the wall. The datasheet is specific about cooling: 0.8 K/W reference heatsink, 40°C ambient limit passively, PSU shutdown at 95°C, and 27 W dissipated at idle with 80% efficiency at rated power.
Sealed inside the tube, the whole cabinet becomes the heatsink. External surface is about 1.84 m², and through the cardboard wall with natural convection on both sides that's roughly 3.4 W/K, which gives:
27 W idle internal air 30°C baseplate 52°C fine
60 W sustained internal air 40°C baseplate 88°C at both limits
120 W sustained internal air 57°C baseplate 153°C shuts down
Bolted straight to MDF with no heatsink it doesn't even survive idle — the module's own baseplate into still air is about 4 K/W, which is 143°C on 27 W. So a real heatsink is going in there regardless.
Where it lands is that idle is comfortable and sustained high output isn't. Bass has enough crest factor that programme material rarely sustains 60 W of dissipation, but that is genuinely the argument I'm making to myself and I'd be interested if anyone has run a 1200AS-series module sealed in a sub long term. The clean fix is an aluminium plate let into the tube wall with the fins in room air, and I don't want to break the cylinder. Compromise is the biggest heatsink that fits, sited under the port's inner mouth but offset from the centreline — that's the only place in the box with real air movement, and it moves hardest exactly when the amp is working hardest.
Construction
Three internal braces: an amp shelf at 16", a brace at the port's free end at 27.4" that grips the tube above the flare rather than at it, and one at the port's mid point at 38.4". Port runs all the way up to the top baffle and exits through it, driver fires down through a 2.25" laminated bottom cap.
The end caps are a flush outer disc at tube OD + 2 mm so the vinyl wrap finishes level with no lip, plus inner discs that slip inside for glue area. I want it reading as one continuous cylinder rather than the usual chunky wood overhang — the legs overhang, nothing else does.
What's next
Cutting discs for this soon. Once it's together I'll run an impedance sweep to find where it actually tuned — the two inputs I'm least sure of are the driver's displacement, which Dayton doesn't publish, and the flares' end correction, which moves Fb by a couple of tenths either way. Worth noting the box air runs 8-18°C above room with the amp inside, and that alone lifts Fb another 0.2-0.45 Hz, so I'll be measuring it warm.
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