Idea: a pipe subwoofer that is triple-use: room sub for a 2.1 system, practice bass amp, and bass guitar travel case that I can put my weird small scale bass guitar in. Made out of 20cm PVC pipe (5mm wall thickness) that I already have and including an amplifier – I already bought a TPA3255 module and an 48V 200W Meanwell PSU for it.
My first idea was a transmission line to extend bass, with one or multiple drivers on one end. For instance, with a single 8″ SB20PFC30-4 driver, a 210cm pipe has its -3dB point around 40Hz and 180cm pipe at around 45Hz. However, at 120cm response gets rather peaky with -3dB around 65Hz. The pipe I have is long enough for the longer options, but the shorter one is much more practical.
Second idea is just a “closed box” – a PVC pipe closed on either end. Perhaps 2×6.5″ would be better than 1×8″, with a driver on either end. My bass guitar is 92cm long, the 6.5″ driver is 8cm deep. Minimum total pipe length would be 108cm so let’s go with 120cm. If I use 2x SB16PFC25, Qtc is about 0.74 which is close to ideal and should be pretty musical. Response is typical for a closed box, with -3dB at around 75Hz and shallow roll-off below that. Max SPL (xmax limited) in half space:
| F3 | max SPL 40Hz | max SPL 60Hz | max SPL 80Hz | Driver weight | Qtc (120cm) | |
| 1x 8″ SB20PFC30 | 75 Hz | 95 dB | 102 dB | 107 dB | 1.43 kg | 0.60 (tight) |
| 2x 8″ SB20PFC30 | 70 Hz | 101 dB | 107 dB | 112 dB | 2.86 kg | 0.94 (boomy) |
| 1x 8″ HW202 | 50 Hz | 95 dB | 103 dB | 108 dB | 1.5 kg | 0.90 (boomy) |
| 1x 8″ HDS-P830869 | 65 Hz | 97 dB | 104 dB | 108 dB | 1.74 kg | 0.65 (ideal) |
| 2x 6.5″ SB16PFC25 | 65 Hz | 94 dB | 100 dB | 105 dB | 1.92 kg | 0.74 (ideal) |
| 1x 10″ SSF102.40 | 50 Hz | 100 dB | 107 dB | 112 dB | 3.5 kg | 0.81 (punchy) |
| 1x 5×8″ SB15SFCR39-4 | 70 Hz | 93 dB | 100 dB | 105 dB | 1.65 kg | 0.53 (tight) |
| 2x 5×8″ SB15SFCR39-4 | 65 Hz | 98 dB | 105 dB | 110 dB | 3.3 kg | 0.76 (ideal) |
| 1x 12″ W250S-4 | 100 Hz | 97 dB | 105 dB | 110 dB | 3.0 kg | 0.68 (ideal) |
With the 1x 8″ option, the driver could be permanently attached and the other side could be an easy-access cap for putting the bass guitar in, or it could house the knobs and connectors and perhaps the amp. For the 2x 8″ option, all the amp things would need to be connected to the side of the pipe. For the 2x 6.5″ option, I could perhaps mount the drivers off-centre and use the extra space for knobs and connectors. In any case, 3d printed end cabs would probably the easiest.
Not sure yet how to make one side accessible to put the bass guitar in but also remain rigid enough as a speaker enclosure, especially if that side also houses a driver. Let’s go with the 1×8″ driver for now. I doubt I need the extra volume, and it just seems a better option than 2×6.5″.
For protection from overexcursion at high volume best pair with a high pass filter to bring xmax down to Pmax. Could probably make a switchable first order high pass filter using capacitors – e.g. 220+470µF in parallel (€9.45 at Reichelt) would be 690µF or 58Hz at 4 ohm.
If I want to pair this pipe speaker with my existing 2×6″ cabinet, I need to take into account sensitivity (in case I want to run parallel) and phase response. Looks like phase doesn’t match very well – matches ok up to 100Hz but switches to 180 degrees out of phase from 300Hz on. Guess I could use a high crossover to match up better. 150µF (€3.97 at Reichelt) at 4 ohm would be 265Hz 1st order: 0 degrees phase at 150Hz and -180degrees phase at 350Hz for 2×6″, 0 degrees and -60degrees for 1×8″ pipe subwoofer. Maybe something else and/or reversing polarity?
Amplifier: TPA3255 according to its datasheet could do 250W into 4ohm at 48V or 445W into 2ohm at 1% THD+N single channel PBTL, or 2x4ohm 235W or 2x8ohm 130W dual channel. I doubt this Chinese board can even reach half of the datasheet rating, but then most of the above options have a program power less than that, so should be enough power even if I connect the 2×6″ cabinet in parallel or on a second channel.
Perhaps for bass guitar, 2×8″ would make more sense SPL wise, but then (besides the boomy Qtc) there would be significant comb filtering because of the 120cm driver distance: cancellation at 143Hz, peak at 286Hz, cancellation at 429Hz etc. Maybe a 1×10″ or 1×12″ option would be better, but that’s hard to mate to the 20cm pipe and even heavier than 2×8″. A 10″ LaVoce SSF102.40 for instance would get about the same max SPL at all frequencies as well as better low end extension, although at 8ohm and lower sensitivity it would need a better amplifier to drive it to its maximum. 2x 5×8″ (SB15SFCR39-4) could work, but side by side are even bigger than 1×10″.
So far 1×8″ HDS-P830869 is looking best, with good F3 and Qtc, low weight, single driver, high xmax. Would need a 46Hz 1st order high pass to bring xmax down to Pmax – a single 470µF capacitor, €5.48 at Reichelt, would be 42Hz, close enough.
Next up: think very hard if I want this project.
- Would be nice for a 2.1 system because it can go very low, would still need some DSP to get response flat but that’s okay. Alternatively, with DSP I can get the same SPL down to 70Hz with my current bass practice amp, but because of the horn reflex the maximum SPL drops quickly below that. Maybe try first if it can already do what I need in a 2.1 system before I continue with this pipe woofer project. Might want to install the TPA3255+PSU in my current practice amp instead.
- For bass practice, again my current practice amp is better suited >75Hz, and this project would only extend bass down at lower volumes. ~100dB max SPL at half space might still be enough for my needs though.
- For bass travel case, I guess this would be great, except without the speaker it would be lighter and easier. Only makes sense if I always want a practice amp together with my bass, which I guess I do most of the time? I don’t currently have a band to play with and if I did play with a drummer, this project would likely not be loud enough (not sure if the current 2×6″ would be either). Carrying both the pipe and the current 2×6″ cab is maybe doable but not ideal with other luggage as well.
Edit:
- SB20PFCS30-7: Qtc 1.20, very peaky
- SB20PFCR30-4: Qtc 0.60, exactly as SB20PFC30-4
- SB23NRXS45-4: Qtc 0.59, nice response, F3 65Hz, +1dB max SPL compared to SB20PFC30-4, expensive
- SB23N(B)ACS45-4: Qtc 0.67, nice response, F3 50Hz, +1dB max SPL compared to SB20PFC30-4, expensive
- SB23CACS45-4: Qtc 0.68, almost exactly as SB23NBACS45-4, expensive
- SB23MFCL45-4: Qtc 0.50, tight response, F3 60Hz, +5dB max SPL compared to SB20PFC30-4. Very heavy & expensive
- SLS-P830667: Qtc 0.98, boomy
Some more options:
| F3 | F6 | max SPL 60Hz | Qtc | Weight | Price | |
| SB20PFC30-4 | 75 Hz | 53 Hz | 102 dB | 0.60 | 1.43 kg | € 36.84 |
| DC200-8 | 55 Hz | 40 Hz | 99 dB | 0.67 | 1.57 kg | € 42.90 |
| HDS-P830869 | 65 Hz | 45 Hz | 104 dB | 0.65 | 1.74 kg | € 56.90 (out of stock) |
| DA215-8 | 70 Hz | 50 Hz | 101 dB | 0.58 | 2.45 kg | € 63.90 |
| CW202 | 65 Hz | 50 Hz | 103 dB | 0.76 | 2.40 kg | € 69.66 |
| GBS-200F35CP02-04 | 70 Hz | 50 Hz | 105 dB | 0.52 (but very peaky resp.) | 2.28 kg | € 69.95 |
| HWB200-4 | 65 Hz | 45 Hz | 104 dB | 0.65 | 2.75 kg | € 79.38 |
| 2x SB15SFCR39-4 | 70 Hz | 50 Hz | 105 dB | 0.76 | 3.40 kg | € 85.42 |
| SIG225-4 | 70 Hz | 50 Hz | 102 dB | 0.61 | 2.63 kg | € 89.00 |
| RS225-4 | 55 Hz | 40 Hz | 102 dB | 0.67 | 2.98 kg | € 89.00 |
| RS225-8 | 55 Hz | 40 Hz | 103 dB | 0.64 | 2.98 kg | € 89.00 |
| DCS205-4 | 60 Hz | 45 Hz | 106 dB | 0.51 (but very peaky resp.) | 3.33 kg | € 89.95 |
| FW222 | 55 Hz | 40 Hz | 104 dB | 0.78 | 2.30 kg | €104.95 |
| SB23NBACS45-4 | 50 Hz | 35 Hz | 103 dB | 0.67 | 2.70 kg | €116.95 |
| 8H2CP | 55 Hz | 42 Hz | 103 dB | 0.72 | 2.70 kg | €121.18 |
| SB23MFCL45-4 | 60 Hz | 40 Hz | 108 dB | 0.50 | 4.70 kg | €137.26 |
HDS-P830869 seems to be the best balance of max SPL, F3 and weight at a decent Qtc, but is currently out of stock likely until December/January. Fountek FW222 would be great, lower F3 and comes with a built in grill, but is a little more expensive and heavier. SB23MFCL45-4 is basically the price, weight and SPL of two drivers in one. It is also the only option which gets possibly high enough max SPL to play with a drummer, without being peaky.
Rough weight estimate:
- PVC pipe: 5 kg
- Baffle & end caps: 1 kg
- Driver: 2-5 kg
- Bass guitar: 4 kg
- Amplifier & PSU: 1 kg
- Total: 13-16 kg
In other words driver weight doesn’t affect total weight a whole lot.
Maybe I should just experiment with the cheap option first, either DC200-8 or SB20PFC30-4 + EQ the low bass response up, and if it’s not loud enough I can always get the heavier more expensive option later. Or wait for the HDS-P830869 to come back in stock.
Idea: make the driver side baffle slanted. Then the pipe can stand up on its back vertically and still point its driver roughly towards listener(s), plus it gives space for knobs and connectors and possibly amplifier + PSU. Could even rotate the drivers 90 degrees and possibly mount 2×8″ vertically on the same baffle. Would be a big 3D printed piece though, and not many options to get a decent Qtc with two drivers. And probably much easier to keep the baffle straight and just point the whole pipe using legs.
Idea: make it a horn reflex cabinet instead. 1×8″ with SB20PFC30-4 and a slightly increasing reflex port from 54cm² to 108cm² brings F3 down to 41Hz, F6 to 37Hz, max SPL 108dB@60Hz/105dB@40Hz – the latter is louder even than the SB23MFCL45-4 ind a closed cabinet. The horn/port could be external, in a 3D printed end cap that inverts for travel. Some max SPL is lost below 30Hz but I don’t need that anyway.
Or a regular reflex cabinet with SB23MFCL45-4, straight 54cm² port brings F3 down to like 25Hz, max SPL 111dB@60Hz/108dB@40Hz. But the port would have to be unrealistically long.
Idea: for home 2.1 sub use, two opposing drivers (push-push) would be ideal to completely cancel cabinet vibrations. Some design ideas: putting the drivers on either end of the tube, or perpendicular to the tube facing outwards, or slot loaded facing inwards. None of these are ideal for frequencies above subwoofer level, because of comb filtering, but maybe not critical for bass guitar practice. Horn reflex 2×6.5″ or even 4×5″ with back-to-back drivers?
Idea: dual speaker baffle fits inside pipe, inverts after travel to give bigger volume. Nice idea but even with 5″ speakers doesn’t fit inside 20cm pipe, can only work with non-opposing driver up to 6.5″, or 5″ if sideways and I also want to include the amp+PSU in the same module. Not ideal with drivers at opposite ends, and/or non-integrated amp because of the cabling. So would only work with a vertical array of 5″ speakers on one baffle, or a single 6.5″ at the top.
Idea: passive radiator on one side. Available PRs are tuned too low to be useful, would need smaller box and 2x PR Sd*Xmax compared to driver Sd*Xmax.
So far dual 6.5″ or 8″ (one on each side of the pipe) with a port seems to make the most sense if I want to keep with the multipurpose idea. Could possibly make one side extendable, uncovering knobs and connectors and housing amp+PSU. In any case, not sure how I would get a non-leaky fit between the baffle and pipe end that can be opened to get the bass guitar out.
Either that, or drop the 2.1 requirement and make it a 1×8″ or even 1×10″ design with less low end focus, more efficiency focus. Use a separate woofer for the 2.1 system which is more focused on low end extension but at low volume and without having to play beyond 150Hz, focused on small box and no vibration (e.g. two opposing 5″ perhaps with opposing passive radiators).