I'm looking at making a subwoofer to pair with a set of small full-range speakers, but the catch is that the only place I can really fit it is underneath the TV cabinet, which gives me maximum ~100mm. I've found what appears to be a decent driver, albeit small, in the Peerless SLS-85S25CP04-04. My thoughts were to use a pair, and have modelled them in a ported enclosure around 4.5 - 5L, tuned to 38Hz. The plan would then be to add a LPF somewhere in the region of 80-100Hz in order to cross effectively with the full-range set.
Anyway, that's the background, but the main question I have is around the placement and orientation of the two drivers. Originally, I was thinking facing opposite walls, horizontal in terms of listening position, but this brings the magnets mighty close to the top and bottom walls (using 12mm ply). The important parts seem to be around 68-72mm on the diagrams, and the enclosure would have an internal height of 78mm max. Would this be a major issue for this application?
Second thought was having them facing up, but this introduces the associated problems with upward-firing speakers (and fires directly into the TV cabinet). It gives them a lot more room to breathe, however, though the magnet is then about 15-20mm from the base.
I've attached some images to illustrate the two options (I'm aware I've probably over-engineered the bracing, but I was aiming to have it CNC milled). The grey cylinders are an approximation of the speaker at it's widest points. The black tube is the port.
Option 1 - the "lid" would be inset, so sitting on top of the cross-beams.
Option 2
Any thoughts or obvious errors? Does it even matter if it's going to be booming straight off the TV cabinet above?
Anyway, that's the background, but the main question I have is around the placement and orientation of the two drivers. Originally, I was thinking facing opposite walls, horizontal in terms of listening position, but this brings the magnets mighty close to the top and bottom walls (using 12mm ply). The important parts seem to be around 68-72mm on the diagrams, and the enclosure would have an internal height of 78mm max. Would this be a major issue for this application?
Second thought was having them facing up, but this introduces the associated problems with upward-firing speakers (and fires directly into the TV cabinet). It gives them a lot more room to breathe, however, though the magnet is then about 15-20mm from the base.
I've attached some images to illustrate the two options (I'm aware I've probably over-engineered the bracing, but I was aiming to have it CNC milled). The grey cylinders are an approximation of the speaker at it's widest points. The black tube is the port.
Option 1 - the "lid" would be inset, so sitting on top of the cross-beams.
Option 2
Any thoughts or obvious errors? Does it even matter if it's going to be booming straight off the TV cabinet above?
The magnet proximity to walls is no concern, unless vents are blocked.
With a LPF in the region of 80-100Hz orientation will be no concern, other than the potential of cone sag over time.
Tuned to 38Hz, a 75.5Hz Fs driver will have be quite a ways down in response at Fb.
Around 1/3 octave above Fb (~50Hz), displacement will be at maximum, a pair of 3.5" could only do around 88dB at 1 meter.
Using something like the Dayton Audio LW150-4 6" Low Profile Woofer (62.6Fs) a much larger cone could be used, increasing output potential to around 96dB, percieved as near four times as loud at low frequencies.
96dB SPL at 50Hz is still not much in terms of subwoofer output, because of our hearing's "equal loudness contours" it would sound around as loud as 70dB at 1kHz, conversational level. 88dB would be closer to ~50dB at 1kHz, about half the volume most would watch a TV at.
Using a LPF in the region of 80-100Hz, location of the subwoofers will make little difference, you might look for alternative locations (flower pot stand, under coffee table, room corners, etc.) that allow more volume to fit larger drivers.
If your TV cabinet is wide, you would have room for a lot more cabinet volume, which could fit a small tapped horn, allowing around 6dB more output.
Art
Very informative, thank you! I think I fell into the trap of believing the numbers without really knowing fully the implications on SPL etc.
There's certainly more room to play with under the cabinet but not loads, probably about 500 × 300mm. I'm always a little scared of horns and TLs and such, just as I don't really know how to calculate them. Do you know of any designs or resources I could take a look at?
I did look at larger diameter, shallower drivers but decided against firing upwards originally.
Larger diameter, shallower drivers could be slot loaded, the slot could be pointed to the sides or forward.
Multiple small drivers with more excursion capability and lower Fs than the SLS-85S25CP04-04 could be used, though in general smaller drivers cost more for a given displacement (Sd times Xmax).
Your port looks small even for the little drivers, keep an eye on port velocity and excursion in your simulations.
Don't know if you have enough space available to gain any real advantage from TL or TH, let us know the total maximum cabinet dimensions you can use.
Multiple small drivers with more excursion capability and lower Fs than the SLS-85S25CP04-04 could be used, though in general smaller drivers cost more for a given displacement (Sd times Xmax).
Your port looks small even for the little drivers, keep an eye on port velocity and excursion in your simulations.
Don't know if you have enough space available to gain any real advantage from TL or TH, let us know the total maximum cabinet dimensions you can use.
As Dan pointed out there's plenty of location options because they only need ~34400/2/100 = 172 cm ctc to couple and lots more with DSP, which may give you larger cab options and if there's a female around; disguising them as decorative planters /whatever helps too, just don't get caught doing too good a job like I did to the point where my then S.O. and her girlfriends and later with daughter/(boy) friend(s) tended to dominate my available free time to 'chill out' with my latest album purchase(s). 
A pair of those drivers in a 5-litre vented enclosure tuned to 38Hz produces the following response. This includes the effect of a 4th-order Linkwitz–Riley low-pass filter adjusted to give a −6dB acoustic response at 90Hz.
With 6.3W re 8ohms of power input, a maximum SPL of only 85dB can be obtained at 40Hz when Xmax is being reached at 30Hz, which is not a lot of SPL. Hence, with the above results in mind, the suggestion by @weltersys to use at least a pair of 6-inch drivers is a very good one.
Placing the drivers on opposite walls would be preferred, as this will result in a good amount of force cancellation. This will produce much less vibration transmission into the TV cabinet if the subwoofer system is mounted within its confines.
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A look at the Thiele–Small parameters for the LW150-4 driver seems to indicate that it really needs a much larger enclosure than a 5-litre one in order to work reasonably well. It has a relatively high Qts of 0.64, and using two drivers doubles the enclosure volume requirements.
Using a 28.8 litre enclosure tuned to 38Hz, we can expect something like the following response for 12.6W re 8ohms of input power, with a maximum SPL of 95dB at 40Hz with Xmax being reached below 30Hz. This seems to be a much more usable result from the SPL perspective and will provide a reasonable low-frequency response. However, the enclosure volume greatly exceeds the desired 5-litre enclosure size.
So I was looking to utilise an SMSL A50 Pro as the amplifier since it has HDMI/optical and a dedicated sub channel, which reportedly works down to 2ohm output, meaning I could run the pair in parallel rather than series in order to make the most of the input power.
Or am I oversimplifying things? (Or just getting them wrong).
Somewhere in the region of 300 × 500 × 100mm.
Another thought, not sure if a good or bad idea: diagonal mounting upwards, so not directly into the cabinet above but still allowing a larger driver...
The filtering on the SMSL A50 Pro amplifier looks like it utilizes 2nd-order (12dB/octave) slopes. I can't find any reference as to whether Butterworth or Linkwitz–Riley filters are used, but the relatively sharp filter knee suggests that they are Butterworth filters. The lowest low-pass filter setting seems to be −3dB at about 120Hz or so. I think that your maximum SPL is still going to be quite limited by the available Xmax of the small 3.5-inch drivers.
Keep in mind that the 38Hz tuning frequency of a 5-litre vented enclosure is going to require a very long port. I'd suggest increasing the vent tuning frequency to at least 50Hz if you really want to use the 3.5-inch driver. Significant chuffing is still likely to occur when using vent tubes of a reasonable length, but it may be possible to face the ports towards the wall.
Below is a simulation using the parallel connection and 20W re 2.0ohms of input power. A 120Hz 2nd-order Butterworth low-pass filter has been included. The driver's Xmax is exceeded below about 44Hz.
What loudspeakers are you using for the mains?
Currently modelled with a 20mm diameter, 120mm long tube. It would be aimed away from the listening position, towards the back wall.
Planning to use CHN-50P in very small enclosures, circa 1.7L due to further space restrictions.
I was considering adding a simple 2nd-order low pass inside the enclosure rather than solely relying on the amp.
I agree with using low profile driver with larger surface area to begin with. Also, if this will be powered separately you can use an undersized sealed cabinet. This will give you a bass bump that can be fixed with EQ. I believe this is called a Linkwitz Transform, but not sure. In any event, it lets you go smaller and deeper than you could otherwise. 🙂
That's around 11 liter gross, not enough volume for a useful tapped horn.
Neither good nor bad, other than the additional reduction of cabinet volume compared to horizontal slot loading.
And the additional difficulty of construction, I suppose.
Another thought - passive radiator instead of a port. Worth investigating?
The only advantage you would see is the reduction of the port volume compared to the PRs.
The PRs must have about double the displacement of the active drivers, and would be too heavy for horizontal mounting, so options are very limited in your particular enclosure shape.
Theoretically, that should work. Practically, you may find that the low-pass passive filter interacts quite a lot with the second impedance peak of the driver in the vented enclosure, and maybe even the first one, as it's not that far below the second peak. Overall, it may prove difficult to get the desired filtered response.
That's a very small diameter tube, so the velocity of the air in the port will likely become excessive at moderate power levels. However, the other design constraints may relegate this issue to a less important level.
I assume that you mean the Scanspeak Discovery 10F/8414G-10 4" full-range driver.
Since you are going to use another driver to extend the low-frequency response of the system, I would suggest using a closed-box enclosure. With a 1.7-litre enclosure, a good amount of filling can be used to attenuate the rear radiation from the driver, and the 2nd-order roll-off may prove to be helpful when trying to blend the output with that of the subwoofer.
A VituixCAD simulation of the 10F/8414G-10 in a 1.7-litre closed-box enclosure is shown below. It only takes 5.0W re 8ohms of input power to hit the Xmax limit below about 100Hz or so, while producing about 94dB SPL in the driver's passband.
To improve the bass output of the subwoofer when it is blended in with the 10F/8414G-10-based main speakers, it might be worth considering increasing the vent tuning frequency of the subwoofer from 38Hz to 45Hz.
A working 12 dB/oct passive filter for a sub will usually need more than just a coil and a cap. Even if you get the values right with the first try, the large parts are close to or more expensive than a small amp with a DSP and power supply. Have a look at the BRU5 amp on Aliexpress or the like. It will cost about the same you have to pay for a large coil... only difference the result will be a light year better.
Don't make things to complicated and don't get picky with a sub amp. This one is OK.
Just construct the largest cabinet you can integrate and present it here. You will get many working options, closed or vented, that will give a pleasant result. The small drivers you picked are the last I would use. There are quite some nice, flat drivers in a reasonable price range.
6" or maybe even 8" is nice and may even be useable with a little larger satellites (later).
https://doc.soundimports.nl/pdf/bra...200F35CP02-04/GBS-200F35CP02-04-datasheet.pdf
A closed cabinet, paired with a DSP and quite some power should be the most simple and probably best sollution.
Just give a volume and dimensions. You have no unsolvable problem for sure.
Construction will not be complicated at all, but you need to use your hands, a jig saw and a drill.
Don't make things to complicated and don't get picky with a sub amp. This one is OK.
Just construct the largest cabinet you can integrate and present it here. You will get many working options, closed or vented, that will give a pleasant result. The small drivers you picked are the last I would use. There are quite some nice, flat drivers in a reasonable price range.
6" or maybe even 8" is nice and may even be useable with a little larger satellites (later).
https://doc.soundimports.nl/pdf/bra...200F35CP02-04/GBS-200F35CP02-04-datasheet.pdf
A closed cabinet, paired with a DSP and quite some power should be the most simple and probably best sollution.
Just give a volume and dimensions. You have no unsolvable problem for sure.
Construction will not be complicated at all, but you need to use your hands, a jig saw and a drill.
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