Hi all
I am just about to build a sub box to match my full range speakers which are loaded with Fane Sovereign 12-250TC.
I will be using a Fane 18XB 8 ohm and I think I will be building a Butterworth B4 ported box which I can change but does need to be reflex style box as I do need to keep the weight down and the driver needs to be shown.
Due to it needing to match the FR speakers and fit where it needs to go the external size will be 960h x 650w x 450d but internally I can build volume to suit. I will be using 18mm plywood for the build.
I just want to confirm that I have it planned correctly with the following internal dimensions.
154.12 litres
F3 = 39.64 Hz
FB = 40 Hz
Port
Dv = 15cm
Lv = 10.39
I am struggling with the ports as well. I would prefer one large rectangular port at the base of the box, but I cannot find any calculators online for that, can I just take the area of the round port and use that?
As you can see I do have a lot of volume to play with internally so I am open to suggestions in how I can get this box to function the best I can.
Thanks
I am just about to build a sub box to match my full range speakers which are loaded with Fane Sovereign 12-250TC.
I will be using a Fane 18XB 8 ohm and I think I will be building a Butterworth B4 ported box which I can change but does need to be reflex style box as I do need to keep the weight down and the driver needs to be shown.
Due to it needing to match the FR speakers and fit where it needs to go the external size will be 960h x 650w x 450d but internally I can build volume to suit. I will be using 18mm plywood for the build.
I just want to confirm that I have it planned correctly with the following internal dimensions.
154.12 litres
F3 = 39.64 Hz
FB = 40 Hz
Port
Dv = 15cm
Lv = 10.39
I am struggling with the ports as well. I would prefer one large rectangular port at the base of the box, but I cannot find any calculators online for that, can I just take the area of the round port and use that?
As you can see I do have a lot of volume to play with internally so I am open to suggestions in how I can get this box to function the best I can.
Thanks
Last edited:
Your Vb, Fb etc all seem to be right and your external dimensions do allow enough space to fit it all in, so that's a solid start.
Couple of things to think about before finalising port sizes though.
Mostly, how loud are you expecting to drive it?
WinISD predicts that if you were applying sine waves, you'd get to maximum excursion at about 58V (equivalent to approx 420W at 8 Ohms). That gets a theoretical 123dB in half space, and would drive your stated vent to about 35ms-1 which is a bit on the high side.
As you have a bit of extra space useable inside your proposed cabinet, and as a rectangular vent isn't constrained by easily available pipe sizes, I'd go a bit bigger and aim to get vent airspeed down to a predicted 20-ish ms-1 if possible.
As for the calculation of length, if you plan on keeping the port well away from the cabinet walls, then just converting the area of your circle into a rectangle and keeping the same length should work.
However, if you want to build it close to the base of the box, that proximity will result in the base of the cabinet effectively behaving like an extension of the length of the port. You'd therefore need to build the port shorter to compensate for this.
If you build it as a slot right across the base of the box, there is a formula to calculate the revised length as per the attached pic.
However if you want the port near, but not hard against the inside wall of the cab, as per your mockup, then you'd have to build your cab with some way of getting at the insides in case you need to change the vent length after construction.
HTH,
David.
Couple of things to think about before finalising port sizes though.
Mostly, how loud are you expecting to drive it?
WinISD predicts that if you were applying sine waves, you'd get to maximum excursion at about 58V (equivalent to approx 420W at 8 Ohms). That gets a theoretical 123dB in half space, and would drive your stated vent to about 35ms-1 which is a bit on the high side.
As you have a bit of extra space useable inside your proposed cabinet, and as a rectangular vent isn't constrained by easily available pipe sizes, I'd go a bit bigger and aim to get vent airspeed down to a predicted 20-ish ms-1 if possible.
As for the calculation of length, if you plan on keeping the port well away from the cabinet walls, then just converting the area of your circle into a rectangle and keeping the same length should work.
However, if you want to build it close to the base of the box, that proximity will result in the base of the cabinet effectively behaving like an extension of the length of the port. You'd therefore need to build the port shorter to compensate for this.
If you build it as a slot right across the base of the box, there is a formula to calculate the revised length as per the attached pic.
However if you want the port near, but not hard against the inside wall of the cab, as per your mockup, then you'd have to build your cab with some way of getting at the insides in case you need to change the vent length after construction.
HTH,
David.
Attachments
Last edited:
FWIW/FYI, bjorno's more recent vent help 😉: https://www.diyaudio.com/forums/att...rt-slot-port-math-effective-port-length-1-jpg
GM
GM
I wouldn't want to see a slot port go above 15 m/s. The effective hydraulic & equivalent diameter is a big problem for these ducts, especially with a high slot ratio due to the boundary layers and turbulence.
A circular is often a more sensible choice as allowable velocities can more than double. Which gives more output, less compression and induced noise!
A circular is often a more sensible choice as allowable velocities can more than double. Which gives more output, less compression and induced noise!