I have a 12" driver and building a ported box of approximately 4.3 cubic feet and tuning to 22Hz. This could be fed as much as 600 watts of clean power from a Crown pro amp XLS 1502. Using WINISD to model. In that program I am concerned about port velocity numbers it is throwing out. Some say don't go beyond 17m/s while others say up to 30 or so will be just fine. I have modeled using two 3" ports and a single 4" port. Not too much difference between the two, so given the choice I would use a single 4". But with either of those choices the port velocity is as follows.
At 300 watts of signal the port velocities for two 3" or a single 4"
at 20Hz 35-40m/s
at 30Hz 16-18m/s
Having two 4" ports solves the problem, but I am thinking two 4" ports might be overkill. Seems like a lot for a single 12" driver, although the Xmax on the driver is 26mm so pretty substantial. I just don't want chuffing during a low rumble in a movie. I will flare the ends of the ports using a 3/4 roundover router bit.
Will a single 4" be fine?
Thoughts?
At 300 watts of signal the port velocities for two 3" or a single 4"
at 20Hz 35-40m/s
at 30Hz 16-18m/s
Having two 4" ports solves the problem, but I am thinking two 4" ports might be overkill. Seems like a lot for a single 12" driver, although the Xmax on the driver is 26mm so pretty substantial. I just don't want chuffing during a low rumble in a movie. I will flare the ends of the ports using a 3/4 roundover router bit.
Will a single 4" be fine?
Thoughts?
That is true, port length approaching one meter brings the resonance around 170Hz, which is only about an octave higher than the traditional home theater crossover point at 80Hz so I'd regard that as the upper limit before too long port for a sub 
Another limit is the box size, which might limit how much of that volume one can allocate for port(s).
If these "limits" are no issue one could choose to use two or more ports to be able to change the tuning by blocking some of the ports.
Another limit is the box size, which might limit how much of that volume one can allocate for port(s). If these "limits" are no issue one could choose to use two or more ports to be able to change the tuning by blocking some of the ports.
OK, so two 4" ports would not be overkill.
But...necessary?
With two ports the internal volume of the box will be reduced plus I will have to make them much longer. Would like to avoid that, but if one port is not sufficient then I will use two. Not sure where resonances play a role, but this will be for woofer/subwoofer use. Integrating into system probably around 80-100Hz.
But...necessary?
With two ports the internal volume of the box will be reduced plus I will have to make them much longer. Would like to avoid that, but if one port is not sufficient then I will use two. Not sure where resonances play a role, but this will be for woofer/subwoofer use. Integrating into system probably around 80-100Hz.
Richie - The reason I included the expected excursions from the WINISD model at 20 and 30 Hz was to highlight that high excursion of the woofer. At 30Hz it doesn't seem to really be a problem, but at 20 there is a ton of movement.
Did your project exhibit chuffing/noise only at very high power levels and very low frequencies? Was it a more persistent problem?
Thanks
Yeah...I figure at high levels that Xmax is going to push quite a bit of air. I guess I am looking at two 4" ports then just to be sure.
A few design issues.
1- I need to use an elbow in the ports. Need ports front facing and box is only 12" deep. Using regular schedule 40 PVC tube and elbows. Any advice about how to calculate length incorporating a typical PVC elbow?
2- Driver is 12.5" outer diameter and I think outer diameter of 4" PVC is 4.5". Face of box is 23 by 30. Due to where these need to fit in the cabinet I will need both woofer and ports to be located on the end. Would it be OK to mount the woofer on one end and the ports on the other? Once I flare the port ends I dont think both ports plus the woofer will fit straight across that 23" width.
How might one do that? Is there an online calculator or website that describes the method? I Googled it real quick and didn't find anything
I am guessing that this would involve being able to take the face off to adjust as the ports will be inside the box itself.
Also - does anybody know if WINISD takes into account the ports in the box volume? I am assuming it does not. For instance if you put 4.2 cubic feet into WINISD as the box size, it is assuming that is the net volume after any ports and drivers have been accounted for.
Re: Port Centre Line, the attached image should show you what you want to be doing I hope.
Re: WinISD and box volume, the Volume used in WinISD (and every other box modeller I know of) is the net volume of air that is free to resonate in the box ie, after accounting for the port, backside of the driver, bracing & any other inclusions in the box.
HTH,
David.
Re: WinISD and box volume, the Volume used in WinISD (and every other box modeller I know of) is the net volume of air that is free to resonate in the box ie, after accounting for the port, backside of the driver, bracing & any other inclusions in the box.
HTH,
David.
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