Help with 4th Order bandpass enclosure for a 15" 600W dual voice coil subwoofer (PA)

[Brian Steele]

you can still work with it if you can find the actual tuning frequencies and then play around with the ports and volumes. Looks like they're too close to each other, hence the peakiness centered in the middle.

I suggest measuring the impedance response with something like the WT3 to determine if the lower and upper (particularly the latter) are correct. Quite likely the lower ports need to be lengthened a little, and the upper ports need to be cut down significantly.

BTW -did you do that FR test with a low-pass filter enabled? The area outside of the sub's passband seems a bit too clean...

 

[col]

I suggest measuring the impedance response with something like the WT3 to determine if the lower and upper (particularly the latter) are correct. Quite likely the lower ports need to be lengthened a little, and the upper ports need to be cut down significantly..

Iv'e constructed the baffle so that I can easily remove it and modify the port lengths. Never thought of testing with WT3. will give that a try tomorrow, cheers.

BTW -did you do that FR test with a low-pass filter enabled? The area outside of the sub's passband seems a bit too clean...

Yes, it was being powered by a Peavey IPR1600 with the crossover set to "Subwoofer" which is a 100hz 3rd order low pass. I used 1 channel per voice coil.

edit: I did a measurement with the amp in "fullrange" as I was curious to how the 100hz low pass could be affecting it. It didn't make much difference to anything below 100hz. I didn't save the image though.......

 

[col]

you can still work with it if you can find the actual tuning frequencies and then play around with the ports and volumes. Looks like they're too close to each other, hence the peakiness centered in the middle.

According to my calculations using WinISD the rear chamber is tuned to 43hz and the front chamber 84hz. I suspect it could be a little out though. I will try testing it with WT3 as suggested by Brian Steele.

Thanks for your input.

col.

 

[Brian Steele]

Iv'e constructed the baffle so that I can easily remove it and modify the port lengths. Never thought of testing with WT3. will give that a try tomorrow, cheers.

I normally do impedance tests before the FR tests, just to confirm that the built sub is as close a match to the target as possible. Using the WT3 makes it very easy to perform these tests.

BTW - 3" vents may be a bit small for a pro audio 15" subwoofer. Have you considered using larger / longer shelf vents instead?

 

[col]

I normally do impedance tests before the FR tests, just to confirm that the built sub is as close a match to the target as possible. Using the WT3 makes it very easy to perform these tests.

Yes, that makes sense, didn't think of it.

BTW - 3" vents may be a bit small for a pro audio 15" subwoofer. Have you considered using larger / longer shelf vents instead?

The vents I'm using are 2 x 4" with a fairly large round over for each chamber. There is quite a bit of air going through them though. Originally, I was going to do 50mm shelf vents at the top/bottom of the cab but then I read that if you do this it significantly lowers the port tuning from the target. That made me change my mind. They don't sound like they are chuffing....

col.

 

[col]

Impedence sweep with WT3

From the Impedence sweep I can see that the front chamber is tuned a bit low, will have a go at adjusting the port lengths and report back later.

Brian, thanks for your suggestions.

 

[OscarS]

both tuning frequencies are lower than expected because the ports are less than 1 diameter away from the walls. This slightly lowers tuning, just as shelf ports tune lower than expected.

 

[col]

both tuning frequencies are lower than expected because the ports are less than 1 diameter away from the walls. This slightly lowers tuning, just as shelf ports tune lower than expected.

yes, spot on. I thought I could avoid the affect the walls would have by going with the round ports, not so. At least using WT3 and the impedance sweep I can see where the tuning is at.

I just shaved 25mm off the front chamber ports and the impedance curve moved in the right direction, not far enough yet though

 

[Brian Steele]

yes, spot on. I thought I could avoid the affect the walls would have by going with the round ports, not so. At least using WT3 and the impedance sweep I can see where the tuning is at.

Note: some box modelling programs over-estimate the vent lengths required for a particular Fb anyway. That's why it's always important to confirm via impedance response measurements that you've get the target resonance point(s).

I just shaved 25mm off the front chamber ports and the impedance curve moved in the right direction, not far enough yet though

Note, it's ok to go slightly too high on the upper Fb, seeing that you're planning to use this with an with a 100Hz LP filter anyway. A slightly higher Fb, in combination with the LP filter could help to flatten and extend the passband.

 

[col]

75mm off the frount chamber vents, quite an improvement.

thanks for the help!

col.

 

[Brian Steele]

75mm off the frount chamber vents, quite an improvement.

thanks for the help!

col.

Three inches off the vents is quite a change, and the shape of the FR curve suggests that cutting them a bit shorter could even further flatten the passband, at the expense of some efficiency at 70~80 Hz (which may not be desirable for a pro audio sub). Nice work.