Enclosure question

[Ben M.]

I really like the forum you all have going here and wanted to get some advise on an enclosure I have nearly completed for a Tumult 15D2 I recently ordered.

It is a 420L sonotube (24"x60") tuned to 16.5hz using a 6" aeroport. I will be powering the Tumult with one channel of a Behringer ep2500 (650W max). I wanted some deep extension, hence the giant enclosure, but since this is my first diy sub I wanted to make sure it won't sound like crap once it's completed.

Like I said, the Tumult is ordered and will hopefully soon be on it's way, so now is the time for any changes to this beast. Thanks.

[JRace]

What is the lenght of your port?

[Ben M.]

total with two flares it is 16".

According to the data I found on aeroport ports, you subtract 1.5" per flare so useable length should be 13".

According to WinISd this should tune to approx. 16.5hz

[AndrewT]

Hi Ben,
I am not sure you have done the flare subtraction correctly.

I think you subtract your 1.5inch from the required length to give the measured length.

Can anyone confirm which way to calculate this?

The ratio of diameter to length seems quite short.

[Collo]

For flared ports you generally subtract half the flare radius from the physical length to get the acoustic length

The Aeroports have a flare radius of about 30mm, so that means subtract 15mm from each end.

For flares each end this gives a total adjustment of 30mm or around 1.5 inches in total

[Ben M.]

Thank you both for your responses. I agree, it seemed like this port wasn't long enough, but I got all my cutting instructions from page 5 of this site:

http://www.rockfordfosgate.com/rftec...4-9317_MAN.pdf

If I am reading correctly they are saying subtract 38mm per flare for a total of 76mm (3+in) whereas you guys are saying 30mm total. Stick with the 30mm?

On another note I think I'm going to cut down the tube from 420L to about 250L. It gives the enclosure a much smoother response, lower group delay, and lower port velocity. Given the port velocity is still high from the high excursion Tumult, but within reason I think. Unfortunately I have to cut my already glued port and extend it to 20.5".



A question about Transfer Function Magnitude graphs...
Flatter response is better, especially for music correct? If I have a large hump around the tuning frequency as I do in the larger tube above, would this make it sound "boomy" or loud at that frequency? Could my BFD easily tame this?

Thanks again for all your help.

[Collo]

Hi again Ben,

I had a look at the pdf you linked to. The three inches they say to deduct from the total length is to get the length of the centre tube on its own. When you add a flare each end, you get the three inches back.

Their formula for calculating length for a particular tuning frequency, should have the end correction already factored in for the aeroport.

In other words, the overall length of the port, including the flared ends should be what their formula gives you.

I would expect this result would be very close to what you get using WinISD to calculate the acoustic length and then adding the 1.5 inches to get the physical length.

Bummer about having to change the length after you've already glued the flares on. If you've got some spare tube and a heat gun, you can make your own joiners like I did...
http://www.users.bigpond.com/bcolliso/PVC-Joiner.htm

With regard to the hump in your response - it's not higher than the resonse higher up, so I don't think it would be "boomy" (not that you would hear it at 18hz anyway!)

regards
Collo

[Ben M.]

Hey thanks Collo for your help. It helped immensely in trying to get this port length correct.

yeah, not looking forward to cutting the pvc, but I'll give your joiner idea a try. Thanks

-Ben