View Full Version : someone please check my math

lord baccus
06-25-2007, 09:34 PM
i measured from the subs to where the mic go's and got 7'
give or take an inch.

so 7'x4=28' gives me the 1/4 wave
devide 1131 the speed of sound by 28'= 40.39Hz
so i port to 30.39Hz - with cabin gain should peek @ 40.39Hz

but what about the ports fireing 90degrees out of faze, ???
subs are pointed forward into the back of the seat & the ports are fireing up along the back window line.
i drive a 1988' Ford F-150 x-cab

06-26-2007, 01:26 PM
ummm...the 1/4 wave theory is a lot different than what your trying to do. say between your sub's face and the mic you have X distance. this distance is equal to the sum of the distance from the back of the sub to the back of the box and from the back of the box to the front of the port. when it comes out of the port, the other 1/2, it ends up being, is equal to X. when doing this, keep in mind it will only work for one freq. each freq. has a different wave length. the freq. should be the resonant freq. of your car. all distances at hand totally depend on the freq. usually this is done not so much by using a port, but a 1/2 wall. most guys that have this set up you can crawl through the "port" and see the sub and everything else. the passenger side will have the sub(s) and the driver's side will be totally open. also making the corners behind the sub rounded will help guide the waves out the front.

the term "1/4 wave" come from the distance from the sub to the back of the box.

http://img79.imageshack.us/img79/121/25waveml9.th.png (http://img79.imageshack.us/my.php?image=25waveml9.png)

06-26-2007, 01:58 PM
Well this is 1/4 wave formula but I don't trust in it :)
Because basically this freq. should be your cabin resonant freq. but it's not.
And also not all box peak 10Hz higher than they are tuned. It can be and 5Hz, 7Hz...
Smaller box peak is 3-5Hz higher than tuning freq, and larger box peak is 5-10Hz higher than tuning freq. But it's also theoreticaly. You need to test boxes, play with tuning freq. etc.

I tryed to build sealed box and find cabin resonant freq. with TL and it wasn't freq. which I got with 1/4 wave formula...

I don't think that 1/4 wave formula works. Maybe in theory yes, but not in practical.
Or maybe I'm wrong.. :)

lord baccus
06-26-2007, 08:53 PM
i have my boxx ported @ 54Hz & it Peeks @ 62Hz on a Hott day & peeks @ 65Hz on a cool day

the resonant freq of my truck is 48Hz but ill be Dammad if i can get it to hitt @ that Freq....
i was going to try porting to 38Hz for Street Beat...

butt according to this formula im hitting too High ???
i meeter on the drivers side Kick - behind the emergency break... only place i can get the mic to stay put

06-27-2007, 02:46 AM
Yes if we trust this formula so you hit too high.
Well as I understant so 1/4 wave theory should tell your your car cabin resonant freq.
But as I said I tryed to put sealed box in my trunk and findout resonant freq. At which frequency I hit most dB's that is cabin resonant freq. So I got ~50-51Hz. And actually I tryed two sealed box, one about 0.85cu.ft. another 4cu.ft :) And with both I got most dB's at 50-51Hz.
And if we use 1/4wave formula so I resonant freq. should be at ~40Hz.
So as you see results are very different. :/

Also you say that your cabin resonant freq. is 48Hz, but with 1/4wave formula you get 40Hz... So also very different results.