View Full Version : Math problem - Port tuning crazy...

08-02-2009, 10:50 PM
Ok, this is giving me a headache...

I dont know the exact volume of the box but it's somewhere between 12-13.5 cuft NET or it should be...

This is what i've got and cant figure out why...

I had a port 25.5" long that peaked at 50hz.

I then had a port 22.25" long that peaked at 51hz
I then had a port 20" long that peaked at 52hz
I then had a port 18" long that peaked at 53hz
I then had a port 16" long that peaked at 54hz

The port extends through a double baffle (1.5" thick) and sticks out of the box 5"

I always cut off the port length on the INSIDE of the box and retest.

My peak frequency is 50hz. My LOUDEST port test was at length - 20" which peaks at 52hz but is loudest on the mic at 50hz...

Anyways, explain this mind-boggling problem-

I tried to use all this data to find actual internal volume of the box...

So, as port length gets cut from the INSIDE, internal volume should increase... As internal volume increases, port tuning will peak closer to the tuning frequency as opposed to smaller boxes port tuning will peak alter on in the frequency range.

So, why is it the shorter i make the port... the higher in tuning it is achieving?

I know you are not suppose to use box building software but this is completely weird...
According to calculations, i gain about 0.35 cuft every 2" i cut off the box or around there.
I know from 25.5 to 16" i gain ~0.9cuft.

So, if i gain 0.9 cuft by shrinkin port length 9", why is the port peaking at a higher note?

On the mic, 50hz is still the loudest note using 16" length but on low volume 54hz is where it is peaking so that means the port is peaking at 54hz...

According to graphs, when i gain internal volume, the port peak should drop BACK, NOT FORWARD!

In my testing, it is acting like every time i cut some port length off, i dont gain anything but actually LOSE volume when i cut port length?!?!

That is how i am having to graph it. ***-backwards.

As it is common knowledge to know that shrinking port length will give higher tuning.. remember, my 25.5" long port is tuned to 29hz... so by gaining 0.9 cuft internally, how is the port wanting to peak at 54hz at 16" long when it is now tuned to the low to mid 30s?