Math problem Help with tuning problem

[shizzzon 461]

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. ass-backwards.

[sadistic 13]

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. ass-backwards.

you got it that how it works. the further you get away from that perfect frequency you lose potential. the bass wave grow weaker in either direction if you have tried lowering the tuning frequency yet it will decrease the out put and make the sub seem like it needs more power. which will only burn up voice coils. this is why they are similar to transmission line boxes just do not have as much output as the t line.

btw have you done a spl measurement ported verses infinite baffle. some subs behave better in sealed then ported or even ib compared to ported you may experience a loss of db going from ib to ported.

as soon as you ported the wall it be came a ported box the rules changed.

now shorter ports = higher tunes,and the inverse longer ports = lower tuned.

.....

[sadistic 13]

also forgot to mention your dealing with a huge air space. a change of .9cuft will not effect the tune very much this is why it tunes higher instead of lower. it's not a drastic change in air space causing a lower tune. although not sure how much it can change by when you go to tune it differently for each port wish i could help more on this. it kinda like the general rule of thumb not to go to much plus or minus what the manufactures spec are. i have heard 20 % plus or minus of them for building small ported / sealed boxes. this is for regular sub with very little x-max and these are boxes of 1 -1.5 cuft average. a box as big as yours with a 20 % change would be a huge amount of like 2.7 cuft it would have to be off but to screw it up. if you notice it's easy to screw up smaller boxes then big for some reason.

[KU40 173]

Are you sure that the tuning frequency isn't increasing as you cut down on the port?

But this is just more evidence that nothing in SPL always goes along with theories.

[shizzzon 461]

i know what the problem is...

I can't prove it because well.. box building software lies in the car audio realm...

I was looking at a response graph in WinISD and looking at how the box likes to peak... This was only true on the very first port i made, the initial port.

All other ports after that i cut, it went way off...

What is happening is my response in the car is actually louder right after tuning then what WinISD is saying... 20hz later...

So, it went all crazy and thats why i was confused.

So, what is happening is even though i am gaining volume by cutting off some of the port, it's still not enough to get it to peak at 50hz AND gain from it every time...

By the time the port was tuned to peak at 54hz, i'm over here burping at 50hz and lost 0.4dbs! I stopped immediately since i knew not to cut anymore off.

I'm gonna create another thread here because i need to go a different route to see if i can get louder another way so i can keep a new thread's topic on topic.

[KU40 173]

I'm not sure why you trust winISD's FR graph, that is based on an anechoic chamber, which a vehicle certainly is not.

I would contend that it's useful as a general guide, but definitely not with the specifics that your project puts forth.

[shizzzon 461]

well, i know that the box's response in an open air environment is different than the car's environment.

I know my car's response down to the T but using that over what WinISD was saying was still throwing it off...

So... i now use WinISD as an extra line in my add\remove programs, hehe.