Boxes, ports, volume, WinISD & head scratching?! - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 6 Old 11-21-2019, 11:04 PM - Thread Starter
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Boxes, ports, volume, WinISD & head scratching?!

Hey guys,
I am just designing a box for my 4x PA460's with WinISD. Now, this might seem like a bit of a dumb question to some, but I'll risk asking it anyway haha.

Is it best to design the box based on GROSS volume (still including driver, port & bracing volumes), or to base it on the NET volume after driver, bracing and ports (I'll get to this in a sec) are subtracted. If this is the case, then won't the port length be directly affected by the space occupied BY the port, and the length then need to be recalculated after working out the space occupied by the port, ending up in a never ending adjustment of figures?

Example:
The GROSS box volume is 200L,
The driver occupies 9.138L,
The bracing occupies say 5L,
So now the net volume (excl. port as is yet uncalculated) is 185.862L.

You then calculate your port length based on this volume and so for a 150mm PVC pipe port tuned to 23Hz that comes out at a port length of 42.80cm. Ok.
So my question is: Is the port length correct at this point then?

OR, do you now subtract the port volume of 7.869L (153mm o/a port diameter) from your box volume and get a NET VOLUME of 177.993L.
But hey, now your box volume is 7.869L less than what you calculated your port length off originally!?
Do you recalculate the port length with this new box volume of 177.993L? But if you do that, then your port will now be longer to compensate for the now smaller box volume. And then you need to recalculate again >>> and you then end up in a vicious cycle between box volume and port length adjustments and you can never getting it exactly perfect! *sigh*

I hope you can follow me on this one... Can anyone bring me some clarity and peace of mind?

Last edited by AudyoVidyo; 11-21-2019 at 11:10 PM.
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post #2 of 6 Old 11-21-2019, 11:20 PM
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Quote:
Originally Posted by AudyoVidyo View Post
...you can never getting it exactly perfect!
Ah, grasshopper, now you truly understand. Welcome to speaker building!

As for your question, I'd say the assumption is box volume is box volume, port size is port size, the port volume has to be added in when you calculate the box dimensions for cutting. Some simulators let you actually 3D a box and then at least with LEAP it is all taken into account IIRC.

But those are all SIMULATIONS. In real life measure twice, cut once, make the port longer than necessary and measure and trim if needed. Cuz it probably will be!
- Also, if you calculated 32 Hertz and your design measure out to 29 Hz, that probably hardly makes a difference. It gets influenced by the barometric pressure anyway...er, I'm pretty sure...hmm, now I'm wondering.
- Don't obsess about the volume taken up by the driver unless it's got a huge magnet stack. Folks forget that the baffle cutout adds back volume, as does the air volume under a half-roll surround. Back when I was doing product planning for car audio I did some meticulous calculations for that and found the issue was rather ignorable as the factors pretty nearly canceled out.
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post #3 of 6 Old 11-21-2019, 11:54 PM
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you are right. cab volume is net after everything (including the effective length that the port extends beyond the port into the cab). lol.

it is an iterative process as you explain where the designer constantly goes back and forth between all the critical features and tries to find a good balance.

as noted above by head unit, the good news is that if you miss your target by 1hz, you will never know when listening. so no need for extra xanax; the standard dose is all you need.
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post #4 of 6 Old 11-22-2019, 01:14 AM - Thread Starter
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Quote:
Originally Posted by head_unit View Post
Quote:
Originally Posted by AudyoVidyo View Post
...you can never getting it exactly perfect!
Ah, grasshopper, now you truly understand. Welcome to speaker building!

As for your question, I'd say the assumption is box volume is box volume, port size is port size, the port volume has to be added in when you calculate the box dimensions for cutting. Some simulators let you actually 3D a box and then at least with LEAP it is all taken into account IIRC.

But those are all SIMULATIONS. In real life measure twice, cut once, make the port longer than necessary and measure and trim if needed. Cuz it probably will be!
- Also, if you calculated 32 Hertz and your design measure out to 29 Hz, that probably hardly makes a difference. It gets influenced by the barometric pressure anyway...er, I'm pretty sure...hmm, now I'm wondering.
- Don't obsess about the volume taken up by the driver unless it's got a huge magnet stack. Folks forget that the baffle cutout adds back volume, as does the air volume under a half-roll surround. Back when I was doing product planning for car audio I did some meticulous calculations for that and found the issue was rather ignorable as the factors pretty nearly canceled out.
Quote:
Originally Posted by LTD02 View Post
you are right. cab volume is net after everything (including the effective length that the port extends beyond the port into the cab). lol.

it is an iterative process as you explain where the designer constantly goes back and forth between all the critical features and tries to find a good balance.

as noted above by head unit, the good news is that if you miss your target by 1hz, you will never know when listening. so no need for extra xanax; the standard dose is all you need.
Haha. I'm glad I'm not the only one! Oh well. Perfection will have to wait.
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post #5 of 6 Old 11-22-2019, 06:52 AM
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Yeah the port length/tune/dimensions shell game can be frustrating but its a worth exercise in patience.

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post #6 of 6 Old 11-23-2019, 07:58 AM
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Quote:
Originally Posted by LTD02 View Post
...the good news is that if you miss your target by 1hz, you will never know when listening. so no need for extra xanax; the standard dose is all you need.
Right! The extra xanax is only needed if you miss by 2 Hz...
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