Reflex ports

[Phil Y]

Hello all,

Am I right in thinking that the important thing about a port is its volume ?
Would it be OK to change the diameter of a port as long as the length was adjusted to keep its internal volume (air mass) the same ?

My reason for asking is that I am building some small speakers for the kitchen (based on an old Wilmslow audio design) and don't want to buy a great length of pipe from a DIY store if I can use something that might be found "in the back of the shed".

Regards, Phil.

[John Caswell]

Hi Phil,
Basically the answer is yes, and bear in mind it does not have to be circular it can be rectangular as long as the volume is close.

John

[Richard]

Hi Phil, good question and I can see your logic!

I did a bit of googling and ran a typical looking small box and port through a couple of online calculators which agreed with each other though the theory seems pretty complex.

For my examples I used 1 cubic foot box, 2 inches diameter round port, and 40Hz tuned f.

I got an end correction factor for a typical on-panel port of 0.73 from another web page which agreed across 2 calculators, one of which calculated it from the results previously calculated so it looks about right.

I then changed the port diameter and finally compared volumes of the ports using another calculator. I found it wasn't simply a case of making the ports the same volume for the same box and tuned f as we might have thought.

Here're the links, it's actually faster and more fun to run them yourself with your own values which should give you the length you need for using the pipe size you have to hand,

http://www.mobileinformationlabs.com...0lenth%201.htm

http://www.calculatoredge.com/new/ventlength.htm

http://www.stevemeadedesigns.com/boa...nd-correction/

http://www.calculatorsoup.com/calcul...s/cylinder.php


Ah, I see John has answered whilst I was playing with the calculators, maybe we can all check some examples and see what we find

[Phil Y]

Thanks for the replies,

I will have a look those links when I get a minuite or two.

Phil.

[Phil Y]

Well, I had a go at this and it is rather counterintuitive. It seems that making a port with a bigger diameter results in a longer tube.

The Wilmslow design has a 32mm dia. tube 100 long but re-calculation with a 39mm (what I have in the coal shed) results in a 156mm pipe according to that calculator.

Is this in line with the results you got Richard ?

Phil.

[Richard]

Yes indeed Phil, and both the calculators agree