Port numbers

[murrmax]

I am currently planning my next speaker box build, and trying to decide on number of ports. Its a 15 inch driver (32-350HZ) approx so will be using 10cm diameter ports.

Question is, is there any reason why you would use a specific number of ports. E.g you could use 1 port at x length, 2 ports at x length, 3 ports at x length.

Given nothing else changes, volumeL, target fs, port diameter (so only port length and number of ports change) - is there any logical reason why you would want to use multiple ports vs a single port? 

 

for example if I use 1 port it will be 12cm, 2 ports @ 32cm each, 3 ports@ 45cm each, 

 

Does it come down to target output ? e.g. xHZ at xDB , .. I must be missing something,  

 

[mwhouston]

The problem with big fat ports is they let midrange through.

 

To use multiple ports keep the length unchanged. Work out the area of the port - Pi r sqd. Or half diameter sqd X Pi. If converting to two port half the area of the port for each port, keep the same length and use two ports. 

[VanArn]

Start your build plan  by obtaining the T & S parameters of the  speaker by measurement or if you trust the manufacturer's specifications, use those. The enclosure internal volume and the Fb  figures will then assist in determining the  port diameter and a suitable length.   A 10 cm diam. port  is on the small side to match a 15'' woofer   as it is approx.  1/10th of the  cone area.  The length of the port tube also needs some  consideration. If  it is too long it can cause  unwanted  pipe resonances and chuffing effects to be heard .

[murrmax]

Yes, I have specs and working to recommended volume&fs; and can determine port diameter & length, based on the volume and fs  Thats all fine, easy.

 

But, there is nothing that specifies the number of ports - this seems to be arbitrary.. given the size of the driver i understand 100mm diameter port is better than a 30mm to minimise port noise. And having (diameter) a big port over multiple small ports to minimise port noise.  

 

There's speakers with 15inch drivers that use 1,2 or 3 ports of the same size 100mm,  my question is that the number of ports isn't related to the volume/fs relationship. 

 

Why would you choose 1 port over 3 of the same diameter is the question I am trying to understand?  

 

Is it to do with resonance? as different length pipes will have different resonances? is it about max spl? 
 

The port diameter will be similar on these subs, lengths will vary based on box size and FS goal, but why has one used 1 port VS 2,3,4 ??? 

 

 

Edited December 12, 2020 by murrmax

[murrmax]

http://www.eminencedesigner.com/EDTopic14.html

 

This article helped, as always it's a compromise between a multitude of variables..