A concentric port is basically a straight port that has been collapsed into itself. Hold a cardboard tube out in front of you, and run as fast as you can into a brick wall and you'll get the idea!

• The simple maths as shown on the Vent Options page. These allow you to get a rough idea of the size a port would end up being
  
  
• The full maths which take into account wall thickness. These are what you need to design your own accurately
  
  
• Equations to determine the acoustic length of your new port
  
  
• Equations to allow you to make a composite tube of whatever size you need
  
  
• I have written a calculator . Tell it what acoustic length you need, what size tube to start with, and the thickness of each of the tubes. It will tell you the diameters of the second and third tubes, the end-gaps, and the total length and volume occupied. It will also do the maths for your DIY composite tubes.

• Check out the Gallery which details the construction of the test port - load time about 20 secs @ 33k
  
  
• Here are some suggested arrangements utilising pipe sizes available in Australia