Sub-woofer port size question?

Question

The recommended port size for my sub-woofer box is 4 inches round and 7.5 inches long.

However the only close size to this that my hardware store had was 4.5 inches round so I bought it anyways.

Will this affect the tuning of the box since the factory recommended these diminsions? If so what can I do.

Answer 1

The fact that you said round ports, tells me that you're not up to date with you're boxes. You should find a box with rectangular vents, as this design will not give you the possibility of turbulence, which results in a low whistle noise like the round ones will.

Answer 2

Chaning the port's cross sectional area (using a different diameter port) will mean you have to change the length of the port to have the same tuning.  

When you increase the diameter of the port you must also increase the length to keep the same tuning frequency.  How much longer you must know make the port will depend on the internal volume of the enclosure (not including the port) and the diameter of the port.  The link below will tell you how long the port will need to be.  

Make sure to round all edges and make sure there is no rough edges.  This will reduce air turbulance which means you will reduce or eliminate port noise.  

Having a larger port will not be a big deal for your situation.  It will actually reduce the air velocity through your port.  This will also reduce or eliminate port noise.  

http://www.the12volt.com/caraudio/boxcal...

Good Luck!!!