Port volume

[lucadelcarlo]

Hi all,

When calculating the amount of internal volume displaced by a port, is the port to be seen as a solid object inside the enlcosure or is it only the wall thickness of the port that counts towards displacing internal volume. I assume its the former, is that correct?

Thanks in advance.

[weltersys]

Originally Posted by lucadelcarlo

Hi all,

When calculating the amount of internal volume displaced by a port, is the port to be seen as a solid object inside the enlcosure or is it only the wall thickness of the port that counts towards displacing internal volume. I assume its the former, is that correct?

Thanks in advance.

The former.
In a small cabinet with a very large port the lines between which is which can become blurred.

[gedlee]

Any air in the box that actually moves is not part of the box as far as its compliance is concerned.

[badman]

Originally Posted by gedlee

Any air in the box that actually moves is not part of the box as far as its compliance is concerned.

Would you consider the air near the internal port termination to be "moving"? Should people be subtracting MORE than the vent volume?

[gedlee]

Yes, just as the port acts longer than it actually is, it takes up more space than it actually does.

[weltersys]

Originally Posted by gedlee

Yes, just as the port acts longer than it actually is, it takes up more space than it actually does.

Earl,

A slotted port, that is a port using the side of a cabinet as one of it's sides, "acts longer" (tunes fB lower) than a circular duct located away from the side walls, and occupies (takes up) less cabinet space than a circular duct providing the same Fb.

What type of port "acts longer than it actually is, and takes up more space than it actually does" ?

Art Welter

[gedlee]

Originally Posted by weltersys

Earl,

A slotted port, that is a port using the side of a cabinet as one of it's sides, "acts longer" (tunes fB lower) than a circular duct located away from the side walls, and occupies (takes up) less cabinet space than a circular duct providing the same Fb.

Art Welter

I'm not sure that this is actually true, to a first order the port shape has nothing to do with the situtation. The acoustic mass depends on length/area with an end correction term that loosely depends on the shape. But if the shapes are similar and then there is still the length/area ratio that determines the port volume for a given box tuning, and hence it can be anything. Where and how it is situated in the enclosure is not a factor. Of course the lower the port volume the more losses there are so this becomes self defeating.