by Jonathan » Thu May 29, 2008 10:59 am
solacedagony wrote:So there is an "optimal" port volume where too high and too low will both negatively effect output through the port.
Not really. It just gets to a point that the bigger you go, the sheer size of the port and box is impractical.
The port itself performs as a solid mass of air, rather than a hole in a box. The air is effectively trapped within the port walls, and while it doesn't have excursion limits, air flow velocity can negatively affect both output and sound quality. You can model this in WinISD and view the port air velocity in different configurations.
The lower you tune or the larger your box, generally the more port area you need because of the velocity created by the two. I don't really go by a set formula for port area in terms of square inches of port area per the box volume, but rather the velocity created around tuning. Remember that at tuning, the port is really doing most of the work, it's "cracking the whip", sucking energy out of the internal air pressure created in the box, loading the subwoofer because it's using it as the fulcrum to bear the force of it's own acceleration. That's why subwoofer excursion is drastically reduced at tuning. The lower the tune, the more excursion the port is required to have thus the added necessity for port area, just like with subs, you'd rather do it with surface area than with excursion.
The amount of output your subwoofer is capable of will also affect the required port area. Take a 4" port which has roughly 12sq.in of port area. If you have a 12" subwoofer (roughly 100sq.in of area), you've got around an 8-1 ratio of cone to port area. If a 12" sub moved .50", the port would have to move around 4 inches. If the sub moves 1", the port has to move 8 inches etc. Obviously that's a lot of velocity and airflow required by a port.
[quote="solacedagony"]So there is an "optimal" port volume where too high and too low will both negatively effect output through the port.[/quote]
Not really. It just gets to a point that the bigger you go, the sheer size of the port and box is impractical.
The port itself performs as a solid mass of air, rather than a hole in a box. The air is effectively trapped within the port walls, and while it doesn't have excursion limits, air flow velocity can negatively affect both output and sound quality. You can model this in WinISD and view the port air velocity in different configurations.
The lower you tune or the larger your box, generally the more port area you need because of the velocity created by the two. I don't really go by a set formula for port area in terms of square inches of port area per the box volume, but rather the velocity created around tuning. Remember that at tuning, the port is really doing most of the work, it's "cracking the whip", sucking energy out of the internal air pressure created in the box, loading the subwoofer because it's using it as the fulcrum to bear the force of it's own acceleration. That's why subwoofer excursion is drastically reduced at tuning. The lower the tune, the more excursion the port is required to have thus the added necessity for port area, just like with subs, you'd rather do it with surface area than with excursion.
The amount of output your subwoofer is capable of will also affect the required port area. Take a 4" port which has roughly 12sq.in of port area. If you have a 12" subwoofer (roughly 100sq.in of area), you've got around an 8-1 ratio of cone to port area. If a 12" sub moved .50", the port would have to move around 4 inches. If the sub moves 1", the port has to move 8 inches etc. Obviously that's a lot of velocity and airflow required by a port.