Thanks everyone... Really appreciate your input. My intention of partially reducing port volume is to...get the bass to go deeper. My speaker port is slotted in shape...approx 10" Long ... 2" high...12" deep. Anyone tried this with some success? What material was used? Thanks.
I would use a block of wood, same height and depth as the port, and vary the width to get the desired tuning. I'd make the block slightly less than the full 2" high, and use a few wraps of electrical tape here and there to give a good enough friction fit that it doesn't rattle. The tape probably isn't necessary unless you're playing real loud. Might as well center the block in the existing slot port, effectively giving you two smaller ports.
Unless you actually have excess bass energy as it now stands, you may not achieve a net benefit by lowering the tuning frequency. What will happen is, the bass rolloff will start higher up but will be more gentle. You won't have more bass energy; you'll just be distributing it differently. That being said...
Personally I'm a big fan of adjusting the tuning frequency for the particular room situation, and provide that capability not only in my home audio speakers but also in my bass guitar cabs.
Here are some ballpark guidelines (real-world results may vary slightly):
If you block off 25% of the port area (for the full depth of the port), the new tuning frequency will be .88 x the original.
If you block off 50% of the port area, the new tuning frequency will be .73 x the original.
If you block off 75% of the port area, the new tuning frequency will be .51 x the original.
So if your current tuning frequency is 40 Hz and you want to see what 30 Hz would sound like, block off about half the port area and you should be real close.
If you want to get fancy, make both ends of the block of wood shaped like the bow of a boat, for smoother airflow (raising the chuffing threshold). Decreasing the port area will have increased the air velocity in the port, so at high SPLs chuffing might become an issue. The tuning frequency will go back up a little bit due to the boat-shape of the wooden insert reducing the effective port length slightly.
Reducing the volume of a port will usually tend to raise the tuning frequency, not lower it.
It's the shape of the port that matters. A long skinny port will have a lower tuning frequency than a short wide port.
If we reduce port length (which reduces port volume), the tuning frequency does indeed go up.
But instead if we reduce port area (which also reduces port volume), the tuning frequency goes
down.
So we need to know the specifics of how the port dimensions are changed before we can say which way the tuning frequency will move.
