Porting issues

Hi there. I don't know if this should be in this forum or the PA system forum, so here goes: I've built some 18" bass cabs using P.Audio speakers, and used Harris Technologies software for the box and porting design. These seem to work reasonably well although I seem to get more bass from behind the cabs than from the front. i think this may be a porting issue. I have the 18's front mounted and have 4 ports in the front panel corners. Does one include the porting volume in the total volume of the cab, or, do you deduct the volume for these ports from the internal cab volume as one would deduct the speaker and other intrusions into the cabinet's volume.:confused:
 

eriksquires

Member
2013-05-10 4:11 pm
Hi John!!

More likely you are running into a room issue. :) Are you walking over to the rear? If so, don't. Listen in the front, and turn the speaker around.

The question of port volume has nothing to do with getting more bass behind a speaker than in front of it. I think in theory you should deduct the port volume, but now that they are built, measure and analyze what you have instead of trying to fix everything theoretically. Theory is good, but no substitute for empirical analysis.

Best,


Erik
 
Take a bit of toilet tissue, a strip, hang it from one of the ports.
Then sweep a sine wave down in freq, when the tissue flaps the most AND the cone moves the least, that's the actual frequency that you ended up tuning the port.

Of course you can also just measure the impedance, or stuff a mic at the port exit and measure it that way too...

It might be good to see if your proposed simulated tuning ended up where you wanted it.

Move the sub toward the center of the room, or nearly so and see if the bass is any different. And/or lay the sub on its back and point the whole thing UP at the ceiling...
etc.

Likely a room effect.

_-_-
 
Brief

Hi there. I don't know if this should be in this forum or the PA system forum, so here goes: I've built some 18" bass cabs using P.Audio speakers, and used Harris Technologies software for the box and porting design. These seem to work reasonably well although I seem to get more bass from behind the cabs than from the front. i think this may be a porting issue. I have the 18's front mounted and have 4 ports in the front panel corners. Does one include the porting volume in the total volume of the cab, or, do you deduct the volume for these ports from the internal cab volume as one would deduct the speaker and other intrusions into the cabinet's volume.:confused:

Here are some things to consider:

1) Cabinet volume occupied by the port structures along with that occupied by the drivers should be excluded to arrive at a net acoustical cabinet volume.

2) The acoustical volume of the port(s) extends well beyond the physical ends of the port.

3) Sound source imaging of bass notes is a function of unit placement in the room and unit proximity to adjacent walls.
For the lower end, the ear has lost its sense of directionality.

Regards,

WHG
 
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