PA Cabinet ports

[Clive Hugh]

I have a pair of PA cabinets, 15" speaker plus horn, the port holes are just that, a couple of triangles cut at the bottom of each cab, approx 4'' x 4'' triangle.
I want to put proper ports in, the ones I have found come in a variety of sizes, from 2" dia (telescoping) thru to 3".
Any recommendations on setting this up?
Clive

 

[Hawking]

There's a few things you need to know before you can put in a proper tuned port. In order for it to work properly, it actually needs to be tuned to both the cabinet and the speaker(s) in that cabinet. You have 3 options at this point.

Option #1: Do nothing. THey designed it that way for a reason (I would think).

Option #2: The correct method of designing a tuned port requires that you know the internal volume of the cabinet and the "Q" number of the speaker (this loosly equates to the tonal qualities and mass of the driver cone). You'll also need to figure out the frequency that the cabinet itself resonates at. All of this is used to get the right diameter port at the right length to give you the added bass response you're looking for without the sound getting muddy.

Option #3 You can also just drill a hole and stick a tube in it.

If you choose option #3, be careful of a tube that's too small. They have a tendency to whistle a bit as air moves through them.

 

[Clive Hugh]

Thanks, the volume bit is easy, next comes the hard part.
There is nothing wrong with the bass response now except that it can distort if pushed hard, I was just under the impression that it was better all round to have a tuned port rather than a simple triangular hole.

 

[Hawking]

If you're looking for tighter bass response (i.e. doesn't distort), you're going the wrong way. Adding a port or increasing the existing port's efficiancy will only make it worse. Have you tried closing the port that's there? It will decrease the bass responce when the volume is turned down low, but when it gets loud, the pressure inside the cabinet will help to restrict the speakers movement. This has the effect of a cleaner and/or tighter bass response. But...(there's always a 'but'). Some speakers aren't designed for sealed enclosures and won't perform well at the higher pressures.

Have you checked to make sure that it's the speakers that are distorting? It could just as well be the amplifier or the signal going into the amplifier that is creating the distortion.

 

[Clive Hugh]

It could well be the signal, I was using my bass thru a DI box, I have noticed that I can put a synth generated bass from Cubasis thru at really high volumes and it causes no problem whatsoever so it could be the initial signal from the bass guitar, I'll have to start experimenting .
Thanks