Adjustable Length Port

[jdybnis]

I'm building a 60 liter MLTL for the RS180. The port is 2" in diameter. The attached graphs show the response of four different tunings, and the driver displacement by frequency. The best tuning depends on the type of music playing. I'd like the flexibility to change the port length without cracking open the box.

I've never seen anything done like this before. Any ideas on making an adjustable length port? When I'm listening to music like Massive Attack or Enya, with lots of sub 25Hz content I want to tune it low to protect the driver from over-excursion. Otherwise I want to tune it flat or give it a low-end bump.

Edit: The very last graph is mislabeled. It should say 6" Long Port.

[Paul H]

I have my suspicions that most people, given an adjustable port, would either think it's too much bother and leave it in the safer higher-tuned position or forget about the port and leave it tuned too low until the driver started 'clanking'.

Also, the graphs you've indicated have an almost flat response for the shorter length ports, which will likely give you a hump in the bass reponse once room gain is factored in - so I would recommend sticking with the longer 6" port as the one that will give a flatter frequency response in-room.

That being said, I can't think of any technical reason why an adjustable port can't be diy'ed, using sections of pipe of differing sizes that fit closely over each other while still allowing sliding.

Don't forget that the port can be partly or almost entirely outside the box to allow easier access. A 'normal' permamnently set port inside the box (using the shorter port length and the larger section of piping) with a pull-out smaller (inner) piece of piping would be one solution.

Paul

[JonMarsh]

PE has sold adjustable ports- you'll find them here, the problem is that they have the adjustable sleeve on the outside- which means adjustable from within the cabinet.

PE Adjustable 2" port



You'll probably get the subjectively flattest inroom response with the 4" or 5" port. The thing about the lower tuning, nothing moves enough air at 20-25 Hz with a 2" port for that to be very usefull, other than moving the port unloading frequency down further. Have you already bought RS180's? I'd try the design concept you're persuing, but with RS225- in a TQWT, it can move enough air in a 3" or 4" port to do some fairly useful low bass levels; the M8ta is tuned to the mid 20's, and it works pretty well.

[jdybnis]

I have my suspicions that most people, given an adjustable port, would either think it's too much bother and leave it in the safer higher-tuned position or forget about the port and leave it tuned too low until the driver started 'clanking'.

That's very true. However this pair I'm building for myself.
[/QUOTE]

Don't forget that the port can be partly or almost entirely outside the box to allow easier access. A 'normal' permamnently set port inside the box (using the shorter port length and the larger section of piping) with a pull-out smaller (inner) piece of piping would be one solution.

Would a sliding piece of pipe be held in strong enough to withstand the air pressure?

[jdybnis]

The thing about the lower tuning, nothing moves enough air at 20-25 Hz with a 2" port for that to be very useful, other than moving the port unloading frequency down further.

That is exactly what my goal is. I don't expect to get 20Hz out of a 7" driver. But the problem I've had with Modula MT's is that some music I listen to has LF content that is practically inaudible but causes the driver to extend past xmax at low volumes.

Have you already bought RS180's? I'd try the design concept you're persuing, but with RS225- in a TQWT, it can move enough air in a 3" or 4" port to do some fairly useful low bass levels; the M8ta is tuned to the mid 20's, and it works pretty well.

I'm far enough along in building my cabinet that I can't switch to a bigger driver. I'm planning on using the Modula MT crossover. The outside width and depth are the same as the PE cabinet but the walls are thicker on my cabinet. The interior width is only 7.75".

Aesthetically I don't like the idea of putting the port tube sticking out of the back so I can get to it. But it may be what I have to go with.

[ThomasW]

But the problem I've had with Modula MT's is that some music I listen to has LF content that is practically inaudible but causes the driver to extend past xmax at low volumes.

A lower tuning is going to effect the output higher up. The best thing for the Modula MT is use a either a subwoofer or a 'rumble' filter to protect the driver from unloading