Detuneing question...

[netzero 10+ year member]

Ok, I've had some people tell me before that if you use a round port, pvc, sono, or flared, it should be a ports width away from the wall it approaches. I have just glanced over those comments and never really thought about it until today. I bought a 8" sono tube im going to use in my box. As of now I had planned it to do a 90* bend to get me to 22".

Then I was messing around in my basement with the tube (making a fool of myself) with my face in it making some low humming. As i pulled the tube an inch or two off the ground i noticed at a certin distance it all of the sudden got louder and reinforced. I assumed this was a way to, "tune the port". To test it, as i got closer to the ground the louder and more reinforced point got to be a lower humming on my part. Sorry for the read but bare with me lol

So my question is, does anyone know if there is a way to strategically tune a box lower by making it shorter, but closer to an inside wall? I'm building a box 18" deep, and the port needs to be 22", can i achieve a tuning of 32 Hz but making it, say 16 inches, which breaks the port width away rule, while tuning the box lower? Thanks for any info you've got!

net

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[netzero 10+ year member]

Anyone? Well I'm going to bed early tonight, had a double practice and swam 11,000 yards so im pretty beat. Hopefully someone has some insight to this... I'll check back tomorrow.

 

[IDSkoT 5,000+ posts]

You can have part of the port outside of the box //content.invisioncic.com/y282845/emoticons/fyi.gif.9f1f679348da7204ce960cfc74bca8e0.gif

 

[netzero 10+ year member]

You can have part of the port outside of the box //content.invisioncic.com/y282845/emoticons/fyi.gif.9f1f679348da7204ce960cfc74bca8e0.gif

I'm working with depth restraints and it's a fiberglass box, so the entrance to the port is molded into the box..

 

[Rashaddd 5,000+ posts]

I've heard restricting port opening (as in what you're talking about) can tune lower, but I can't confirm this, I'm sure someone can tell you though...

 

[Immacomputer 10+ year member]

It sounds like you made a mass loaded transmission line by bringing the tube close to the ground. You basically made a port for a transmission line that you were having trouble exciting due to the difference in surface area of your mouth to the line.

First off, the port's width away rule doesn't always apply. In reality, 1/2 the ports width (or the port radius) would increase the port length by about 1/2 of the radius. By putting the port about 2" away from the back wall would add about 4" to the effective length of the port.

If you want to get the tuning down lower, you can do something like this keeping the port away from the port wall by about 1.5-2":

Or you could have one of the sides be a side of the box too. That would basically decrease the tuning by increasing the air mass in the port. If you did a disk of about 12"-14" diameter, you should have a tuning that would be closer to what you want and maybe even a little bit lower.

In the enclosure, the tube won't really act like a transmission line, like it did when you were humming into it, except for frequencies above about 150hz so you should be fine.

 

[netzero 10+ year member]

Immacomputer, you're amazing! That's exactly what I would like to do. I'm still pretty noob with all this so I don't really understand the transmission line thing, but i think i understand the increasing the port length by about 1/2 of the radius to add relative port length.

What exactly does the 12-14" disk do? And will tuning my box this way create more turbulence in that portion of the port? Thanks

 

[Immacomputer 10+ year member]

The ring will help lower the tuning and allow a more even pressure transfer to the port. Rounding over the side facing the enclosure wall will also help in pressure transfer.

 

[netzero 10+ year member]

The ring will help lower the tuning and allow a more even pressure transfer to the port. Rounding over the side facing the enclosure wall will also help in pressure transfer.

Ok great, thanks a bunch. I'll have to give this a shot!