Pipe resonances.

[perpetual]

I have two questions about pipe resonances.

1. I know they affect long ports, but what is the length:frequency ratio. I've read that a 30" port will create pipe resonances at about 220Hz or something like that.

2. When I asked someone at PE about pipe resonances, he answered that if I was worry about them, I sould dampen the ports. I'm not sure what he meant. Did he mean dampen the outside of the tube? I hope is not the inside or the dampening material might fly out.

Thanks for all your help.

[planet10]

Originally posted by perpetual
2. When I asked someone at PE about pipe resonances, he answered that if I was worry about them, I sould dampen the ports. I'm not sure what he meant. Did he mean dampen the outside of the tube? I hope is not the inside or the dampening material might fly out.

I'm sure by damping the port he means inside. If you damp it enuff you will get an aperiodic box (i've not had a commercial BR in my room that ddn't sound better with a damped port)

dave

PS: expect to get some ribbing for the use of "dampen" which means to get wet. Pouring water in your port will only make a mess

[Bill Fitzpatrick]

Yes, he meant stuff the inside of the pipe. Just the right amount.

[perpetual]

what kind of damping should be used for it not to fly out of the port.

[GM]

Greets!

Ignoring any end correction calcs that gives a good approximation of the port's acoustic length (which will be longer), port resonances begin at ~ 2x its length, so ~13560"/2/30" = ~226Hz, though it will be a bit lower depending on the port's radius and how it's terminated at each end. Since it's classified as a 1/2WL open cylinder, it will have both even and odd harmonics.

The best way to damp a port is to add a resistive cap over the exit, i.e. aperiodic loading. If you stuff the vent then you'll need to put a grill over it anyway in case it should be energized enough to pop it out.

GM

[planet10]

Originally posted by perpetual
what kind of damping should be used for it not to fly out of the port.

I've used rolled up foam sheet... currently have a rolled up pad of poly fluff in the tannoy D50s.

dave

[Mr Evil]

Originally posted by planet10
PS: expect to get some ribbing for the use of "dampen" which means to get wet. Pouring water in your port will only make a mess

Well, according to the OED, dampen not only means to make wet, but also:

Make less strong or intense.

Reduce the amplitude of (a sound).

[DIAR]

Gradient uses a helmholtz resonator in the middle of the tube (pressure max point).

http://www.gradient.fi/design/highlights/
http://www.gradient.fi/products/intro/index.html

[planet10]

Originally posted by Mr Evil
Well, according to the OED, dampen not only means to make wet, but also:

Make less strong or intense.

Reduce the amplitude of (a sound).

Makes no nevr mind... i've seen the beatings over & over again

dave

[perpetual]

Do I have to stuff the whole port, or just part of it? Like for example half the length of the port.

Do I have to just cover the walls of the port or all the diameter?

Will the stuffing change the tuning frequency?

GM,

Are there 6" resistive caps? where can I get them?

Thanks you've all been very helpfull.