# Pipe resonances.

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#### 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

Paid Member
perpetual said:
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

Paid Member
perpetual said:
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

Re: Re: Pipe resonances.

planet10 said:
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).

#### planet10

Paid Member
Re: Re: Re: Pipe resonances.

Mr Evil said:
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.

#### perpetual

My port is 6" in diameter and about 33" long.

Do you think I'll have pipe resonance problems if is crossed at 80Hz on the receiver?

Will damping the port following all your directions solve the problem?

Sorry for all the questions.

#### planet10

Paid Member
perpetual said:
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.

Play... when it sounds right you have the appropriate amount.

dave

#### Ron E

perpetual said:
My port is 6" in diameter and about 33" long.

Do you think I'll have pipe resonance problems if is crossed at 80Hz on the receiver?

I believe you are asking about one of my posts. I remember doing the calculation anyway. If you are using teh sub only to 80Hz, why worry about a 220Hs port resonance? It will only be excited if you feed the sub 220Hz material. Best would be to put the port firing down or back or to the side and disperse any spurious noise. Sometimes when you ask a vague question (IIRC you were deciding between 2-4" or 1-6" port - no box or woofer info provided), people add lots of info that may not apply to your situation just to be helpful.

Stuffing the port will reduce output, worst case to the extent that you won't get any useable output and might as well have made a sealed box.

When ports get long, you are usually better off using a PR. Not that the port won't work....

#### perpetual

I'm asking all these questions because, from my understanding, the 80 Hz cross over is not a brick wall, and the sub might actually play the 200Hz at some significant volume. I'm not really sure how loud it will play at this frequency, or if will play at all. I'm hoping is not that loud so that I won't have to do anything to the port.

#### ThomasW

Lining the walls of the sub with damping material (fiberglass is best), absorbs the higher frequencies that bleed through the crossover

#### RJ

I used 3 1/2" thick fibreglass and peeled off the paper backing. I then glued the sheets on the back and all four sides with Gorilla Glue.
I left a cavity around the port area so the tube could do it's resonating thing. I also covered the cavity area with polyester batting to help keep the fibreglass particles from being thrown out into the room.
Depending on the size of the box thinner sheets of fibreglass could be used. Mine was a pretty big box.
Before I added fibreglass I could definately hear midrange and vocals coming out of the port. Very annoying. After I added the fibreglass it was almost dead quite inside. Only the bass could be heard. I crossed over at 80 hz 12db and yes there was midrange in the box.

#### Attachments

• fiberglass, wool & miraflex-fig43.gif
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#### RJ

Thanks for the heads up Dave, I removed the graphic and replaced it with the bookmarks to those sites.
I was wondering where I got that graph. I think it was from a Miraflex discussion many months ago.
Thanks again for the links. That is quite an article on stuffing!!!!

#### Gary P

Does anyone here have any experience using a Helmholtz resonator to null out port resonances?

I helped my brother build a mini sub for his son. He wanted something that would put out real bass in a small bedroom or dorm room.

The design uses the 6.5" Tangband sub driver in a .75 cf vented box with a 100 watt plate amp in the back. Box size is a guess as I don't have it in my notes. The down side to building a very small sub tuned low is the port size needed to get a small box to resonate at 30 hz tends to be on the silly side.

To get the box tuned with a port we ended up with a 3" port that is close to 5' long. When they were modling the sub the question came up of what to do with the port. After many silly ideas and good laughs I suggested that they use the port to make a base for the box. And so the mini sub was born...

An externally hosted image should be here but it was not working when we last tested it.

The box tuning is a 6th order electronic filter assisted alignment. Their were 2 reasons for the eq filter.

1. Allowed for a lower tuning frequency.
2. Allowed for low freqency cutoff to keep the sub from excursion limiting when there is very low frequencies in the music program.

The eq filter was built on a small piece of vector board and installed inside the plate amp between the crossover board and the amp section.

Performance is much better than I was expecting with solid output down to 30 hz. The main draw back to this design is the long port. The first port resonance is at ~120hz and quite high Q. We tried damping the port with both dacron and long fiber wool but by the time their was any results at the 120 hz resonance the port had stopped working as a port. With all the bends in the port there is enough resistance that the port resonance at box tuning is already low Q and adding any more damping is not a valid way to deal with the problem.

So, we want to try a Helmholtz resonator tuned to 120 hz and mounted in the port at the half way point. My thoughts are that with the port size of 3" the port on the Helmholtz resonator should be 1 to 1.5" in diameter so that it can work independantly of the 3" port it is attached to. Does this sound like a good starting point?

Any input is welcome.

Gary

#### GM

Greets!

====
>Does anyone here have any experience using a Helmholtz resonator to null out port resonances?
====
These are called band-stop filters and will only produce a deep notch at one frequency. To damp a series of modes requires a low pass filter, which is an expansion in the line like an exhaust resonator.
====
>The design uses the 6.5" Tangband sub driver in a .75 cf vented box with a 100 watt plate amp in the back. Box size is a guess as I don't have it in my notes. The down side to building a very small sub tuned low is the port size needed to get a small box to resonate at 30 hz tends to be on the silly side.

>To get the box tuned with a port we ended up with a 3" port that is close to 5' long. When they were modling the sub the question came up of what to do with the port. After many silly ideas and good laughs I suggested that they use the port to make a base for the box. And so the mini sub was born...

http://www.pacifier.com/~gpimm/minisub.jpg
====
Correct, you basically wind up with a zero expansion BLH. Cute design BTW.
====
>The first port resonance is at ~120hz and quite high Q. We tried damping the port with both dacron and long fiber wool but by the time their was any results at the 120 hz resonance the port had stopped working as a port. With all the bends in the port there is enough resistance that the port resonance at box tuning is already low Q and adding any more damping is not a valid way to deal with the problem.

>My thoughts are that with the port size of 3" the port on the Helmholtz resonator should be 1 to 1.5" in diameter so that it can work independantly of the 3" port it is attached to. Does this sound like a good starting point?
====
OK, maybe a BSF will suffice: http://www.kettering.edu/~drussell/GMI-Acoustics/Filters-Frame.html

That said, rather than 'band-aid' this design, I recommend you build an end loaded reverse tapered TL, which if you think about it is where you're intuitively headed with wanting to use a somewhat smaller pipe in series with the larger one. It also has the benefit of being shorter for a given Fp than a straight pipe, the length decreasing with increasing compression ratio (CR) and provides more loading around Fp so you may be able to tune it a bit lower.

GM

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