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-   -   Multiple ports with different tunings. (http://www.diyaudio.com/forums/subwoofers/144331-multiple-ports-different-tunings.html)

chris661 18th May 2009 06:50 AM

Multiple ports with different tunings.
 
This was just an idea buzzing around in my head, but now I've got some time, I can ask about it.

Let's say I have a speaker, with a port tuned to 30Hz. The driver will unload <30Hz.

So what if I put a port tuned to, say 20Hz in, to stop some of the unloading, and reduce cone excursion around that frequency.

Would that work - anyone with any experience of this?

winISD won't let me use lots of different ports, so I decided I should ask. Maybe it's a good idea.

aznboi3644 18th May 2009 07:45 AM

That is not how a helmholtz resonator functions. Failed idea.

If you want to have two enclosure resonances look up the aperiodic bi-chamber enclosure.

Patrick Bateman 18th May 2009 11:56 PM


Earl Geddes had a bandpass box at RMAF in 2005 that worked like that I believe. I would contact him, or see if you can get his attention in this thread.

Moondog55 19th May 2009 03:44 AM

I know that Colin Whatmough of Whatmough Monitors/A1 Audio; used to tune one of his sub-woofers to two close but different frequencies, and now I can't find the old catalogue to check on the details.

What is the difference between two different sized pipes and a shelf port cut a 45 degree angle?? I tried that once and it seemed to improve performance.

Josh Ricci 19th May 2009 06:03 PM

I've done a little bit of rough testing on having 2 unequal length ports in an enclosure and they act like a port or ports that split the difference. Basically if you have a 16" port and an 10" port they will act like a pair of 13" ports at least as far as the tuning is concerned. Perhaps it could be useful for diminishing pipe resonances or something. I believe that Polk used to do this and called is ARC or something like that.

I've thought about having a slot port that tapers from a short length on one end to a long length on the other. Like 1" long that increases to 20" long on the other end within say 20" width. In theory I think that it would act like a 10.5" port for tuning purposes.

Patrick Bateman 19th May 2009 06:22 PM

Quote:

Originally posted by Josh Ricci
I've done a little bit of rough testing on having 2 unequal length ports in an enclosure and they act like a port or ports that split the difference. Basically if you have a 16" port and an 10" port they will act like a pair of 13" ports at least as far as the tuning is concerned. Perhaps it could be useful for diminishing pipe resonances or something. I believe that Polk used to do this and called is ARC or something like that.

I've thought about having a slot port that tapers from a short length on one end to a long length on the other. Like 1" long that increases to 20" long on the other end within say 20" width. In theory I think that it would act like a 10.5" port for tuning purposes.

Josh,

Do you know how to use hornresp? One really cool feature, that's not well known, is that it can model vented and bandpass boxes.

Now that isn't a big deal, because a lot of programs can. The *cool* part is that horn response can model vents like you describe, where they're tapered. Horn response can also model vents that are flared at both ends.

It is the only program I am aware of (besides akabak) that can do this.

It's really interesting when you get into it, because you start to realize that you can get a huge bump in efficiency from a vented or a bandpass box if you use VERY large vents. The big drawback are resonances. But the original posters idea would deal with that.

I explored that to some extent in this thread.

If anyone needs me to clarify how you can model a vented/bandpass in hornresp, just let me know.

http://www.diyaudio.com/forums/showt...hreadid=128937

GM 19th May 2009 07:02 PM

Quote:

Originally posted by Josh Ricci
I've done a little bit of rough testing on having 2 unequal length ports in an enclosure and they act like a port or ports that split the difference.

I've thought about having a slot port that tapers from a short length on one end to a long length on the other.

That's been my experience too, i.e a more broadband (lower Q) tuning which can be approximated via a single Karlson Coupler (pipe terminus slashed at an angle). This type of vent was patented some time ago as a new, unique breakthrough in venting, so probably legally strictly for personal use for now.

Can't remember the brand now, but IIRC it was either a small boutique vendor or start-up company, so may not even be around anymore as I don't recall reading about them since someone on one the forums asked about their performance claiams 10-12 yrs? ago.

GM

gedlee 19th May 2009 08:30 PM

Quote:

Originally posted by Josh Ricci
I've done a little bit of rough testing on having 2 unequal length ports in an enclosure and they act like a port or ports that split the difference. Basically if you have a 16" port and an 10" port they will act like a pair of 13" ports at least as far as the tuning is concerned. Perhaps it could be useful for diminishing pipe resonances or something.



This is precisely what happens both theoretically and in practice. I typically use three, have used five, all different lengths. Just take the average if they are all the same diameter. (Thats not exact, but close enough).

j.michael droke 19th May 2009 11:31 PM

unequal multiple ports
 
Hi there: If unequal multiple ports act "nearly" the same as a single port of the averaged size, why not use use the single average port? .... regards, Michael

Patrick Bateman 20th May 2009 12:33 AM

Re: unequal multiple ports
 
Quote:

Originally posted by j.michael droke
Hi there: If unequal multiple ports act "nearly" the same as a single port of the averaged size, why not use use the single average port? .... regards, Michael
Because a long, large port has a severe resonance. And the larger and longer the port is, the lower in frequency and the higher in SPL the resonance is.

For instance, let's say you have a subwoofer that covers a range of 20hz to 80hz. Due to a port resonance, there's a peak at 160 hz. That *peak* can be more audible than the sub itself, particularly due to the Fletcher Munson curve.

It's not a small problem.


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