tuning a port to 1/4 wavelength of the speaker resonance - diyAudio
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Old 13th September 2012, 11:45 PM   #1
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Default tuning a port to 1/4 wavelength of the speaker resonance

I'm designing a large area port for a small box, and thinking of tuning it to 1/4 wavelength of fs. What am I getting into here? Seems like a long port may as well be tuned thus a way...
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Old 14th September 2012, 12:08 AM   #2
tvrgeek is offline tvrgeek  United States
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Wrong. Please read a bit of theory on how a ported enclosure works. The box functions as a filter to control the resonance of the driver.
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Old 14th September 2012, 12:09 AM   #3
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Not sure what the negativity is for.

Answer: Yes. Although I don't think you were envisioning this, it sounds like a back-loaded horn: a "small" box attached to "large" port. which is often tuned to have a quarter-wave resonance roughly in the neighborhood of the driver's Fs (more or less). And the port does have a large area. The trick is that the port is often horn-shaped.

Another simpler approach would be to forget the small box, and just utilize the large port as the entire cabinet -- make a long, skinny tube/box with a length set to a quarter wavelength of Fs (more or less), then drill a hole in one side and put the driver in there (classic transmission line). Martin King has a free spreadsheet for that purpose. Transmission Line Theory
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Old 14th September 2012, 12:15 AM   #4
Einric is offline Einric  United States
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I am planning just that with a 5" cardboard packing tube.

I found some inspiration here.
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Old 14th September 2012, 12:20 AM   #5
sippy is offline sippy  United Kingdom
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Sounds like a BVR (Big Vent Reflex) to me.........
Not that I know a lot.
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Old 14th September 2012, 12:33 AM   #6
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Sorry about the vague question. My library computer was timing out by the second...
I'm tuning the box to an Fb for a 15th Alignment according to Keele, from his paper "A New Set.. etc." I've recently been gobbling up Thiele and Small's AES Journal papers, as well as a couple of Keene's, and just now Salvatti et al in their port paper.
Basically that last paper put the fear into me and I'm looking at how to make as large a port as I can manage. When I found the duct length approaching the 1/4 wavelength of Fs, I thought huh... and busted out my old 1957 book that describes the Acoustic LabyrinthTM. I note the Labyrinth is lined with absorbent material, not what I have in mind for this big duct, so far.

Basically the current idea is a line array with a big old chimney type duct exiting the top of the cab- or maybe the bottom. This seemed to me to be the way to get the longest unbent duct of the largest area. Then I saw it was nearly at 1/4 wavelength of Fs and thought... I get into a lot of trouble thinking though, so I thought I'd ask.

So Fb, the box tuning, will be one frequency and the length of the duct will be equal to 1/4 wavelength of Fs.
I don't think I'm after a transmission line, but that keeps coming up so I'll take a peek when I have another moment...
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Old 14th September 2012, 01:21 AM   #7
GM is offline GM  United States
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Not familar with 15th alignments, but vents are 1/2 WL resonators, so if it's a 1/4 WL long at Fs, then its strong pipe harmonics will comb filter with the driver's HF output, ergo the vent will need damping for best overall performance. In this scenario though, best to use a passive radiator [PR] or two.

A high aspect ratio cab such as a line array OTOH will have 1/4 WL pipe resonances, so won't conform to a reflex's simple Helmholtz design math. Instead, for a given Fb [Fp] the length of the cab will effectively shorten the vent due to its 1/4 WL resonances damping it.

This type of alignment we refer to as a mass loaded transmission line [MLTL], so must be simmed with a horn design program such as Hornresp or MJK's MathCad software, who came up with the MLTL moniker.

The latter is preferred for newbies though as it predicts how polyfil stuffing will impact performance plus allows some basic in-room simming.

GM
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Old 14th September 2012, 11:22 PM   #8
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Quote:
Originally Posted by GM View Post
Not familar with 15th alignments, but vents are 1/2 WL resonators, so if it's a 1/4 WL long at Fs, then its strong pipe harmonics will comb filter with the driver's HF output, ergo the vent will need damping for best overall performance. In this scenario though, best to use a passive radiator [PR] or two.
I wonder where this HF (high frequency, right?) comb filtering kicks in- this is a bass guitar cabinet where I'm really unconcerned about what happens above 800-1000 hz- there seems to be hardly anything there. I will investigate passive radiators- Small mentioned those as well.
Quote:

A high aspect ratio cab such as a line array OTOH will have 1/4 WL pipe resonances, so won't conform to a reflex's simple Helmholtz design math. Instead, for a given Fb [Fp] the length of the cab will effectively shorten the vent due to its 1/4 WL resonances damping it.

This type of alignment we refer to as a mass loaded transmission line [MLTL], so must be simmed with a horn design program such as Hornresp or MJK's MathCad software, who came up with the MLTL moniker.

The latter is preferred for newbies though as it predicts how polyfil stuffing will impact performance plus allows some basic in-room simming.

GM
Wow, glad I asked! I've gone ahead and built a 36" long line array cab, 6 speakers, and it actually is behaving quite nicely. The tuning is a bit off though- I saw a miscalculation I made that could account for that, but clearly there's more at work than just that.

Unfortunately, I have no means to run any kind of software- only have computer access at the library here. Is there some kind of other resource that will allow me to do the calculations here?

Thanks!
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Old 15th September 2012, 12:02 AM   #9
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For this application it seems I am not after a transmission line. I've found the sound I'm after comes from only a couple patches of absorbance (long haired wool) on the inner walls of the cab. What I have going as a prototype sounds great, but I would dearly love to improve on it without going in a whole different direction.

I don't have a long duct in this prototype, and hearing about the frequency dependant phase cancellations this would create, I don't think I will pursue a duct beyond a 12th of the wavelength of FS. Although I note that Thiele in Part I writes that the goal is provide a consistent air mass load (I think I got that right) at all frequencies the box needs to reproduce- as I mentioned I not after anything above 800-1000 hz, so I wonder if I can push the duct length at all beyond a 12th wavelength. I definitely don't want to screw with the lows, though.

I'm getting away with a 36" long cab with 6 speakers with Fs=38 hz or so, which if I figure right has a 1/4 wavelength 41-1/2" long. I'm not hearing anything I can't live with, but I intend to build a similar cabinet for 6 better but similar drivers, and I'd like to better understand the issues I'm getting into.

"A high aspect ratio cab such as a line array OTOH will have 1/4 WL pipe resonances, so won't conform to a reflex's simple Helmholtz design math. Instead, for a given Fb [Fp] the length of the cab will effectively shorten the vent due to its 1/4 WL resonances damping it."

This is consistent with the tuning discrepancy I'm hearing. I wonder what the math is for compensating for the "effectively shortened" vent, to yield the intended tuning.

I think with this short line I'm wobbling on the cusp as it were- It's only slightly taller than many commericial bass reflex cabs I've encountered, although it's definitely tending towards narrower. Right now the prototype is 36"x6-3/8"x11-3/4" interior, although that's interrupted by a number of shelf-like braces. Possibly there are enough interruptions of the height to mitigate the effect.

I've also thought of putting in a narrow wedge on the top or bottom to eliminate the vertical standing wave, send it bouncing among the shelves.

Thanks again for any clues!
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