Vented Divider To Cure Standing Waves In A Tall, Narrow Enclosure? - diyAudio
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Old 24th February 2003, 02:38 AM   #1
Wizard of Kelts
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Default Vented Divider To Cure Standing Waves In A Tall, Narrow Enclosure?

Suppose we want to build a tall, narrow enclosure that is 46" high by 8.5" wide by 4.5" deep inside dimensions. It will have a base so it doesn't fall over.

The illustration shows a Ported enclosure, but it could be a Closed Box.

Such a box will have a huge standing wave, very narrow band, (like a "crack"), at 280 Hz, which is the wavelength of 46".

In order to prevent this, we decide to build a divider that prevents the standing wave from hitting the speaker at 280 Hz. This divides the speaker volume into two chambers-The Small Chamber with the woofer, and The Large Chamber with the port, (if it's a Ported system).

The idea is keep the speaker in a smaller enclosure until the frequency goes below 280 Hz, then have The Large chamber open up for full volume around the bottom octave or so. This means that The Small Chamber would not have any standing waves that were not very short wavelength, therefore very easily absorbed by the inch or so of stuffing on the walls of The Small Chamber.

Let's say the tuning frequency will be 50 Hz.

A) Will this idea work without response irregularities?

B) If the idea does work, what would be a good frequency to tune the interior divider?

The illustration as shown shows The Small Chamber as being one fourth of the total box volume. The ratio is not set in stone. You can make the The Small Chamber one third the total box volume if you want. Just want a divider to prevent standing waves of low frequency from forming.

My guess is to make the vent of a size to tune The Large Chamber to 200 Hz, which is half an octave beneath the standing wave of 280 Hz. That should free up the total box volume when you need it.

With more and more tall, narrow enclosures being built for aesthetic and other reasons, I think this is a good way to cure the standing wave problem.

For the advanced course, we can discuss two or more dividers. However, for right now, let us deal with the guidelines to make just one work.
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Old 24th February 2003, 02:48 AM   #2
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PS: I know one way to do this is to have the divider be built at an an angle, to cut down on standing waves. We can discuss that. But I would also like to discuss the divider installed as shown, breaking the box into two rectangular chambers.
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Old 24th February 2003, 12:23 PM   #3
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Aperiodic enclosures are worth investigating.

Eric.
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Old 24th February 2003, 01:08 PM   #4
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Mr. Feedback:

Thank you for responding. Yes, there is a kind of aperiodic enclosure where a port to another chamber opens up that chamber at certain frequencies. There is also the double chamber reflex which had a thread devoted to it some time ago.

But what I am interested in here is to establish some guidelines as to how to tune a chamber so that the bass reflex or closed box acts identical to a non-divided reflex in the bass region, only the standing waves are broken up.

I realize there are alternative enclosures, such as the double chamber aperiodic, that have merit in and of themselves, but I want to put them aside momentarily and just concentrate on making the Reflex/Closed Box work normally with a divider.

With many builders no longer satisfied with the squat enclosure, and increasing attention being paid to tall enclosures, (not necessarily Transmission Lines), I thought that would be a good thing to establish for future reference.

The dividers can perform double duty both as braces and standing wave preventers. I just want to establish how big the vent in the dividers, (which vents might be no more than a hole in the dividers-no pipes), should be not to interfere with the bass action.

We know that in Ported enclosures, the vent action only extends about an octave above the tuning frequency. After that, it is essentially equivalent to a Closed Box.
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Old 24th February 2003, 01:34 PM   #5
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Before moving on to the low-pass filter concept I'd be interested to know how much this 280 Hz standing wave could be mitigated by having the bottom at an angle - for instance putting a wedge shaped piece of MDF 8.5" x 4.5" at the bottom. This wedge could go from 2" thick at the thickest down to nothing.

In theory, this should give an even spread of the standing wave frequencies from, say, 280 Hz to 293 Hz.

What do you think?

Steve
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Old 24th February 2003, 01:47 PM   #6
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7V:

My initial reaction would be that such a piece would not give enough spread to mitigate the standing wave.

On the other hand, I saw a response curve in a test where Audio magazine tested a Pioneer speaker, 4 feet tall, that had this problem. The "crack"-a large dive upward followed by a large spike downward-was very narrow band. So such a small difference in the bottom might spread the phenomenon across enough frequencies to mitigate it with a decent amount of stuffing.

So, offhand, I would say that something like your approach has a chance for success.
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Old 24th February 2003, 02:17 PM   #7
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What I am trying to say is, if we are going to build speakers that look the illustration below-and a lot of us are headed in that direction-we might as well use the dividers to break up the standing waves. At least one of the dividers, anyway.

Even speakers less extreme, but built along similar lines, would benefit from these guidelines.

By the way, the illustration below is from a speaker project by Dan Cramer. The entire site is very instructive and worth checking out. Give the page a minute or two to load.

LOL, those random blue spots are not on the original picture!

http://www.hilberink.nl/codehans/tannoy7.htm
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Old 24th February 2003, 02:20 PM   #8
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So, having (possibly) added a wedge at the bottom, my next question would be: what if we then had a ledge (at an angle of course!) with a gap in it for the port, rather than a tube? The ledge could be 1/2" or 1" thick (which would be the equivalent port length). Is there a gap size that would be right for a suitable low-pass filter?

I suppose that, given that it's inside the cabinet, a tube wouldn't cause any audible problems through air-flow but still, let's do without it if we can.

Steve
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Old 24th February 2003, 02:25 PM   #9
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And yes Keltic (having just seen your last post with the photo) I know where you're coming from and I think you're absolutely on track with this approach.

I'm going in a similar direction myself.

Steve
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Old 24th February 2003, 02:32 PM   #10
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7V:

I put the divider with the port in it just to illustrate the concept. By all means, if we can make the port just a hole in the divider, let us do so. After all, there are plenty of speaker enclosures, especially PA enclosures, where the port is just a hole cut in the wall of the enclosure.

I mentioned the idea of a slanted divider earlier. While I think that is definitely the best way to go sonically, for ease of construction purposes, I would like to investigate the straight divider as well. Still, let's go with the slanted divider for now.

Okay, then we have 2 questions.

A) Where the should the divider go-that is, how big is The Large Chamber compared to The Small Chamber-or should both chambers be the same?

B) In our example-a 46" high enclosure with a standing wave of 280 Hz-how big a hole in the divider should we make to break up the standing wave while leaving the port action unchanged from an undivided enclosure?
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