More than one frequency for constructive backwaves? - diyAudio
 More than one frequency for constructive backwaves?
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 5th October 2015, 02:01 AM #1 diyAudio Member   Join Date: Nov 2014 More than one frequency for constructive backwaves? I am wondering if it is possible to tune an enclosure to have multiple constructive backwaves. Consider the following situation: Place a 15" subwoofer with long throw into a sonotube. IF the back wave is directed through a large port of length equal to 1/2 20hz wavelength + some arbitrary distance X, and the front of the sonotube ahead of the woofer is equal to that distance X, then the two waves will add together constructively. Could this be done with a system of more than one port set (front and rear) to create constructive addition of backwaves through the entire useable range of the subwoofer?
 5th October 2015, 11:03 AM #2 diyAudio Member   Join Date: Nov 2003 Location: Brighton UK Hi, The short answer is no. Your theory is very dodgy. A port by definition acts acts as a lumped mass, not a transmission line with phase changes. Multiple ports in parallel don't work. 1/2 20Hz wavelength = 8.6 metres. rgds, sreten. __________________ There is nothing more practical than a really good theory - Ludwig Boltzmann When your only tool is a hammer, every problem looks like a nail - Abraham Maslow
 5th October 2015, 03:51 PM #3 diyAudio Member   Join Date: Nov 2014 Actually, since posting this, I found a military patent for a weaponized subwoofer.... and it used this concept. So yes, it is possible.... we'll see if I can replicate it on a small scale. JG
 5th October 2015, 04:13 PM #4 diyAudio Member   Join Date: Nov 2014 Question along the same line.... We took our prototype out into a field and measured SPL at one meter. We had almost 115 db at 5 hz, and less than 4% thd. We were hoping for over 125 db, however. How much room gain can we expect in a listening room? We can't get the system into a house or even a garage at the moment... and if our current build @10kW can't hit more than 125db with in-room gain, then we'll call it quits.
 5th October 2015, 04:55 PM #5 diyAudio Member     Join Date: Apr 2007 You have to consider the building structure and "how" the boundaries are elastic; with some 10 kW hitting the right spot, the collapsing of a wall takes moments
 5th October 2015, 04:57 PM #6 diyAudio Member   Join Date: Nov 2014 Good point. The application of this design is noise pollution management in nuclear power plants. I think the walls there are strong enough.
 5th October 2015, 05:12 PM #7 diyAudio Member   Join Date: Nov 2014 Post 10 here was pretty clear on how room gain would work. http://www.avsforum.com/forum/155-di...d-project.html It looks like this endevour is totally worth it based on the ground plane measurements. I'll keep this thread updated.
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Quote:
 Originally Posted by bobthedespot I am wondering if it is possible to tune an enclosure to have multiple constructive backwaves. Consider the following situation: Place a 15" subwoofer with long throw into a sonotube. IF the back wave is directed through a large port of length equal to 1/2 20hz wavelength + some arbitrary distance X, and the front of the sonotube ahead of the woofer is equal to that distance X, then the two waves will add together constructively. Could this be done with a system of more than one port set (front and rear) to create constructive addition of backwaves through the entire useable range of the subwoofer?
This is actually a pretty popular question. I wondered the same thing when I was new - why not have a port (or a whole other ported enclosure) for each note, or each octave?

The problem is phase. With a single port you don't get a huge spiked port output that is very narrow bandwidth like WinISD suggests. You get a small spike that has lower amplitude output and spiky garbage resonances for several octaves above the port tuning like 1/4 wave software suggests. With multiple outputs you get vast overlap and consequently massive phase problems so they don't sum the way you would like.

When you put multiple ports in a single ported box it doesn't work like you suggest. See these links, in particular post 2 in the first link and post 4 in the second link. I've never experimented with this but in lieu of that I'll put my money on John Kreskovsky.
multiple ports all tuned to different Hz
What happens if you have different length ports in a cabinet?

While it is possible to fill in a single hole in response with resonant 1/4 wave enclosures (like a tapped horn or like a Bose Wavecannon which IIRC is the name of what you describe in post 1), trying to do more than fill in a single hole in response gets really tough, maybe impossible.

I was quite surprised to learn that this concept DOES actually work with sealed enclosures. A couple of years back I simulated a Hegeman subwoofer for a guy. That's a sealed box with 4 separate stubs connected at the throat chamber, and all 4 of the stubs are sealed.

Each stub needed quite a bit of stuffing in the first few inches to settle the response spikes down to what you see above. Now compare that to a simple sealed box of the same volume with the same driver as shown below. As you can see the sealed stubs worked but did nothing except trade a squashed impedance spike for a slightly less flat response curve. No advantage for the extra complexity that I can see.

So while that does work for sealed boxes I was unable to make it work for unsealed stubs. To be fair I didn't try very hard or for very long and I should probably try again.

BUT unless the goal is MORE than 3 octaves of bandwidth there's no point. That's what horns already do naturally.

Quote:
 Originally Posted by bobthedespot Actually, since posting this, I found a military patent for a weaponized subwoofer.... and it used this concept. So yes, it is possible.... we'll see if I can replicate it on a small scale. JG
No, for the reasons listed above I don't think it's the same concept. But please post a link to this military patent so I can see it.

Quote:
 Originally Posted by bobthedespot Question along the same line.... We took our prototype out into a field and measured SPL at one meter. We had almost 115 db at 5 hz, and less than 4% thd. We were hoping for over 125 db, however. How much room gain can we expect in a listening room? We can't get the system into a house or even a garage at the moment... and if our current build @10kW can't hit more than 125db with in-room gain, then we'll call it quits.
Depending on the room you can expect a lot or a little. Please show some details of this prototype - either a picture, a drawing or at least a description.

Last edited by just a guy; 5th October 2015 at 05:32 PM.

 5th October 2015, 05:49 PM #9 diyAudio Member   Join Date: Nov 2014 Wow thank you for the detailed response. Here is the military design. http://www.dtic.mil/dtic/tr/fulltext/u2/a399241.pdf Applying the variable Helmholtz resonator to each port, we can limit the output of harmonics and other annoying distortions. Furthermore, the phase lag can be corrected in this way. I can't disclose our design in full (because I am working on it with someone else), but I will say that it has the sum effect of a rotary subwoofer mounted in a sonotube (sp), but it has less chuffing, uses less power, and has the potential for greater volume. Our design that we just tested in a field was simply to test the additive backwave, and our equivalent xmax was 12 meters through a 12" diameter pipe @ 5 hz.
 5th October 2015, 05:59 PM #10 diyAudio Member   Join Date: May 2006 Can you at least state your goal? What bandwidth at what spl? 5 hz - ??? hz?

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