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Discussion Starter #1
Maybe this is a bit off topic, but since everybody here is interested in subwoofers.....

I did the folowing thought experiment:
In a shoebox-shaped room a subwoofer is placed at one side of the room and another one at the opposite site of the room. The subwoofers are of the closed box type and are perfect (ideal frequency- and phase response. That is: ruler flat frequency response and zero degree phase shift for every frequency). Let's assume that the two subwoofers are 10 meters apart. The second subwoofer is driven with the inverted signal of the first one (180 degrees phase difference). This set-up can be compared with a single driver placed in an open baffle with a diameter of 5 meter (where there is a path of 10 meters between the two sides of the driver). The output of the front and the output of the back of the driver will start to cancel each other out somewhere at 17 Hz.
Now we remove the enclosures of the subwoofers and replace them with a duct with the same diameter as the drivers. The duct connects the backside of each driver with the backside of the other driver. Sound emanating from the first driver will "collide" with the sound of the second driver halfway the duct. Since the two sound waves have an 180 degree phase difference they will cancel out each other. So: no reflections in or at the end of the duct, each driver "sees" an infinite duct.

This would mean: no more large subwoofer enclosures, just place two drivers and some piping between them that can run through the cellar.

This is to good to be true, so please help. Where did I go wrong??

Thanks
 

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Well, your duct is a big box, but I suppose you are now putting it in a more convenient place. Thats an interesting question you pose.
 

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Discussion Starter #4 (Edited)
Fred,
thanks for your reply. You are right, the duct is indeed a box. The volume some 1000 liters, i.e. 500 liter per driver.
But: if it works, shouldn't it work also with a tapered duct behind the drivers and a duct with a much smaller diameter in the middle? What is your opinion

Regards

Joost
 

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Discussion Starter #5
Steve,

Thanks for your answer.
I think that the room is not an ideal duct. In an ideal duct sound will travel in parallel plane waves. I think that in a room the set-up will act more like an open baffle (see the site of the revered Siegfried Linkwitz (linkwitzlab). So cancelation will occur, but only in the lower / bass regions. What is your idea on this?

Regards

Joost
 

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This reminds me of the DBA (Double Bass Array) concept, but with an IB style enclosure. Apparently it's supposed to be this awesome thing, but I have yet to really read up on it and see how it works. I'm guessing, just like anything else, it's fairly room dependent.
 

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jhmkoop said:
I think that in a room the set-up will act more like an open baffle (see the site of the revered Siegfried Linkwitz (linkwitzlab). So cancelation will occur, but only in the lower / bass regions. What is your idea on this?
If you stand directly at the side of an open baffle speaker, don't you get a lot of cancellation?
 

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Discussion Starter #8
Steve,

indeed at the side of an open baffle there will be a lot of cancelation, but how much and at what frequencies?

I decided to do some measurements. I have a copy of Room EQ Wizard and a Radio Shack SPL meter, so I can do measurements. I will build two closed bass-boxes, drive them with a 180 degree phase difference and make measurements when they are 1 meter apart, two meters and so on.
I will post the results.

Regards

Joost
 

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Discussion Starter #9
Looney,

thanks for pointing me to the Double Bass Array. I never heard about it before, but it is very interesting. I think the purpose is to get a plane wave in the room and placing the point of cancelation at the end of the room. Combining my original idea and the double bass array set-up might be interesting.
I want to do some measurements (see my answer to Steve) and it might be a good idea to use a digital time-delay to create the distance between the boxes instead of moving them before every next measurement

Regards,

Joost
 

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Will the mic be positioned directly between the two drivers?

If so, my prediction would be that if you have the subs very close to each other, firing towards each other, you will get a near complete cancellation. As you move the subs further apart, room reflections will start to play a role, and it will no longer be a complete cancellation, but there will still be a lot of cancellation. By the time you move the subs all the way against the side walls, assuming the room isn't perfectly symetrical, you will start to get something resembling a poor FR, but again, it would be comprised primarily from reflections.
 
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