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jmiyake 25th April 2002 01:48 AM

1000 liter sub
 
I am considering building a raised platform for some recliners in my home theater. It occurs to me that the platform could be used as a subwoofer enclosure. It would be about 10' wide 4' deep and 10" to 12" tall.

So my question is this.
Is there an advantage to have a 36 cubic foot volume for the subwoofers? Also you would be seated very nearfield. Possibly even on top of the drivers. What is the downside to being in such close proximity to the sub?

Any thoughts on what type of woofers, or alignment would be best suited to this application?

James

Pete Mazz 25th April 2002 03:15 PM

Depending on how many drivers you install, it would function like an infinite baffle or a low Q sealed alignment. Look for drivers with low Fs and good efficiency. Adire's 15" Tempest is a good choice. You probably want to keep the Xover low for nearfield listening, but it should be fine. Won't need tactile transducers, either! :)

Pete

Bill Fitzpatrick 25th April 2002 03:31 PM

You could wind up with a dip in response around the crossover frequency if you don't consider that the distance between the listeners and main speakers will be different that the distance between the listeners and the subs.

For example, if you cross at 100Hz and the listening distance to the main speakers is 10 feet you get a difference of 360 degrees which is OK. If the main speakers are located at 15 feet your subs will be 180 degrees out of phase and a phase reversed connection to them will be needed.

This is all based on the idea of a properly designed crossover in the first place.

I certainly recommend bi-amping.

jmiyake 25th April 2002 04:48 PM

Tactile bass
 
Hi Pete,
you bring up some very interesting points.

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You probably want to keep the Xover low for nearfield listening, but it should be fine.
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A low active crossover seems like a good recommendation.

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Won't need tactile transducers, either!
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Well this is an interesting comment. I sat in a car once where the woofers were mounted in the trunk firing through the seat. That was an interesting effect. I imagine movies would benefit.

How do you think this approach would compare against using tactile transducers?

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It would function like an infinite baffle or a low Q sealed alignment
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Do you feel a large single enclosure of 36 cubic feet, would be superior to three 12 cubic foot enclosures, or say six 6 cubic foot enclosures?

James

jmiyake 25th April 2002 05:15 PM

Another good point
 
Hi Bill,
some really good points.

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You could wind up with a dip in response around the crossover frequency...
I certainly recommend bi-amping.
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Sounds like a adjustable crossover would be required.

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For example, if you cross at 100Hz and the listening distance to the main speakers is 10 feet you get a difference of 360 degrees which is OK. If the main speakers are located at 15 feet your subs will be 180 degrees out of phase and a phase reversed connection to them will be needed.
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Hmmm. Interesting. I hadn't thought of that.
So a steep slope in an active crossover should minize the effect, right?
Also if there are multiple drivers what is the effect?
For example given the area we are talking about what if one woofer were 10' and another woofer at 15' would they cancel? Also there is the possiblity that the distance from the right speaker and the left speaker would be dramatically different, how does that effect the equation? I am just trying to understand the dynamics here.

James

Bill Fitzpatrick 25th April 2002 06:45 PM

Jimiyake:

Room influence issues aside, there are always compromises for home theater set-ups because you can't get everything right for every listener. By placing the subs under the listeners you are just introducing another compromise.

I'm a big fan of a coherent first arrival wavefront. That is why I always place subs under the main speakers and use a properly integrated electronic crossover. For situations where there are multiple listeners this becomes even more important. As we say on our website, commercial subs which are sold with the idea that you can place them anywhere you please are only good for the unwashed masses. Worse, the usually uninformed crossover design for these things prevents the buy from finding any really great spot for them. I assume that you want something considerably better.

If you think I'm trying to talk you out of your idea, YOU'RE RIGHT!

Yes, one woofer at 10' and another at 15' would tend to cancel at about 100Hz if there were NO room involved. Room standing wave modes and boundary reflections change the picture considerably so you could never have complete cancellation.

There are only two good ways to cross subs to satellites, whether active or passive, if you want to maintain waveform integrity. The first is 6db/o low pass and 6db/ high pass. The second is 24db/o low pass and 24db/o high pass. The 6db/o scheme doesn't work very well for subs because you would have to crossover at a high enough frequency so that the low frequency roll-off of the enclosed satellite system fades from the picture. This pretty much leaves the 24db/o as the only reasonable choice.

If your primary consideration is uncompromised performance then it's subs under sats at 24db/o.

Our website HERE has some tutorial style information about our crossovers in the YGBSM link.

jmiyake 26th April 2002 01:56 AM

Ok sounds like it would be good for special effects, but not so for accurate sound. So I should pass.

James

mrfeedback 26th April 2002 03:44 AM

Sensuround shook the cinema goers listening position - good effect but can get tiresome.

Regards, Eric.

Bill Fitzpatrick 26th April 2002 04:08 AM

Great idea. Built a shake table for a sub-sub. Build it good enough and you can make those pesky visitors run for the hills!

Let's see, 1 HP x 5Hz x 12" (xmax) + 2 beers = Every listener dumped onto the floor.

:D

jmiyake 26th April 2002 04:33 AM

New plan forming
 
So it sounds like the thing to do is to build kick a** subs with the main system, AND do a transducer think with the platform.

Hey I could like this.

Maybe DIY transducers...

I have seen someone on the web do them out of old 8" drivers. I have a pair of old 15" woofers with 80oz magnets and rotted out surrounds that might be good for this application.

Anyone know anything about DIY transducers?

James


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