[John Ashman]

I'm still digesting some of this because I'm thinking of trying to build bass traps into my future HT room when I replace the roof (the WHOLE roof!), but what about this:

"Active" bass trapping.  What would happen if you were to have a sub or two up front, in phase, but then put a subwoofer at the back of the room that is out of phase and then delayed the distance between the subs so that it cancels some of the bass when it attempts to bounce off of the back wall, and then adjust the cancellation effect to taste?

I figure that this is either a very good idea or has so many other unimagined consequences that it becomes actually stupid.  

I mean, isn't the idea of a bass trap to vibrate at the frequency of the room modes so that it turns acoustic energy into heat?  Or am I way off?  I'm starting from the end of the thead and moving backwards!  I've never had any major bass problems in any of my rooms, but I'm sure it can always be better.

[John Casler]

I'm still digesting some of this because I'm thinking of trying to build bass traps into my future HT room when I replace the roof (the WHOLE roof!), but what about this:

"Active" bass trapping.  What would happen if you were to have a sub or two up front, in phase, but then put a subwoofer at the back of the room that is out of phase and then delayed the distance between the subs so that it cancels some of the bass when it attempts to bounce off of the back wall, and then adjust the cancellation effect  ...

Hi John,

Look here: http://www.audiocircle.com/index.php?topic=15708

This chronicles some of my "push/pull" adventures, and I think it is the bass system of the future.

I think the key is getting the "phase" right.  While I am currently running 180 degrees "out of phase", I think the phasing will vary according to several parameters, the most important being the distance the subs are apart and their relative distances from front, back and side walls.

I know there is a "formula" that will present the best performance, but am not sure what it is.

Now this config "will not" reduce ringing (in fact it might even add to it), but it has the potential at the listening area to "rebalance the actual wave force, by "increasing" the power of the direct and decreasing the power of the reflected.

That said, the concept I support of "attempting" to make all room boundaries as if they were "acoustically transparent", is still the goal.

[John Ashman]

Hi John C

   I read the thread, but in order to do active cancellation, you would definitely have to run a delay of 15-25 ms depending on the room.  The idea is to "catch" the waveform with a perfectly canceling waveform (delayed in time to wait for the original) just as it was about to strike the back wall.  In fact, an in-wall sub (s) placed more or less in the middle of the back wall would be ideal.  In this way, you could time align it so that as the original bass reflected, it would simultaneously send out a cancelling, out of phase signal that would interact and "suck out" the reflection before it even had a chance to pass by you.  The big thing is it would have the advantage of also cancelling, not just a peak, but the in-room "waterfall" by preventing those reflections from rebounding countless additional times.

Digital room correction simply reduces music in order to create a "hole" for the distortion to slip into and, while pschoacoustically may sound similar, I doubt it would be nearly as good as literally cancelling out the wave.  Of course, we are used to "hearing" the room, so it could actually made it sound like  you were outside, for better or worse.  

In *theory* it would work, but I'm just curious if anyone has attempted it with a delay.  It would take an outboard digital delay to pull it off, really.

[Ethan Winer]

Guys,

> an in-wall sub (s) placed more or less in the middle of the back wall would be ideal. <

One big factor is that sound waves strike the entire rear wall, and a couple of subs can "push back" only a small portion of that. So I imagine that any improvement will be much more localized than you'd get from a larger passive bass trap.

There are active bass traps - at least I've seen them "announced" if not actually being sold. An active bass trap, properly designed, could operate over a wide range and to a very low frequency. The problem is it still has to be very large, and such a device would be very expensive to produce.

--Ethan

[klh]

Thanks everyone for the help. The idea of one subwoofer at each midpoint was discussed in the Harmon white paper. I've asked people about that, including e-mailing an author, but replies where all negative. Most were skeptical of dealing with phase issues, and the author who responded said he wasn't sure how to deal with it (but suggested side midpoints and not front and back). That didn't make sense to me, especially if the front and back walls are shorter than the side ones. I would think it would be much easier to align the phases for more places in the room if they were front and back with those walls shorter, not to mention it would be much easier to align them with the mains. John's idea of timing the front and back wave so they are perfectly out of phase is very interesting and probably a better explanation of what they were getting at.

[John Ashman]

Ethan,

     Well, what I'm thinking, of course, is that there are two goals with active bass cancellation.  One is to cancel the first reflection before it gets to the seating postion, so the active bass module(s) would have to be mounted pretty close to ear level behind you, maybe just above and that would create a zone that would more perfectly cancel several feet or more from the wall for a limited  area.  But the second goal would be to cancel the bass wave by the time it gets to the front wall to be reinforced.  For instance, if some reflected bass passed by you that would be fine if it didn't actually hit you, but if, by the time it reached the front wall for the second reflection, it synchs up with the cancellation wave and cancels (or is simply greately reduced) before it can be, then there is nothing to come back at you.  So, if you concentrate on the sweetspot, the rest of the situation will largely be fixed by the time it hits the front wall again anyway.  

Does anyone know what the volume reduction of the typical reflection is?  For instance, if it is 10dB in a 20ft deep room, at 90db volume, you'd be at over 150ms before the wave is absorbed - this would be worse than the impulse response made of 24" woofer with almost no damping.  But with active cancellation, you might be able to get that down to 10-20ms, improving the "tightness" of the room.  Anyway, that's my theory, at this point in time.

[Ethan Winer]

John,

> there are two goals with active bass cancellation <

I agree with the premise, but I can't see why that's any better than just putting foot-thick of rigid fiberglass over the entire rear wall, and more rigid fiberglass in the corners.

--Ethan

[John Ashman]

1 foot?  How much broadband bass reduction can you get with 1' of fiberglass?  Unfortunately, I'll have a large door/window on my back wall, but it's not completely out of the question.

[Ethan Winer]

John,

> How much broadband bass reduction can you get with 1' of fiberglass? <

A lot. But if we're talking about practical bass trapping, rather than theoretical devices, the first place to put bass traps is in all of the corners, including the ceiling corners.

--Ethan

[John Casler]

John,

> How much broadband bass reduction can you get with 1' of fiberglass? <

A lot. But if we're talking about practical bass trapping, rather than theoretical devices, the first place to put bass traps is in all of the corners, including the ceiling corners.

--Ethan

That is the type of "suggestions" I am looking for, priortity trap placement and why.

Which leads me to  my next question.

Multiple Complimenting Bass Treatments.

I am of the opinion that a combination of LENRD (Low-End Node Reduction Device) and Real Traps in the corners would compliment each other in total effectiveness.

I know that the LENRD devices as only good for bass above 75Hz, but it would still seem to offer a great compliment to the "dynamic impact" at 75Hz and above.

I can only assume that this might "improve" the efficiency of the "Real Traps" in some way.

Any feedback??

[nathanm]

I thought it would be cool if there was a triangular-shaped Real Trap that could be fit into a corner, offering a bit neater and more finished appearance than a rectangular shaped one.  Maybe with the frame sides angled inwards at a 45 degree angle.  Just a thought.

[John Casler]

I thought it would be cool if there was a triangular-shaped Real Trap that could be fit into a corner, offering a bit neater and more finished appearance than a rectanglar shaped one.  Maybe with the frame sides angled inwards at a 45 degree angle.  Just a thought.

Exactly!!!

Real traps, bundled into a single package with LENRDs that could be placed the full length of the corners.

[Ethan Winer]

John,

> Multiple Complimenting Bass Treatments <

While it's true that adding one bass trap in front of another has an additive effect, the increase in absorption is very small when one bass trap absorbs a lot more than the other. So in practice very little would be gained by that particular combination.

As for triangle shaped tri-corner traps, believe me, we considered that. The problem is they'd then be only half the size, and would absorb only half as much. Bass trapping is all about coverage - the more surface area you have, the better the results.

--Ethan

[nathanm]

Actually this could be made much larger than it appears.  Even with 48" sides if necessary.  It seems that placing treatments in the corners is less obtrusive both visually and functionally.  It's likely that the loss of ceiling space will not be a practical hindrance.  

I mean, Auralex offers that 12" cube chunk which you can butt 3 LENRDs against, but personally I thought that shape looked a bit awkward.  Ideally if each corner was lined completely with so there was no wall\foam discontinuity, but the triangle is a good compromise.  I'd imagine that the "slanted ceiling" look might even be an aesthetic improvement over plain 'ol straight corners.  Perhaps some kind of felt standoffs could be stuck on the back of the trap so that instead of being tight against the wall there would be that nice airspace which Ethan recommends.  It might create a nice shadow too.  Perhaps a lightbulb could even be stuck back behind there for an interesting reflected lighting effect.

[markC]

Looks like a cross between a g-string and something off of star trek

[Ethan Winer]

> Looks like a cross between a g-string and ... <

When I see one of those made of thin fabric, I refer to them as a "bikini"

--Ethan

[John Casler]

> Looks like a cross between a g-string and ... <

When I see one of those made of thin fabric, I refer to them as a "bikini"

Bass trapping is all about coverage - the more surface area you have, the better the results.
 

Looks like these two are far, far apart.

One works best when it covers a large "surface area", and the other works best (at least IMHO ) the less suface area it covers  

[8thnerve]

Actually this could be made much larger than it appears.  Even with 48" sides if necessary.  It seems that placing treatments in the corners is less obtrusive both visually and functionally.  It's likely that the loss of ceiling space will not be a practical hindrance.  

I mean, Auralex offers that 12" cube chunk which you can butt 3 LENRDs against, but personally I thought that shape looked a bit awkward.  Ideally if  ...

Hey, have you been sneaking into our factory?

[nathanm]

You aren't gonna pull a Monster Cable on us now and claim you own the rights to any triangular object are ya!?   (kidding)

Well, a 4" thick, 4' wide chunk of fiberglass will absorb more than the little pillow.  No offense to 8th Nerve, I know the little pillows are more WAFerific, but let's face facts, them suckers are tiny!

Okay, so my first sketch wasn't the greatest.  The scale was made it look  like the triangle pillows.  Here's a side view of what I meant.  The red trap is a 2' triangle which yes, would have less area than Ethan's Microtraps.  But I'd suggest going for the green version which is 4' or so, large yes, but as you can see it still isn't bonking the audiophile in the head.  The purple one is obviously excessively and impractically large (6'), however it would be the most effective.  I am trying to show how putting these large chunks in the ceiling corners doesn't necessarily intrude on the floor\walking\sitting area of the room.



So come on Ethan, get crackin' on these 4' Tri-Traps!   (joke)

[ctviggen]

Actually, if you put in both the red and the green one, I wonder if you'd get improved response as opposed to just the green one?   If so, you might as well do this, as no one could see the red one anyway.  Also, although the 8th Nerve stuff is small, in my case, I'm definitely thinking of using it for various locations where I can't fit anything massive (even 2 feet would be too big).