complete wall as a sub driver ( planar? ) ... impossible ?

[JinMTVT]

Just a quick thought,
and it may has been discussed already here or elswhere, if so please link me!

What about using a very very light diaphragm of ridiculous dimension ( complete wall section of a room)
for sub bass reproduction ?

( following up on my planar wave project for my next HT , wich came up from IB setup and DBArray thread )

my quick idea would be using mylar as a diaphragm,
on a carbon fiber composite mesh that would probably maintained in correct place by some carbon composite suspension ( probably a sliding teflon lined carbon composite plate with calculated spring action )

and using similar motors as used on any subwoofer drivers, would need to think of suspension, but there would be a need to "seal" the big diaphragm on all the walls, probably some glued light weight suspension

is this possible ?
would it achieve a serious planar wave ?
if once would use a sealed Electrostatic driver of a defenite size XY, in a box that has a surface XY,
throwing on the Z axis, would it achieve a perfect planar wave ?


just to see if there was any work done on that yet

thanks all for your time

[Calvin]

Hi,

kind of such Idea is described in Ron Wagners book about Electrostats.
It´ll be very difficult to implement this thing in practise though...starting with the prob to get sufficintly wide mylar and so on.

jauu
Calvin

[JinMTVT]

ok
i do posess this book and i don't recall this passage...i'll get intoo it later then

what other technical difficulties does it present?

mylar is usually in 1m rolls, and i can make the composite
structure to match this width without any problems...


what aboout surface irregularities due to tension
and deflection ?
non lineair control of the movement ?

room accoustic complications ?

[lne937s]

"using similar motors as used on any subwoofer drivers..."

This wouldn't be a true planar-magnetic, but either like a large NXT driver (bending wave), or a large subwoofer driver (maybe with multiple motors). If bending wave, it really wouldn't work well as a subwoofer (you want some kind of pistonic action to create bass frequency wavelengths). If it is a really large driver, I would skip the mylar and just use a rigid carbon/honeycomb sandwhich.

If you were to do a planar-magnetic, it is totally doable- Maggie 20.1's play to below 25 hz and are dipoles (with the associated dipole cancellation). If you take away the dipole cancellation effect in a simulation and then simulate an infinite baffle, it works fine. Now, you would need to place them at an angle to avoid standing waves in a rectangular room. And you would need to limit excursion in the middle of the diaphragm with either anchoring points or by creating an array of smaller drivers- but it can be done. Carbon fiber would not work well for the magnet structure, as it is non-magnetic, but still conductive (and unnecessarily expensive). I would use cast iron.

[david yost]

If you build an identical wave generator on the opposite side of the room, invert the phase and delay the driving signal just right, you will acoustically "absorb" the plane wave and there will be no standing waves or modes in the room at all...assuming a rectangular room.

Unfortunately, my room isn't built this way, otherwise I would love to try this.

David

[lne937s]

http://www.school-for-champions.com/...ncellation.htm

What you are talking about is active noise cancellation, which will not only cancel out the reflection, but the original wave as well. Air moving with no increase in pressure. You would probably do better to use the front and back wall in phase, with delay to have the soundwaves reach the listening position at the same time- compounding the pressure.

I'm sure that there are a number of applications for active noise cancellation in listing rooms however...

[david yost]

The method you are talking about will not deal with some of the room modes, but will tend to smooth out bass response simply due to the increase in the number of sources. The plane wave, OTOH, will also not allow floor-to-ceiling or side wall-to-side wall modes to develop...at least in theory... but with rear wall cancellation, the end-to-end modes will not develop either...and the sound in the room will be identical (or close enough) to the anechoic response. The major down side is doubling the cost of bass generators, amps, and crossover/delay units. I'd still like to try it one of these days!

David

[lne937s]

We need to keep in mind with a subwoofer how long the wavelength is. At 20hz, the wavelength is ~17m (55ft) (halving with every octive). As such, if you had a room smaller than that, with identical drivers at identical volumes time alligned so that the driver at the rear cancels out the one at the front, the pressure wave never fully develops. Not like anechoic output, more like virtually no output. You could adjust the volume, (however) to only eat up what would have hit the wall, but the mechanical losses (no speaker is perfect) would make this less than ideal and very expensive (as well as eliminating boundary reinforcement, which can be a good thing) absorbtion may work better. Also, given the wavelength, you would need an enormous driver to have a directional wave at bass frequencies (probably larger than your room). Due to the wavelength, the sound wave would still be bending and you would have floor/ceiling and sidewall cancellations/reinforcement.

With bass, the more even the pressure you can create throughout the room, the better, and it is really difficult to eliminate all room interactions.

[david yost]

I'm not sure about your idea that below a certain frequency bass would tend to decrease faster than anechoic response because pressure wouldn't build in the room...in fact, this is the very concept behind the active wave cancellation theory, and is why no (or vastly weakened) modes are stimulated. Alas, the bass needs to be mono for this to work at all. Please see the link provided.

David

http://www.avsforum.com/avs-vb/showt...4&page=1&pp=30

[lne937s]

That's pretty interesting stuff- using a mono sub with a delayed inverted signal to cancel out what would have been reflected back as a secondary reflection. I think with DSP, something like this is doable commercially. (program it in for each sub using some kind of bass management program)

At lower frequencies, what I was talking about is related to this quote from the thread you noted:
"At lower frequencies the room works as a compression chamber and the sound will not be reflected by the rear wall anymore."

With the driver at the opposite side of the room, time delayed polarity reversed, same volume, you are counteracting the pressure that the original driver is trying to place into the room.

Another way to think about it is a speaker creates sound by pressing up against a mass of air, that presseses up against another mass of air, etc., creating a wave of pressure. What you are recommending counteracts the mass of the air- if at equal volume, eliminating the sound pressure completely.

Now, you could do some sort of back wall noise cancellation to leave only a simulation of the mass of the air: creating the equivalent of of anechoic (in true planars, the mass of air is often greater than the mass of the diaphragm). However, as the diagphragm is not much more than a sheet of plastic wrap with somethin wires glued to it, it would do little to dampen any sound hitting up against it from behind- you would need to have a large, well damped room behind it. Practically speaking, it would probably be better just to leave it open.