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Author Topic: Subtract the playback room acoustics  (Read 3641 times)

syntheticwave

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Subtract the playback room acoustics
« on: June 22, 2010, 04:04:32 pm »

One of the main problems of the wave field synthesis is the need of source free volume as playback room.

I am working at different approach. One of the main advantages of the synthesis is, we can determinate the time for synthesize direct wave, first reflections and reverberation separately.

If we take advantage of that fact, a way for subtract the additionally detours regarding each of those wave fronts arise. Finally, that opens a possibility for subtract unwanted playback room influence from transmitting chain. Not in frequency domain, in Time and level!

The way is described in the forth chapter of my http://www.syntheticwave.de website at help of some understandable animations. But the procedure is not realised in practise until.

What will be the obstacles?

Regards H.
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Thomas Jouanjean

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Re: Subtract the playback room acoustics
« Reply #1 on: June 23, 2010, 03:47:48 am »

Not sure this is meant for a studio acoustics forum. This will likely never find an application in studios.

Your request is unclear. You want to remove the effect of the room in wich your WFS is played, so you can optimize it? Please formulate it better.

If I understood you right, my guess is that this can be done by either "active cancellation" or room treatment. The cost of implementing WFS alone is rendering WFS useless, at least for now. So adding thoses costs to it...

WFS is a bit like DSP Room Correction. Not a big fan here.
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Thomas Jouanjean
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bruno putzeys

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Re: Subtract the playback room acoustics
« Reply #2 on: June 23, 2010, 06:12:50 am »

The obstacle is fairly simple. If the WFS is only one "wall" of the room, it can only reproduce wave fronts traveling away from that wall into the room. The unwanted wavefronts are traveling into other directions and can't be canceled by any waves you generate from the "WFS wall". Your only solution would be to cover all six surfaces with transducers. Then you could do it.
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Warp Drive. Tractor Beam. Room Correction. Whatever.

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syntheticwave

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Re: Subtract the playback room acoustics
« Reply #3 on: June 24, 2010, 01:55:51 am »

Hello Thomas,

the goal of the procedure is differently system approach of conventionally the procedures, but also regarding the commonly known WFS approach.  In any case the transmitting chain ends at the loudspeakers. The "Holophony" procedure wants to include the playback room properties. Final goal of audio transmittion should be, restore the signals at he ears of a listener in recording room at the ear of a listener in playback room, not any loudspeaker signals.  

bruno putzeys wrote on Wed, 23 June 2010 05:12

The obstacle is fairly simple. If the WFS is only one "wall" of the room, it can only reproduce wave fronts traveling away from that wall into the room. The unwanted wavefronts are traveling into other directions and can't be canceled by any waves you generate from the "WFS wall". Your only solution would be to cover all six surfaces with transducers. Then you could do it.


...that would be perfect without all doubt, but we have to consider the spouse acceptance factor. Smile

Such huge WFS Loudspeaker field will radiate very directed. At WFS we have separate access at any of the signal components, we can change time and level for direct wave, first reflections and reverberation separately and in different manner. That opens the possibility for common systen from recording room and playback room. Per example, we can delay the radiation of the first reflections, radiate in playback wall direction. That will enlarge phe perceived size of the recording room. On the other hand, we can send the first reflections in wall dirrection before the direct wave is synthesized. Than we will perceive smaller room.

For subtract the playback rooms influence regarding first reflections, show this littel animation:  
http://www.syntheticwave.de/pictures/WFS_transformation_prin ciple.swf

Of course that wave fronts will be reflected in playback room again. But in the near field of the speaker wall that will have not very disturbing effect, I hope.


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bruno putzeys

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Re: Subtract the playback room acoustics
« Reply #4 on: June 24, 2010, 03:22:34 am »

The insight that the 4 adjoining walls provide mirrors that virtually extend the front array is a valuable one. What it cannot do is offer separate control over waves travelling from the back wall to the front wall. The idea seems to work for reflections off the ceiling, floor and side wall, but I can't see how you control anything coming off back wall.
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syntheticwave

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Re: Subtract the playback room acoustics
« Reply #5 on: June 24, 2010, 03:58:40 am »

bruno putzeys wrote on Thu, 24 June 2010 02:22

The insight that the 4 adjoining walls provide mirrors that virtually extend the front array is a valuable one. What it cannot do is offer separate control over waves travelling from the back wall to the front wall. The idea seems to work for reflections off the ceiling, floor and side wall, but I can't see how you control anything coming off back wall.


...rear is a problem. Seems possible as ceiling- rear wall double reflection, yet only above possibly 400...600 Hz at a 2,7m speaker field. As long as no primary source in rear range that`s possibly sufficient. In other case we would have to buy 3 times a bundle of Flowers. Because of acceptance factor for two additional rear boxes. Can become simply included into the time management.


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bruno putzeys

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Re: Subtract the playback room acoustics
« Reply #6 on: June 24, 2010, 07:25:43 am »

OK I think we're lined up then. Anyhow, to use the "mirror images" of the transducer array to extend it outside the room is a seriously cute idea Smile
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syntheticwave

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Re: Subtract the playback room acoustics
« Reply #7 on: June 26, 2010, 10:24:29 am »

bruno putzeys wrote on Thu, 24 June 2010 02:22

What it cannot do is offer separate control over waves travelling from the back wall to the front wall. The idea seems to work for reflections off the ceiling, floor and side wall, but I can't see how you control anything coming off back wall.


... I have found in my archiv a little animation for source behind the listener:

http://www.syntheticwave.de/pictures/behind.swf

Unfortunately, distinction between theory and practise is, in theory no distinction between theory and practise exist, yet in practise though.
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AndreasN

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Re: Subtract the playback room acoustics
« Reply #8 on: June 26, 2010, 10:56:39 am »

syntheticwave wrote on Sat, 26 June 2010 16:24

Unfortunately, distinction between theory and practise is, in theory no distinction between theory and practise exist, yet in practise though.


QFT. Nicely put!
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bruno putzeys

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Re: Subtract the playback room acoustics
« Reply #9 on: June 27, 2010, 02:26:30 am »

If I understand the animation correctly the source is behind and somewhat above the listener and the transducer array creates a wavefront that passes above the listener's head so he only hears the reflection. Essentially you're using one of the rear mirror images to create the wave and hope the original wave doesn't hit the listener. That would work for sources that are not directly behind the listener. Unfortunately the same restriction applies for sources dead in front of the listener because the unwanted mirror image that you will want to cancel is then behind the listener.

Conversely, the amount of space that is kept free from waves the listener shouldn't hear becomes quite small, a new sweet-spot so to speak.

I also have the impression you are presuming that the wavelength is very short (especially your last simulation looks like ray tracing) so as to be able to pass beams above the listener unheard. Achievable beam width is diffraction limited, which means you can't cut out a sharp "listener shadow" at low frequencies, say 1kHz.
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Warp Drive. Tractor Beam. Room Correction. Whatever.

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syntheticwave

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Re: Subtract the playback room acoustics
« Reply #10 on: June 28, 2010, 08:39:07 am »

Yes, i agree. The rear source would only work correctly at shorter wavelengths. My prediction is -6db/ Oct below 1 kHz. We have to consider the huge dimensions of the loudspeaker field, the 1296 Loudspeakers would have 2,7 meters in diagonal. Much more as the sound projectors, which are able for producing rear sources in certain degree.

The difference of the www.holophony.net approach compared with the soundprojectors is, the principle doesn`t faking loudspeakers, but the source itself and her reflections in recording room.

Without all doubt, the playback room will produce unwanted own reflections. However, the listener is situated in near field of the loudspeaker screen. That lessens the influence of the playback room considerably. See the animation, how different the first reflections in playback room regarding the  first reflections in recording room at undirected loudspeakers:

http://www.syntheticwave.de/pictures/Stereo50.swf
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