small room diffusion?

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Postby bert stoltenborg » Sat Jul 31, 2004 7:30 pm

Ido

This is a bit of line, but there is an electronical equivalent to your approach to open up a room:

A way of opening up a room is using the wave field synthesis method.
Diemer de Vries from the toegepaste technische natuurkunde lab at the Delft university showed this (Delft is near Gouda, your partner Ram should visit the lab when he is still in the Netherlands, as far as I kno they like boast about the system :-) ); they measure the impulse responses of a particular room/hall as a matrix and then, using digital signal processing and a shell of loudspeakers, generate a holistic sound field.
Sounds like you are really there. Only when you are very close to the speakers you notice that you are being fooled.

Isn't your approach aleady implemented? I seem to remember such a room, but I'm not shure where. (Probably drink to much, I'm a bit of a musician, rmeber? :-) )
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Postby Bob » Sat Jul 31, 2004 8:10 pm

I read somewhere recently that a wall with small absorbers on it (i.e. 2' absorber, 2' gypsum, 2' absorber, 2' gypsum, ...) actually acts as a diffuser.
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Postby Eric.Desart » Sat Jul 31, 2004 8:39 pm

Bob,

That's diffraction/scattering caused by a difference in impedance.

That's in fact partly how the Helmholtz principle of the John Sayers group is based on.
For high frequencies those slats reflect.
When the wavelength matches the slat width it diffuses.
And for the low frequencies it's the principle you find in the ChrisW file.

And it's gradual.
But for me the Scott polys still do.
Image
divinely-inspired
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Postby Scott R. Foster » Sat Jul 31, 2004 11:52 pm

Bob:

Polys aint semicylinders

Bert:

If a wall behind you reflects 'X" enegry across the 20 to 20k bandwidth to your ears within +/- 3 ms of theoretical flat panel behavoir... and a diffuser is installed effective from 20 to 20k that divertes the return path of say 66% of said energy sample X so that it hits the sides walls instead following a return path +/-3 ms to yoru ears... doesn't ther room now sound bigger given that 2/3rds of the rear walls "flat panel" energy now returns buy a significantly longer path [seems further away]?

Eric:

Quit beating your head on that wall... it can't be good for you

;-]
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Postby bert stoltenborg » Sun Aug 01, 2004 12:16 am

Scott,

Hate to be a pain in the ass, but I still think/have the feeling that a 6 dB decrease in reflected energy does nothing to making the room sound bigger. Sound different, OK, but bigger?? The bat flying in the room would not be fooled, I believe. :-)
we have to do double blind tests to check it. Maybe we can find a nurse school or something full of volunteers.

Dan has to give us more smilies.

regards,

Bert
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Postby Scott R. Foster » Sun Aug 01, 2004 12:57 am

Bert:

I aint so sure the energy level will diminish... merely that a large fraction of the rear wall reflection energy will be delayed due to it being diverted from a flat reflectance path... and a bat would notice that.... our theorectical bat would "know" that the rear wall was not monolithic, but rather would - I think - assume it was an array of materials of greatly varied distance [like a copse of trees several meters deep, as opposed to a flat wall].

Hmm... it may be time to check my bat boxes for volunteers. :-)
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Postby bert stoltenborg » Sun Aug 01, 2004 11:37 am

Scott,

Not to nag, just to be shure we understand each other:

when direct sound from fe a speaker hits the back wall it will reflect and after a time defined by the distance between wall and listener come to the listener.
when the back wall is a qrd, theoretically the sound is spread evenly over a cylindrical space. this would mean the reflected sound level decreases with 3 dB compared to a reflection from a normal wall.
with a 2-D difusser the sound is spread half space, the reflection is 6 dB decreased in level.
but the part of the sound that is reflected will not be delayed in time.
only in level, but that doesn't matter in relation to perceived room dimensions, IMHO.
and the part of the sound that is reflected away from the listener will be delayed, but when it comes as a reflection from another wall our sensory perception system will recognize it as such, and will not judge it as making the room bigger.
Maybe a stupid example, but when I screw around with a waves trueverb in a sequencer, I have the feeling that I have to alter the first reflection parameter to make the room sound bigger or smaller, influencing the reverberation time only influences the definition of the sound.


Maybe we should ask Ethan, he'll know the answer :-)

Bert
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Postby Eric.Desart » Sun Aug 01, 2004 12:23 pm

Bert,

While delay is certainly a very important parameter, I still am sure that what we perceive is more than physical analyzis.

Our brain post processes signals, which is a combination of information of different senses.

As such there are optical and auditive illusions.
We can hear and see things which are physically not existant, just because our brain concludes they are there, or ought to be there within those specific circumstances.

The comparison with the bat is one, but not necessarily all explaining.
For a bat those frequencies are indeed measurement equipment.

Eric
being on slippery ice now.
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Postby Bob » Sun Aug 01, 2004 2:41 pm

Eric:

Our brain post processes signals, which is a combination of information of different senses. As such there are optical and auditive illusions. We can hear and see things which are physically not existant, just because our brain concludes they are there, or ought to be there within those specific circumstances.

An example of two senses combining like that from home theatre is where to place the center channel speaker.
Humans, possibly due to a pre-historic need to hunt small animals, are very good at locating by sound alone things that are lower than our heads. Humans are not very good at all at locating things by sound alone that are higher than our heads.
Thus the center channel speaker should be placed either behind an acoustically transparent movie screen or above it. If behind the screen then the audio clues are enough to locate the action. If the speaker is above the screen then the brain can't tell where the sound is coming from, but it sees the car chase on the screen, and the brain convinces the listener that the sound is coming from several feet lower than it really is.

I would imagine that a bat, who has to fly under objects like tree branches, or quick moving small insects (food), is better at locating things above them than we are. A bat's ears have a complex collection of folds that help it determine an insect's vertical position.
Room size is determined by the time of the first echo, and although a diffuser may make it quieter, and knock the sound around the room, some of the echo is coming right back at ya, and that's how a bat would know how big the room is. A diffuser would have little effect on that perception. Bats are pretty good at muliple echoes, and with two ears are able to determine where the object is, how big it is and in what direction it is moving. An absorber or diffuser might make the bat think the wall is smaller than it really is, but not further away.
Regards
Bob Golds
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Postby Scott R. Foster » Sun Aug 01, 2004 3:05 pm

Eric:

At Bert's suggestion I have consulted with Ethan, and he and I are training bats to discern and correlate the "swooshiness" of more or less out of phase waves reflected at 1/4 wave distances from [mostly] the rear wall using 1 Hz wide pink noise in order to prove our "new school "odd numbered wall theory of studio acoustics and I'll thank you not denigrate the bat's status as bona fide measurement instruments [watch your mouth or you'll be in trouble with the bat union].

Bert:

Whoo, that was fun... but seriously...

I hear ya.. but I think the matter does somewhat relate to the dreaded "psychosis acoustics" as Eric warns... things like how we [at least in the mid band] process phase coherence between our left and right ear inputs to take locational cues... and how ear shape alters the input of sounds made forward versus those coming from behind. As such I too must admit I am uncomfortable making statements that purport to be any more that a general sense. But as you know better than I - in the critical listening environment - the vaguest of notions becomes important.

All I can tell you for sure is that after installing a giant grating on the rear wall of my control room, I was surprised at how subtle the change was... I expected a major and noticeable difference in the "feel" of the room's reverb content. But there wasn't one. The only firm impression I had of a change was that the room sounded "bigger" behind my head. I aint saying this wasn't welcome... I like it... it seems to help with clarity when trying to visualize instrument placement in the sound field when mixing... but it was not what I expected.

The best I can describe it is that there seems to be a much larger space behind me than before. Oddly, I can't say it is a reverb thing... not like the cues I would use to estimated that you had led me into a gym blind folded... making this guess because of the long hollow echoes of our footsteps. It is more a sense of openness - like shouting out into the yard from a front porch vs. shouting back into the house. All the same echoes are there in either case.. just in different proportions and there is a dorectional quality to it... the front of the room is enclosed.. like any normal listening space... the back of the room is much less so - its bigger, more open than seems right when you turn around and look at the dimensions involved in the physical space [as opposed to the perceived acoustic space].

Oh shit! Now I'm doing it... this subjective stuff is so vauge it scares me. I hope some of this makes sense.

:bang

Anyway - I understand your point about reverb and near reflections, all I can say is that the sense of larger space I perceive from a large grating at the back of my CR aint like that... there are still similar near [short] reflections - it just seems that there is also a sense of an open wall toward the back of the room.... it makes the room seem bigger.

In any event - this explanation satisfies the casual curiosity of many visitors, and that alone is valuable. Sort of like when folks ask me if my Pinzgauer is amphibious - "Certainly Not! We carry far to much ammunition for the truck to float - instead we drive along the bottom until we get to the other side."
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Postby bert stoltenborg » Sun Aug 01, 2004 3:48 pm

Yo Scott,

On the bats and studiotips discussion,
I agree, of course, on the feeling of a more open sound in a room with difussers on walls.
Was only trying to be critical about the explanations we get from the qrd-industry.
I'm from a dutch region called the Achterhoek, litteraly translated: the backcorner, kind of equivalent to South Park in America, I guess.
So you have to invite me for wasting some of your ammo some time. :-)
I probably could bring Marlon and Philip, also achterhoekers/rednecks, with me.

Now it' s vamalos a la batta queba, as I heard Batman say in a Venuzuelian episode.

Regards,

Batman, oh no, Bertman.
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Postby Scott R. Foster » Mon Aug 02, 2004 1:11 am

Anytime Bertman... bring lawyers, guns and money.... we'll have a high old time.
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