Re: [Sursound] Distance perception

[Marc Lavallée]

Mon, 25 Jul 2011 20:18:42 -0400,
I wrote :

> Imagine two rooms with proper acoustic characteristics and treatments
> for ambisonics reproduction: the first is 3mX4m and the other is four
> times larger in surface (9mX12m).

Oops! This is of course 9 times larger, not 4... Anyway, what is a the
effect of room sizes and speaker distances on distance perception? 

--
Marc
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[Sursound] Distance perception

[Marc Lavallée]

After reading this difficult thread (I'm replying with a new title), 
I have simple questions about room sizes and speaker distances.

Mon, 25 Jul 2011 18:35:31 +0100,
Dave Hunt  wrote :

> The Distance Compensation (aka NFC, and not the shelf filters)  
> attempts to correct for the loudspeakers not producing plane waves
> at the listener.
>
> True the "Classic Ambi rig or soundfield mike . record & present  
> distance as presented to them". The concept of a 'unit circle' only  
> appears in the encoding equations, which describe how to 'pan' mono  
> sources to produce B-Format signals. When you try to include
> distance in these there is a different behaviour outside the radius
> of the speakers, than inside. Direction is determined by coordinates
> limited to being inside the unit circle, whereas distance (and its
> effects on amplitude, time of arrival, and changes in reflections
> and reverberation) must use unlimited coordinates. Then it is useful
> to consider the radius of the speaker rig as unity, and all distances
> as being relative to that.

Imagine two rooms with proper acoustic characteristics and treatments
for ambisonics reproduction: the first is 3mX4m and the other is four
times larger in surface (9mX12m). In both rooms there's a
horizontal hexagon of speakers, and 5 speakers are against a wall.

When NFC is applied in both rooms, do they sound the same in terms of
distance perception when playing the same recording? Or is the same
"sound object" appear to be twice as far in the largest room?

Apart from widening the listening sweet spot, are larger rooms "better"
at reproducing distance cues when using the same speaker configuration?
Is distance perception directly related to speaker distances?

--
Marc
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[Sursound] Previously un-noted UHJ recordings

[Paul Hodges]

I recently came across a number of recordings made in UHJ format in the 
early 1990s by a small company called Mirabilis.  The man who started this 
broke off the recording business when he took over the old British pipe 
organ company Henry Willis and Sons, which has naturally kept him fully 
occupied ever since.  Most of the recordings he made are, unsurprisingly, 
of organs - but there are a number of others.  In fact, the only two disks 
still available to buy are one of chamber music by modern Scottish 
composers and one of a program of music sung by a male close-harmony group 
from Ely.


All the information I have been able to unearth about these recordings is 
gathered here: .  I have got copies of 
more than half of the disks, which sound superb; but I am still hunting for 
the rest.  (If anyone here happens to have any of the recordings I am 
missing, I would love to know, of course!)


Paul

--
Paul Hodges


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[Sursound] Sursound] the recent 2-channel 3D sound formats and their viability for actual 360 degree sound

[Dave Hunt]

Hi,


Date: Mon, 25 Jul 2011 08:24:55 -
From: Richard Lee 


Mr. Hunt, I hope Sampo & Fons have been sufficiently enlightening.  A
Classic Ambi rig or soundfield mike has no concept of a "unit circle".
 They record & present distance as presented to them.  The mike cos
Helmholtz etc and the Classic Ambi rig with tricks like NFC.  Even  
simple

1st order Classic Ambi rigs with NFC do a good job at plane wave
reconstruction at LF.


Both have been exemplary in their clarity, unlike myself.

I was so involved in thinking about what I was trying to say, and the  
relationship between Distance Compensation in a decoder and Near  
Field Compensation in NFC. that I didn't re-read what I'd written  
objectively enough before sending something with so many errors. I  
apologise, though the result has been very interesting.


The Distance Compensation (aka NFC, and not the shelf filters)  
attempts to correct for the loudspeakers not producing plane waves at  
the listener.


True the "Classic Ambi rig or soundfield mike . record & present  
distance as presented to them". The concept of a 'unit circle' only  
appears in the encoding equations, which describe how to 'pan' mono  
sources to produce B-Format signals. When you try to include distance  
in these there is a different behaviour outside the radius of the  
speakers, than inside. Direction is determined by coordinates limited  
to being inside the unit circle, whereas distance (and its effects on  
amplitude, time of arrival, and changes in reflections and  
reverberation) must use unlimited coordinates. Then it is useful to  
consider the radius of the speaker rig as unity, and all distances as  
being relative to that.


It has ben suggested that W = S(1 - 0.293(sq(x) + sq(y) + sq(z)),  
where S is source amplitude, be used to increase W to compensate for  
X,Y & Z tending to zero and perceived loudness decreasing instead of  
increasing with proximity. Again this is only applicable inside the  
'unit circle'. This is possibly only to be used if there is no other  
amplitude/distance law in operation.


The term NFC can be used in two different contexts: decoding, as in  
your above paragraph, and encoding, as in NFC-HOA, though that would  
also seem to include the decoding.


For a synthetic source to replicate this serendipitious situation,  
you have

to

1)	Add proximity for close sources or motorcyles as the Encoding  
Eqns in
Appendix of "Is my Decoder Ambisonic?"  This is the most important  
(only)

cue available for close sources in anechoic conditions


Agreed, but quantifiable only relatively i.e. for moving sources. For  
an unknown static source it tells you very little.


2)	Add a suitable reverb pattern as MAG's "Distance Panners".  You  
need to

do this not cos 1) dun wuk but cos real life distance perception is
TERRIBLE under anechoic conditions.  Ambisonics is probably the  
best "I am
there" system cos it's isotropic nature reproduces reverb and other  
diffuse
fields 'accurately'.  This was one of MAG's obsessions, even with  
stereo.


Anechoic conditions are fairly rare, and reverberant conditions  
common, especially in urban societies. More reverberation, or rather  
the variation in the ratio of direct to reverberant sound levels,  
suggests greater distance, but again it is only relative, and the  
nature and level of the reverberation is not simply related to source  
distance.


Reverberation is also used compositionally, to suggest distance, for  
effect, and to unite disparate sources in a common 'acoustic'.


3)	For very far souces, you might want to add HF absorption etc but  
this is

probably out of the realm of the sources you want to simulate.


As you say, HF absorption is only perceptible at large distances, and  
again is only relative.


Another factor is time of arrival of sound from a distant source, not  
directly perceptible unless accompanied by a visual event, or a  
source which is travelling towards or away from you, the Doppler  
effect.  It can be almost deleterious to model this, though it is  
commonly experienced.


Ciao,

Dave Hunt

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