From: daw@cray.com (Dave Wright)
Subject: Re: QRD's
Organization: Cray Research, Inc.
Date: 20 Nov 92 14:57:34 CST

In article <20NOV199213495834@cs.umass.edu> weems@cs.umass.edu (CHIP WEEMS) writes:
>Does anybody have experience using Quadratic Residue Diffusers? I'm
>interested in criteria used for determining the number and placement
>of them in different environments. I've read the discussion in the
>Master Handbook of Acoustics, and am looking for information gained
>from practical experience, especially small studio, control room, and
>listening room setups. Personal opinions on their effectiveness are
>also welcome.
>
>Chip

I am a big believer in good diffusion, especially in small listening spaces
like small studios and control rooms. RPG DIffusors is the king of
manufacturing Diffusors of many types. Now they even have abfusors and
concrete block diffusors I would use the the shell of the litening room.
They also offer diffusor technology for home listening environments.
Look in Acoustical Society of America Journal and AES Journals for the last
10 years. Dr Manfred Shroeder and Dr Peter D'ANtonio are the best authors
to check on. You can build your own if you have the math and the cabinet
making skills. RPG diffusors simply are able to produce quantities.
Dr Antonio is also chair of the AES committee for the Characteristics of
Acoustic Materials and has written a white paper on diffusion and 
measurement techniques for random incident diffusion. I have heard several
rooms with QRD diffusors and prefer it. I am not one who likes to solve
acoustic problems by absorbtion. My recording studio was named
Ambient Sound. I most certainly did not want the image of a room filled
with fiberglass or anechoic.

Dr Antonio first made his QRD diffusors for LEDE control room design. The
room I heard was the Oak Ridge Boys Studio back in 86. In this environment
it was LEDE. The front half of the room is dead so that the room ITDG or
initial time delay gap was longer than the Studio part so that you hear the
relection pattern of the recording not your room. The back half of the room
was then to be as diffuse as possible. When I was in the Oak Ridge boys
studio we were doing localization listening experiments covering up
surfaces with sonex (which was also first used in LEDE for acoutic
purposes). We felt that we needed the front of the room dead so that the
flat response of the speaker was heard. ie. that early reflections were not
comb filtering the speaker response. We also needed the rest of the live
room to be as live as possible to make up for the lack of reflections from
the dead end, (these could have been 3rd order or higher reflections).
Diffusion was the best anwser and Dr Antonio being in X Ray Diffraction
with the Navy applied his background to his own persuit of a custom studio.
We also felt that the control room needed flat reflectors as well. We
called these Hass Kickers and were Plexiglass reflectors that could be
aimed. All of this was then looked at with TEF (or now MLSSA) and tuned
Reflection patterns became reality. TEF measurements of a flat surface,
polycylinder diffusors etc are very revealing. What we wanted with LEDE
was basically a holographic room where you could hear the localization cues
in the recording not the listening room. In our listening tests many felt
that they could hear the test chirp go left/right/front back but also
up and down, like the kick drum and the High hat be positioned properly.

The larger the room, the longer the time delay gap and the more likelyhood
of multiple order reflections. These reflections are also lower in
frequency content. Any room is broken between modal and reflective
properties. A small room does not have low frequecy rays but waves. As the
room gets larger the crossover between modal and ray tracing goes down.

If I were to design MY ultimate listening room, It would be designed with
EASE to look at the Ray Tracing, NASTRAN to look at the modal responses
and contain a proper balance of Reflection/Diffusion/Absorbtion to get a
clean reflection pattern with a decent ITDG a few hot 1st order reflections
and then a diffuse sound field.

In small listening rooms, there is no such thing as RT60. There is not
enough sound path travel time to develop a diffuse sound field. Two rooms
with the same RT60 can sound completely different. You need MLSSA or TEF
to quantify the difference.

Call RPG Diffusors if you are interested in gettng more info from them.
SYN-AUD-CON is also a good source for listening experiences etc. Their #
is 812-995-8212

301-249-5647


Dave Wright


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