Recording - September 2014 - 62

By Bruce Black

So how do we pacify this unrepentant hooligan? The same way we would treat any other
room surface! The first and hardest step is realizing that the ceiling is part of the problem. We
then apply solutions.

Tools of the trade

The forgotten Room Surface

-The Ceiling!
We never think about it... until it ruins our room's acoustics.
Here's how to tame it.
When we think of acoustical treatments for a critical listening space like a studio, we usually think in terms of putting acoustical materials on the walls. There
are absorptive panels, foam panels in all sorts of shapes, my favorite Helmholtz
resonators as described in December 2013 and March 2014 (actually, these
can go on the floor), diffusors in a breathtaking selection of shapes, sizes, and
effectiveness, and a host of other devices that are limited only by someone's
fevered imagination. Time and time again, though, I find in my practice that
there is a room surface that is almost universally forgotten.
This poor, lost waif of a room surface, this abandoned stepchild of your studio, taken for granted yet always overlooked, is the ceiling. In a way it's odd
that it is so neglected so often, as it is the largest, most reflective unobstructed
surface in your room. All of the issues that come from highly reflective surfaces-comb filtering, the smearing and shifting of localization, flutter echoes,
excessive reverberation, and so on-are present in abundance via your ceiling.
Other surfaces have some obstructions of one sort or another, such as bookcases, tables, furniture, and the like, and these all break up the sound by adding odd
reflections and absorption. But of all the surfaces that are reflective, the ceiling is
actually the worst offender because it is simply the biggest unimpeded flat surface.
It's the Renegade of Reflection, the Felon of Flutter, the Culprit of Comb Filtering!

The first thing that might come to mind is
absorption, the ever-popular go-to guy when
thinking of room acoustics... but there's a catch
here. Absorption takes acoustical energy out of
the air, so it reduces the amount of sound being
reflected, but it doesn't actually scatter or redirect
sound. You do get some improvement, but it's not
the best answer.
The best way to do this is with diffusion. Diffusion
breaks up the specular ("hard") reflections you get
off highly reflective surfaces like drywall or glass.
The best diffusors are Quadratic Residue Diffusors,
or QRDs; they are also known in some circles as
Schroeder Diffusors. Figure 1 shows some QRDs in
a room I designed for Paul Massey; they're shown
on the walls rather than the ceiling, but at least you
can see what they look like.
These are very effective and would make
quick work of ceiling reflections, but they have
one drawback-they're really heavy, and
require a particularly robust attachment to the
ceiling. Personally, I wouldn't want one of these
hanging over my head here in earthquake-prone
California. So that takes us to the next best diffusor-a polycylindrical.

A well-rounded solution
A polycylindrical diffusor is essentially like a
section of a tube's length that has a convex outer
surface. (We don't use the concave inner surface; it can create acoustical problems by focusing sound.) A polycylindrical doesn't perform as
well as a QRD, but something this light will do a
lot less damage if it falls, and it still does a
decent job. Polycylindricals are useful on walls
as well as ceilings; I wasn't able to photograph
a studio where I'd installed any on a ceiling, but
Figure 2 shows some wall-mounted examples I
built for Captain Audio.
You can make them by cutting along the length
of a thick cardboard concrete form tube to make
a quarter or third section of a tube. These tubes

Figure 1. Arrays of Quadratic Residue Diffusors at the back and side walls of Paul Massey's mix room, Ojai, CA.
Room designed by the author, QRDs provided by RPG.
62

RECORDING September 2014

Recording - September 2014

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