Recording - November 2016 - 51

Figure 2: Early reflections arriving from the side walls damages clarity and imaging.
Reflections from the rear wall are also considered early if that wall is closer than
about ten feet behind you. The small rectangles on the walls are acoustic panels.

Figure 3: Early reflections also arrive from the floor and ceiling, so those surfaces
should be treated as well for best results. The center of the reflection area is
halfway between you and the loudspeakers.
imaging due to the comb filter frequencies shifting differently at each ear. After
absorbing the reflections, both imaging
and frequency response are more stable
versus position, making it easier to nail
down a pleasing mix. Mixing in a reflection-free environment also lets you hear
smaller changes in applied reverb and
EQ. Even at my age (67) I can easily
hear EQ changes of half a dB at
midrange frequencies in both of my well
treated listening rooms.
Where to place the panels
Figure 2 shows the three main paths by
which sound from a loudspeaker arrives
at your ears: 1) the direct sound from the
speakers, 2) early reflections from the left
and right side walls, and 3) additional
reflections from the rear wall behind you.
Since sound travels at approximately one
foot per millisecond, reflections from a
rear wall closer than about ten feet behind
your head are considered early because
the round-trip distance is 20 feet.

The side-wall reflection points are
between you and the front of each speaker,
slightly forward of center-how much forward depends on the room's width and the
distance between your speakers. But the
ceiling and floor reflection points are exactly halfway between you and the speakers if
the tweeters are at ear height, which they
should be. This is shown in Figure 3.
The left and right side acoustic panels
should be symmetrically placed, assuming
your speakers are centered left-right in the
room, which is required for good stereo
balance. With the help of a mirror and
an assistant, you can identity the exact
location for the center of each panel.
While you sit in your usual place, have a
friend hold a hand mirror flat against the
wall and slide it around. When you can
see the tweeter in the mirror, the mirror is
at the center of the area to be treated.
It's important to understand that the
range of frequencies absorbed affects
the quality of a Reflection Free Zone.
Since early reflections are mainly about

mid and high frequencies, you can use
relatively thin panels, say, one or two
inches thick (as described by Michael
Nickolas elsewhere in this issue).
However, using thicker panels extends
the absorption to even lower frequencies, which is never a bad idea. This is
especially true for absorbers on the rear
wall, where the peaks and nulls extend
all the way down to the bass range.
Most people consider the corners the
most important place for bass traps, but
the rear wall is just as important.
If your budget allows for dedicated
construction, early reflections can be
avoided by angling the side walls outward, and sloping the ceiling upward,
toward the rear of the room. With an
angle of at least 35 degrees, reflections
are directed behind the listening position
without having to apply absorbing material to the walls or ceiling. This lets you
retain more room ambience because you
don't need additional absorption just to
get rid of the reflections. But most people
don't have the luxury of building new
walls. So the only option is to apply
absorption at the reflection points, and
that works perfectly well. Smaller rooms
sound better with less ambience anyway.
A reflective couch or seat back can
also cause severe comb filtering. I made
the thick seat cover shown in Figure 4
from two layers of half-inch thick cotton
batting, covered with plush fabric from a
local fabric store.
Early reflection myths
Some people believe that reflections in
a home-size studio or listening room
enhance spaciousness. By "home-sized"
I mean a room less than about 25 feet
wide, where reflections from nearby surfaces arrive soon after the direct sound
from the speakers. On the surface this
seems plausible, but it's incorrect.
The ambience and reverb you add to
your recordings is typically that of a

Figure 4: Ethan's trusty assistant Mrs.
Emma Gray demonstrates this thick
seat cover that absorbs damaging
reflections from a high chair back.
RECORDING NOVEMBER 2016

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Table of Contents for the Digital Edition of Recording - November 2016
