Recording - November 2016 - 50

By Ethan Winer

This article explains what "early reflections" are, and why it's important to treat
them in order to enjoy accurate monitoring and make portable mixes. I'll also
debunk some related myths along the
way. But first, it's useful to understand that
all acoustic problems are caused by
reflections off the walls, floor, and ceiling.
At bass frequencies, below around
300 Hz, we use bass traps to absorb
reflections, which reduces peaks, nulls,
and ringing. At higher frequencies
broadband absorption is used to tame
excess reverb, ambience, and echoes.
Early reflections are a specific type of
echo-those that reach your ears within
about 20 milliseconds of the direct sound
from the loudspeakers-and they concern mainly mid and high frequencies.
Early reflections make it difficult to mix,
because sound from the loudspeakers
arrives at your ears at different times and
from different directions. You hear the
direct sound from the speakers first, then
the reflected sound arrives slightly later
after reflecting off one or more nearby
surfaces. When an echo arrives within
about 20 milliseconds or less, the ear
doesn't identify the echo as a separate
sound source, so you don't hear it as an
echo. But sound coming from two locations-the loudspeaker and a side wall-
reduces clarity and degrades stereo imaging. You can tell when an instrument is
panned fully left or right, but in-between
positions are more poorly defined.
Worse, in an untreated room, reflections
often arrive from more than one nearby
surface, which confounds imaging further.
Comb filtering
Further, when early reflections combine
in the air with direct sound from the
speakers (or any other source), a specific
type of skewed frequency response
known as comb filtering is created.
Depending on the difference in arrival
times, some frequencies are boosted (in
phase) while others are cancelled (out of
phase). Figure 1 shows the comb filter
frequency response caused by a delay of
50

RECORDING NOVEMBER 2016

1 millisecond. This is equal to about one
foot of difference between path lengths,
which is typical with nearfield listening
when sound bounces off your desk.
A delay of 1 ms gives 180 degrees of
phase shift at 500 Hz and 360 degrees
at 1 kHz, so the first null is at 500 Hz,
then there's a peak at 1 kHz. Because
the peak and null frequencies are related to the delay time, which depends on
distance, the response isn't even the
same at both of your ears. Worse, the
frequencies change with even very small
head movements, which further harms
localization. Note that this graph uses a
linear frequency scale rather than the
more usual logarithmic spacing. Comb
filter peaks and nulls occur at equal frequency multiples, rather than by octaves
as with musical overtones or harmonic
distortion products.
The solution for all of these problems
is to create a Reflection-Free Zone (RFZ)
at the listening position (a term coined
by Dr. Peter D'Antonio of RPG). This is
accomplished by putting absorber panels at specific places called reflection
points on the side walls and ceiling.
Adding thick carpet can also help if
there's bare floor between you and your
monitor speakers. Listening to music free

of reflections is similar to listening with
headphones-musical instruments and
voices sound much clearer, and their leftright placement is better defined. The
stereo field is also wider, and can even
seem to extend beyond the physical leftright speaker boundaries.
Note that the term reflection "point,"
while common, is a bit of a misnomer
because it's really an area. Sound radiates in an arc outward from a source, so
by the time it reaches the wall or ceiling it
covers more surface and that entire area
should be treated. The goal is for early
reflections to be at least 15 dB softer than
the direct sound from the speaker at all
frequencies, and 20 dB or more is better
still. Software such as the fabulous (and
free) Room EQ Wizard (www.roomeq
wizard.com) can identify individual reflections and assess their relative strength.
When mixing music you need to hear
everything as clearly as possible. If
music or dialog is obscured by reflections and other room acoustic anomalies, mixes you think sound good now
may not sound so good later, or in your
car, or through other playback systems.
As mentioned, when reflections are
allowed, moving your head even an
inch or two changes both tonality and

Figure 1: Comb filtering is characterized by a repeating pattern of equally spaced
peaks and deep nulls. The response shape resembles a hair comb, hence the name.

http://www.roomeqwizard.com http://www.roomeqwizard.com

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