Recording - March 2017 - 30

W

hen deciding where to place speakers in a room relative to the listening
position, the first consideration is usually the stereo image. There are

BY JAMES LINDENSCHMIDT

numerous strategies for optimizing this sort of placement. My favorite is the method recommended by nearly every
monitor manufacturer: begin with the listening position, and then place the speakers so that they are in an equilateral
triangle with the listener's head. This strategy consistently results in excellent stereo imaging, particularly if there is some
nice thick absorption at the reflection points on the side walls and ceiling.
However, there are other considerations at play when it comes to maximizing the sound of speakers with good
placement in the room. How big, for instance, should the equilateral triangle be? Surely the stereo imaging will
change as the size of this triangle changes-that's pretty intuitive. But there is another phenomenon to consider that
greatly affects what we hear, one that is not so straightforward...

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SBIR stands for "speaker boundary interference response,"
and it describes an acoustic phenomenon that occurs between
speakers and nearby boundaries such as walls, floor, and
ceiling. Bass frequencies behave differently than treble; while
treble frequencies are somewhat directional and "beam" in a
room, longer-wavelength bass waves tend to be omnidirectional,
leaving the speaker like a pebble in a pond (or more accurately,
like a sphere in 3 dimensions rather than the 2-dimensional
pond surface). So if you imagine a bass wave leaving the
speaker, some of the energy will go backward toward the
front wall, reflect off the front wall, and then come back to the
speaker, where it will interfere with the new bass wave that's
just leaving the speaker. This creates a peak and/or a null at a
specific frequency, related to the distance between the speaker
and the front wall. SBIR occurs on all nearby surfaces, such as
the side walls, the floor, and the ceiling.
The effects of SBIR can be somewhat mitigated by placing
a bass trap (usually a broadband absorber, as thick as
possible) directly behind the speaker, or in the path the
reflected sound travels to get back to the speaker (these traps
should be in addition to the other bass traps in the room,
commonly in the room's corners). But we can also change
the SBIR peaks/nulls by moving the speakers, changing the
distance between them and the boundary causing the SBIR.

As a way to explore the SBIR phenomenon, we will look
to a very useful function in Room EQ Wizard (REW), the
popular acoustic measurement software freely available on
all platforms. REW is great at measuring rooms, but for this
article we will be using one of its newer functions (introduced
in version 5.1): the Room Sim mode.
The Room EQ Wizard software is available for Mac,
Windows, and Linux. You can download it, as well as
see extensive documentation with an active forum and an
easy donation tab to support this great software, at www.
roomeqwizard.com.

30

RECORDING MARCH 2017

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

Table of Contents for the Digital Edition of Recording - March 2017

Contents
Recording - March 2017 - Intro
Recording - March 2017 - Cover1
Recording - March 2017 - Cover2
Recording - March 2017 - 1
Recording - March 2017 - 2
Recording - March 2017 - 3
Recording - March 2017 - 4
Recording - March 2017 - 5
Recording - March 2017 - Contents
Recording - March 2017 - 7
Recording - March 2017 - 8
Recording - March 2017 - 9
Recording - March 2017 - 10
Recording - March 2017 - 11
Recording - March 2017 - 12
Recording - March 2017 - 13
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Recording - March 2017 - 15
Recording - March 2017 - 16
Recording - March 2017 - 17
Recording - March 2017 - 18
Recording - March 2017 - 19
Recording - March 2017 - 20
Recording - March 2017 - 21
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Recording - March 2017 - 24
Recording - March 2017 - 25
Recording - March 2017 - 26
Recording - March 2017 - 27
Recording - March 2017 - 28
Recording - March 2017 - 29
Recording - March 2017 - 30
Recording - March 2017 - 31
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Recording - March 2017 - 33
Recording - March 2017 - 34
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Recording - March 2017 - Cover3
Recording - March 2017 - Cover4
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