If you have been following this series, in Part 1 you saw
how monophonic sound progressed into stereophonic sound and then into what is presently
called the high-end. From Part 2, you know what happened in the audio industry
as the 3-dimensional soundstage developed favor and why rectangular listening
rooms are preferred. You also learned how moving speakers influences bass
prominence and how the listening room itself influenced the overall sound. From
Part 3 you learned how to minimize room bass resonances by mathematically
positioning your speakers based on your room dimensions and how high off the
floor they should be. You also know that floor rugs are good and that the
listening chair ("sweet spot") are about the same distance from the
rear wall as the speakers are from the front wall.
In this part, we will explore how to treat your room acoustics so you can achieve the biggest soundstage possible in your listening
room regardless of its size or shape. You will also understand better what
helps to enhance the size of this soundstage and what to avoid. Here we will
address issues above 300Hz, those frequencies where directional imaging cues
occur. So with this introduction, let's roll up our sleeves and get to work.
STANDARD SIGNAL
SOURCE: At this point, you should be listening to your favorite music
in your sweet spot becoming familiar with its new attributes. You may have
already noticed a change in the size and shape of the soundstage and maybe not.
What we need to do next is all get on the same page and start listening to what
readily-available recordings can tell us about what to do next. The first
conventional (AKA Red Book or 16 bit/44 KHz) recording that well reveals a
soundstage's dimensionality is:
Norah Jones Come Away
With Me (Red Book)
If you do not have this recording, you need to buy one
now since I refer to it constantly to explain what to listen for in
specific passages. While it may be tempting, DO NOT use the MP3 version
of this recording in this positioning process since the MP3 compression masks
the potential three-dimensionality of an uncompressed Red Book version. However,
you should play an MP3 version later to understand the differences and
compromises between these two popular digital recording formats.
The Red Book version will suffice to a point but another
round of tweaks requires a better signal source. To permit reliable A-B
comparisons and assure yourself you are moving in the correct direction, it is
best to use the same program material for both positioning rounds. I
will use a high-resolution version of this same Norah Jones album and I suggest
you purchase the $30 download it here:
Norah Jones Come Away With Me
(high resolution)
Download the highest resolution your player/streamer can
handle (use the pull-down menu to see what format your streamer can play). This
description uses the 24-192 FLAC version. Other similar Red Book and high-res
recordings may be used once you better understand how to listen or rather what
audible cues to listen for (something else that you should begin to understand
as a result of this exercise).
Tweaking is an iterative
process, one that takes a lot of time and patience, along with critical
listening. As we progress, movements in speaker positions may vary from as much
as 4” to as little as ½” with each move making gains in some respects and
losing gains in others. The final positions will most likely fall into “your
personal preference” category as they should to leverage those strengths you
prefer. Allow yourself time to embrace the sound of the relocated speaker and
listen to a variety of familiar music after making a location change, typically
over several days. You may already be intimately familiar with many different
recordings (your personal “favorites”) and I suggest that you listen over and
over to those pieces focusing primarily on soundstage width and height during
this time, especially the extreme outer edges. Before we begin listening, help your
room reveal more of what your stereo can produce by eliminating simple physical
impediments, here being first-reflections.
IDENTIFY FIRST
REFLECTIONS: First reflections are the surfaces the sound from your
speakers strike on the way to your ears. Like balls bouncing off the cushions
of a pool table, sound bounces off the surfaces of every object in your
listening room. These bouncing sounds arrive at your ears at different times
than the direct sound from your speakers. When the level of reflected sound is
high, it interferes with the ear-brain interpretation of the location of the
direct sound. In other words, reflected sound messes up the soundstage and
should be absorbed or redirected to minimize this interference. This is a
concept in room treatment called the “live-end, dead-end” or LEDE where the
part of the room in which the speakers reside is deadened to absorb these
undesired first reflections.
Before we start identifying where these first reflections
are, you must remove all furniture from the speaker half of the
listening room. Do this now. Doing so allows you to easily identify
exactly where on the walls, floor, and ceiling these reflections originate by
simple line-of-sight observations. You can return these items to that side of
the room later and observe what compromises you have made by re-introducing
this furniture near your speakers.
USING A MIRROR:
Begin by identifying where the first reflections are in your listening room as
observed from your sweet spot and then damping them out. This part is pretty
easy but requires some masking tape, a friend, and a mirror (a flat 12” by 12” square
will do just fine). All you have to do is move this mirror against the flat
wall until you can see one of the speakers and then put tape around that visually-identified
area. To identify the areas on these surfaces where first reflections originate
(and where sound absorption should be placed in the dead-end of your listening room),
do the following:
- Sit down at the sweet spot.
- Have your friend hold the mirror flat against the left wall.
- Move the mirror around on the wall until you see the LEFT speaker.
- Put masking tape on the left wall framing the area where you can see any part of the left speaker.
- Continue moving the mirror around on the left wall until you can see the RIGHT speaker.
- Put masking tape on the left wall framing the area where you can see any part of the right speaker.
- Repeat steps 2-6 for the right wall.
- Repeat steps 2-6 for the front wall, left side.
- Repeat steps 2-6 for the front wall, right side.
- Repeat steps 2-6 for the floor.
- Repeat steps 2-6 for the ceiling.
If there are multiple seats at the sweet spot, repeat all of
these eleven steps for all alternate seating positions.
The taped-off areas on the walls, floor, and ceiling are the
locations where sound absorption is necessary. In your decorating decisions for
this room, place sound-absorbing
panels or hang tapestries at these locations so that the sound is reflected
less at these critical points. Hang the tapestries from 1”x2” boards to permit
adding acoustic batting behind them. If the first-reflection areas are windows,
put soft but heavy curtains on the windows that can be easily raised and
lowered. There are many suggestions for both DIY and professional room
treatments in the Rives Audio section at http://www.audioasylum.com/forums/rives/bbs.html.
Use a Mirror to Locate First Reflections in the Dead End
For aesthetic reasons, ceiling treatments are usually the
hardest to include in your listening room. You can disguise such a treatment as
an indirect-lighting ceiling fixture, especially if there is already a ceiling
light nearby. Build an absorber panel as you would for a wall panel (use an
Owens-Corning #703 2’x4’x2” fiberglass panel). Frame it at the edges and cover
with coordinated cotton fabric. then suspend it uniformly from the ceiling with
wires and hooks. Take down the panel. Connect a rope light to the existing ceiling
fixture. If the ceiling fixture is not above the absorber, use some plastic
self-adhesive electrical channel to help hide the wire from the fixture to
above the absorber. Re-hang the absorber. Hide the rope light on top of the
absorber and you now have a sound absorbing room light.
If you choose not to build your own panels, you can also buy
sound absorbing panels from many sources including a thrifty manufacturer at http://www.readyacoustics.com/diy_acoustic_panels.html.
So far you have taken care of the DE half of the LEDE
concept. The LE portion identifies the first reflections in the sweet-spot half
of your listening room. Unlike the DE half, the LE half must disperse the sound
at these identified locations to randomize their otherwise predictable
interference patterns. Use what is called a diffuser to break up these
undesired uniform reflection patterns. Identify where this diffuser should be
by performing similar steps with the mirror but this time while looking at the
back wall from the sweet spot at all listening positions. Tape off the area for
the diffuser.
Use a Mirror to Locate First Reflections in the Live End
DIFFUSERS:
In keeping with the Live-End-Dead-End (LEDE) approach to room design, the live
end (sweet-spot half) should be reflective and the rear wall is critical in properly
contributing to this reflectivity. Here the rear wall should not only reflect
sound, it should disperse it in a controlled manner. The human ear is most
sensitive those frequencies produced by the human voice (300Hz-6000Hz) so it
makes sense to focus on dispersing these frequencies. However, diffusers with a 300Hz
cutoff will stick out into the listening room quite far. A good compromise is
one octave above the lowest frequency, or a diffuser dispersing sound between
the 600Hz-6000Hz range. This LE reflective area identifies the smallest
diffuser you should use.
You can buy diffusers in various sizes from many
manufacturers such as Overtone Acoustics,
but you can also easily make one yourself if you have the time and are so
inclined.
Grid Assembly
Below is a pattern for four heights of 1”x1” square blocks
that easily diffuses frequencies within the range of 600Hz-6770Hz. Assembly is
simple gluing these blocks to a piece of ½” thick plywood in the positions
determined by the number in the table below.
The diffuser below makes one panel 12”x12”. Make multiple
panels at least the size of the masked area on the back wall to properly
diffuse the sound from the rear wall (a 2’x4’ panel needs 8 of these, 4’x4’
panel needs 16 of these, etc.). These finished panels are very heavy and must
be attached to the wall studs for proper support.
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Block Heights:
1=2.823”, 2=5.642” 3=8.465”, 4=11.287”
At this same time, it is best to get whoever is involved in
decorating your listening room to understand why things are placed where you
have decided to put them. Compromise is the key to a successful relationship
and in your listening room such compromises should be weighed against audible
effects. Keep your spouse (substitute the appropriate politically-correct term
here that describes your relationship) involved in your decorating decisions
now instead of having him/her move things around later for aesthetic reasons or
otherwise.
Keep to the LEDE plan and you will have a good listening
environment, one that can reveal the full potential of your system. In Part 4
of this series, we will describe how to use the Norah Jones signal sources to
fine tune the location of your speakers. In this part, I will explain what to
listen for in various passages so movements and changes for the good or bad can
be correlated.
HOMEWORK: Make sure you have the reference signal sources for Norah
Jones album Come Away with Me. Add
the absorbers and diffuser to the room now at the appropriate locations and get
used to the soundstage with your favorite songs and tracks. This will help you
in Part 5 of this series when we start systematically moving things around.
Yours for higher fidelity,
Philip Rastocny
I do not use ads in this blog to help support my efforts. If you like what you are reading, please remember to reciprocate, My newest title is called Where, oh Where did the Star of Bethlehem Go? It’s an astronomer’s look at what this celestial object may have been, who the "Wise Men" were, and where they came from. Written in an investigative journalism style, it targets one star that has never been considered before and builds a solid case for its candidacy.
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Copyright © 2015 by Philip Rastocny. All rights reserved.
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