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Ed's AV Handbook
Batting practice for the AV Professional
and primer for the novice

Chapter 6  Page 3

The Room, Speaker, & TV
The Room Continued

House of mirrors

House of Mirrors

Imagine a room of mirrored boundaries -- the floor, ceiling, and walls.  Then envision viewing a large screen television in this mirrored room with images reflected and repeated from all directions.  This chaos of images can cause visual disorientation and a splitting headache. 

Similarly, room boundaries are acoustic mirrors.  Acoustical-reflections from all room boundaries follow the direct sound from a speaker from all directions.
The reflections of the middle and high frequencies can cause acoustic disorientation of the sound fields and distort harmonic detail.  The control of these reflections is critical for high fidelity sound reproduction.  Taming these reflections is a function of absorbing and or diffusing their amplitude.

1st Reflections

First reflections 

The most significant reflections are the first arriving reflections.  Locate the source of these reflections with a mirror.  Place a mirror at a room boundary. 
While sitting at a listening position, have someone move the mirror along the room boundary.  The 1st reflection points are at all speaker reflections viewed in the mirror from the listening position.

Place acoustical-absorption at the first points behind, above, and in front of all speakers to dampen the reflections.  Use heavy drapery, plush furniture, acoustic foam panels, and carpet with extra padding.

Acoustic option

Lateral reflections can add a subjective element to the situation.  More lateral absorption creates the acoustical-image of a smaller space.  Less lateral absorption recreates the impression of a larger acoustical-space.  You may produce the acoustical stereo image that you prefer by applying more or less lateral absorption.  However, too much absorption will create a dull sounding room.

Note for the smaller room
If a seating position must be at the rear room boundary, then place acoustical absorption at ear level to dampen the acoustic mirror behind the seating.  Then place acoustical diffusion to the left and right of the absorption to psycho-acoustically move the sound farther away.  Open-cell foam absorption panels, diffuser panels, furniture, book filled bookcases, and plants provide effective options.

Uninvited Noise 

If undesirable noise from adjoining areas merges with your music or movie soundtrack, it will obscure sound fields and harmonic detail.  Conversely, sound exiting the sound room can compete with an adjoining area's activities.  Therefore, suppress the transmission of noise too and from the room.

Trapped air and mass are the two components that are most effective at stopping the transmission of sound through room boundaries.  The measurement of how effective any material blocks sound through itself is the Sound Transmission Class or STC.  The higher the STC value, the better its noise suppression.  Many manufactures and industrial associations offer STC data on their construction and acoustical product. 

However, all construction and acoustical product have a unique resonant frequency, a natural ring, at which they virtually become an open window to that resonating frequency.  Therefore, consider the construction of an acoustic sandwich of different materials such as Sheetrock, acoustic insulation, acoustic vinyl, and air space.  Each layer closes another's resonant window and adds to the STC value of the walls, floor, and ceiling.

As an example, construct two stud-framed walls with one inch spacing between the walls.  Hang dense mineral fiber insulation between the studs. 
Cover the walls with a 1/8 inch dense, thick limp-mass vinyl blanket.  Consider adding a sandwich layer of Sheetrock, the vinyl blanket, and another layer of Sheetrock.  Attach this sandwich to the wall with isolating metal channel fasteners.

The sum of the layers close the resonant windows and substantially improve the STC rating of the boundary.  Consider the same techniques for the floor and ceiling.  Additional floor isolation is available via dedicated floor-isolating products from several manufactures. 

Next, reduce noise originating from the heating and cooling system.  Install a dedicated, larger ventilation duct.  Line the vent duct with lightly painted fiberglass board.  Suspend it with rubber tie-down straps; rather than loose metal straps and nails.  Wrap the metal vent in a noise suppression material such as the viny blanket mentioned earlier.

How about the windows?  The perfect answer is "what window"?  However, if windows are necessary, select a dual-pane or install two separate parallel windows.  Float the window structure on isolating vinyl material as used on the wall.  Next, select an exterior grade solid core door.  Finally, sound as air and water will flood through any small gap.  Therefore, seal, dampen and, isolate gaps in windows, doors and, electrical outlets.

Remaining Issues 

Three significant room issues remain.
1. Room color
2. Room lighting
3. Speaker and listener placement.
The balance of this Handbook chapter addresses each each issue via the "The Speaker System, and The TV.

Shut the Door

That closes the door on 'The Room'.
-- We have drawn curtains over acoustical mirrors.
-- We have identified room mode peaks and nulls.
-- We have sited safe harbor from peak-null acoustical swells.
-- We have shown our noisy guest the door.
Now let's put speaker in the room.

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Ed's AV Handbook   
Copyright 2007 Txu1-598-288 Revised 2021


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