Ed's AV Handbook.com
Home Theater & High Fidelity Stereo Audio


Chapter Six
The Room, Speaker, & TV
Page 3

Batting practice for the audio/video pro and a primer for the novice 



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House of mirrors 
          Imagine a room of mirrors.  All room boundaries, floor, ceiling, and walls are mirrored. Then imagine viewing a large screen TV in this mirrored room as the image is reflected and repeated from all directions.  This can cause visual disorientation and a headache.
          Similarly, with regard to the middle and higher frequencies of sound, room boundaries are acoustic mirrors.  Direct sound from a speaker is followed by its reflections from the room boundaries.  Acoustical reflections of the middle and high frequencies can cause acoustic disorientation of the sound fields and distort the harmonic detail of sound.  
The control of these reflections is critical for HiFi sound reproduction. Taming these reflections is a function of absorbing and/or diffusing their sound energy.


First reflections
          The most significant reflections are the first arriving reflections.  The source of these disruptive reflections can be located with a mirror as it is placed at and moved along a room boundary.  The first points of reflection are at any point at the room boundary where the speaker’s reflection can be viewed in the mirror from the listening position.  
          Therefore, with the assistance of a full-length mirror, locate and then dampen or absorb these first points of reflection. Place acoustic 'pillows' at the first points behind, above, and in front of the speaker.  Use heavy drapery, plush furniture, manufactured acoustic foam panels, and carpet with extra padding. 

          Lateral reflections add a subjective element to the acoustic situation.  More lateral absorption creates the acoustical image of a smaller stage or theater; less creates the image of a larger acoustical space. Therefore, create the acoustical stage that you prefer by applying more or less lateral absorption.
          Finally, keep in mind that too much absorption can create an acoustically dull sounding room. Therefore, moderate and limit the use of absorption to the first points.

Note for the smaller room
          If a seating position must be at the rear room boundary (this situation should be avoided), then place acoustical absorption at ear level to dampen the acoustic mirror behind the seating. Hang acoustical diffusion to the left and right of the absorption to psycho-acoustically move the sound farther away.  Manufactured open cell foam absorption panels, diffuser panels, furniture, book filled bookcases, and plants are very effective.

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 to 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 construction materials and acoustical products.

          However, all construction materials have a unique resonant frequency, a natural ring, at which they virtually become an open window to resonating sound.  Therefore, consider the construction of an acoustic sandwich of different materials such as sheet-rock, 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 dense 1/8” thick limp-mass vinyl blanket.
          Then build a sandwich with a layer of sheet-rock, the vinyl blanket, and another layer of sheet-rock.  Attach this sandwich to the wall with isolating metal channel fasteners.  The sum of the layers closes resonant windows and substantially improves the STC rating of the boundary.  Consider the same techniques for the floor and ceiling.  Additional isolation of the floor can be achieved with isolating products from several manufactures.
          Next, reduce noise originating from the heating and cooling system.  Use a dedicated trunk line with a larger than normal duct.  Line the vent with lightly painted fiberglass board. Suspend the duct and vent with rubber tie-down straps rather than loose metal straps and nails.  Wrap the metal vent in a noise suppression material such as the vinyl blanket mentioned earlier.
          How about the windows?  The perfect answer is “what window?”  However, if windows are necessary, select a dual pane type 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 room issues
          Three
significant room issues still remain: the Room color, the Room lighting, and the Room placement of the listener and the speaker system.  The balance of this chapter will address each within the context of
The Speaker System,
& The TV.

Shut the door
          That closes the door on The Room.  We have drawn curtains over the acoustical mirrors.  We have sited predictable safe harbor from acoustical swells.  We have shown our noisy guest the door.  Now let’s put speakers in the room.

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Ed's AV Handbook.com
Batting Practice for the AV Pro and a Primer for the Novice.
Copyright 2007 Txu1-598-288   Revised 2018