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.
The most significant reflections
are the first arriving reflections. Locate the source of
these reflections with a mirror. Place a mirror at a
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.
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
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.
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
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.
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.
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.