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

Chapter Seven
An Acoustical Strategy
for the small room

Page 4

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

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3. The transition zone
          The 3rd Region is a difficult region to illustrate.   It begins above 300Hz.  It is dominated by wavelengths that are too long to be described by specular acoustics.  And this region is too short for wave acoustics.  

Acoustical tactics for the 3rd region
          The tactics of this region are dominated by acoustical diffusion and diffraction.  A room full of furniture provides both adequate diffraction and/or diffusion.  The tactics of Region 4 will provide the balance of any additional needed treatment.

4. The region of short wavelengths
          Region 4 is the region where the concern of specular reflections prevail.  Room boundary reflections compete with the direct sound from a speaker.  This competing sound distorts sound field imaging and spatial impression.  

Acoustical tactics for the 4th region
          Locate the 1st points of reflection at all room boundaries.  Specular reflections act as light reflecting off a mirror.  Hence, a 1st point is at the reflection of a speaker in a mirror placed at a boundary that is observed from the listening position.
          Mount acoustical absorption on the wall at ear level between the 1st points of the front left and right speakers in a stereo arrangement; and the left and right rear speakers of a multi-channel arrangement.    
          Absorb the 1
st points at the floor with carpet and extra padding.  Absorb the 1st points at the ceiling with acoustic foam or ceiling tile.

          Experiment with the 1st points of the left and right side walls.  Increasing lateral absorption creates the sound of a smaller less spacious room.  Less absorption creates a more spacious image.  Adjust the amount of absorption to your subjective desire. 

Noisy loose end

          A very significant concern remains - noise.  If noise from an adjoining area merges with your room, it will obscure sound fields, musical harmonic detail, and compromise dynamic range.   Therefore, suppress the transmission of noise through walls, the ceiling, and the floor.
          Begin by simply closing windows and doors.  Then consider the following construction instructions. Trapped air and mass are the two components that are most effective at stopping the transmission of sound through room boundaries.  However, all construction materials have a resonant frequency at which they virtually become an open window to 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 further suppresses the transmission of noise.
          Sound as air and water will leak through small gaps.  This includes gaps in windows, doors, and electrical outlets.  Seal all gaps.  The HVAC system can also produce noise.  Use a dedicated trunk line with a larger than normal duct.  Line the vent with lightly painted fiberglass board.  Suspend the ducting 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 mention earlier.  In addition, simply closing a door or window can have a huge effect.

          In summary, the 2nd and 4th Regions are the regions of most interest in small room acoustics.  The management of turbulent standing waves, the evasion of room mode peaks and nulls, the absorption of the 1st points of reflection, and the suppression of noise are the most significant concerns. If these acoustical issues are successfully managed, the acoustical distortions that affect timbre, imaging, spatial impression, and dynamic range will be minimized.  The result will be a first-rate sound room. 
Additional Note for the 2nd Region: Experiment with sub woofer placement at a null location.  This may in some situations create a more even distribution of the sub woofer’s energy.  It may also require a second  sub woofer to generate adequate loudness

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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