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

Chapter Seven
An Acoustical Strategy
for the small room

Page 3

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



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Acoustical tactics for the 2nd region
          Problematic room modes cannot be eliminated in the acoustical small room.  They can only be managed. This can initiate by designing or selecting a room with dimensions that produce a more even distribution of room modes.  The following ratios from the Master Handbook of Acoustics will assist:
 

 1:1.14:1.39                1:1.26:1.59                 1:1.28:1.54                  1:1.30:1.9
 1:1.40:1.9                  1: 1.50:2.5                  1:1.60:2.3                   1:1.6:2.6*

(*The Golden Ratio per George Cardas - not from the Master Handbook)
HandbookNote:
Do not panic if your room does not meet this room ratio criteria.  You can still make it work.


          If you are of a curious nature, draft a room mode analysis of the proposed room to expose any potential room mode issues.  Locate modes below 300 Hz and analyze their spacing.  Identify coincidences and isolated modes spaced more than 20 Hz.  For the less curious skip to the next paragraph.  



 
          Create an illustration of the room mode peak and null points of F1
through F3 on the proposed floor plan. Note that the points are equal to the length and width dimensions divided and even number.  The floor plan sketch will be used for speaker and listener placement. 


This is it -- the Mounkes speaker listener placement method  
           Steve Mounkes was my friend and a reference for the word audiophile.  He is missed.  
Long ago he taught me this simple speaker/listener placement method.  This plus a modest dose of room treatment (carpet, drapery, book filled shelves) delivers substantial audio improvement.  If I had to pick one practical audio application from this Handbook -- this is it.

          1. Divide the room width by the values 3, 5, 7, and 9.  With the results; measure and mark these points along the width of the room from the front left corner and then the right corner.

           2. Next,
divide the room length by the same values.  Mark these points along the room length from the front left and right corners.

Then select the stereo corner stones
          O
bserve the on-floor coordinates established by the width and length points.  While avoiding the room mode peaks and null points of your F1 thru F3 sketch; select a pair of practical coordinates for the left and right speakers.  Then place the listening position in an equilateral triangle with the speakers.

Surround sound
          A multi-channel surround sound system is a circular configuration that introduces additional speakers onto the stereo arrangement.  Its radius is the distance from the listening position to the left/right speaker locations. Therefore place the center channel speaker on the radius mid-point between the front left and right positions.


Choose rear speaker positions.
- In a 5.1 surround sound arrangement; place rear speakers at 110
   left and right of the front center speaker. 

- In a 6.1 arrangement; add an additional speaker to the 5.1 arrangement at 180
   of the front center speaker.

- In a 7.1 THX arrangement; add two additional rear speakers to the 5.1 arrangement at 150
   left and right of the front center channel.


          The listening position of the equilateral triangle is the ideal seating location.  However, additional seating is probably desirable.  Therefore, carefully select additional positions that are comfortable and practical, but avoid room mode peaks and null points.

Subwoofer placement
          Finally, while evading peak and null points; select the placement for the sub woofer.  If the sub woofer is placed sufficiently away from the room boundary, but less than of the dimension, it will minimize stimulating modes F1 thru F4.  Chapter Six page 5 offers more detailed instructions.


          If troublesome distorting room modes audibly persist; then absorb them with the placement of a broad bandwidth bass trap in the corner nearest each speaker.  

          I
n the final analysis subjectively fine-tune all speaker and listener positions with the organic sound meter that is attached to your head.  It is still the most sensitive listening device available.
 

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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