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

Chapter Six
The Room, Speaker, & TV
Page 2

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

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The Room
          Your room can become an unwanted inharmonious instrument playing along with
your music or movie soundtrack.  In addition, your room has an uninvited guest, noise.
Tame the unwanted effects of this unwelcomed instrument and uninvited guest with a practical selection of room construction choices.  This includes the room dimensions, the management of their subsequent room modes, the control of reflected sound within the room, and the suppression of noise from adjoining areas.

Room dimensions
         Each dimension of the room creates bumps in the audio system's low frequency response.  In effect, each dimension acts as a bass flute or acoustic bass cannon that
produces resonating waves of sound referred to as
standing waves or room modes.  
Room modes muddy sound.  
          Various ratios of room height, width, and length have been proposed to tame room modes.  Here are several such sets of ratios from the Handbook of Sound Engineers:

     1 : 1.14 : 1.39               1 : 1.4 : 1.9          1 : 1.28 : 1.54
     1 : 1.30 : 1.90               1 : 1.5 : 2.5          1 : 1.60 : 2.33*

                  * Golden Ratio per George Cardas                
These are guidelines. Do not panic if you cannot accommodate these ratios.
 But at a minimum, avoid the square room or dimensions that are multiples of each other.

Room modes and their harmonics
          As mentioned, a room mode can muddy sound.  The primary room mode or first resonant frequency of each dimension equals the speed of sound (1130 ft/sec) divided by twice the room dimension.   In addition, each primary mode is followed by its harmonics. Each harmonic is a multiple (2x, 3x, 4x, 5x, ….) of the primary room mode.  And each harmonic is successively lower in amplitude.
          Primary room modes and their harmonics produce acoustical peaks and nulls,
acoustical swells, throughout the room.  For example, the first primary mode F1 peaks at its opposite boundaries but nulls at the mid-point of the room dimension.  F2, the first harmonic, peaks at each boundary and at the mid-point of the dimension, but nulls at the mid-point between each peak.  F3 peaks at each boundary, the middle, and at the & points, then nulls at the mid-point between each peak.  This peak/null pattern continues for F4, F5, F6, F7, …. etc.
          If a speaker is positioned at the end of a dimension it will excite the first mode and its harmonic series.  Position the speaker at the middle of the dimension and only the even numbered harmonics are stimulated.  Position the speaker at a third of the dimension and only the third, sixth, ninth, and so on are stimulated.  Position it at the quarter of the dimension and only the fourth, eighth, twelfth.  And the pattern continues.

          Given the above, you can deduce that careful speaker placement can manage room modes of any room dimension. We will deal with this issue in detail in The Speaker System.  
If room modes continue to be a problem after proper room dimensions have been considered and the speakers 
are properly placed, then control or “suck out” room mode peaks with bass traps placed in the corners of the room. Bass traps can be constructed from scratch or obtained from several manufacturers.

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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