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Ed's AV Handbook
Batting practice for the AV Professional
and primer for the novice

Chapter 6  Page 2

The Room, Speaker, & TV
The Room

Couch

        Your room can morph into a discordant instrument playing along with your music and movie soundtracks.  Also, you room my grant access to an uninvited noisy guest.  Tame the unwanted effects of your potential disruptive instrument and uninvited guest with a practical selection of room construction and configuration choices.  Choices include room dimensions, room-mode management, the absorption and diffusion of reflected sound, speaker and listener placement, plus the suppression of noise from adjoining areas and within the room.

Room Dimensions

Each dimension of a room creates deviations in an audio system's low-frequency response.  Like the pipe of a pipe organ, each room dimension generates resonating waves of sound.  The resonating waves, defined as standing waves or room modes, distort audio.

Select from many recommended ratios of room height, width, and length that minimize room mode distortion.  Here are several such sets of room dimension 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

The ratios are guidelines.  Do not panic if you cannot accommodate a recommended ratio.  Potential distorting room modes still exist with any residential room.  The proposed ratios minimize the issue.

Room modes and their harmonics

The primary room mode or 1st resonant frequency of each dimension equals the speed of sound (1130ft/sec) divided by twice the room dimension.  Each primary-mode produces a series of harmonics.  Each is a multiple of the primary mode (2x 3x, 4x, 5x, etc.x the primary mode).  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 primary mode F1 peaks at its opposite boundaries but nulls at the mid-point of the room dimension. 

F2 is the 1st harmonic.   F2 peaks at each boundary and at the mid-point of the dimension. 
But F2 nulls at the mid-point between each peak. 

F3 is the 2nd harmonic.  F3 peaks at each boundary, the middle point, the 1/4 point, 3/4 point.
And F3 nulls at the mid-point between each peak.

This peak/null pattern continues through F4, F5, F6, F7, etc.
Room Modes

Room Modes & Speaker Placement

A speaker placed at a room boundary excites 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 the speaker at a quarter of the dimension:
   and only the fourth, eighth, twelfth, and so on.

-- And this pattern continues.

Rm Spk Placement

Given these facts, we can conclude that speaker placement has the potential to manage the room modes of almost any room. 
This will be discussed in more detail later in this Handbook chapter.

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Ed's AV Handbook   
Copyright 2007 Txu1-598-288 Revised 2021


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