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.
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'.
: 1.4 :
1.9 1 :
1 : 1.30 :
: 1.5 :
2.5 1 : 1.60 :
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
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.
The primary mode F1 peaks at its opposite
boundaries but nulls at the mid-point of the room
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,
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
-- Position the speaker at a third of the dimension;
and only the third, sixth, ninth, and so on are
-- Position the speaker at a quarter of the dimension:
and only the fourth, eighth, twelfth, and so on.
-- And this pattern continues.
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