User Manual

Multichannel Monitoring Tutorial Booklet (M2TB) rev. 3.5.2
Masataka Nakahara : SONA Corporation
©2005 YAMAHA Corporation, ©2005 SONA Corporation
44 / 74
[Fig. 32] shows a comparison between a small room and large room, illustrating how the playback level
from speakers decreases by distance and how movement of 25 cm (one head) from the listening point will
affect the playback level from each speaker[4],[8]. We assume that the speakers are flush-mounted into
the wall (directivity coefficient Q=2), and that they are placed according to Rec. ITU-R BS. 775-1 (L/R;
30˚, LS/RS; 110˚).
We assume a monitoring distance of 1.5 m for the small room and 4.0 m for the large room, and the
conditions of each room are as follows.
•Small room
3.5 m
W
x 4.0 m
D
x 2.2 m
H
, floor area 14 m
2
, room volume 31 m
3
, total surface area 61 m
2
Average absorption coefficient α
ave
= 0.6
•Large room
10.0 m
W
x 15.0 m
D
x 6.0 m
H
, floor area 150 m
2
, room volume 900 m
3
, total surface area 600 m
2
Average absorption coefficient α
ave
= 0.6
[Fig. 32] Variation of the monitoring level caused by the movement of the listening position; Small
room (r=1.5m) vs. Large room (r=4m)
The solid line of the graph plots the decrease in playback sound pressure level for the Small room
according to the distance from the speaker, and the dashed line indicates the decrease in playback sound
pressure level for the Large room. When we leave the listening point, the distance to each speaker is no
longer identical, meaning that we lose the playback sound pressure level balance between the channels.
Differences in playback level between speakers caused by forward/backward movement (+/-25 cm) are
plotted by circles “O”. In the Large room where the monitoring distance is 4.0 meters, the difference
between speakers is approximately 0.8 dB. However in the Small room where the monitoring distance is
1.5 meters, it is greater (1.8 dB). In this way, the playback level balance between speakers tends to
become unstable in a small playback environment, leading us to consider ways to broaden the coverage
area. This tendency occurs even more markedly if the room is more dead, and if the speakers are free-
standing rather than flush-mounted.
To summarize the above, considerations related to monitoring distance can be grouped into the following
three situations, with appropriate measures to be taken for each situation.
3 meters or more Ideal. Stable. Attention to room acoustics is important.
2–3 meters Typi cal. Measures to reduce instability should be taken as appropriate for the
specific case.
Less than 2 meters Most likely to be unstable. It is desirable that the coverage area of the surround
speakers be expanded.
However, the monitoring distance is often restricted not only by the size of the room but also by the
capabilities of the speakers.
L
C
R
LS
RS
110deg
30deg
SPL(r)
SPL(r)
SPL(r)
SPL(r)
SPL(r)
(+25cm)
(-25cm)
L
C
R
LS
RS
SPL(r’)
SPL(r’)
SPL(r’)
SPL(r’)
SPL(r’)
Monitoring distance; r [m]
Small room; r = 1.5m
Large room; r = 4.0m
0
-35
-30
-25
-20
-15
-10
-5
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.5 5.0
Distance from the loudspeaker [m]
Reduction of SPL; SPL(r’) [dB]
4.0
∆SPL=1.8dB
∆SPL=0.8dB
Q=2
Small room
r=1.5m,
3.5m
W
x4.0m
D
x2.2m
H
,
α
ave
=0.6
Large room
r=4.0m,
10m
W
x4.0m
D
x2.2m
H
, α
ave
=0.6