User Manual

Multichannel Monitoring Tutorial Booklet (M2TB) rev. 3.5.2
Masataka Nakahara : SONA Corporation
©2005 YAMAHA Corporation, ©2005 SONA Corporation
43 / 74
3-7. Monitoring distance
The playback sound field becomes more stable as the monitoring distance (the distance from the listening
point to each speaker) increases. In other words, surround playback tends to be more stable in a larger
room and less stable in a smaller room. However as the monitoring distance increases, the influence of the
room also increases, so it is important to pay attention to the acoustics of the room.
[Fig. 31] shows calculations for each speaker simulating an off-axis deviation of 25 cm (one head)
forward and backward from the listening point relative to the Rec. ITU-R BS. 775-1 speaker
placement[4],[8]. Even if speaker angles are adjusted precisely (L/R; 30˚, LS/RS; 110˚), the L-R spread
will narrow (
θ
L,R
= 30˚ → θ ’
L,R
) if the listener moves backward from the listening point, causing the
surround speakers to change from a rear placement to a sideways placement (
θ
LS,RS
= 110˚ → θ ’
LS,RS
). The
graph in [Fig. 31] describes such changes. The upper half shows the angle difference when moving
forward 25 cm, and the lower half shows the angle difference when moving backward 25 cm. The dashed
lines show the angle difference for L and R (∆
θ = θ ’
L,R
- θ
L,R
), and the solid lines show the angle
difference for LS and RS (∆
θ = θ ’
LS,RS
- θ
LS,RS
). The horizontal axis indicates the monitoring distance.
[Fig. 31] Variation of the placement angle of the loudspeaker caused by the movement of the
listening position
[4],[8]
From [Fig. 31] we can determine the following points regarding how forward/backward movement will
affect the speaker placement angle.
1. As the monitoring distance is shorter, the angle deviation increases rapidly
=> Instability in the playback environment is more likely to occur in small rooms than in large rooms.
In other words, the listening area becomes smaller as the monitoring distance becomes shorter.
2. The LS/RS angle deviation is greater than the L/R angle deviation.
=> Sound field instability is more likely to occur for the surround speakers than for the front speakers.
3.The L/R angle deviation is greater when moving forward than when moving backward.
=> It is desirable that the front speakers be placed for broad coverage in front.
4.The LS/RS angle deviation is greater when moving backward than when moving forward.
=> It is desirable that the surround speakers be placed for broad coverage in the rear.
From the above points, we can conclude that the playback sound field will tend to become unstable
particularly for the surround speakers that are placed in a small room, and that it is therefore important to
give broad coverage area to surrounds.
In the experience of the author, a fairly stable playback environment can be obtained with a monitoring
distance of 3 meters or more, and monitoring distances of less than 2 meters tends to produce an unstable
sound field. Most studios have a monitoring distance between these two, in the range of 2--3 meters, and
this is the same as the values recommended in ITU-R BS. 1116-1.
θ
L,R
30deg
L
C
R
LS
RS
θ
LS
,RS
110deg
Monitoring distance; r [m]
(+25cm)
(-25cm)
L
C
R
LS
RS
θ ’
L
,R
θ ’
LS,RS
θ ’
L,R
θ ’
LS
,RS
Off-axis error ; ∆
θ
=
θ
’-
θ
[degree]
Monitoring distance; r [m]
12 3 4 51.5 2.5 3.5 4.5
0
+5
+10
+15
-5
-10
-15
LS, RS
L, R
Forward 25cmBackward 25cm
robust
critical