Dynaudio AIR 15, Reference Manual

Page: 115 / 129

Appendix: Acoustics
Dynaudio Professional AIR reference manual – 2014-09-28 112
In a box-shaped room, the frequencies can be calculat-
ed as follows:
where
►
f = frequency in Hz
►
c = speed of sound (approx. 340 m/s or 1130 ft/s)
►
l = length of the room
►
w = width of the room
►
h = height of the room
►
n = integer from 0 and up
Fig. 69: Maxima of standing waves
The curves shown here express the area of the room
where the respective frequency is audible. At the min-
ima, that frequency’s level is much lower (sometimes
-40 dB compared to the maximum).
If the room’s length, width and even height are identical,
obtaining an even sound distribution can be very hard.
Prohibiting standing waves
Parallel walls in the room should be prevented. This will
suppress the strongest modes.
When placing monitors, it is important that as few
modes as possible are excited. This is why no monitor
should be placed at the maximum of a standing wave.
When monitors are built into walls, you should therefore
make sure that the opposite wall is not parallel to the
wall with the monitor.
At low frequencies, a monitor can be considered as ra-
diating its sound energy in all directions. This is also
called a
4 π radiation .
Fig. 70: 4 π radiation
When the monitor is placed close to a solid boundary
(for instance a wall), the sound energy that should have
been radiated in the direction of the wall is radiated into
the free half space instead. Hence the sound pressure
is
doubled in the half space, which 6 dB. This is
also called a 2 π radiation .
Fig. 71: 2 π radiation
When the monitor is placed against two boundaries (for
instance in a corner limited by two walls), it is now ra-
diating its energy to the quarter space. Accordingly, the
sound pressure is doubled twice, which 12 dB.
This is also called a
π radiation .
Fig. 72: π radiation
When the monitor is placed against three boundaries
(for instance in a corner limited by the floor and two
walls), the sound is radiated into 1/8 of the space.
Compared to free space, the sound pressure now is
increased by 18 dB. This is also called
π/2 radiation .
In practice, placing a monitor close to boundaries will
influence the frequency range below 125 to 150 Hz.
Fig. 73: π/2 radiation