Ad 4.
If the rear and side walls (behind or at the side of the speaker) are not indefinitely far away (over 5m), their effect
must be considered. The physical basis of these considerations is wavelength. If 2 waves of the same wavelength
are added with the same phase, the result will be louder by 6 dB; if they meet with reversed phases (phase offset
180°), they will be cancelled out completely.
Here, close limitation surfaces are added to the equation. Side and rear walls form the perfect reflection surface
for low sounds with wavelengths of 10m to 3m. If our bass chassis radiates sound spherically – all bass chassis
radiate spherically, no matter if they are installed in the speaker at the rear, side or front –, the wave reaches the
ear right from the chassis. At the same time, this wave is also radiated against the walls and reflected to the ear
from there. These waves kind of take a detour via the wall to reach the ear "around the corner". This detour causes
a phase displacement in the wave.
An example: A 3m long wave is
a.) radiated directly to the ear and
b.) reflected by the rear wall before reaching the ear. The distance between the speaker and the rear wall is 0.75m
Now the two waves add up in the listener's ear: the direct and the "detour" wave with a detour of 1.5m. ( Detour:
Speaker to rear wall = 0.75m + rear wall to speaker = 0.75m, together 1.5m) 1.5m is exactly half the wavelength,
cancelling the wave directly radiated as shown in the illustration: Of course this does not cause complete cancelling,
since the wave reflected from the rear wall is already dampened and many other reflections weaken this cancelling
effect. This tone is nevertheless lower in the listening place than adjacent ones. The effect grows even more extreme
if not only the rear wall, but also the side wall is at a distance of 0.75m, and even more so if the same situation
is true for both speakers. This leads to a very simple recommendation: The distances from the speakers to the rear
and side wall should never bet he same, and the distance of the left speaker to the left side wall and the right
speaker to the right side wall should also be different.
Low and high shelving
As we can see here, the energy reflected from the side walls plays a role after all. If the speakers are placed freely
in the room, this is the "neutral position" where all filter controllers should be in the "neutral – 12 o'clock position".
Setup close to a wall (distance wall-speaker less than 2m) increases bass energy. This can be reduced using the
low shelving controller. A corner setup increases this effect, and low shelving can be used more strongly. Accordingly,
our speakers' high tone energy radiated can be adjusted to the spatial situation as well.
Ad 5.
Finally, we would like to say something about mode formation in the listening space. Modes are standing waves
appearing because the listening space forms too narrow a cage for the radiated wavelengths (low tones). Generally,
there is nothing to be done about this. One can only attempt to place this mode formation so that the unpleasant
results are not extreme in the listening position. We do not need to care whether or not there is a considerable
bass overshoot at the rear wall of the room, i.e. if the bass sounds too loud there. After all, we are not listening
from there. A good mode distribution in the room is achieved if the room modes are not generated at a single
position. Therefore, we offer many speakers in which the bass membranes are installed at different heights. This
generates modes in a more chaotic state and not as strongly. The advice in item 4 on the distance from rear and
side walls also has a beneficial influence of mode distribution and intensity. One or several subwoofers may
considerably decrease the problem of stationary modes at the listening position.
KSdigital
Referencemonitor
