A critical factor in the bass response
of a speaker in a room is the distance of nearby walls (or boundaries) from the
speaker. The wall behind the speaker reflects the omnidirectional low frequency
sound energy back towards the listening position.
When the total distance travelled by this reflected sound is half a wavelength
it destructively interferes with the direct sound causing a notch to appear in
the bass region of the sapeaker. This notch will be typically about
2/3rd octave wide and cause a significant decrease of the total bass
energy in the room. You must check that the speaker is not at a distance from
the wall behind it that will cause this cancellation notch to appear in the
frequency range that the speaker can reproduce. For example, if a speaker is
placed so that the front of it is 86cm from the wall behind it the cancellation
frequency will be approximately 100Hz.
There are two different ways to solve this problem. It will depend on the room
as to which one is most suitable and can be used:
The speaker should be moved as close to the wall behind it as possible so
that cancellation frequency is very high. At these higher frequencies the
speaker has far less omni-directional energy so the cancellation effects are
greatly reduced. One side effect of this method is that the bass tilt should be
set correctly to compensate for the additional loading presented to the bass
driver by the wall. One final tweak to minimise any remaining cancellation
effect is to pack any remaining space between the speaker and the wall with
absorbing material such as rock wool.
The speaker should be moved further away from the wall behind it so that the
cancellation frequency is well below the low cut off frequency of the speaker.
In practice this is not always possible, as the room may not be large enough
especially for speakers with a deep low cut-off frequency.
Reflections of other boundaries
Other sources of bass
cancellation are from reflections off the floor, ceiling, side walls and
rear wall. The cancellation is still based on half wavelength cancellation of
the path difference but, as the sound does not travel back on itself, a slightly
different formula is required. In fact this is the general equation for
cancellation frequencies and can also be use for desk reflections that occur in
the mid frequency range.
Solution
There are some different ways to
solve these reflection problems
Change the room shape so that the reflections are not directed towards the
listening position. This is not always possible for physical and financial
reasons.
Ensure that the wall behind the listening position (rear wall) is more that
3m away from the listening position to avoid low frequency cancellation
problems. This is often a problem in rooms that are less than 5m in length.
Add absorbing materials to reduce the level of the reflected sound. Although
generally a cheaper solution this has the disadvantage that the reverberation
time, T60, can be adversely affected.
The wall behind the speaker
