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Reflections
Practically all rooms suffer somehow from interference between the direct and reflected sound.
The first floor reflection is usually present, especially for the center loudspeaker when mounted
below the screen. Attempts to absorb the reflected sound are usually not very successful, espe-
cially for the floor reflection. One should note that even with an absorption coefficient of 0.9 the
reflected wave is only 10 dB below the direct sound. To get 90 % absorption at 80 Hz about 1 m
of porous absorbent is needed. To properly deal with boundary reflections, reflective surfaces
should be minimised and some absorption should be placed near the loudspeakers. Also, by
using Genelec controlled directivity loudspeakers, the ratio of direct versus reflected sound is
significantly increased.
Wall Behind the Loudspeaker Cancellations
Another boundary interference to consider is the so called 'rear wall' cancellation effect, gener-
ated by the single reflection from the wall behind the loudspeaker. When two identical signals are
in anti-phase, they cancel each other. If the loudspeaker is a quarter wavelength away from a rear
reflective wall, the reflected wave will come back to the loudspeaker with half a cycle phase dif-
ference and thus cancel the original signal at that frequency. The importance of the cancellation
depends on the distance and the reflection coefficient of the wall, but it is usually very audible. For
a bass-managed Home Theater set-up using an 80 Hz crossover between main loudspeaker and
subwoofer here is a set of practical placement solutions.
First, the distance between the radiating subwoofer driver and the wall behind the subwoofer
providing part of the low frequency loading must not exceed a conservative 90 cm (35"). If the
subwoofer is placed further than that, cancellation notches will degrade the subwoofer response.
Secondly, there are three practical alternatives for the main loudspeakers (reproducing frequencies
above 80 Hz in our example):
•
First, the loudspeakers could be flush mounted in a hard wall (or 'infinite baffle') eliminating
the rear wall reflection problem.
•
Second best is placing the loudspeaker very close to the wall, which raises the cancellation
frequency to a value where the loudspeaker is more directional and so the cancellation
becomes inaudible. In most cases a distance around 20 cm (8") between the front baffle and
the wall behind can be considered as appropriate.
•
The third logical cure is to move the loudspeaker away from the wall, and in our example this
means at least over 1.1 m (3'7"). In doing so, the cancellation frequency drops down below
the 80 Hz cut-off of each main loudspeaker.
5.3
Home Theater Performance - The Right Combination of Products
The final performance of a quality Home Theater is intrinsically linked to the equipment selection, the
room construction, the entire system integration and the project planning and management. The aim of
this chapter is to provide brief guidelines concerning the basic concept behind audio equipment selection
so that the best possible result is achieved.
First, the best audio equipment will only reach its optimum performance if properly installed in a quality
room. This means that the first consideration when building or installing a Home Theater is the room
acoustics. This is the foundation, but often the most neglected part of the project. Then, in the audio
equipment chain, cabling and source equipment will certainly play an important role in the final sound
quality, but the most noticeable changes will come from the loudspeaker performance. Of course, the
product selection should fit each room. If the audio is undersized then the whole Home Theater expe-
rience will be spoiled. Surprisingly, a screen that is slightly too small combined with a fantastic audio
system will feel much more comfortable and impressive than vice-versa.
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