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How can these low frequencies be controlled? There is one traditional but effective way of controlling low
frequency energy and that is to build what is called a "¼ wavelength bass trap." This bass trap is built
over an area greater than 66% of a reflecting surface (typically the hard back wall). The idea is to convert
the acoustic air movement (velocity) into heat energy and hence absorb the sound. This process is most
efficient when the absorbing material (typically rock-wool or mineral wool) is positioned where the air move-
ment is largest; unfortunately, this is often in the middle of the room. However, placing the absorbing mate-
rial on the wall allows the sound wave travelling towards the wall to be absorbed but ALSO the remaining
sound that bounces off the hard wall travels back though the absorbing material before re-entering the
room at a considerably lower (and hopefully insignificant) level. In a good case, more than 95% (>25 dB)
of the sound should be absorbed.
Unfortunately, there is a trade-off. When the absorbing material is placed against the wall the depth of
the material defines how low a frequency is trapped. To be effective at low frequencies the absorber
material needs to be thick... very thick! For example, if the absorber needs to trap down to 50 Hz then
the thickness should be ¼ of 6.88 m (25'8"), which is 1.72 m (6'5"). The bass trap's effectiveness will be
less at lower frequencies than this.
Clearly, this can be a problem in many large Home Theaters, as this is equivalent to about one row of
seating. In smaller rooms, this loss of space is probably impossible to consider. Panel resonators can
save space and still have attenuation properties down to quite low frequencies but they are more difficult
to design and construct. Alternatively, corner traps can control a reasonable amount of low frequency
energy and take up less useful internal room space. Further information on these acoustic treatment
techniques is available in textbooks and good acoustic design firms should have little problem working
out good solutions for problematic spaces.
5.1.2
Mid and High Frequency Treatment
Unlike low frequencies, the midrange and high frequencies are far easier to control because the wave-
length is so much shorter; 1 kHz is approximately 30 cm (1') and 10 kHz about 3 cm (1"). It is very easy
to add back into an over damped room some midrange and high frequency reflections required for natu-
ral sounding acoustics by using diffusers (even picture frames and decorative thin wood panels can be
effective if used cleverly).
The reverberation time (RT) gives and indication of how well controlled the room is at various frequen-
cies. A good sounding Home Theater should have a flat RT of about 0.3 s to 0.4 s, however, it may drop
away slightly at higher frequencies and rise up at lower frequencies. Below are examples of a good
quality room and a bad quality rooms.
•
The room with the RT shown in Graph 2 has a very tight and punchy bass but the mid and
high frequencies also sound natural and spacious. This was entirely due to efforts to control
the low frequency reverberation time using panel absorbers on the ceiling and 1.4 m of
damping material at the rear of the room!
•
In the room with the red curve in Graph 3 the sound was very boomy even though the
frequency response of the loudspeakers was very flat. This case is hard to solve, as thick
damping material needs to be added to control the low frequencies.
•
In the room with the blue curve in Graph 3 the sound was very bright and so the treble
tilt control had to be turned down considerably to compensate. This case is easy to solve
by adding some absorbing material, such as heavy curtains, around the room to control the
high frequency reflections.
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