signal it should (shown in black) but it
must also produce a signal that
counter-acts the ringing in the room
mode. This is shown in the lower plot
in blue. As can be seen there (most
easily in the ringing after the signal
has stopped), the loudspeaker’s
compensation signal (the blue curve) is
the mirror image of the room’s
“misbehaviour” (in red). If you add
these two curves together, the result is
that they cancel each other out, and
the result is the black curve.
If you would like to calculate a
prediction of where you’ll have a
problem with a room mode, you can
use the following equation:
•
metric version:
frequency = 172 / (length in m)
•
imperial version:
frequency = 558 / (length in feet)
This calculation will produce the
fundamental frequency of the room
mode in Hz for the dimension of the
room represented by “length”. Your
most audible modal problems will be at
the frequencies calculated using either
of the equations above, and multiples
of them (e.g. 2 times the result, 3
times the result, and so on).
So, for example, if your room is 5 m
wide, your worst-case modes (for the
room’s width) will be at 172 / 5 = 34.4
Hz, as well as 68.8 Hz, 103.2 Hz and so
on. Remember that these are just
predictions – but they’ll come pretty
close. You should also remember that
this assumes that you have completely
immovable walls and no absorption – if
this is not true, then the severity of the
actual problem will vary accordingly.
Sadly, there is not much you can do
about room modes. There are ways to
manage them, including, but not
exclusive to the following strategies:
•
make sure that the three
dimensions of your listening
room are not related to each
other with simple ratios
•
put up membrane absorbers or
slot absorbers that are tuned to
the modal frequencies
•
place your loudspeaker in a node
– a location in a room where it
does not couple to a problematic
mode (however, note that one
mode’s node is another mode’s
antinode)
•
sit in a node – a location in a
room where you do not couple to
a problematic mode (see warning
above)
•
use room correction DSP software
such as ARC in the BeoLab 90
16.3
Reverberation
Reverberation is what you hear when
you clap your hands in a big cathedral.
It’s the collection of a lot of reflections
bouncing from everywhere as you go
through time. When you first clap your
hands, you get a couple of reflections
that come in separated enough in time
that they get their own label – “early
reflections”. After that, there are so
many reflections coming from so many
directions, and so densely packed
together in time, that we can’t
separate them, so we just call them
“reverberation” or “reverb” (although
you’ll often hear people call it “echo”
which is the wrong word to use for this.
Reverb is what you get when you have
a lot of reflective surfaces in your room
– but since it’s so irregular in time and
space, it just makes a wash of sound
rather than a weird comb-filter effect
like we saw with a single reflection. So,
although it makes things “cloudy” – it’s
more like having a fog on your glasses
instead of a scratch, or a soft-focus
effect on a kitschy photograph of a
field of flowers.
16.4
Solutions
As we’ve seen, if your listening room is
normal, you have at least these three
basic acoustic problems to deal with.
Each problem has a different solution...
The first solution has already been
started for you. As is explained in the
section on
, the final
tuning of every Bang & Olufsen
loudspeaker (including the BeoLab 90)
is voiced in at least four rooms with
very different acoustical behaviours
ranging from a very “dead” living room
with lots of absorptive and diffusive
surfaces to a larger and very “live”
space with a minimalistic decorating,
and large flat surfaces. Once we have
a single sound design that is based on
the common elements those rooms,
we test the loudspeakers in more
rooms to ensure that they’ll behave
well under all conditions.
The second solution is BeoLab 90’s
Active Room Compensation which will
correct the effects of boundaries
(walls) and room modes on the timbre
of the loudspeaker at the listening
position(s). Using measurements of the
characteristics of the loudspeaker at
the listening positions, the ARC
algorithm then creates a filter that is
used to “undo” these effects. For
example, if the loudspeaker is close to
a wall (which will generally result in a
boosted bass) then the filter will
reduce the bass symmetrically.
Similarly, ringing caused by room
modes will be actively cancelled by
both BeoLab 90’s. That way, the loss in
the filter and the gain due to the room
will cancel each other.
The third solution is unique to the
BeoLab 90 – Beam Width Control. This
allows you to customise the relative
levels of the direct sound and the
reflected sound at the listening
position. The result of this is that, even
if you have acoustically reflective side
walls, the BeoLab 90 can still deliver
an accurate and precise representation
of the spatial presentation of your
stereo recordings.
16.5
Conclusions
Of course, this section does not cover
everything there is to know about room
acoustics. And, of course, you can’t
expect a loudspeaker to sound exactly
the same in every room. If that were
true, there would be no such thing as a
55
