Some Extra Information - Bang & Olufsen BeoLab 90 Technical Sound Manual

make a filter that "undoes" the e ect
of temperature on the loudspeaker's
response. For example, if the woofer's
voice coil gets 160 C above room
temperature (where we originally
measured it), it drops 3.2 dB at 20 Hz,
the BeoLab 90 knows this and adds 3.2
dB at 20 Hz. In order to do this, the
processing of the BeoLab 90 includes a
set of filters (one for each driver)
whose response varies in time with the
temperatures of the the drivers. The
temperature-dependent filters for the
front woofer are shown in Figure 20.5.
4.5
4
+ 180º C
3.5
+ 160º C
+ 140º C
3
+ 120º C
2.5
+ 100º C
2
+ 80º C
1.5
+ 60º C
1 + 40º C
+ 20º C
0.5
0 + 0º C
−0.5
10 100
Frequency (Hz)
Figure 20.5: Magnitude responses of
the compensating filter for BeoLab 90's
front woofer vs. the temperature of its
voice coil.
It's important to note three things here.
This can only be done because
we know how the response of the
woofer changes at di erent
temperatures (this behaviour
was found as part of the
development process).
This can be done because the
loudspeaker "brain" (the DSP)
knows the temperature of the
voice coil in real time as you're
playing music
This particular filter shown in
Figure 20.5 should only be
applied to the appropriate
loudspeaker driver. The other
woofers and the other drivers
have di erent behaviours and
should be processed with their
own correction curves. In other
words, this filtering can only be
done because the BeoLab 90 is
an active loudspeaker with
independent filtering for each of
the 18 loudspeaker drivers.

20.5 Some extra information

You should be left with at least one
question. I said above that, as the
music gets loud, the woofer heats up,
so you lose output, so we add a filter
that compensates by putting more
signal into the driver. However, this
means that the problem is caused by
1,000
the signal being too loud, and the
result is that we make the signal
louder.
However, there is one more trick up
our sleeve. Appendix 6: ABL - Adaptive
Bass Linearisation
90's Thermal Protection algorithm. This
means that the DSP brain knows the
temperature of the drivers and, in a
worst-case situation, turns the levels
down to protect things from burning
up. So, if we go back to the example of
a BeoLab 90 playing at full volume,
let's see what's happening to the
signal levels.
describes BeoLab
64
Figure 20.6: The gains (in dB) applied to
the signals sent to the drivers in a Beo-
Lab 5 as a result of playing pop music
at full volume. The X-axis is the time in
minutes.
These curves in Figure 20.6
gains applied to the front woofer in a
BeoLab 90 at the same time as the
measurements shown in Figures
and 20.3 were being made. In fact, if
you look carefully at Figure
around the 5 minute mark, you'll see
that the temperature dropped – which
is why the gain in Figure 20.6
(because it can!) in response.
Now, don't panic. The BeoLab 90 isn't
messing about with the gains of the
drivers all the time. Remember that
this example was done at full volume –
which, for a BeoLab 90 is extremely
loud. The gains shown in Figure
are a "last-ditch" e ort of the
loudspeaker to protect itself from a
very mean customer (or the very mean
children of a customer who is away for
the weekend). This is the equivalent of
the airbags deploying in your car. You
can guess that, if the airbag is outside
the steering wheel something
significant has occurred.
Many thanks to Gert Munch for his help
in writing this section.
show the
20.2
20.2
increases
20.6