Loudspeaker Response Changes; The Solution - Bang & Olufsen BeoLab 90 Technical Sound Manual

Figure 20.2: The temperatures (in C)
of the voice coils of the drivers in a Be-
oLab 5 as a result of playing pop music
at full volume. The X-axis is the time in
minutes.
As you can see in Figure 20.2, while
playing music, the woofer varied from
a maximum temperature of about
200 C down to about 110 C.
This means that the worst-case
variation in temperature of the woofer
was about 90 C whilst playing music,
and peaked at about 180 C above
room temperature (which we'll assume
is 20 C).
Unfortunately, this temperature cannot
be measured directly, since we cannot
put thermal sensors directly on the
drivers' voice coils. Instead, we
measure the temperature of the
loudspeaker driver magnets, and use
that real-time data input in addition to
the signal that we're sending to the
drivers to calculate the temperatures
of the voice coils based on thermal
models of each of them. As you can
see in Figure 20.3, the magnet
temperature reacts much more slowly.
These measurements were taken at
exactly the same time as the ones
shown in Figure 20.2.
Figure 20.3: The temperatures of the
magnets of the loudspeaker drivers in a
BeoLab 5 as a result of playing pop mu-
sic at full volume. The X-axis is the time
in minutes.
20.3 Loudspeaker response
changes
So, now the question is "what does this
change in temperature do to the
response of the loudspeaker driver?".
As I mentioned above, the thing that
changes most in the model shown in
Figure 20.1 is the loudspeaker driver's
voice coil resistance. For those of you
with a background in reading electrical
circuits, you may notice that the one
shown in Figure 20.1 has some reactive
components in it which will result in a
resonance at some frequency. For
those of you without a background in
reading electrical circuits, what this
means is that a loudspeaker driver (like
a woofer) has some frequency at which
it "wants" to ring – if you thump it with
your thumb, that's the note that you
will hear ringing – a little like a bell with
a low pitch.
As the voice coil resistance goes up, its
resistance increases, and we generally
lose sensitivity (i.e. level or loudness)
from the woofer. In other words, the
hotter it gets, the quieter it gets.
However, this only happens at the
frequencies where the resistor is not
"overridden" by another component –
say the mechanical resonance of the
woofer or the inductance of the voice
coil.
The total result is shown for various
temperature di erences in Figure 20.4.
Notice that these plots show the
change in magnitude response of the
63
driver with changes in temperature.
So, the curve at the top is the change
in the woofer's magnitude response
(which is 0 dB at all frequencies – in
other words no change) when the
loudspeaker is playing at the same
temperature at which it was measured
(let's say, 20 C or room temperature).
As the temperature of the voice coil
increases above that temperature, you
can see that you lose output in two
frequency bands on either side of a
"bump" in the response – that bump is
at the resonant frequency of the
loudspeaker driver.
So, the louder you play, the more low
end you lose, apart from a peak in the
response (which also rings in time) at
the resonant frequency of the driver.
0.5
0
+ 0º C
−0.5
+ 20º C
−1
+ 40º C
+ 60º C
−1.5
+ 80º C
−2
+ 100º C
−2.5
+ 120º C
−3
+ 140º C
−3.5 + 160º C
+ 180º C
−4
−4.5
10 100
Frequency (Hz)
Figure 20.4: Sensitivity of BeoLab 90's
front woofer vs. the change in temper-
ature of its voice coil.

20.4 The solution

Interestingly, everything I said above is
true for every moving coil loudspeaker.
So, if you're the kind of person who
believes that the only "proper"
loudspeaker is one where you have
nothing but a loudspeaker driver (in a
cabinet of any kind, or not) and an
amplifier – and no active filtering, then
you'll have to live with the kind of
unpredictable behaviour that you see
above. However, since a BeoLab 90
"knows" the temperature of the voice
coil of its loudspeaker drivers, and
since it has been programmed with the
curves like the one shown in Figure
20.4, we can actively linearise its
response, making it much more
predictable.
In essence all we need to do is to take
Figure 20.4, flip it upside down and
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