Appendix 7: Thermal Compression Compensation; Introduction; Voice Coil Temperature - Bang & Olufsen BeoLab 90 Technical Sound Manual

Appendix 7: Thermal Compression Compensation

20.1 Introduction

Take a woofer and put it in an
appropriately-sized cabinet, connect it
to an amplifier. Set the room
temperature to 20 C. When everything
in the room is the same temperature,
measure the woofer's on-axis
magnitude response.
Turn up the room temperature to
100 C. When everything in the room is
the same temperature again, measure
the woofer's magnitude response once
more.
You will notice that these two
measurements look very di erent – but
why?
When you read a magazine review of a
loudspeaker, it will include a
measurement of its "frequency
response" (more accurately called its
"magnitude response") which shows
(ignoring many things) how loud
di erent frequencies are when they
come out of the loudspeaker assuming
that they all came in at the same level.
Unfortunately, this is only a small part
of the truth.
We can explain a loudspeaker driver's
electromechanical characteristics by
breaking it down into di erent
components (both actual and
analogical). For example, the
suspension (which is comprised of the
surround and the spider) can be
thought of as a spring. The electrical
analogy for this is a capacitor. If you
take all of the moving parts in the
loudspeaker driver, they all add up to a
mass that has to be moved – the
electrical analogy for that mass is an
inductor (since an inductor has some
electrical "inertia"). Some of the
components are not an electrical
analogy – they are real electrical
components. For example, the voice
coil, since it's a coil, acts as an
inductor. Since it is a thin bit of wire, it
also has some resistance to the flow of
electrical current through it, so it's also
a resistor. A simple version of this
breakdown is shown in Figure 20.1.
Voice Coil Voice Coil Energy Losses in
Resistance Inductance Suspension
Amplifier
Electrica Components
Figure 20.1: A simplified version of the
actual electrical and electrical analogies
of mechanical components in a loud-
speaker driver.
This shows the components of a
moving coil dynamic loudspeaker as a
very simplified "circuit" . If these
components don't look familiar to you,
don't worry, it's not that important for
now. Some components in the circuit
are actual electrical things and others
are analogies – electrical
representations for a mechanical
component in the system.
If you know how each of these
components behaves, and you know
the correct values to put in for a given
loudspeaker, and you know how to do
the right math, then you can come
close to getting a prediction of the
response of the loudspeaker that
you're modelling with the circuit.
However, if you just put in one value
for each component, then you're
assuming that they never change – in
other words that you're dealing with a
"linear" system.
The problem is that this assumption is
incorrect. For example, the voice coil
resistance – the amount that the wire
in the voice coil resists the flow of
current through it when the
loudspeaker driver is not moving –
changes with temperature. The hotter
the wire gets, the higher the resistance
goes. (This is a normal behaviour for
most resistors.) If the voice coil
resistance changes, then the whole
system behaves di erently, since it
isn't the only component in the circuit.
So, as we change the temperature of
the voice coil, the total response of the
loudspeaker changes.
Sadly, the temperature of the voice coil
isn't only dependent on the room
62
temperature as it seemed to be in the
beginning of this discussion. As soon
Inertia of Mass of Springiness of
Moving Parts Suspension
as you start playing sound using the
loudspeaker, it starts heating up. The
louder the signal, the hotter it gets. So
as you play music, it heats and cools.
Mechanical Components
The speed with which it heats up and
cools down is dependent on its
"thermal time constant" – a big woofer
with a large voice coil and magnet will
take longer to heat up and cool down
(and therefore have a longer thermal
time constant) than a small tweeter.
This raises at least four questions:

20.2 Voice coil temperature

A typical loudspeaker driver is, give or
take, about 1% e cient. This means
that approximately 1% of the power
you push into the loudspeaker from the
amplifier is converted into sound. The
remaining 99% is lost as heat – almost
all of it at the voice coil of the
loudspeaker. So, the louder your
music, the hotter your voice coil gets.
Of course, if you have a way of cooling
it (for example, by using other parts of
the loudspeaker as a radiator to your
listening room) then it won't get as
hot, and it will cool down faster.
For example, play pop music that has
been mastered at a high level and play
it at maximum volume on a BeoLab 90
whilst monitoring the temperature of
the voice coils. What you'll see if you
do this is something like the Figure
20.2.
How much does the temperature
vary when I play music?
How does the response of the
loudspeaker change with
temperature?
How much does the response of
the loudspeaker change with
temperature?
What can we do about it?