Appendix 6: ABL - Adaptive Bass Linearisation
19.1
A General Introduction
to ABL
Almost all loudspeakers made by Bang
& Olufsen include Adaptive Bass
Linearisation or ABL. This includes not
only our “stand alone” loudspeakers
(the BeoLab series) but also our
smaller “BeoSound” loudspeakers and
televisions. The only exceptions in the
current portfolio are our passive
loudspeakers, headphones, and the
BeoLab 5.
There is no one technical definition for
ABL, since it is in continual evolution –
in fact it may change from product to
product as we learn more and as
different products require different
algorithms. Speaking very broadly,
however, we could say that it reduces
the low frequency content sent to the
loudspeaker driver(s) (e.g. the woofer)
when the loudspeaker is asked to play
loudly – but even this is partially
inaccurate.
It is important to note that it is
not
the
case that this replaces a “loudness
function” which may (or may not) be
equalising for Equal Loudness Contours
(sometimes called “Fletcher-Munson
Curves”). However, since (generally)
the bass is pulled back when things get
loud, it is easy to assume this to be
true.
When we are doing the sound design
for a loudspeaker (which is based both
on measurements and listening), we
ensure that we are operating at a
listening level that is well within the
“linear” behaviour of the loudspeaker
and its components. (Typically, the
sound design is done at a standard
playback level where a -20 dB FS
full-band pink noise produces 70 dB (C)
at the listening position.)
This means that
•
the drivers (usually the woofers)
aren’t being asked to move too
far (in and out)
•
the amplifier is operating within
its limits
•
the power supply is operating
within its limits, and
•
nothing (not the power supply,
the amplifiers, nor the voice
coils) is getting so hot that the
loudspeaker’s behaviour is
altered.
This is what is meant by “linear” — it’s
fancy word for “predictable”. In
addition, it should be stated that if we
were listening to loudspeakers at high
levels daily, we would get increasingly
bad at our jobs due to hearing loss.
So, we do the tuning at a listening level
where we know things are behaving –
remember that we always do it at the
same calibrated level every time for
every loudspeaker so that we don’t
change sound design balance due to
shifts associated with equal loudness
contours. (If you tune a loudspeaker
when it’s playing loudly, you’ll wind up
with a loudspeaker with less bass than
if you tuned it quietly. This is because
you’re automatically compensating for
differences in your own hearing at
different listening levels.)
After that tuning is done, then we go
back to the measurements to see
where things will misbehave. For
example, in order to compensate for
the relatively small cabinet behind the
woofer(s) in the BeoSound 8 / BeoPlay
A8, we increase the amount of bass
that we send to the amplifiers for the
woofers as part of the sound design. If
we left that bass boost in the tuning
when you turn up the volume, the
loudspeaker would go up in smoke – or
at least sound very bad. This could be
because:
•
the woofer is being pushed or
pulled beyond its limits, or
•
because the amplifier clips or
•
the power supply cannot supply
more current or
•
something else.
So, after the tuning process is
complete, we put the loudspeaker in a
small torture chamber roughly the size
of a clothes closet, put on some dance
music (or modified synthetic test
signals) and turn up the volume. While
that’s playing, we’re continually
monitoring the signal that we’re
sending to the loudspeaker, the driver
excursion, the demands on the
electronics (e.g. the amp’s, DAC’s,
power supply, etc.) and the
temperature of various components in
the loudspeaker, along with a number
of other parameters.
Armed with that information, we are
able to “know” how those parameters
behave with respect to the
characteristics of the music that is
being played (e.g. how loud it is, in
various frequency bands, for how long,
in both the short term and the long
term). This means that, when you play
music on the loudspeaker, it “knows”:
•
how hot it is at various locations
inside,
•
the loudspeaker drivers’
excursions,
•
amplifier demands,
•
power supply demands,
•
and so on. (The actual list varies
according to product – these are
just some typical examples...)
So, when some parameter gets close
to a maximum (e.g. the amplifier starts
to get too hot, or the woofer is nearing
maximum allowable excursion) then
something
will be pulled back.
What
is pulled back? It depends on the
product and the conditions at the time
you’re playing the music. It could be a
band of frequencies in the bass region,
it could be the level of the woofer. In a
worst-case-last-ditch situation, the
loudspeaker might even be required to
shut itself down to protect itself from
you (or the guests attending your
party). Of course, there is no
guarantee that you cannot destroy the
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