Features
9.1
Resonance-based Sound
Design
A very large part of the sound tuning of
the BeoLab 90, like many other Bang &
Olufsen loudspeakers, is based on
acoustical measurements performed at
many locations around, above and
below the loudspeaker.
Figure 9.1: The first BeoLab 90 pro-
totype undergoing acoustical measure-
ments in The Cube.
One of the important aspects of these
measurements is to find the behaviour
of the loudspeaker in time. For
example, if a sound is sent to the
loudspeaker, and then stopped
suddenly, does the loudspeaker also
stop, or does it “ring” at some
frequencies (in exactly the same way
that a bell rings when struck). Ringing
in the time response of a loudspeaker
is an indication that it has a resonance
– a frequency at which it “wants” to
move. This resonance has a
detrimental effect on the overall sound
of the loudspeaker, since it “smears”
sounds in time.
For example, if you have a loudspeaker
that has a natural resonance at 110 Hz
(two octaves below a “Concert A”, to
musicians) then it will naturally ring at
that note when it is “hit” with an
impulsive signal such as a kick drum. If
the song that the band is playing is not
in A (major or minor), but in B-Flat
instead, then there will be a
dissonance between the notes played
most often in the song, and the note
that is “singing along” with the kick
drum. This can contribute to the
loudspeaker sounding “muddy” (to use
only one word...).
This is why the measurement-based
portion of the filtering of all current
Bang & Olufsen loudspeakers is
primarily designed to counteract the
natural resonances in the system. So,
for example, if one of the woofers in
the BeoLab 90 has a natural resonance
at 110 Hz, then that resonance is
mirrored with an equal, but opposite
phase behaviour in the Digital Signal
Processing engine. The total result of
the filter in the DSP and the behaviour
of the woofer is that there is no
unwanted ringing in the entire system.
This, in turn, means that the
loudspeaker’s response is controlled
not only in the frequency domain but in
the time domain as well.
This is only possible with an extensive
set of measurements of each
loudspeaker driver’s mechanical and
acoustical behaviour and a
custom-created set of filters for it.
9.2
Phase-Optimised Filtering
Like all audio devices, in order for the
BeoLab 90 to deliver its level of sound
performance, filters are used in the
Digital Signal Processing (DSP).
Generally, an audio filter is a device
that changes the overall response of
the the audio signal. In the case of
BeoLab 90, these are used for various
reasons such as controlling the
relationship between the different
loudspeaker drivers, acting as
crossovers to distribute the correct
signals to the tweeters, midranges and
woofers, and optimising the overall
magnitude response of the total
system.
An audio filter has an effect on the
behaviour of the signal’s magnitude
(how loud it is at a given frequency)
and/or its phase (a type of measure of
the amount of time it takes a given
frequency to get through the filter).
Since the BeoLab 90 uses digital
instead of analogue filters, we are able
to choose the characteristics of each
filter’s phase response independently
of its magnitude response. For
example, a filter can be implemented
to have a “minimum phase” or a
“linear phase” (the two most common
responses) characteristic, regardless of
the magnitude response it is required
to deliver.
The phase response of each filter in
BeoLab 90’s processing chain have
been individually tailored according to
its particular function. For example,
some of the crossover filters have been
implemented as linear phase filters.
Most filters in the Active Room
Compensation algorithm are
implemented as minimum phase filters
(since room resonances have a
minimum phase characteristic). The
Beam Width Control filters have
customised phase responses that are
dependent on the particular
frequency-dependendent
characteristics of the individual
loudspeaker drivers that they control
and are therefore neither minimum
phase nor linear phase.
9.3
Automatic Bass
Linearisation (ABL) and
Thermal Protection
Almost all loudspeakers in the Bang &
Olufsen portfolio (including BeoLab 90)
feature Automatic Bass Linearisation or
“ABL”. This is an algorithm that was
patented by B&O in 1991 and is
custom-tuned for each of our products.
Its purpose is to ensure that, when the
physical limits of a component of the
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