What is the Doppler-Effect and why
is it so disturbing for a good
Each moving loudspeaker membrane generates
because of the varying
distance to the listener a phase modulation a.k.a. Doppler-Effect.
The generated frequency components are non-harmonic and very
disturbing to a good listening experience. Silbersand offers an
analogue signal processor, which eliminates the phase modulation of
the moving loudspeaker membrane completely. This analogue signal
processor is combined with a motion feed back system for the
loudspeaker which removes the non-linearity of the mechanical system
and allows the lower corner frequency to be selected at will, e.g. at
What is the Doppler-Effect:
During a recording session high quality
condenser microphones are
used. They convert the sound pressure by tiny movements of their
membranes into voltages, which are then further amplified and mixed
down. The amplitude of the movement of the microphone membrane is in
the order of 1 µm. One can say that the sound pressure is
at exactly one point in the sound field.
During the reproduction of the sound the
loudspeaker membrane needs
to move significantly to reproduce especially deep notes with
sufficient sound pressure. The amplitude of that movement lies in the
order of 5 to 10 mm.
What happens to the sound, when it is
reproduced at different
positions by the speaker membrane contrarily to the recording
Let us do a mental experiment: A deep note is
reproduced with an
amplitude of the loudspeaker membrane of e.g. 10mm. The membrane
moves back and forth by that amplitude and changes the distance to
the listener periodically by ± 10mm.
We add now a high note to the sound. It becomes
that the high note is reproduced periodically at different distances
to the listener by the moving speaker membrane. That creates the
well-known effect, that C. Doppler, an Austrian scientist described:
When the speaker membrane moves towards the listener, the high note
changes its original frequency to a higher frequency and when the
speaker membrane moves back from the listener, the frequency of the
high note gets lower than the original frequency. Everybody has made
this experience when a police car with its siren is approaching and
leaving – the siren frequency changes heavily from a high
tone to a
The above-described effect is not only there
for the high note, but
for each frequency, which a moving loudspeaker membrane reproduces.
In fact this is a Phase Modulation created by the different distances
at which the sound is reproduced with respect to the listener. The
figure below illustrates that effect again.
Why is the Doppler-Effect so
disturbing for a good listening
Each loudspeaker shows a certain non-linearity.
mixes all frequency components, which are reproduced, and creates in
this way alias frequencies. Well known are the harmonic distortions,
which are often played down with the argument, that each music
instrument shows a certain harmonic frequency content already. Less
well known, but simultaneously created, are the intermodulation
products, frequency component that are non-harmonic and therefore
significantly disturbing. The non-linearity of a loudspeaker can very
well and significantly be reduced by a motion feedback (MFB) systems,
a technology that Silbersand offers for High-End users. The movement
of the membrane is exactly measured, compared whit the driving signal
and continuously corrected. That motion feedback system works at the
speed of light and reduces all non-linearity-caused distortions to an
However, there remains the phase modulation
created by the movement
of the loudspeaker membrane, which is a well audible distortion. This
is a physical effect, which would be created even by an ideal
The Doppler-Effect creates spectral lines
(frequencies) which are
e.g. the sum and the difference of the different frequencies
contained in the sound. Mathematical this is described by the Bessel
functions. These newly created frequency components are non- harmonic
and therefore well audible. This answers the question asked in the
The acoustic spectrum below shows the Doppler
effect, when a 30 Hz
tone with 80% amplitude and a 750 Hz tone with 20% amplitude
reproduced by a loudspeaker with a total membrane amplitude of 2 mm.
30 Hz tone is adjusted to -20 dB (top of the spectrum plot). The 750
Hz tone is at -12 dB corresponding to the 20/80 ratio. Next to the
750 Hz tone the Doppler lines at frequency distances of – 30
+ 30 Hz are well visible. They are in this example 37 dB below the
amplitude of the 750 Hz tone and disturb significantly a good
How to eliminate the Doppler-Effect?
There are two ways to get rid of the
uses a very large area loudspeaker membrane with an amplitude equal to
the recording amplitude – i.e. 1 µm. This is
obviously not very practicable.
calculates continuously and precisely the position of the membrane at
any point in time and compensates the phase modulation created by the
varying distance to the listener by an inverse phase modulation of the
electrical signal feeding the loudspeaker. That is a practical
solution, which can be done completely in the analogue signal domain.
The second acoustic spectrum below demonstrates
this compensation of
The two Doppler-lines at 750 Hz ± 30
Hz are completely gone. The
Doppler-compensation works perfectly.
both acoustic spectra there are the same harmonic spectral lines of
the 30 Hz tone with 2 mm Membrane amplitude: K2
at -45 dB and k3
at -55 dB. This extremely high linearity of the loudspeaker is
enabled by the applied motion feed back (MFB) technology.
Why has this Doppler compensation never be done
in the past – even
by the most well-known manufacturers of Hi-End loudspeakers?
The answer to the question lies in the fact,
that only motion feed
back speakers achieve a linearity even at very low frequencies, which
do not mask the Doppler spectral lines. Even High-End speakers of
well-known manufacturers show non-linear distortion in the 50% range
when low frequencies are to be reproduced at sufficient sound
pressure level. Those distortions would mask the Doppler lines. A top
performance speaker needs both, the motion feed back as well as the
Doppler compensation. The resulting product is then unique on the