reviewing the current scene
of Bending Wave Loudspeakers (BWL), one sees mainly two distinct
implementations using flexurally (semi) rigid membranes - the cone type
and the planar. For the purpose of the following discussion, only
planar BWLs are considered. The attempt will be to present the planar
BWL as one capable of the highest fidelity possible, with the full
audible range handled by just one planar driver. Surely, it goes
without saying that the above can be achieved only if the design and
construction of the BWL is done capably and with a fuller understanding
of the theories behind it.
planars, by which audiophiles and reviewers set much store by, are the
electrostatic and the electromagnetic loudspeakers, depending on
whether it is the electrostatic forces that drive the film
diaphragm, or the electromagnetic. The main distinguishing
between such common 'area driven' planars and the BWL are two in the
main - on the one hand, how the driving force is applied to the
diaphragm membrane, and the structure of the membrane itself
the other hand.
widely believed among
many interested in audio, that driving the thin and stretched foil of a
conventional planar loudspeaker membrane over its whole area
at least along tightly mounted conductive lines in case of a planar
magnetic – will result in an ideal 'pistonic' kind of motion
where arbitrary points on the membrane move with same amplitude and
things worse, that
assumed - but not really existing - pure pistonic motion, is thought of
to be the optimal requirement for natural music reproduction - again a
widely held belief. Those commonly held fallacies, and sometimes
deliberately spread disinformation, have to be examined afresh for what
they are truly worth.
common-sense thinking to driving the thin membrane diaphragm. The
membrane itself is far from rigid, and applying force - even a
uniformly distributed one - over the area of a stretched thin structure
cannot induce a pistonic motion of ALL the points on the membrane with
the same amplitude; the situation is worse when you consider the edges
of the membrane that are securely, and immovably, anchored.
physical or analytical knowledge at hand, this is intuitively known to
everyone who has blown soap bubbles as a child. When blowing softly
against the film, it will bend with a maximum excursion in the middle,
and with reduced displacement towards the periphery, and none at the
edges where the film is attached to the ring.
typical motion of such a
"film stretched on a frame" structure in case of transient dynamic
excitation is similar that of a drumhead. This has several vibrational
modes, which are excited depending on the frequencies present in the
driving signal. These vibrational modes are typically distributed in an
intermittent and imbalanced manner, especially over the lower audible
frequency range. This is the cause for irregularities in frequency
response and dispersion.
But then a drum, and for that matter every drum, has its
'sound' and characteristic. What is good to give a certain 'snap' or
'colour' to the sound of a drum is precisely what is unwanted
a loudspeaker, which has to have a 'neutral' sound. The vibrational
modes occuring in a musical instrument give its characteristic colour,
while those in a speaker colour and thus degrade its sound output, and
thus its performance. Designers of conventional planar magnetic and
electrostatic speakers had to find ways to deal with those 'drumhead
modes' by damping and 'taming' the membrane diaphragm.
bewildering array of techniques
have been developed up to now to deal with the above situation. A
common method is to have membranes of planar or line source (long
ribbon) speakers fixed or damped at certain support points. Again it is
obvoius, that even with such techniques, there can be virtually no
pistonic motion of the membrane as a whole. Another ploy adopted is the
application of a cloth 'filter' with defined flow resistance behind the
membrane, or using the perforated stators themselves in an
electrostatic loudspeaker as a damper accordingly.
It may be
noted that in
typical ESL or magnetic planars, the lowest usable frequency (or the
low cut-off frequency) is determined by the lowest vibrational mode of
the membrane, where the center region has maximum ampitude and the
fixed edges have close to zero amplitude.
again of the soap film of a
bubble which started to emerge, but is relaxing and oscilating for a
short time after you have stopped blowing. That is the base
vibrational mode, which is present inevitably in all flat membrane
structures, be it a soap film, a trampoline, a drum head - or the
membrane of a planar loudspeaker. For higher frequencies there will
occur more complex modes, having more than one maximum of excursion.
These complex modes were investigated systematically for the first time
of a rectangular membrane is called Mode-1,1 since there is one maximum
of membrane excursion to be found in either direction, height and width
of the membrane. It should be made clear, that trapezoidal membranes
have similar vibrational modes too, just their exact frequencies being
somewhat more difficult to calculate.
decision to use
electrostatic force to drive a large area foil has been made, applying
that force over the whole area is not necessarily a decision for the
best acoustical solution. Merely driving the whole area at least for
the low frequencies in an electrostatic loudspeaker is the only way to
gain sufficent force with a given maximum voltage and membrane stator
spacing, which both are strongly restricted by practical and safety
limits. Likewise the only way to gain sufficient force in a planar
magnetic speaker is to have conductors of sufficent length, which are
located in the magnetic gap of the motor assembly.
other hand, a planar
BWL behaves in a manner far different from its electrostatic or
magnetic planar cousins. A bending wave planar transducer with either a
single actuator or multiple actuators can undergo what is called
"driving point optimization". Driving point optimization means, that a
membrane of sufficient size, which is capable of vibration is excited
at one ore more dedicated points using a spezialized and highly
efficient (dynamic) exciter in such a way that the distribution of
excited modes over frequency is optimally balanced. Well defined
accurately excitation of a membrane can be turned into a major
advantage over the whole area of excitation when aiming for balanced
excitation of vibrational modes and continuous dispersion of sound over
the whole audio range.
properties of a panel
form membrane in a BWL differs from other planar speakers using film or
foil. Unlike the 'floppy' membranes of the ESL or the magnetic planar,
here since the membrane has some flexural rigidity by defined material
properties, as also some thickness and damping, it can be optimized for
the propagation of bending waves with defined speed and damping, as
well as efficient radiation of sound into the surrounding air. As there
is no tension of the membrane necessary like in conventional planars,
the problem of 'de-tensioning' with age also does not occur and the
system can be built to have outstanding long-term stability of
speakers aiming for
pistonic membrane movement, a BWL is not a 'moving mass' loudspeaker.
The membrane can be seen instead as a two-dimensional medium for
bending wave propagation, allowing the radiation of frequencies even
above the audio range. In fact the highest usable frequency depends
mainly on the properties of the exciter. The travelling speed of
bending waves, the dimensions of the membrane and damping can be
optimized for a seamless and very dense distribution of vibrational
modes over a wide frequency range.
vibrational modes overlap each other in their frequency ranges ('Modal
overlap' factor) in such a way, that single vibrational modes do not
show up in the frequency response of a well designed BWL. Modal density
and modal overlap in a high-end bending wave transducer can as a matter
of fact be made much higher than in typical musical instruments,
allowing the speaker to "emulate" the spectral fingerprint of, say, a
violin with extraordinary precision, or the transients of drums and
cymbals with equal fidelity.
modal overlap factor
is one major key for 'high fidelity' sound reproduction. From the
perspective of BWL design, a conventional cone midrange driver which is
used in the range of modal cone breakup (which is the usual case in
common multiway speakers) appears just like a bending wave transducer
having very poor modal overlap, and consequently, it is in fact a very
deficient sound transducer.
the Bending Wave
Loudspeaker offers the only solution where the properties of the
diaphragm membrane, the driving motor (exciter) and the driving point
can be matched and optimized with more degrees of freedom than in
conventional planar speakers.