Conventional Planar v. Bending Wave Planar Loudspeakers

A brief comparison between different principles of
operation in planar loudspeakers

© Oliver Mertineit

While 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.

The best-known conventional 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 marks 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 on the other hand.

It is widely believed among many interested in audio, that driving the thin and stretched foil of a conventional planar loudspeaker membrane over its whole area – or 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 phase.

To make 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.

Let us apply some 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.

Even with no specific 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.

The 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 typical 'sound' and characteristic. What is good to give a certain 'snap' or 'colour'  to the sound of a drum is precisely what is unwanted in 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.

A 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.

Think 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 by Ernst Chladni.

The basic vibrational mode 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.

Once the 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.

On the 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.

The 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 parameters.

Unlike 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.

In doing so, single 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.

A high 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.

In short, 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.