Technology
The stiff bending wave loudspeaker
Only a stiff bending wave loudspeaker is qualified for a sound reproduction that features a very broadband frequency / phase linearity, and an impulse fidelity without any sound-focusing effects. As mentioned above, the propagation of bending waves on the membrane is dependent on the frequency, that has been inducted. This effect is called the "dispersion of the bending wave" and it causes a rise of the phase velocity on the membrane while the frequency increases.
The frequency, where the phase velocity on the membrane and the phase velocity in the air are the same, is called "coincidence frequency". It can be controlled by the bending stiffness (E-Modul) and the mass density.An ordinary stiff bending wave loudspeaker bypasses the changes of air pressure, which are being produced by bending waves below this coincidence frequency. This means that the bending wave loudspeaker works only as a normal piston loudspeaker in this range. Above the coincidence frequency the efficiency factor increases by leaps and bounds and the bending wave starts to detach at an angle of > 0 degree.
