The klystron, tube waveguide, is a beam tube including at least two apertured cavity resonators. Yttrium iron garnet (YIG) spheres can act as tunable microwave resonators. These panels use a combination of three Helmholtz resonators and a wooden resonant panel. Figure 6 shows a filter using disc flexural resonators and magnetostrictive transducers. Unstable laser resonators (not used in most lasers) produce fractal-shaped beams. Here formula 55 denotes energy of high frequency electromagnetic field stored by both resonators. Modeling of acoustic resonators often occurs with equivalent electrical circuits. The grey dots represent molecules in the laser cavity that act as resonators. In other music cultures some versions have gourds that act as Helmholtz resonators. Figure 8 shows a filter using torsional resonators. The beam produced by stable, paraxial resonators can be well modeled by a Gaussian beam. These intrinsic properties could lead to applications such as NEMS as pressure sensors and resonators. A frequency comparator measured the beat frequency of the combined outputs of the two resonators. The classical sensing application of quartz crystal resonators is microgravimetry. Resonators or pipes suspended underneath the bars amplify their sound.