Imagine sound emanating from a source (e.g. a speaker), which you can hear when you are facing towards one side of the source, but is inaudible when you are facing towards the opposite side of the source. Researchers at the California Institute of Technology have now developed an “acoustic rectifier” that allows certain tones to travel in one direction through it, but not in the opposite direction.
Sound is basically a ‘pressure wave’, which travels though a medium by inducing periodic regions of compression (where the particles in the medium are pushed closer to each other) and rarefaction (where the particles in the medium are pulled away from each other). The rate at which these regions of compression and rarefaction ‘vibrate’ sets the tone or frequency of the acoustic wave.
The scientists created a linear array (chain) of 19 stainless steel spherical particles stacked end-to-end. All the spheres had the same mass (30 grams) and radius (1 centimeter), except the second one from the left that had a smaller mass (6 grams) and radius (0.6 centimeters). The arrangement acts like a ‘nonlinear medium’, where different acoustic frequencies can ‘mix’ with each other due to the large coupling between the particles. This is because the force acting on one sphere affects the neighbouring spheres also.
Acoustic waves were generated by an ‘actuator’ that periodically compressed the array from one end, at a certain rate or frequency. A constant force pushed the array from the other end. Two configurations were studied – forward (the actuator or source at the left end and the constant force on the right end) and backward (the actuator at the right end and the constant force on the left end). Sensors were embedded on a couple of spheres (one near each end) to measure the compressive force acting on them in real time, in order to study the passage of the acoustic wave through the chain of spheres.
The chain of spheres acts like a low-pass filter, allowing low-frequency or more bass tones to propagate through the medium, while blocking more treble tones having frequencies higher than the ‘cutoff frequency’. The inhomgeneity of the chain (due to the presence of the smaller sphere) also creates a ‘localized’ mode at a frequency higher than the cutoff – this tone falls off in intensity exponentially fast away from the small sphere.
The actuator is used to generate an acoustic wave at a frequency close to that of this localized mode. For the backward configuration, there was no acoustic energy detected at the other end and the acoustic wave was completely attenuated. For the forward configuration, the ‘localized’ mode of vibration is excited (since the small sphere is very close to the actuator), which then transfers energy to more bass tones due to nonlinear mixing. The sound at the high tones is thus essentially converted to frequencies lower than the cutoff, which can then propagate through the chain. The forward transmission of acoustic energy is thus much higher compared to the bacward transmission. The range of tones transmitted can be easily controlled by the strength of the constant force acting on the chain, at the end opposite to the source.
The authors propose that such acoustic rectifiers may be used to build ‘logic circuits’ to process sound directly, and could potentially be employed in ultrasound imaging for biomedical applications, in addition to enhancing energy harvesting technologies.
Boechler, N., Theocharis, G., & Daraio, C. (2011). Bifurcation-based acoustic switching and rectification Nature Materials, 10 (9), 665-668 DOI: 10.1038/nmat3072