Absorbing sound through controllable loudspeaker nonlinearities

What is it about?

Loudspeaker membranes can be used as natural sound absorbers, as any mass-spring-damper resonator. We show how we can further improve the sound absorption performance by forcing a nonlinear response to sound waves. Their nonlinear behavior is controlled by electric means through an active control scheme, allowing a nonlinear restoring force (the restoring force is not proportional to the membrane displacement anymore). It is shown that such a nonlinear spring allows for improving the sound absorption towards the low-frequency range. We also show that it does not require high sound pressure level to trigger such nonlinear behavior.

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Why is it important?

Sound absorption at very low-frequency (<100 Hz) is rather impossible to achieve with small devices. According to the "quarter-wavelength rule", it needs at least 1 m thick absorber layer at 80 Hz (and up to 4 m at 20 Hz!). Therefore using rather small loudspeakers as sound absorbers can be advantageous, and several concepts of such "electroacoustic absorbers" have been reported in the literature. Here we go a step beyond conventional (linear) electroacoustic absorbers and show that using loudspeaker nonlinearities helps improving the absorption performance towards the low-frequency regime, as was already introduced with Nonlinear Energy Sinks using passive (nonlinear) elastic membranes. We show that the nonlinear behavior can be totally controlled with an active control architecture, and that such nonlinearities it can even be triggered at very low pressure amplitude (around 1 Pa, whereas it requires more than 100 Pa for other passive nonlinear membranes concepts). It is also shown that the nonlinear products that are likely to be generated do not contribute to increase the net sound pressure levels in front of the absorber, Experimental assessment of the achieved acoustic performance validate the concept.