Acoustic Coupling between Active Oscillators Allows for their Synchronization and Explains Identical-Frequency Sounds Emitted from the Two Ears

Our ears emit sounds spontaneously, known as spontaneous otoacoustic emissions (SOAEs). It is unclear, however, why some animals emit sounds with identical frequencies from their two ears. In most nonmammalian tetrapods, acoustic coupling between the eardrums through the head cavity might influence SOAE production.

We record SOAEs simultaneously from the two ears of the tokay gecko and find several synchronized SOAEs. A model describing the eardrums as acoustically coupled, stochastic, active oscillators also produces synchronized SOAEs. In agreement with our experimental observations, the model captures the frequency-dependent phase differences between ears, the complex alterations in emissions upon suppression of the contralateral ear by static pressure, the change in emission frequency as the contralateral oscillator’s peak frequency is varied, the synchronization of emissions as the peak frequencies converge, and the dependence of some SOAEs on the emission strength of the contralateral ear.

Moreover, the model predicts that the ears’ activities enhance the localization of weak sound sources. The two ears of a gecko evidently function together as a single active system that is sensitive to the location of weak sound sources.