Comparing external tympanum vibration and spontaneous otoacoustic emissions

Generated inside the inner ear, spontaneous otoacoustic emissions (SOAEs) are the most salient evidence for an “active” ear. These emissions propagate back through the middle and external ears, setting the tympanic membrane in motion such that it effectively acts as a “speaker”. Better characterization of this motion would be useful for the quantification of power produced by the associated active processes. Towards this end, we took a comparative approach, examining spontaneous activity in both a lizard (green anole, Anolis carolinensis) and an invertebrate (tree cricket, Oecanthus nigricornis). Using a laser Doppler vibrometer and sensitive OAE probe, we measured both tympanum motion and the acoustic output. For lizards, using a single point measurement system (whose focal point was not well localized), roughly similar behavior was observed in both the mechanical tympanum motion and the acoustic SOAE. For a relatively large spontaneous vibration peak (at 3.9 kHz) on the lizard tympanum, the amplitude of displacement was approximately 60 pm. Differences were however noted between SOAE and vibration, and several possible explanations for such are discussed. For the cricket, only “open” coupling of the microphone could be implemented. It was observed that while spontaneous vibration on the tympanum could be detected above the noise floor (single peak about 3-4 kHz, with displacements of 10-35 pm), SOAEs could not. When a stimulus was presented to a elicit a stimulus frequency emission (SFOAE) using a suppression paradigm, no residual could be detected either in the vibration or acoustic response. Lastly, when two tones were presented, distortions were readily observable in the tympanum motion but no acoustic DPOAEs were detected. It is suggested that the relatively small size of the cricket tympanum and open coupling employed could explain these inconsistencies (e.g., it is too small to effectively radiate acoustic power above the noise floor).