Is there a risk for the astronauts' hearing in a microgravity environment? The Acoustic Diagnostics experiment on board the ISS

Acoustic Diagnostics is an experiment dedicated to monitoring the hearing function of the astronauts on board the International Space Station (ISS). The Italian Space Agency (ASI) provides access to the ISS as a laboratory in space to the Italian research community, through agreements between ASI and NASA for its use and between ASI and the European Space Agency (ESA) for the "BEYOND" mission, and via the Italian participation to the ISS as a member state of ESA. The main aim of the "Acoustic Diagnostics" experiment on board the space station ISS is investigating possible dysfunction of the human auditory system related to long-term missions on board the station. Some astronauts reported indeed mild hearing damage after long-term permanence in orbit. Probable hazard sources might be noise and micro-gravity, which affects intracranial fluid pressure. Before and after the mission, the astronauts will be subjected to full audiological examination, including pure tone audiometry, ABR, tuning and time-resolution measurements, impedance measurements, and otoacoustic emission (OAE) tests, which are a particularly sensitive tool for the early detection of mild hearing loss. The OAE test employed will be the measurement of distortion product otoacoustic emissions (DPOAEs) monitored during the flight, as a function of their permanence, in at least three in-flight sessions. OAE-based measurements are objective, non-invasive, and fast tests. The particular technology used for this type of tests on board will allow measurements relatively unaffected by the high noise levels of the ISS environment, and special precautions will be used to limit the EMI disturbances. High reproducibility of the measurements is guaranteed thanks to the forward-pressure calibration using the Thevenin methodology and a substantial (10dB) SNR increase will be provided by a customized time-frequency filtering procedure based on the wavelet transform.