Characterization of a Dynamic Loudspeaker Headphone in hypobaric conditions

Objectives/Hypothesis: The frequency response of a dynamic loudspeaker is well known at sea level atmospheric pressure condition and corresponding equivalent electric-mechanic-acoustic models have been formulated. A lack of knowledge concerns the behavior of these loudspeakers in hypobaric conditions. This in an important issue involved in the study of human hypoxic effects. Methods. Measurements of input electrical impedance magnitude of a Telephonic TDH-39P headphone have been carried out at altitudes simulated in a hypobaric chamber. White noise has been supplied when the headphone was in free field or coupled to an artificial ear. The acoustic pressure levels generated by the headphone have been also measured in both configurations. A fit of impedance data has been carried out by means of an equivalent electric-mechanic-acoustic circuit model, in order to characterize the loudspeaker elements as function of the environment pressure. Results. In free field, by increasing altitude, the loudspeaker input electrical impedance shows a resonance condition around 2 kHz, which is a decreasing function of pressure and is reflected in the acoustic pressure levels. In the artificial ear condition, the ear resonance frequency appears at lower altitudes. Moreover, the sound pressure transmission dependence on air density as an increasing function also causes the same trend on loudspeaker electrical impedance at higher frequency. Conclusion. The analysis carried out shows that the dependence of a dynamic loudspeaker frequency response on environment pressure is an essential parameter to be considered when auditory tests employ this type of headphone in order to study hearing effects under hypobaric hypoxia.