Microphone calibration estimation for mobile audiological tests with resonating bottles.

OBJECTIVE Audiological tests on smartphones require consistent microphone recordings across device types with a reasonable standard uncertainty (2-3 Decibel (dB)) of the sound pressure level at the microphone. However, the calibration of smartphone microphones by the non-expert user is still an unsolved issue. We show that whistling on standardized glass bottles permits a coarse sound level calibration with an uncertainty that is smaller than the standard uncertainty of clinical audiograms (4.9dB) and enough for mobile health (mHealth) products. DESIGN We define and test a calibration procedure with bottle-whistles for smartphones. The empirical sound pressure levels are used to calculate the mean and standard deviation of a single measurement. STUDY SAMPLE Two uncalibrated studies with a total of 30 participants, one calibrated study with 11 participants. RESULTS The mean maximal sound pressure level of 330 ml Vichy-shape bottle-whistles at 50 cm distance is 92.8 ± 1.6dB sound pressure level (SPL). The sound pressure level variation of a single measurement is 3.0dB SPL. CONCLUSIONS In comparison to other possible ways of level calibration estimates for smartphones (e.g. level of own voice, level of common environmental sounds), the current method appears to be robust in background noise and easily reproducible with glass bottles of defined dimensions.