Evaluation of Accuracy and Reliability of a Mobile Screening Audiometer in Normal Hearing Adults

Quantifying hearing thresholds via mobile self-assessment audiometric applications has been demonstrated repeatedly with heterogenous results regarding the accuracy. One important limitation of several of these applications has been the lack of appropriate calibration of their core technical components (sound generator and headphones). The current study aimed at evaluating accuracy and reliability of a calibrated application (app) for pure-tone screening audiometry by self-assessment on a tablet computer: Audimatch app installed on Apple iPad 4 in combination with Sennheiser HDA-280 headphones. In a repeated-measures design audiometric thresholds collected by the app were compared to those obtained by standardized automated audiometry and additionally test-retest reliability was evaluated. Sixty-eight participants aged 19–65 years with normal hearing were tested in a sound-attenuating booth. An equivalence test revealed highly similar hearing thresholds for the app compared with standardized automated audiometry. A test-retest reliability analysis within each method showed a high correlation coefficient for the app (Spearman rank correlation: rho = 0.829) and for the automated audiometer (rho = 0.792). The results imply that the self-assessment of audiometric thresholds via a calibrated mobile device represents a valid and reliable alternative for stationary assessment of hearing loss thresholds, supporting the potential usability within the area of occupational health care.


Total Harmonic Distortion Purpose
At maximal sound pressure level, the headphone should be able to output pure tones without creating more than 2.5 % of harmonic distortion at the levels defined in IEC 60645-1:2001.

Method
The measurement of the total harmonic distortion is executed by a professional and certified calibration company according to IEC 60645-1:

Linearity Purpose
The reduction or increase of the sound pressure level by changing the amplitude of the digital signal in the device shall correspond to the same reduction or increase of the acoustical sound pressure produced by the headphones.

Method
The measurement of linearity is executed by a professional and certified calibration company. For this measurement the device allows to present tones with 10 dB difference in the sound pressure level: The corresponding screen of the device for measuring its linearity

Microphone Calibration Purpose
The hearing screening device's microphone must be calibrated because it is used to measure the sound level in the hearing test room to determine if the sound level is low enough to execute a hearing test according to ANSI-ASA standard S3.1-1999 "Maximum Permissible Ambient Noise Levels for Audiometric Test Rooms".

Method
The calibration of the device's microphone is executed at Sonormed by a qualified employee with a technical university degree. In order to lower the effect of standing waves that could influence the sound pressure of pure tones depending on slight differences of the device's physical location, wobble tones are used for the calibration of the device's microphone.
During the microphone calibration, a narrow wobble tone is presented with centre frequencies of 125,250,500,750,1000,1500,2000,3000,4000,6000, and 8000 Hz. The wobble tone at 12000 Hz is not used in this calibration procedure.
The calibration of the device's microphone is challenged by the dynamic nature of the wobble tones and by possible sounds in the environment. The calibration is therefore executed in a very silent room.
During the calibration of the device's microphone, its sensitivity is adjusted using the device's "+ sensitive" and "-sensitive" buttons so that the sound level in dB SPL (which is displayed between these buttons) matches the sound level displayed on the sound level meter: Screen of the device for the microphone calibration