Distortion product otoacoustic emissions measured as vibration on the eardrum of human subjects

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 104; no. 5; pp. 1546 - 1551
• Main Authors: Dalhoff, E, Turcanu, D, Zenner, H.-P, Gummer, A.W
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 30.01.2007; National Acad Sciences
• Subjects: Adult; Audiology; Audiometry - instrumentation; Audiometry - methods; Biological Sciences; Cochlea; Ear, Middle - anatomy & histology; Eardrum; Eardrums; ears; Emission measurements; emissions; Female; Hearing; hearing disorders; Hearing loss; Hearing Loss - diagnosis; Hearing Tests - methods; Human subjects; Humans; Input output; Male; Otoacoustic Emissions, Spontaneous - physiology; Reflectors; Reproducibility of Results; Sensitivity and Specificity; Sensory thresholds; Signal reflection; Sound; Sound field; Sound pressure; Supernova remnants; Tympanic Membrane - anatomy & histology; Tympanic Membrane - physiology; Vibration
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0610185103

It has previously not been possible to measure eardrum vibration of human subjects in the region of auditory threshold. It is proposed that such measurements should provide information about the status of the mechanical amplifier in the cochlea. It is this amplifier that is responsible for our extraordinary hearing sensitivity. Here, we present results from a laser Doppler vibrometer that we designed to noninvasively probe cochlear mechanics near auditory threshold. This device enables picometer-sized vibration measurements of the human eardrum in vivo. With this sensitivity, we found the eardrum frequency response to be linear down to at least a 20-dB sound pressure level (SPL). Nonlinear cochlear amplification was evaluated with the cubic distortion product of the otoacoustic emissions (DPOAEs) in response to sound stimulation with two tones. DPOAEs originate from mechanical nonlinearity in the cochlea. For stimulus frequencies, f₁ and f₂, with f₂/f₁ = 1.2 and f₂ = 4-9.5 kHz, and intensities L₁ and L₂, with L₁ = 0.4L₂ + 39 dB and L₂ = 20-65 dB SPL, the DPOAE displacement amplitudes were no more than 8 pm across subjects (n = 20), with hearing loss up to 16 dB. DPOAE vibration was nonlinearly dependent on vibration at f₂. The dependence allowed the hearing threshold to be estimated objectively with high accuracy; the standard deviation of the threshold estimate was only 8.6 dB SPL. This device promises to be a powerful tool for differentially characterizing the mechanical condition of the cochlea and middle ear with high accuracy.