Transmission properties of bone conducted sound: Measurements in cadaver heads

Abstract

In the past, only a few investigations have measured vibration at the cochlea with bone conduction stimulation: dry skulls were used in those investigations. In this paper, the transmission properties of bone conducted sound in human head are presented, measured as the three-dimensional vibration at the cochlear promontory in six intact cadaver heads. The stimulation was provided at 27 positions on the skull surface and two close to the cochlea; mechanical point impedance was measured at all positions. Cochlear promontory vibration levels in the three perpendicular directions were normally within 5 dB. With the stimulation applied on the ipsilateral side, the response decreased, and the accumulated phase increased, with distance between the cochlea and the excitation position. No significant changes were obtained when the excitations were on the contralateral side. In terms of vibration level, the best stimulation position is on the mastoid close to the cochlea; the worst is at the midline of the skull. The transcranial transmission was close to 0 dB for frequencies up to 700 Hz; above it decreased at 12 dB/decade. Wave transmission at the skull-base was found to be nondispersive at frequencies above 2 kHz whereas it altered with frequency at the cranial vault.

Estimation of bone conduction skull transmission by hearing thresholds and ear-canal sound pressure

“…3 the difference between ECSP and thresholds originated in the threshold measurements where a dip in the transcranial transmission was seen at 0.5 kHz whereas the ECSP were similar irrespective if stimulation was ipsilateral or contralateral. The relatively higher sensitivity from ipsilateral stimulation was not easily explainable when similar results were neither seen in the current ECSP measurement nor in cochlear vibration measurement executed previously in cadaver heads (Stenfelt and Goode, 2005a). However, in the Stenfelt and Goode (2005a) study it was concluded that the response vibration direction of the cochlea was different when the stimulation was close (ipsilateral mastoid) than when it was further away (e.g.…”

“…Recently, the transcranial attenuation was estimated for two stimulation positions (one close to the ear-canal opening and the other further back) by hearing thresholds (Stenfelt, 2012). In the same study the results were compared with estimations from vibration of the cochlea in cadaver heads using one-dimensional velocity measurements (Eeg-Olofsson et al, 2011) and three-dimensional acceleration measurements (Stenfelt and Goode, 2005a). According to Stenfelt (2012), average perceptual measures were close to the vibration estimates at frequencies between 0.8 and 6 kHz.…”

“…Recently, there have been several studies investigating sound transmission in the human skull, also known as bone conduction (BC) sound transmission, by measurements of skull vibrations (Eeg-Olofsson et al, 2011;Håkansson et al, 2010;Stenfelt and Goode, 2005a). The rationale for these studies is that the vibration of the cochlea is related to a hearing perception, and the measurement of cochlear vibration enables a fast estimation of the BC excitation with greater level and frequency resolution compared with the same measurements done by hearing thresholds.…”

“…Another caveat is that most studies of cochlear vibrations were done in cadavers (Eeg-Olofsson et al, 2008;Eeg-Olofsson et al, 2011;Stenfelt and Goode, 2005a) or dry skull specimens as the vibration of the cochlea is generally not easily accessible in live humans.…”

“…According to Stenfelt (2012), average perceptual measures were close to the vibration estimates at frequencies between 0.8 and 6 kHz. In Stenfelt and Goode (2005a) the vibration data of the cochlear promontory was compared to the proposed sensitivity difference between ipsilateral mastoid and forehead BC stimulation according to the standard ISO:389-3 (1994). The worse sensitivity at the forehead of 8 to 14 dB corresponded well with the cochlear promontory vibration data: at the lower frequencies (below 3 kHz) one-dimensional cochlear vibration predicted the response, while at higher frequencies three-dimensional vibration data gave a better estimate.…”