Abstract
The occlusion effect is commonly experienced by in-ear device wearers as an increased loudness sensation of bone-conducted low frequency sounds. A widespread theory proposed by Tonndorf and based on a simplified electro-acoustic model describes the phenomenon as the removal of the open earcanal high-pass filter effect due to a perfect or partial occlusion. However, this filter has not been clearly defined and several ambiguities remain. Revisiting the model, a second order high-pass filter effect for the volume velocity transferred between the earcanal wall and the eardrum is highlighted. This filter remains for partial occlusion but vanishes for perfect occlusion. In the latter case, the volume velocity transferred from the earcanal cavity to the middle ear through the eardrum drastically increases, which explains the predominance of the occluded outer ear pathway on the hearing by bone-conduction at low frequencies.
Highlights
The occlusion effect (OE) is experienced as an increase of the auditory perception to bone-conducted sound when covering or blocking the earcanal (EC) [1]
The open EC high-pass filter effect is caused by the acoustic radiation of the open EC entrance, which is governed at low frequencies (LF) by the acoustic mass of radiation Lrad
The EA model proposed by Tonndorf has been clarified, and used to highlight a second order high-pass filter for the volume velocity transferred between the EC wall and the tympanic membrane (TM) in the open EC
Summary
The occlusion effect (OE) is experienced as an increase of the auditory perception to bone-conducted sound when covering or blocking the earcanal (EC) [1]. The OE is usually described as one’s own voice perceived as “hollow” or “talking into a barrel” [2] This perception is the conjunction of the bone-conducted transmission increase and the air-conducted transmission decrease due to the occlusion device [3]. A common objective measurement of the OE is defined as the difference between sound pressure levels measured close to the tympanic membrane (TM) in occluded and open EC This phenomenon is most prominent at low frequencies (LF), typically below 1 kHz, and for shallow occlusions [1]. Since the first descriptions of experimental findings underlying the OE in the 1830s, several theories were developed to explain this phenomenon over the course of the century. At LF, Tonndorf rather suggested that the OE resulted from the removal of the open EC high-pass filter effect due to the occlusion [5,6,7]
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