Bone conduction

Bone conduction, also called bone conduction, refers to the transmission of sound vibrations or vibrations through the skull bone surrounding the organ of hearing, bypassing the middle ear. The perception of " sound bone " is usually covered by the transmitted as airborne noise signals due to the high sound wave resistance of the skull bone.

Background

Sound, the spread of the smallest pressure and thus variations in density in an elastic medium dar. true sound from a gaseous medium, here the air to a head, so there are not only the tympanic membrane and the ossicles added subsequently to vibrate, but a certain amount of air-borne sound is converted at the appropriate intensity in sound body and leads to oscillations of the entire skull. Similarly, the vibrations of a vibrating solid body ( vibrator ), such as a tuning fork or air - transmitted sound generator ( headphones ). The vibrations can have the following physical waveforms:

• Surface waves at the interfaces between the skin and skull soft tissues;
• Bending waves along the skull;
• Translation waves across the cranial cavity;
• Deformation vibration in the stronger bone structures of the skull base, eg in the temporal bone.

The incident on the temporal bone different waveforms overlap in their respective amplitudes and wave phases. From this it can be generated in a complex way " sound impressions ." The vibrating bones of the temporal bone stimulate the air column in the external auditory canal and the tympanic cavity. The ossicular chain but also the inner ear fluid through the vibrations due to so-called Releativbewegungen these structures. The verformungsschwinungen in the area of the temporal bone may in turn lead by deformations in the bony labyrinth and membranous labyrinth in the deflections. To follow all the sections of the hearing organ are involved in the development of the sound impression of the bone conduction in each different ways.

Clinical measurement

Putting an air - sounder (headphones) directly to the skull, so you have to increase the sound level by about 50 decibels to achieve the same perceived loudness as over the ear. For Hörmessungen via bone conduction, a special bone conduction hearing aid with standardized support surface and standardized bearing pressure on the mastoid process of the temporal bone is set ( bone directly behind the ear ) and held in place with a headband. In order to determine in this way the normal hearing threshold, the threshold power level of the votes from the bone conduction receiver vibration must be significantly higher than for air conduction hearing aid. The audiometer but is calibrated so that the normal threshold of hearing is also indicated when measured using the bone conduction handset than zero decibels. If the bone conduction receiver on the forehead placed a ten decibels higher threshold level of force is necessary to achieve the threshold of hearing. If the ear via bone conduction excited closed (through headphones or ear muffs ) results in a closed volume of air in the external auditory canal, which leads to a lowering of the Hörschwellenpegels.

The bone conduction is of medical relevance, because, in contrast to the air line bypasses the middle ear. Patients with damage to the middle ear or the outer ear can therefore perceive normally via the bone conduction sound while on air line show significant deficits in hearing.

The audiometric examination of the bone conduction when Rinne test and the pure-tone audiometry uses this for the differential diagnosis of a hearing impairment. In normal hearing via bone conduction but impaired hearing about air line, there is a conductive hearing loss, at the same deterioration of hearing by air conduction and bone conduction to an inner ear deafness.

Of any practical use of bone conduction or bone conduction hearing impaired with a conductive hearing loss can be. In certain cases, bone conduction is used with a bone conduction hearing aid to correct the hearing impairment. The air sound is transmitted to the skull bone, the inner ear then can receive the sound vibrations.

In everyday life, the phenomenon by the habit of conductors and choral conductors is known to put on the tuning fork on the skull, and so to hear the pitch directly through the bone. The use of hearing protection or in-ear monitoring bone conduction results in certain musical instruments (vocals, brass instruments ) to a sound distortion.

While listening to your own voice of the subjective sensation of the current conducted through the bone conduction shares is significantly influenced. After elimination of this bone portion of sound, for example when the acoustic reproduction of your own voice from a magnetic tape or other record, you realize clearly that belonged to change.