Bone conduction
Bone conduction , even acoustic bone called, referred to the forwarding of sound - oscillations or vibrations through the skull bone surrounding the organ of hearing, bypassing the middle ear . The perception of the “bone sound ” is normally masked by the signals transmitted as airborne sound due to the high acoustic wave resistance of the skull bone .
backgrounds
Sound represents the propagation of the smallest pressure and thus also density fluctuations in an elastic medium. If sound from a gaseous medium - in this case air - hits a skull , not only the eardrum and consequently the ossicles are set in vibration there, Instead, a certain amount of airborne sound is converted into structure-borne sound with the appropriate intensity and leads to vibrations of the entire skull. The vibrations of an oscillating solid body ( vibrator ), such as a tuning fork or air sounder (headphones), are transmitted in a similar way . The vibrations can take on the following physical waveforms:
- Surface waves at the interfaces between skin and soft parts of the skull;
- Flexural waves along the skullcap;
- Translational waves across the interior of the skull;
- Deformation vibration in the area of the stronger bone structures of the skull base, for example in the petrous bone.
The different waveforms hitting the temporal bone are superimposed in their respective amplitudes and wave phases. From this, “sound impressions” are generated in a complex manner. The vibrating bones of the temporal bone stimulate the air columns in the external auditory canal and in the tympanic cavity. The ossicular chain but also the inner ear fluid lead due to the vibrations to so-called relative movements of these structures. The deformation vibrations in the area of the petrous bone can in turn lead to deformations in the bony labyrinth and thus to deflections in the membranous labyrinth. Thus, all sections of the hearing organ are involved in the creation of the sound impression from the bone conduction in a different manner.
Clinical measurement
If you put an air sound generator (headphones) directly on the skull , you have to increase the sound level by about 50 decibels in order to achieve the same volume sensation as via the ear. For hearing measurements via bone conduction, a special bone conduction receiver with a standardized contact surface and standardized contact pressure is placed on the mastoid process of the temporal bone (bone directly behind the auricle) and held in place with a headband. In order to be able to determine the normal hearing threshold in this way , the threshold force level of the vibration emitted by the bone conduction receiver must be significantly higher than with the air conduction receiver. However, the audiometer is calibrated in such a way that the normal hearing threshold is again indicated as zero decibels even when measured with the bone conduction receiver. If the bone conduction receiver is placed on the forehead, a threshold force level that is around ten decibels higher is required to reach the hearing threshold. If the ear stimulated via bone conduction is closed (e.g. with headphones or hearing protectors), a closed volume of air is created in the external auditory canal, which leads to a lowering of the hearing threshold level.
Bone conduction is of medical relevance because, unlike air conduction, it bypasses the middle ear . Patients with damage to the middle ear or the outer ear can therefore perceive sound normally via bone conduction, while they show clear deficits when hearing via air conduction.
The audiometric test of bone conduction in the Rinne experiment and in tone audiometry use this for the differential diagnosis of a hearing impairment . Normal hearing via bone conduction, but impaired hearing via air conduction , is conductive hearing loss, while hearing deterioration via air and bone conduction is the same as inner ear hearing loss.
Bone conduction or bone conduction can be of practical use for hearing impaired people with conductive hearing loss. In certain cases, bone conduction is used with a bone conduction hearing aid to correct the hearing impairment. The airborne sound is transmitted to the cranial bone, and the inner ear can then receive the sound vibrations.
In everyday life, the phenomenon is known from the habit of conductors and choir leaders to put the tuning fork on the skull and thus hear the concert pitch directly through the bone . When using hearing protection or in-ear monitoring , the bone conduction leads to a falsified sound in certain musical instruments (vocals, brass instruments).
When listening to one's own voice , the subjective hearing impression is significantly influenced by the sound components conducted through the bones. After switching off this bone-borne sound component, for example with the acoustic reproduction of one's own voice from a magnetic tape or another sound carrier , the change heard is clearly noticeable.
Graphic representation of the measurement results, interpretation
After the air conduction measurement has been carried out in both ears using the attached headphones, the patient is given a bone conduction receiver and sits directly on the skull bone. In this way, the hearing thresholds of the bone conduction can be determined analogously to air conduction. In this relative representation, the so-called hearing threshold curves for air conduction and for bone conduction are then obtained separately for each ear - that is, right and left ear - by connecting the auditory threshold points. Basically, the findings of the be r real ear in r oter color (symbols for air line "O" right "X" left; symbols for bone conduction "<" right ">" left) in the l entered inke field, the findings of the left ear in b l except color in the right field. In the case of audiograms in which the curves entered are only shown in blackened form, the symbols used provide additional information as to which graphic stands for the right and which for the left ear. For example, the hearing threshold via headphones, the so-called air conduction, is entered with a circle “O” for the right ear and a cross “x” for the left ear.
| - | right ear | left ear |
|---|---|---|
| Air duct | O | X |
| Air duct with masking | Δ | ẝ |
| Bone conduction | < | > |
| Masked bone conduction | [ | ] |
| Missing response | ↓ | ↓ |
Conductive disturbance
If the tones offered in audiometry are poorly perceived via the air conduction but well perceived via the bone conduction, this is referred to as a sound conduction disorder . The reasons for this are simplified, everything that hinders the sound on its way from the outer ear to the inner ear.
Sensory sensation disorder
In comparison, there is the sensorineural disorder , with its origin in the inner ear or the related neural connections. The audiograms show hearing curves that run in parallel for both bone and air conduction, but the individual hearing thresholds are well above the limits of normal hearing; see picture of 13 year old boy with Alport syndrome.
Audiogram Hearing threshold curves of an inner ear hearing loss (sensorineural hearing disorder ) in a 13-year-old boy with Alport's syndrome .
: Bone conduction
ẝ air conduction
Conductive hearing loss in the audio audiogram of the right ear ("red" lines; compared to the left ear "blue" lines)
<: bone conduction, left
X: air conduction
The difference between air conduction and bone conduction audiogram is typical for conductive hearing loss
See also
Web links
- Airborne sound and bone sound Air and bone conduction Why does your own voice sound so different when you hear it from a recording?
Individual evidence
- ↑ G. Böhme; K. Welzl-Müller: Audiometry. Hearing tests in adults and children. Hans Huber, Bern / Göttingen / Toronto / Seattle 1993, ISBN 3-456-82296-0 , p. 40 f.
- ↑ How to Interpreter an Audiogram. December 19, 2014 betterhearingjax.com
- ↑ How do I read an audiogram? S031 - Audiogram reading aid, V01, January 10, 2005 meditech.biz
