Principle
There exist two mechanical acoustic pathways to the
inner ear:
·
the regular one via
the ear canal and middle ear
·
the other via the
body tissues, especially those in the head, the so-called “bone” conduction
pathway.
The sound vibrations in the air evoke to and fro
vibrations of the whole body. Self-evident, those of the head are most
important. They are transmitted via the skin to the deeper tissues. Inside the
head, due to differences in mechanical and acoustic properties between the head
tissues, the tissues vibrate with different amplitudes and phases. This gives
rise to the second pathway, the bone conduction pathways. The vibration in the
skull bone stimulate the middle ear bones and at the same time small vibratory
deformations of the bony encapsulation of the cochlea emit waves in the basilar
membrane and tectorial membrane. However, which anatomical structures, soft
tissues and skeletal structures under what conditions are precisely involved
and to what extent is still enigmatic.
Bone conduction tends to amplify the lower frequencies,
and so most people hear their own voice with a lower pitch than it actually is.
Application
Some hearing aids and non-clinical devices employ bone
conduction, achieving an effect equivalent to hearing directly by means of the
ears. A headset is ergonomically positioned on the temple and cheek and the
electromechanical transducer, which converts electric signals into mechanical
vibrations, sends sound to the internal ear through the cranial bones.
Likewise, a microphone can be used to record spoken sounds via bone conduction.
Most common are products such as ears-free headset or headphone. An example is
the "bonephone" that is designed for stereo presentation of
sounds, rather the traditional bone-conduction vibrator used in clinical
audiology settings.
Hearing aids and assistive listening devices, such as bone-anchored
hearing aids. The latter uses a
surgically implanted abutment to transmit sound by direct bone conduction to
the inner ear, bypassing the not-functional middle ear. A titanium ”plinth” is surgically embedded into the skull with a
small abutment exposed outside the skin on which a sound processor is attached.
This transmits sound vibrations, finally
vibrating the skull and so the inner ear.
Further there exist specialized communication products
(i.e. underwater & high-noise environments). An example is a bone
conduction speaker is a rubber over-molded piezo-electric flexing disc about
Bone conduction transmission can be used with individuals
with normal or impaired hearing. There are several products on the market that
exploit the bone conduction transmission pathway for sound. Some of these include
the AquaFM and the SwiMP3 which are devices for transmitting sound to swimmers.
Bone conduction products have following advantages over
traditional headphones:
· Ears-free, thus providing extended use comfort and safety.
· No electromagnetic waves, eliminating the potential effect of such waves on
cerebrum, if any.
High sound clarity in very noisy environments can be used
with hearing protection.
Disadvantages of some implementations is the requirement
of more power than headphones and the less clear recording & playback than
traditional headphones and microphone due to the reduced frequency bandwidth.
More
Info
Fig. 1. Basilar
membrane (BM) excursion due to translatatory (a) and compressional (b)
vibrations. O oval window, R round window, V vestibule. See for further
explanation under text of More Info.
(Modified from von Békésy, 1960.)
Bone conduction is a highly complicated process (e.g.
Taschke and Hudde, 2006). Despite this, it is certain that the pathway of the
stimulation with a tuning-fork on the scalp is different from the pathway when
for instance the head is submerged and an underwater source generates the
stimulus. For
instance, the air filled head
cavities are irrelevant with the tuning-fork, but they play a role when hearing
with a submerged head, since then the cavities reflect the sound waves. In air, with air-born sound, the bone
conduction pathway is again different but now its contribution in hearing is
strongly overruled by the canal-tympanic pathway. For frequencies up to about 1
kHz and
The impact of sound waves on the head results in three
types of bone conduction:
1. the direct movement of the whole head producing an inertial reaction of
the auditory ossicles;
2. the penetration of sound waves into the head (with a theoretical loss of
some 66 dB in air);
3. the deformation of the skull by the mass reaction acting during the
vibrations.
For frequencies up to about 1 kHz (1) dominates and
the theoretical calculated loss is about 50 dB (von Békésy, 1960). For higher
frequencies skull deformations become important. They result in translatatory
and compressional vibrations indicated by panel (a) and (b) respectively of
Fig. 1, which causes excursion of the basilar membrane. In (a) it is due to
unequal mobility of the both windows and (b) mainly by compressibility of the
vestibule and semicircular canals.
Fig. 2 gives the result of measurements of the
vibration of the basilar membrane, stapes and forehead with an air born sound
and the head in the free field with open ear canals and a strong air-born
source of 100 Pa ( = 1000 dyne/cm2,
equivalent to 134 SPL. The lower curve
is measured at the threshold for bone conduction.
Fig. 2. From
von Békésy (1960). See text for explanation.
Recent research
suggest that the 'classical' bone conduction theory with a vibrating
tuning-fork as stimulator should be modified to include a major pathway for
cochlear excitation which is non-osseous: the bone vibrations may induce
audio-frequency sound pressures in the skull contents (brain and cerebro-spinal
fluid) which are then communicated by liquid-like channels to the liquids of
the inner ear (Freeman et al. 2000). A similar conclusion was drawn from
results of experiments with the bullfrog (Seaman, 2002).
We
may conclude that the classical theory that the “bone-conduction” pathway is
dominated by conduction via skull bones is questionable. A fluid pathway is probably
at least as important.
References
G. von Békésy. Experiments in hearing, McGraw-Hill
Book Company, 1960.
Freeman S, Sichel JY, Sohmer H. Bone conduction experiments in
animals - evidence for a non-osseous mechanism. Hear Res.
2000;146:72-80.
Reinfeldt S, Stenfelt S, Good T, Hĺkansson B. Examination of bone-conducted
transmission from sound field excitation measured by thresholds, ear-canal
sound pressure, and skull vibrations. J Acoust Soc
Am. 2007;121:1576-87.
Seaman RL. Non-osseous sound transmission to the inner
ear, Hearing Research 2002;166:214-215.
Taschke H and Hudde H. A finite element model of the
human head for auditory bone conduction simulation. ORL J
Otorhinolaryngol Relat Spec. 2006;68: 319-323.