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.
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.
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 kHz, the tympanic pathway is ca. 55 dB more sensitive, and and 8 kHz about 45 dB (Reinfeldt et al. 2006), rather well in accordance with earlier measurements up to 2,.5 kHz.(von Békésy, 1960).
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.
G. von Békésy. Experiments in hearing, McGraw-Hill Book Company, 1960.
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.