External ear canal
The external auditory canal ( meatus acousticus externus ) begins at the auricle and extends to the eardrum . It is therefore part of the outer ear . The external auditory canal in humans has a length of about 2 to 2.5 cm and a diameter of about 7 mm and is slightly inclined forward and downward, which is why in the study with the otoscope to better view a straightening by rolling train at the ear after trying to reach back and above.
The outer third of the wall of the external auditory canal is formed by a cartilage ( Cartilago meatus acustici ), which with a part ( Lamina tragi ) also forms the protrusion on the front of the auditory canal entrance ( tragus ). The wall of the remaining two thirds forms the bony ear canal of the temporal bone. The outer auditory canal is completely lined with skin, which is relatively thick in the outer part, but in the bony part, especially in the vicinity of the eardrum, it is very thin and immovably connected to the periosteum. At the eardrum, the skin of the ear canal merges into the outer layer ( stratum cutaneum ) of the eardrum. The auditory canal is sensibly innervated by the external meatus acustici externi , a branch of the auriculotemporal nerve , and the auricular branch of the vagus nerve . The blood supply takes place via the deep auricular artery .
Only in the outer, cartilaginous part of the ear canal are there special protective hairs, the tragi . It is a special form of bristle hair . The secretion of the special glands in the ear canal, the so-called cluster glands ( glandulae ceruminosae , ceruminal glands), together with sebum , which comes from glands associated with hair, and epidermal scales form the ear wax ( cerumen ).
The ear canal amplifies the incoming sound in the range from 2000 to 4000 Hz through its natural resonance , which is why humans are more sensitive to these frequencies than to others. This effect, also called Open Ear Gain , is also responsible for the frequency-dependent volume perception , among other things .
In addition, the external auditory canal acts like a λ / 4 resonator because it is flush with the eardrum, i.e. the auditory canal acts like a closed organ pipe . As with closed pipes, the resonances are at λ / 4, 3 λ / 4, 5 λ / 4 etc. and can be calculated as follows:
First resonance:
λ / 4 = 0.025 m = length of the ear canal
→ λ = 0.10 m
→ f = c / λ = 343 / 0.10 = 3430 Hz
The second and third resonance result analogously:
| Wavelength λ | Frequency f = c / λ | ||
|---|---|---|---|
| 1. Response | λ / 4 = 0.025 m | λ 1 = 0.100 m | f 1 = 343 / 0.100 = 3430 Hz |
| 2. Response | 3 λ / 4 = 0.025 m | λ 2 = 0.033 m | f 2 = 343 / 0.033 = 10394 Hz |
| 3. Response | 5 λ / 4 = 0.025 m | λ 3 = 0.020 m | f 3 = 343 / 0.020 = 17150 Hz |
This means that the third resonance is already at the outer end of the human hearing range - all other resonances are outside the hearing range.
In the "curves of equal volume level ", the first two resonances are noticeable through increased sensitivity at around 4 kHz and 12 kHz. It must be noted that even the smallest differences in the length of the ear canal (as is common in humans) cause large differences in the position of the resonance frequencies.
See also
literature
- Barry J. Anson, James A. Donaldson: Surgical Anatomy of the Temporal Bone . Saunders, Philadelphia 1981, ISBN 0-7216-1292-X .
- Uwe Gille: Ear, Auris . In: Franz-Viktor Salomon (Ed.): Anatomy for veterinary medicine . Enke, Stuttgart 2004, ISBN 3-8304-1007-7 .