What Is the Order of the Ossicles From the Tympanic Membrane to the Cochlea
The middle ear transmits airborne sound waves to cochlear fluids
Sound waves picked up by the external ear vibrate the eardrum, which in turn mobilizes the ossicular chain of the eye ear. The footplate of the stapes applies a mechanic pressure on the oval window and the vibration reaches the perylymphatic fluid of the cochlea.
Centre Ear: scheme
The eardrum (4) or tympanic membrane separates the external auditory canal from the middle ear which communicates with the nasopharynx via the Eustachian tube (6). The oval window (hidden by the stapes footplate: three) and the round window (5) separate the centre and inner ears. The ossicular chain: malleus (ane), incus (2) and stapes (3) links the eardrum to the oval window. The surface ratio of eardrum to oval window (twenty/i) allows an adequate free energy transfer of the audio pressure between the air and the fluids of the inner ear. The middle ear can be considered as an impedance adapter - without it most 98% of energy would be reflected back.
Eardrum
| Thou. Mondain | Human eardrum (in situ)Ossicles, especially the malleus anchoring in the middle of the membrane, are seen by tranparency. |
| | SEM picture of a guinea grunter eardrum.The surface of the tympanic membrane is observed from the eye ear and the fastened arm of the malleus is seen in the eye. |
Ossicular reflex
An intense sound (> lxxx dB) trigger this reflex. A feed-back loop (midbrain) contract mussels (in Homo, simply stapes), which tighten ossicular concatenation and results in less energy transfered to the cochlea. This reflex ( more than details elsewhere) only works for depression frequencies (< 2000 Hz).
| | - (1) Malleus ; |
This internal view of the heart ear cavity allows understanding of how the ossicular reflex may reduce the transfer function of the ossicular chain. This reflex protects the cochlea confronting loud sounds, but this type of protection is quite limited:
- as it does not play a pregnant role above 1 or 2 kHz ;
- equally it is absolutely ineffective for impulse racket (fireworks, gunfire etc.)
Another office of the ossicular reflex is to attenuate perception of ane'south own voice: this is peculiarly important for singers.
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| This 3D animation shows clearly the ossicular joints. The surface ration between the tympanic membrane (blue) and the stapes footplate (overlaping the whole oval window) explains the distension of the middle ear. |
Role of the Middle Ear
The middle ear (ME) transmits acoustic free energy from the tympanic membrane (TM) to the inner ear, by allowing aligning of the difference in impedance between an air surround and a fluid surroundings.
If the sound pressure waves in the air were applied directly to the inner ear fluid, 99.nine% of the acoustic energy would exist lost considering of their reflection at the air /fluid interface (equating to a loss of approx 30dB).
The middle ear acts as a force per unit area amplifier: in this mode it is able to "capture" the bachelor audio-visual energy in the air, and broaden the amplitude of the mechanico-acoustic stimuli in the inner ear.
Because of the relationship between the surface areas of the TM (surface area S1 = 0.half-dozen cm2) and the stapes footplate (area S2 = 0.03 cm2), and because of the interaction of the ME levers (the axis of the ossicular chain passes very close to the incudomalleolar joint, but the two artillery of this lever are of unequal length, d1/d2 = ~ ane.3), the pressure distension is theoretically in the order of x26 (approx 28 dB)
Beware, yet! This calculation must exist used with circumspection, because, due to its mechanical characteristics, the behaviour and the efficiency of the ME varies greatly with varying frequency of sound (f). In consequence, ME function (similar to any type of mechanical system) depends on the friction (R) of the ossicular joints, the mass of the drum/ ossicular chain, and the rigidity (1000) of the diverse membranes, ligaments, air book then on.
More about physics (s ee beneath)
Center Ear transfer office
The mechanisms described above go obvious when i studies the transference office of the ME, ie the circuitous interaction between amplitude and phase that exists betwixt the audio-visual pressure at the entrance to the inner ear (Pv: pressure within the perilymph at the base of the scala vestibuli) and the pressure level at the TM (Pt): Pv/Pt.
In humans, the maximum sound amplification possible is 20dB, and this varies greatly with frequency, for example there is a maximum of 13dB amplification at 200 Hz, xx dB at 1000 Hz, 12 dB at 8000 Hz. ( graph: P. Minary)
Auditory thresholds and transfer function of external and middle ear
The shape of the auditory thresholds (shown here in man in a straight line) is equivalent to the global transference properties of the external and middle ear together (dotted line). This holds truthful for all mammals. ( graph: P. Minary)
Nosotros can therefore arrive at 2 conclusions:
- It is the external ear and eye ear together which "model" the curve of the sensory auditory threshold, according to the quantity of audio-visual energy transmitted at each frequency.
- The inner ear is a detector whose threshold for each frequency is abiding beyond most the whole of the audible range (about one.1 – 18 W in human)!
Source: http://www.cochlea.eu/en/ear/middle-ear
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