The Outer & Middle Ears
1. The outer ear has two roles in transmitting sound to the tympanic membrane or the eardrum. It aids sound localization by altering the spectrum of the sound, in a way that depends on the direction of the source. It also, by resonances, increases the sound pressure at the tympanic membrane.
2. The middle ear apparatus couples sound energy from the tympanic membrane to the oval window of the cochlea. the sound is transmitted by three small bones, the ossicles, called the malleus, the incus and the stapes. The middle ear acts as an acoustic impedance transformer, coupling energy from low impedance air to the higher impedance cochlear fluids, thus reducing the reflection of sound energy that would otherwise occur.
3. The middle ear transformer uses two principles.
4. Transmission through the middle ear depends on the frequency of the stimulus. Greatest transmission is produced in the range around 1-2kHz. Below this frequency, transmission is reduced by the stiffness of the middle ear structures and by compression and expansion of air in the middle ear cavity. Above this frequency, many factors, including the mass of the ossicles and less efficient modes of vibration of the structures, reduce transmission. There are also dips in the response arising from acoustic resonances in the middle ear cavity.
5. Transmission through the middle ear is affected by the middle ear muscles that reduce the transmission of low-frequency sounds. They may serve to protect the ear to some extent from noise damage, reduce the masking effects of low-frequency stimuli on higher-frequency stimuli, act as an automatic gain control for low-frequency stimuli over a narrow range of intensities, and reduce the perturbing effects of middle ear resonances.
2. The middle ear apparatus couples sound energy from the tympanic membrane to the oval window of the cochlea. the sound is transmitted by three small bones, the ossicles, called the malleus, the incus and the stapes. The middle ear acts as an acoustic impedance transformer, coupling energy from low impedance air to the higher impedance cochlear fluids, thus reducing the reflection of sound energy that would otherwise occur.
3. The middle ear transformer uses two principles.
- The area of the oval window is smaller than that of the tympanic membrane, increasing the pressure.
- The lever action of the ossicles increases the force and decreases the velocity.
4. Transmission through the middle ear depends on the frequency of the stimulus. Greatest transmission is produced in the range around 1-2kHz. Below this frequency, transmission is reduced by the stiffness of the middle ear structures and by compression and expansion of air in the middle ear cavity. Above this frequency, many factors, including the mass of the ossicles and less efficient modes of vibration of the structures, reduce transmission. There are also dips in the response arising from acoustic resonances in the middle ear cavity.
5. Transmission through the middle ear is affected by the middle ear muscles that reduce the transmission of low-frequency sounds. They may serve to protect the ear to some extent from noise damage, reduce the masking effects of low-frequency stimuli on higher-frequency stimuli, act as an automatic gain control for low-frequency stimuli over a narrow range of intensities, and reduce the perturbing effects of middle ear resonances.
Pinna Related Disorders
- Macrotia (Too large)
- Microtia (Too small)
- Prominent Ears (Too sharp of an angle)
- Lop Ears (folded over)
- Anotia (missing or incomplete)
Head Related Transfer Function
Frequency changes with elevation, filtering by the pinna provides cues to sound source localization
Temporal Bone
Squamous Part
Petrous Part
Petrous Part
Gain
the increase caused by an amplifier, especially the ratio of output over input
average gain of the human ear is tuned to frequencies between 2-10kHz, begins building at around 500Hz with peak amplification around 2.7kHz (speech), tapers off and dips at 10kHz
Pressure gain of the outer ear boosts frequencies particularly important for understanding consonants. Vowels are low frequency, and consonants are high frequency
average gain of the human ear is tuned to frequencies between 2-10kHz, begins building at around 500Hz with peak amplification around 2.7kHz (speech), tapers off and dips at 10kHz
Pressure gain of the outer ear boosts frequencies particularly important for understanding consonants. Vowels are low frequency, and consonants are high frequency
Middle Ear Function
- Impedance matching - transformer/amplifier of sounds to overcome the difference in impedance between the air of the External auditory canal and the fluid of the inner ear
- transformer ratio
- transfer function
- efficiency
- Filtering - resonant frequency is approximately 1000Hz, functions as a bandpass filter
- Acoustic Reflex - contraction of the stapedius muscle in response to loud sounds
- Middle ear is an impedance transformer
- couples energy from low impedance air to the high impedance cochlear fluid
- without this amplification hardly any sound would reach the inner ear
- the impedance is determined by the flow between the flexible windows and interaction with cochlear mechanisms
- The impedances must be "matched"
- the TM is about 18 times larger than the oval window
- the ossicles produce a lever action that further amplifies the signal
- the malleus and incus vibrate together transforming sound waves into mechanical energy at the stapes footplate
- increased pressure over area + increased force (lever action) = impedance matching
- 18.75 x 4.4 = 82.5!
- The middle ear transfer function is a band pass filter
- maximum increase in gain happens at ~ 1-2kHz
- low frequencies are attenuated, otherwise would hear everything, like brownian motion