By: Matthias Scholz
User Interface Designer
PhD Applied Acoustics
Brüel & Kjær
The outer ear collects sound, the inner ear transfers these vibrations into neurological signals that can be processed by the brain, and the middle ear provides the coupling between them. In this issue, we follow sound through the ear to the tips of the hair cells, where vibrations turn into neurological signals. What happens then is the material for a future chapter.
In its normal state, the ossicles have an amplifying effect to efficiently excite the fluid in the inner ear. However, muscles in the middle ear can change this to actually attenuate the oscillations, thus providing a protective mechanism in case of excessive sound pressure. However, the adjustment is too slow to protect against impulsive events, such as explosions
The Outer Ear
The outer ear consists of the pinna and the auditory canal. As already discussed in ‘Listening in 3D’ (Waves, October 2017), the pinna plays an important role in the auditory source location. In addition, its horn-like shape provides a smooth transition from the ‘infinite’ space around the head, funneling the sound into the narrow auditory canal. The canal then guides the sound towards the eardrum, a thin membrane separating the outer from the middle ear.
The Middle Ear
The middle ear is a small air-filled chamber between outer and inner ear. The purpose of this chamber is twofold. First, it contains a mechanism of three bones, called the auditory ossicles, connecting the eardrum and the inner ear. This gear box like mechanism is needed since the inner ear is filled with a fluid, making direct excitation by the eardrum inefficient.
Secondly, the middle ear is needed to equalize pressure across the eardrum. A healthy eardrum is completely airtight, preventing airflow from the outer ear into the middle ear. The pressure difference between the two chambers moves the membrane in and out, which is exactly what is needed to pick up the rapid pressure fluctuations of sound.
Outer ear dimensions and amplification
The outer ear is especially sensitive to frequencies between 1 and 5 kHz. Not coincidentally, this range is important for communication, with 3 kHz being the frequency around which our hearing is most sensitive. Acoustically, the outer ear works like a tube resonator, with the strongest first resonance around 3 kHz, where a quarter wavelength of sound in air (10 cm / 4 = 2.5 cm) fits the length of the auditory canal. In contrast, sensitivity drops significantly at lower frequencies where the wavelengths are large compared to the ear’s size.
Cochlea with basilar membrane
Even when excited by the sound of a pure tone, the entire basilar membrane will be set into motion. However, the area associated with the frequency will react the most; that is, the lateral oscillations will peak around this section
However, a problem can arise when the atmospheric (static) pressure in the outer ear differs from the pressure inside the middle ear.
This mechanism is not that evident in everyday life but is easily experienced during lift-off and landing on an aeroplane, where the ambient pressure changes significantly due to the change in altitude. The pressure in the outer ear follows the ambient pressure in the aeroplane, whereas the pressure on the inside of the eardrum remains unchanged. The constant pressure difference applies a pre-tension to the membrane, pushing it either in or out, which gives an unpleasant sensation and leads to sound being perceived duller.
The Eustachian tube, which connects the middle ear to the throat, helps to equalize this pressure. When we swallow, the tube opens briefly causing the static pressure on the inside of the eardrum to equalize to that of the outer ear, resetting the eardrum to its neutral position. The eardrum will have its normal sensitivity and sound will again be bright.
The Inner Ear
The inner ear is the most complex element in the chain. It is a fluid-filled chamber and consists of two parts: the vestibular labyrinth, which functions as part of the body’s balance mechanism, and the cochlea, containing the basilar membrane and the organ of Corti, a sensory element that converts sound into nerve impulses so that our brain can process the information.
Sound that has been funneled into the auditory canal, will set the eardrum into motion. The auditory ossicles in the middle ear pick up these oscillations and transfer them to the fluid through the oval window, one of two flexible surfaces between the cochlea and middle ear. Exciting this membrane generates waves in the fluid-filled inner ear that travel along the basilar membrane, thereby setting it and the organ of Corti in motion.
Static pressure equalization in condenser microphones
To convert sound pressure into an electrical signal, Brüel & Kjær’s condenser microphones use a delicate diaphragm stretched across a backplate with a very narrow gap between them, forming a capacitor. Impinging sound deflects the diaphragm, and the variation in distance to the backplate produces an electrical signal proportional to the sound pressure.
The diaphragm seals the microphone at the top so that a variation in the static, ambient pressure would change the diaphragm’s neutral position relative to the backplate. The ear solves this problem with the Eustachian tube, and condenser microphones use a similar design. A narrow air channel at the side or rear of the microphone ensures that the internal cavity’s static pressure equalizes with the environment.
This organ contains thousands of small hair cells, which are connected to the acoustic nerve. The oscillation pattern of the basilar membrane is quite complex, with different areas being stimulated more or less by different frequencies. For each of these areas, a different group of hair cells will be activated and send impulses through the nerves to the brain. Thus, the organ of Corti splits up the sound into its spectral components, similar to rain drops splitting up sunlight into individual colours.
Well, at least this is the short version. The long version is much more complex, but also exciting, explaining many of the phenomena in our perception of sound. It deserves a separate chapter, so stay tuned...
More articles about the physics of sound and vibration