Biophysics of HearingBiophysics of Hearing

Ján JakušJán Jakuš

Hearing – is very important human sense because through the ear we can receive sound, music or noise.Sound - is a kind of mechanical (pressure) energy, that results from vibration of air molecules. Thus, in a vaccum there is no sound, because no molecules that could undergo the vibration. Velocity of sound in air is approx. 330 m/ s. However, in liquids the velocity of sound is 5-times higher, in solids there is a velocity even 20-times higher, but its intensity (amplitude ) is lower.  

Frequency of the sound –Frequency of the sound – is the number of is the number of waves that pass a given point, in a given time. waves that pass a given point, in a given time. The The shortershorter is the is the wavelenghtwavelenght ( λ( λ ), the higher ), the higher is is the the sound frequencysound frequency, and , and vice versa.vice versa. Most sounds are mixtures of several frequenMost sounds are mixtures of several frequen--ciescies.. Harmonic soundsHarmonic sounds with the same frequency with the same frequency enable the perception of enable the perception of musicmusic..There areThere are 4 subjectively perceiving 4 subjectively perceiving sound sound propertiesproperties : : PitchPitch,, ColorColor, , StrenghtStrenght,,Duration Duration of the soundof the sound.Watch! For physical properties of sound, also a

light, electricity or magnetism, look Nave &Nave

Among theAmong the subjectively percieved subjectively percieved properties of sound,properties of sound, the most important is the most important is the the StrenghtStrenght . The human ear can dis. The human ear can dis--tinguish the sound within the frequency tinguish the sound within the frequency range from range from 16 Hz – 20 000 Hz16 Hz – 20 000 Hz, but it is the , but it is the most sensitive for sound frequenmost sensitive for sound frequen--cies cies between 1500 – 4000 Hz. between 1500 – 4000 Hz. The The higher is the frequencyhigher is the frequency,, the bigger is the bigger is the pitchthe pitch of the sound, and of the sound, and vice versavice versa..The The LLoudness of the soundoudness of the sound closely co closely co--rrelates withrrelates with the the Amplitude Amplitude of the sound.of the sound.The The higher is thehigher is the amplitudeamplitude, the , the louder louder is the sound, andis the sound, and vice versavice versa..

IntensityIntensity of the sound in created in environment is mea-of the sound in created in environment is mea-sured in units named sured in units named decibels (dB).decibels (dB). The ear threshold for the The ear threshold for the minimal sound intensityminimal sound intensity coming coming from environment is from environment is 00 dB. dB.Subjectively percieved soud intensitySubjectively percieved soud intensity is measured in is measured in units named units named Phons (PhPhons (Ph)). . At frequency of 1 kHz the scale At frequency of 1 kHz the scale for sound intensity (given in dB for sound intensity (given in dB )) equals to scale for equals to scale for subjectively percieved intensity (given in Phonssubjectively percieved intensity (given in Phons))The perception of sound changes into the The perception of sound changes into the pain sensa-pain sensa-tiontion at the sound intensity around 130 dB (the pain at the sound intensity around 130 dB (the pain threshold).threshold).The hearing thresholdThe hearing threshold is examined by a diagnostic is examined by a diagnostic method – method – AUDIOMETRYAUDIOMETRY. . Audiometer is an electronic Audiometer is an electronic generator of a sound frequencies at different intensities. generator of a sound frequencies at different intensities. AudiometryAudiometry enables to obtain a graphical record of hea-enables to obtain a graphical record of hea-

ring loss for right and left ears.ring loss for right and left ears.

EAR - schemeEAR - scheme

EAR ANATOMY. EAR ANATOMY. 

Ear consists of: I. Outer Ear – composed of the Pinna (auricula), and the Auditory canal (Ear canal) II. Middle Ear – composed of Tym-panic membrane (Ear drum) + 3 ossicles: Maleus

(Hammer), Incus (Anvil), Stapes (stirrup) + Oval win-dow and the Round window + Two tiny musles (the stapedius muscle and tensor tymphani muscle) being connected with the ossicles + Eustachian tube (3.5 cm long tube connecting the middle ear with pharynx, thus balancing the air pressures within the middle ear and in the atmosphere. (Inflammation can enter from nasopharynx to the middle ear in children, because ET is short) III. Inner Ear - Cochlea with Organ of Corti

SOUND PERCEPTIONSOUND PERCEPTION

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Composition of Inner Ear

Organ of Corti - compositionOrgan of Corti - composition

Mechanism of hearing:Mechanism of hearing:

The pinnaThe pinna captures the sound energy from the envi- captures the sound energy from the envi-ronment and directs it through theronment and directs it through the auditory canal toauditory canal to the the eardrumeardrum. Then, the sound passes. Then, the sound passes three small ossic-three small ossic-lesles, , being located within the midle ear. being located within the midle ear. Next, when pas-sing the last ossicle – stapes - the sound comes throu-ght the Oval window into the Cochlea of the inner ear.The Cochlea is divided into three parts: SCALA VESTIBULY (is filled in with a perilymph, that consists of a high concentration of Na+ cations, and Cl -anions and higher amount of proteins) , SCALA MEDIA (filled with endolymph consisting a high concentration of K+ cations and lower concentration of Na+ and Cl- ions) and SCA-LA TYMPANI (filled again with perilymph). The different concentrations of ions (mainly K+) play a key role in

production of local electrical changes (GP and AP).  

Within the Scala Media there is the Corti´s  Organ, placed on the basilary membrane, consisting of one row of the INNER HAIR CELLS (they serve as proper

receptors for hearing) + three rows of OUTER HAIR CELLS (serving as pre-amplifier) both equipped with stereocilia. Entering the Inner Ear, the sound pushes both the perilymph and endolymph, resulting in the liquid movements. Such liquid oscillations bend the stereocilia of the hair cells within the Corti´s Organ of the Scala Media. One direction of bending causes the depola-rization of the Hair cells, an opposite bending results in their hyperpolarization. GP rises up within the filaments of Inner Hair cells. AP is created in the axons leaving the Inner Hair Cells.

These APs are transmitted throughThese APs are transmitted through thethe Cochler nerve ultimately reaching the Temporal lobe of the Auditory brain cortex, where they are processed and perceived as a sound.

According to BEKESY´S HEARING THEORY: we are able to perceive the high frequencies of sound at the base (at a botom) of the cochlea. Contrary, the sound with a low frequencies stimulates the hair cells near the top of the cochlea, named the helicotrema. Thus, the high pitch sound (e.g. 10,000 Hz) is detected by the hair cells placed on the basilary membrane at the base of the Cochlea, whereas the low pich sound (e.g. 200 Hz) is detected by the receptors near the helicotrema.

ANIMATIONANIMATION

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