Sound and human ear

Sound and human ear 7.6.12

Revision • Properties of waves intensity, amplitude, frequency, wavelength, time period • Properties of sound pitch, loudness, quality • Pitch and quality of sound is related to frequency of sound • Loudness is related to intensity/amplitude of sound wave

Revision • Sound, speed of sound in various mediums, breaking the sound barrier, echolocation • Standing waves • Fundamental frequency and its harmonics • Acoustic resonance in a flute (standing waves in an open tube) • Acoustic resonance in a partially closed tube (standing waves in a tube open at one end and closed at other)

Basic parts of human ear • Outer ear • The Pinna • The Ear Canal • The Eardrum (tympanic membrane) b. Middle ear c. Inner ear

Outer Ear Ear Canal Pinna Ear Drum

Outer Ear • Pinna collects the sound and directs it to ear canal • Because of the length of the ear canal, it is capable of amplifying sounds with frequencies of approximately 3000 Hz. Ear canal acts as a resonator for this fundamental frequency • As sound travels through the outer ear, air molecules inside the ear canal propagates the sound wave with an alternating pattern of high and low pressure regions • The arrival of compression and rarefaction pulls the eardrum inward and outward respectively setting it into vibration

Basic parts of human ear a. Outer ear b. Middle ear • Auditory Ossicles • The Tympanic Cavity • The Eustachian Tube c. Inner ear

Middle ear • The middle ear is an air-filled cavity that contains a linkage of three bones called “ossicles” that connects the eardrum to the inner ear • The three bones are called the malleus, incus and stapes, or, reflecting their shapes, the hammer, anvil and stirrup • The hammer is attached to the inner surface of the eardrum and the stirrup is connected to the oval window which is a membrane covered opening to the inner ear

The ossicles are the three smallest bones in the human body!

Auditory Ossicles • The function of the auditory ossicles is to transmit sound from the air striking the eardrum to a fluid-filled labyrinth inside the inner ear (Cochlea). • The bones are connected by small ligaments and transmit the vibratory motions of the eardrum to the inner ear.

Transmission of sound wave by ossicles to inner ear • Being connected to the hammer, the movements of the eardrum will set the hammer, anvil, and stirrup into motion at the same frequency of the sound wave. • The stirrup is connected to the inner ear; and thus the vibrations of the stirrup are transmitted to the fluid of the inner ear and create a compression wave within the fluid

Sound amplification by middle ear • Ossicles amplify the sound reaching eardrum by lever action With a long enough lever, you can lift a big rock with a small applied force on the other end of the lever. The amplification of force can be changed by shifting the pivot point

Sound amplification by middle ear • The three tiny bones of the middle ear act as levers to amplify the vibrations (pressure) of the sound wave. • The pivot point or fulcrum is located farther from the tympanic membrane than from the stapes. This means that the displacement at the oval window is less than at the tympanic membrane but the force at the oval window is amplified. The mechanical advantage is 2 • The resulting vibrations would be much smaller without the levering action provided by the bones

Sound amplification by middle ear • The area of the eardrum is about 30 times larger than the oval window. Therefore, the pressure on the oval window is increased by the same factor • This feature enhances our ability of hear the faintest of sounds

Amplification of sound with frequency of 3000Hz • In the frequency range around 3000Hz, there is an increase in the pressure at the eardrum due to the resonance of the ear canal. This amplifies the sound pressure by a factor of 2 • Lever action amplifies by another factor of 2 • Smaller area of oval window amplifies the sound by a factor of 20 • Total amplification= 2 x 2 x 20=80 • This accounts for the high sensitivity of ear to this frequency range

The ossicular chain converts the large displacement, low pressure signal in air to the small displacement, high pressure signal needed for transmission through the cochlear fluid. • It does this by acting as a lever system and by exerting all the force at the tympanic membrane on a smaller surface.

The Tympanic Cavity and the Eustachian Tube • The tympanic cavity is an air chamber surrounding the ossicles within the middle ear • The Eustachian tube is a membrane lined tube (approximately 35 mm long) that connects the middle ear space to the back of the nose (the Pharynx) • The Eustachian tube does not directly relate to the mechanical process of hearing

Ossicles Ear Drum Eustachian Tube

Functions of the Eustachian tube • Pressure equalization: Air seeps in through this tube to maintain the middle ear at atmospheric pressure A rapid change in the external air pressure such as may occur during an airplane flight causes a pressure imbalance on the two sides of the eardrum. The resulting force on the eardrum produces a painful sensation that lasts until the pressure in the middle ear is adjusted to the external pressure • Mucus drainage: The tube drains mucus, keeping ears from becoming “stuffy.” Mucus stuck in the tympanic cavity can develop a high level of pressure, and can lead to ear infections

Volume control by muscles of middle ear • The ossicles are connected to the walls of the middle ear by muscles that also act as a volume control • If the sound is excessively loud, these muscles as well as the muscles around eardrum stiffen and reduce the transmission of sound to the inner ear

Why do our own voices sound different to us when we hear them on a recording vs. when we hear them as we speak • This is because there are two different ways in which we hear sounds. One is through air conduction, and the other is bone conduction. • Everyday sounds we hear are primarily hear through air conduction, which is basically sound waves traveling through your ear canal and impacting your eardrum, and eventually to the cochlea of the inner ear.

When we speak, however, we hear our voice through both air conduction and bone conduction. Bone conduction is the vibrating of our bones and body tissue which transmits sounds directly to the cochlea. • When we hear our voice on a recording, that's how it sounds to everyone else, as we are then hearing it through air conduction only • You can note the difference in your voice by talking with the ears plugged

Sound and human ear