12.3 Hearing and the Ear

12.3 Hearing and the Ear

• Waves spread through a medium.
• When a wave encounters an obstacle, it spreads into the region behind it.
• The spread of the wave depends on the wavelength.
• If the size of the obstacle is smaller than the wavelength, there will be significant diffraction into the region behind it.

• A person sitting behind a pillar in an auditorium can hear the performer because of the long wavelength sound waves behind the pillar.
• Because the wavelength of light is smaller than the pillar, the light cannot diffract into the region behind the pillar.

• There is no significant reflection when objects are smaller than the wavelength.
• Diffusion is to blame for this too.
• The wave diffracts around the small obstacle, like the water spreads around a stick.

• Light and sound waves can be focused with curved reflectors.
• There is a limit to the size of the focused spot.

• Waves have consequences in the process of hearing and seeing.

• The sensation of hearing is caused by the response of the nerves in the ear.
• Most of the skin is pressure-sensitive, and the nerves in the ear are not the only ones that respond to pressure.
• The ear is more sensitive to pressure variations than any other part of the body.

• There is a drawing of the ear.
• The outer ear, middle ear, and inner ear are the main parts of the ear.
• The inner ear contains sensory cells that convert sound to nerve impulses.

• The purpose of the outer and middle ears is to amplify the sound.

• The pinna helps the animal locate the source of sound by rotating it toward the source.
• The pinna is small and fixed in humans, so it doesn't seem to contribute much to the hearing process.

• A drawing of the ear with various structures cut away and simplified to show the basic relationships more clearly is a semidiagrammatic drawing.
• The middle ear muscles are not present.

• The ear canal of an average adult is about 0.75 cm in diameter and 2.5 cm long, a configuration that is good for sound waves.
• The ear is sensitive to sound waves in this range.

• The sound has to be coupled from air to the sensory cells in the inner ear in order for an animal to hear it.
• Most of the sound energy is reflected at the interface, so it's inefficient to put sound waves into a fluid.
• The middle ear has an efficient path for sound waves to enter the inner ear.

• The hammer is attached to the eardrum and the stirrup is connected to the window in the inner ear.

• When sound waves hit the eardrum, the ossicles send the sound waves to the window, which in turn causes the fluid of the inner ear to change.
• The muscles that control the volume of the middle ear are connected to the ossicles.
• The transmission of sound to the inner ear is reduced if the sound is loud.

• The middle ear is used for more than one purpose.
• The person's own voice, chewing, and movements of the head can cause noise in the inner ear.
• The sound of the vocal cords is transmitted through the bones into the inner ear, but it is not as strong as it could be.
• We hear ourselves talking mostly from the outside.
• Talking with ears plugged can show this.

• Air enters through this tube to keep the middle ear moist.
• The movement of air through the Eustachian tube is aided by swallowing.
• A sudden change in the air pressure can cause a pressure discrepancy on the two sides of the ear.

• The force on the eardrum causes a painful sensation until the pressure in the middle ear is adjusted.
• If the Eustachian tube is blocked, the pain is even worse.

• The cochlea is shaped like a snail shell.
• The wide end of the cochlea has an area of about 4mm2.
• The cochlea is formed into a spiral.

• There are three ducts.
• The two canals contain the cochlear duct.

• The sound wave came from the fluid in the canal.
• The sound wave, which travels along the tympanic canal and through the helicotrema into the vestibular canal, stimulates the auditory nerves to send electrical signals to the brain.
• The round window at the end of the tympanic canal is where the excess energy in the sound wave is dissipated.

• The sensation of sound in the brain is evoked by nerve impulses.

• The physical properties of sound such as intensity and Frequency are difficult to relate to the subjective responses.
• Some of these relationships are well understood, while others are still subjects for research.

• The sound wave patterns produced by instruments and voices are very complex.
• If the response to each sound pattern had to be analyzed separately, it would be impossible to evaluate the effect of sound waves on the human auditory system.

• The problem is not that complicated.

• J. Fourier showed that simple waves of different frequencies can be analyzed.
• A complex wave pattern can be created by adding a sufficient number of waves at appropriate frequencies and amplitudes.

• We can evaluate the ear's response to a wave pattern if we know the response of the ear to a broad range of frequencies.

• Figure 12.7 shows an analysis of a wave shape into its components.
• One sound source is different from another.

• The wave shape shown in (a) is a result of the point-by-point addition of the fundamental frequencies.

• The human ear can detect sound at a range of frequencies.
• The ear's response is not uniform within this range.
• The ear is most sensitive to higher frequencies and less sensitive to lower frequencies.

• There are many different frequencies in the response of individuals.
• A few people can hear sounds above 20,000 Hz, while others cannot.
• Hearing gets worse with age.

• The pitch is related to the sound.
• The pitch goes up with Frequency.

• Wave forms of sound come from different musical instruments.

• There is no simple relationship between pitch and Frequency.

• The lowest intensity that the human ear can detect is about 10-16 W/ cm2.
• The intensity of the loudest sound is 10-4 W/ cm2.
• Permanent damage to the eardrum and the ossicles can be caused by sound intensities above the threshold of pain.

• A sound which is a million times more powerful than another does not evoke a million times higher sensation of loudness because the ear does not respond linearly to sound intensity.
• The ear's response to intensity is close to being linear.

• It is convenient to express sound intensity on a scale because of the large range of inten sities involved in hearing.

• Table 12.1 shows the intensity of some common sounds.

• It was thought that the ear would respond to sound intensity in a certain way.
• The power of the street sounds is a million times greater than the power of the rustle of leaves.

• The assumption of a logarithmic response still provides a useful guide for assessing the sensation of loudness produced by sounds at different intensities, despite the fact that the intensity response of the ear is not exactly logarithmic.

• The ear is sensitive.
• The ear can detect a sound intensity of 10-16 W/ cm2 at the threshold of hearing.