PS222: Hearing I

What is the central challenge of the auditory system, parallel to the visual system's challenge?

• Dividing the incoming mix of sounds into auditory objects: figuring out what sound is what, and what sound goes with what

• In any scene (the fireworks, a quiet room) many sounds arrive together, each with its own identity and location. The auditory system must sort them into auditory objects.

Sound

• A vibration transmitted when particles in a medium crash into each other, forming a wave of compressions (particles close together) and rarefactions (particles far apart)

• A tuning fork pushes air particles together (compression) and then lets them spread (rarefaction). That vibration travels outward as a wave.

Sound vs light in a vacuum

• Light travels through a vacuum, but sound needs a medium of particles and cannot travel through a vacuum

• Sound is carried from particle to particle, so no particles means no sound. Light can cross the vacuum of space (that's how we see distant stars).

How does the speed of sound depend on the medium?

• It changes with the medium: about 344 m/s in air, faster in water (about 1,462 m/s), and faster still in denser materials like wood, iron, and stone

• Particles that are closer together start crashing into each other sooner, so denser media transmit sound faster. The speed of light, by contrast, is constant.

What does it mean that sound can be reflected or absorbed? Which example illustrates this?

• Sound bounces off hard surfaces (like bare walls) and is soaked up by soft ones (like couches or foam); an empty apartment sounds echoey because the absorbing furniture is gone

• To soundproof a room, add absorbing material (like foam) in the walls.

Why does sound get weaker farther from its source?

• The same energy spreads out over a larger and larger area as the wave moves away

• Unlike light (which keeps its energy until interrupted), sound loses energy with distance because it is a chain reaction spreading outward.

What happens to sound energy when sound passes from one medium to another (such as through a closed door)?

• It is lost: differences in density cause some of the sound to be reflected or absorbed, so transmission is less efficient

• Standing just inside a doorway versus just outside with the door closed sounds very different. This energy loss is why the middle ear exists.

Frequency of a sound wave

• Perceived as pitch; measured in hertz (cycles per second)

• Higher frequency means higher pitch. Frequency is how often the wave goes up and down per unit time (cycles per second = Hz).

Amplitude of a sound wave

• Perceived as loudness; measured in decibels

• Amplitude is the difference between the wave's peaks and valleys (the pressure change). Bigger amplitude means louder sound. Think of an amplifier.

Waveform

• Perceived as timbre (quality)

• Timbre is what makes a flute sound different from a violin or one voice from another, even with the same pitch and loudness.

How do pure tones differ from natural sounds?

A pure tone has a simple waveform, while any natural sound source produces a complex waveform

On a sound wave diagram, how do you recognize a change in amplitude versus a change in frequency?

• Amplitude: the height of the peaks and valleys

• Frequency: how many up-and-down cycles occur per unit time

• A taller wave is louder (higher amplitude). A wave with more cycles in the same time is higher pitched (higher frequency).

What range of frequencies can humans hear, and where do speech sounds fall?

• About 20 to 32,000 Hz, with speech concentrated in the middle range where we are most sensitive

• Speech sounds sit in a mid-range band, which is also where hearing is most sensitive.

How does human hearing range compare to other animals?

• Hearing ranges vary by species: bats, porpoises, and moths hear much higher frequencies, while snakes and crickets hear lower, narrower ranges

• Like different cone types and color vision, hearing ranges reflect each species' environment (for example, snakes and crickets detect low-frequency vibrations through the ground).

What is a decibel (dB), and what are the key landmarks on the decibel scale?

• A unit of sound pressure level (amplitude)

  • 0 dB is the threshold of hearing, a whisper is about 15-20 dB, and about 130 dB is the threshold of pain

• Other landmarks include a quiet rural setting, normal conversation, a passenger car, a heavy truck, a jackhammer, a rock band, and a jet or 747 at takeoff.

What is the 'damage zone' on the decibel scale, and how can you protect your hearing?

• About 90-130 dB and above, where prolonged exposure can cause hearing loss; wear earplugs at concerts and use noise-canceling headphones instead of turning up the volume on planes

• On an airplane, cranking up your audio above the engine noise raises amplitude into the danger zone. Also, don't stick a Q-tip in your ear canal.

Audibility curve

• Shows the minimum amplitude (dB) needed to detect each frequency (Hz): the absolute threshold, with lowest thresholds (best hearing) at mid-range frequencies

• Sounds above the curve are audible; sounds below it are not. We need much louder sounds to detect very low and very high frequencies.

How is 0 dB defined?

• The minimum amplitude required for a young adult with typical hearing to perceive a 1,000 Hz tone

• 0 dB is a reference point tied to the absolute threshold for a 1,000 Hz tone, not true silence.

According to the audibility curve, what happens with a 125 Hz tone at 10 dB?

• It is inaudible: it falls below the curve, because 125 Hz needs about 20 dB or more to be detected

• Low frequencies need more amplitude to be heard. Anything under the curve cannot be heard, even though 10 dB is above 0 dB.

How does the audibility curve change over a lifetime?

It changes with age and a lifetime of loud sound exposure, so louder sounds are needed to detect anything, particularly high frequencies

What are the fundamental frequency and harmonics of a natural sound?

• The fundamental is the main frequency you hear as the note (e.g., 392 Hz for G4)

• Harmonics are additional waves at multiples of the fundamental, each with its own amplitude

• Playing a note produces the fundamental plus a set of harmonics. All of these waves add up to one complex waveform.

A flute and a violin play the same G4 at the same loudness. Why do they sound different?

• Their harmonics occur at the same frequencies but with different amplitudes, which changes the waveform and therefore the timbre

• The same applies to human voices: two people singing the same note sound different because of their harmonic amplitudes.

What three things must the auditory system do to let us hear?

• Capture and detect sound vibrations

• Detect differences in frequency, amplitude, and waveform while keeping the information organized

• Transduce those signals into neural signals

• This follows the general steps: physical stimulus → sense organ → receptor cells (transduction) → neurons/brain → perceptual experience.

What is the external auditory canal, and what does earwax do there?

• The tube leading from the pinna to the eardrum

• Earwax softens the canal so sound is absorbed a bit rather than echoing

• Sound captured by the pinna travels down the canal to the tympanic membrane.

Tympanic membrane (eardrum)

• A thin, taut membrane that separates the external ear from the middle ear and vibrates in response to sound

• Sound waves in the canal make it oscillate back and forth. It is attached to the first ossicle, the malleus.

The ossicles

• Three tiny bones in the middle ear (malleus/hammer, incus/anvil, stapes/stirrup) that transmit and amplify vibrations from the tympanic membrane to the cochlea

• The malleus is attached to the eardrum, and the footplate of the stapes attaches to the oval window of the cochlea. They work like a piston.

Why does the middle ear amplify sound?

• The cochlea is fluid-filled, and sound moving from air into fluid would lose a lot of energy

• The ossicles' leverage and the large eardrum-to-small stapes footplate area ratio boost the pressure about 20-fold to compensate

• The eardrum is about 55 mm² and the stapes footplate about 3.2 mm², which plus lever action by the malleus and incus raises pressure about 22 times.

Acoustic reflex (reflexive dampening)

• Muscles (tensor tympani and stapedius) tighten and restrict the ossicles' movement in response to very loud sounds, reducing amplification to protect the ear

• It takes a few hours to relax, which is why hearing is muffled after a loud concert

• One muscle attaches to the malleus and one to the stapes. Tightening them stiffens the chain, so less sound is passed on.

The eustachian tube

It connects the middle ear to the throat and equalizes pressure between the middle and outer ear

Why do your ears feel muffled on a plane?

• The eustachian tube

• When cabin pressure differs from the middle ear's pressure, sound transmission is less efficient until you pop your ears

• Chewing gum, yawning, or blowing against a pinched nose opens the normally closed tube and equalizes the pressure.

What is the cochlea, and how does it relate to the semicircular canals and auditory nerve?

• The coiled, fluid-filled inner ear structure that holds the receptor cells where transduction happens

• The axons of those receptors leave via the auditory nerve, while the semicircular canals are for balance, not hearing

• The cochlea is embedded in the skull and shaped like a shell.