Hearing and Touch

Physical Definition of Sound

• pressure changes in a medium (water, air, liquid etc)

• sound is a vibratory disturbance that occurs in air or some other medium

• sound consists of waves-local changes in pressure, travel through the medium

Perceptual Definition of Sound

the experience of hearing

Condensation

increase in pressure pushes particles together

Refraction

decrease in pressure, particles spread out

What is the sound perception of changes in frequency?

Pitch

Units of Frequency

• measured in Hertz

• a 1 Hz sound is one cycle per second

Frequency

• the lower the frequency, the more bass the sound has, and conversely, the higher the frequency, the more treble the sound has

• humans typically hear sounds in the frequency range of 20 Hz to 20,000 Hz

Pitch

the perceptual experience of frequency ( it is a high or low a sound seems to us)

Tone Chroma (Pitch Class)

refers to the quality of a pitch that makes it recognizable as belonging to a particular musical note

What is the sound perception of changes in amplitude?

loudness

How is amplitude measured?

• measured in decibels (dB), which refer to the sound pressure level or intensity

• the decibel scale is logarithmic, meaning that an increase of 10 dB represents a tenfold increase in sound intensity

What is the sound perception of changes in waveform?

timbre

Waveform

• the shape of a sound over time

• tells us how the air pressure changes as the sound travels

Timbre

• what makes sounds different even when they have the same pitch (frequency)

• it’s why a violin and a piano playing the same note

Complex Waves

• a combination of simple waves (sine waves) happening at the same time

Fundamental Frequency

the main pitch you hear

Harmonics (Overtones)

• extra frequencies that give sound its richness and timbre

The Life Cycle of a Sound

• Attack: how fast a sound builds up from silence to its full volume

• Decay: how quickly a sound drops in volume after the attack

• Sustain: how long a sound stays at a steady volume after the initial attack and decay

• Release: how long it takes for a sound to fade away after you stop playing it

Binaural Cues

• Interaural Time Difference

• Interaural Level Difference

Monaural Cues

• Spectral Cues

• Reverberation and Distance Cues

Interaural Time Difference (ITD)

• sound reaches one ear slightly earlier than the other

• usually less than a millisecond

• sound engineers use ITD cues to create 3D in games

Interaural Level Difference (ILD)

• the head blocks some of the sound from reaching the far ear, making it quieter

• when a sound is closer to one ear, that ear hears it louder, while the other ear hears it quieter due to the head shadow

• ILD is strongest for high pitched sounds (like birds chirping)

Acoustic Shadow

• your head blocks sound waves, making them quieter on the far side

• low frequency sounds have long wavelengths and can wrap around your head, so they don’t create much of a shadow

Cone of Confusion

• imaginary cone-shaped region extending from each ear, where sounds from different locations can create the same Interaural Time Differences (ITD) and Interaural Level Differences (ILD)

Spectral Cues

• the outer ear (pinna) changes how sound waves bounce before they enter the ear canal

• listeners were measured for performance locating sounds differing in elevation; they were then fitted with a mold that changed the shape of their pinnae

• right after the molds were inserted, performance was poor for elevation but was unaffected for azimuth

Reverberation and Distance Cues

• Reverberation occurs when sound waves reflect off surfaces in an environment before reaching the listener

• echo is a specific case of reverberation where the time delay is long enough for the brain to separate the original sound from its reflection

• reverberation is essentially a dense collection of echoes, where multiple reflections overlap and blend together before the sound fully decays

What does the outer ear consist of?

• Pinna

• Auditory canal

• Tympanic membrane

Auditory Canal

• The ear canal acts like a natural amplifier, especially for sounds in the 2,000-5,000 Hz range

• Its shape helps channel and focus sound waves toward the tympanic membrane (eardrum)

• The curved shape of the canal helps prevent debris, dust, and insects from reaching the delicate structures of the middle and inner ear

What structures make up the middle ear?

• Tympanic membrane'

• Ossicles

• Eustachian Tube

• Oval Window

• Round Window

Tympanic Membrane

• cone shaped membrane that vibrates when sound waves hit it

Malleus (Hammer)

• connected to the eardrum, receives vibrations first

Incus (Anvil)

• acts as a bridge between the malleus and stapes

Stapes (Stirrup)

• transfers vibrations to the oval window of the cochlea

Eustachian Tube

• connects the middle ear to the throat (nasopharynx)

Oval Window

• receives vibrations from the stapes and passes them into the fluid filled cochlea

Round Window

• helps dissipate excess sound energy, preventing damage to the inner ear

What are the two middle ear muscles?

• Stapedius Muscle

• Tensor Tympani Muscle

Stapedius Muscle

• reduces excessive vibration of the stapes to protect the inner ear from loud sounds

Tensor Tympani Muscle

• Contracts in response to loud sounds, chewing, or self-generated noises

• Helps regulate the tension of the eardrum to adjust sound sensitivity

What does the inner ear consist of?

• Vestibular System (Balance Center)

• Cochlea (Hearing Center)

Vestibular System

• helps you stay upright and sense movement

Cochlea

• a spiral-shaped, fluid-filled structure

• converts sound waves into electrical nerve signals

• different parts of the cochlea respond to different frequencies

What are the three chambers of the cochlea?

• Scala Vestibuli (Upper Chamber)

• Scala Media (Middle Chamber)

• Scala Tympani (Lower Chamber)

Scala Vestibuli

• filled with perilymph (fluid), receives vibrations from the stapes at the oval window

Scala Media

• filled with endolymph and houses the Organ of Corti (the actual hearing organ)

Scala Tympani

• also filled with perilymph, ends at the round window, which releases excess pressure from sound waves

How the Cochlea Processes Sound

• Sound waves enter through the oval window via the stapes, creating waves in the perilymph

• These waves travel along the basilar membrane, stimulating specific hair cells based on frequency

• The Organ of Corti detects vibrations and converts them into electrical signals

The Organ of Corti

• located inside the scala media and contains hair cells that convert mechanical sound vibrations into electrical signals