Perception: Loudness & Pitch

Vocabulary practice flashcards covering acoustic stimuli, outer/middle/inner ear anatomy, auditory transduction, loudness coding, and pitch perception based on lecture notes by Dr. Neil Roach.

Sound (Physical Definition)

Pressure changes in the air or other medium caused by the vibration of an object.

Sound (Perceptual Definition)

The subjective experience an organism has when hearing.

Pure Tone

A sound that occurs when changes in air pressure form a perfect sinusoidal wave.

Rubens' Tube

An experimental apparatus comprising a perforated metal tube supplied with gas and coupled to a loudspeaker and frequency generator, used to visualize acoustic standing waves as varying flame heights.

Amplitude

The magnitude of variation in air pressure, measured as the difference between peak and trough of a sound wave; physically relates to the perception of loudness.

Frequency

The number of pressure cycles per second, measured in Hertz ($1\text{ Hertz} = 1\text{ cycle/s}$); physically relates to the perception of pitch.

Complex Sounds

Acoustic stimuli composed of two or more superimposed sine waves of differing frequencies.

Fundamental Frequency

The lowest frequency component of a complex sound wave, which typically dictates its perceived pitch.

Harmonics

Additional higher-frequency waveforms superimposed upon the fundamental frequency in natural complex sounds.

Human Ear (Three Subdivisions)

The anatomical organization of the human auditory periphery consisting of the outer ear (pinna, auditory canal), middle ear (tympanic membrane, ossicles), and inner ear (cochlea, semicircular canals).

Pinnae

The visible external anatomical structures of the outer ear that funnel sound into the auditory canal.

Auditory Canal

A tube-like structure approximately $3\text{ cm}$ in length within the outer ear that protects the middle ear.

Tympanic Membrane

A cone-shaped membrane (eardrum) separating the outer and middle ear that vibrates when sound waves create pressure differences across it; larger amplitude sounds cause larger vibrations, and higher frequency sounds cause faster vibrations.

Ossicles

The three smallest bones in the human body—the malleus (hammer), incus (anvil), and stapes (stirrup)—located in the middle ear cavity ($\sim 2\text{ cm}^3$) that amplify tympanic vibrations and transmit them to the oval window.

Oval Window

A membrane-covered opening beneath the footplate of the stapes through which vibrations are transferred into cochlear fluid.

Cochlea

A snail-like, liquid-filled organ of the inner ear consisting of three parallel canals: the vestibular canal, middle canal, and tympanic canal.

Basilar Membrane

The membrane separating the middle and tympanic canals within the cochlea whose mechanical motion triggers auditory transduction.

Organ of Corti

The auditory receptor structure extending along the surface of the basilar membrane that contains specialized hair cells.

Hair Cells

Specialized sensory receptor cells within the Organ of Corti that generate a voltage when mechanically bent, generating impulses in auditory nerve fibers sent to the brain.

Decibel ($\text{dB}$)

A logarithmic unit used to quantify sound amplitude and pressure level; a change of $20\text{ dB}$ corresponds to a $10\text{-fold}$ increase in amplitude.

Rate Code (Loudness)

The mechanism by which sound amplitude is encoded in the auditory system via the firing rates of auditory nerve fibers, where higher amplitudes yield increased firing.

Auditory Nerve Rate-Intensity Functions

Response profiles showing that some auditory nerve fibers have high spontaneous rates and saturate rapidly, while others have low spontaneous rates and saturate slowly, facilitating loudness discrimination across broad amplitude ranges.

Loudness Doubling Requirement

The acoustic principle that loudness is not directly proportional to amplitude; for a sound to be perceived as twice as loud, its amplitude must increase by a factor of approximately $3.16$ ($10\text{ dB}$).

Equal Loudness Curves

Plotted curves indicating the sound pressure levels in decibels ($\text{dB}$) required at various frequencies to produce an identical subjective loudness perception.

Human Auditory Frequency Range

The normal frequency limits of human acoustic perception, extending from $20\text{ Hz}$ (below which pressure waves are felt rather than heard) up to approximately $20{,}000\text{ Hz}$ ($20\text{ kHz}$).

Audibility Curve

A U-shaped curve defining the threshold of hearing across frequencies, showing poorer sensitivity for extremely low and extremely high frequencies.

Place Code

The tonotopic pitch coding mechanism wherein different sound frequencies induce maximum vibration at distinct anatomical locations along the basilar membrane.

Apex vs. Base (Basilar Membrane)

The tonotopic differential of the basilar membrane where high sound frequencies maximally deflect the base (near the oval window) and low sound frequencies maximally deflect the apex.

Timing Code

The pitch coding mechanism whereby sound frequency is signaled by the temporal synchronization of auditory nerve firing relative to the pressure cycle of the acoustic waveform.

Phase-Locking

The tendency of auditory nerve fibers to fire impulses synchronously with specific phases of a sound pressure wave, functioning for frequencies up to approximately $4000\text{ Hz}$.

Timbre

The subjective tonal quality of a sound that differentiates musical instruments playing the same fundamental pitch, governed by the number, frequency ratios, and relative amplitudes of their harmonics.

Missing Fundamental Illusion

A psychoacoustic phenomenon where listeners continue to perceive the pitch corresponding to a fundamental frequency even after it has been removed, demonstrating that pitch is inferred centrally by the brain rather than solely resolved in the cochlea.