Audition

Sound Creation and Propagation

• Sounds are generated when objects vibrate.

  • Object vibrations lead to the vibration of surrounding molecules in a medium, creating pressure changes.

  • Fluctuations in air pressure occur over time.

  • The vibrating source causes air molecules to pack at different densities.

  • Changes in air pressure propagate as a longitudinal wave.

Characteristics of Sound Waves

Speed of Sound

• Sound waves travel at a speed that depends on the medium.

  • Example: Speed of sound is approximately $340 ext{ m/s}$ (about 1100 feet per second) in air.

  • In water, the speed of sound is approximately $1500 ext{ m/s}$.

  • Time taken: Sound travels 1 mile in about 5 seconds. (Used for estimating the time between lightning and thunder.)

Wave Properties

• Sound waves consist of compression and rarefaction.

• Represented visually as sine waves.

  • Example Image:

  • Compression: Peaks of the sine wave.

  • Rarefaction: Troughs of the sine wave.

Frequency and Amplitude

• Frequency:

  • Measured in cycles per second (cps) or Hertz (Hz).

  • Frequency relates to pitch.

• Amplitude:

  • Intensity of compression (height of a wave cycle).

  • Amplitude relates to loudness.

• Most sounds consist of multiple frequencies and amplitudes.

Perception of Sound

Physical Dimensions

• Two fundamental physical dimensions of sound:

  • Frequency and Amplitude.

Perceptual Dimensions

Pitch

• Higher frequencies perceived as higher pitch.

• Human hearing ranges from 20 Hz to 20,000 Hz.

• Frequencies examples:

  • 200 Hz Sine tone.

  • 400 Hz Sine tone.

Loudness

• Higher amplitude results in louder sounds, measured in decibels (dB).

• The threshold of hearing is 0 dB.

Loudness in Everyday Sounds

• Sound Decibels with Associated Sound Pressure Ratios:

  • Rustling Leaves: 20 dB (10 times pressure ratio)

  • Ambient Office Noise: 40 dB (100 times pressure ratio)

  • Conversation: 60 dB (1000 times pressure ratio)

  • Auto Traffic: 80 dB (10,000 times pressure ratio)

  • Lawn Mower: 100 dB (100,000 times pressure ratio)

  • Concert: 120 dB (1,000,000 times pressure ratio)

  • Jet Engine: 140 dB (10,000,000 times pressure ratio)

  • Each increase of 10 dB indicates a tenfold increase in sound pressure.

• Loudness varies significantly in daily environments.

Human Hearing Sensitivity

• Human hearing can detect frequencies from about 20 Hz to 20,000 Hz across various intensity levels.

• The threshold of hearing varies by species, e.g., elephants can hear lower frequencies while dogs perceive higher frequencies.

Sine Waves and Complex Sounds

Pure Tones vs. Complex Sounds

• Pure Tones:

  • Simple sine waves with a single frequency.

• Complex Sounds:

  • Mixtures of different frequencies and amplitudes.

• Sound quality can depend on the combination of frequencies involved.

  • All complex sounds can be described mathematically using Fourier analysis.

  • Combining sine tones produces complex tones (Fourier synthesis).

Timbre

• Timbre refers to the quality of sound, dependent on higher frequency harmonics.

  • Harmonic series differentiates musical instruments.

The Auditory System

Basic Structure

Outer Ear

• Includes the pinna and ear canal.

  • Sounds are collected from the environment by the pinna, funneled into the ear canal.

  • The ear canal's shape enhances specific sound frequencies while protecting the tympanic membrane (eardrum).

Middle Ear

• Comprised of the tympanic membrane and ossicles (the smallest bones in the body).

  • Ossicles amplify and transmit sound to the inner ear.

  • Contains three tiny bones:

  • Malleus (hammer)

  • Incus (anvil)

  • Stapes (stirrup), which connects to the oval window of the cochlea.

Inner Ear

• Converts fine changes in sound pressure into neural signals.

• The cochlea:

  • Has spiral structure filled with watery fluids and houses the organ of Corti.

  • Contains the basilar membrane, which helps in sound transduction.

Hair Cells and Signal Transduction

Hair Cells

• Support stereocilia that transduce mechanical movements into neural activity.

  • Bending of hair cells produces action potentials in the auditory nerve.

• Stereocilia:

  • Hair-like extensions that initiate neurotransmitter release when flexed.

Neural Coding of Sound

• Coding of amplitude and frequency occurs in the cochlea through:

  • Place Code: Different parts of the cochlea tuned to various frequencies.

  • Temporal Code: Neurons’ firing rates match the sound wave cycles.

  • Phase locking: Refers to the firing of a single neuron in relation to the sound wave cycle.