The Human Ear and Sound Frequencies

Sound Stimulus

• Sound as a physical stimulus begins with the movement of an object, which sets off waves of vibration in the form of miniature collisions between adjacent molecules.
• Movement produces outwardly moving bands of high and low pressure.
• Sounds vary in amplitude, frequency, and complexity.

Sound Intensity and Frequency

• Intensity is measured on a logarithmic scale based on the decibel (dB).
• Frequency is measured on a scale based on hertz (Hz).
• Human hearing ranges from 20 Hz to 20,000 Hz.
• Infrasound refers to frequencies below the range of human hearing, while ultrasound refers to frequencies beyond the upper range of human hearing.

The Auditory System

The Outer Ear

• Pinna: collects and focuses sound.
• Auditory canal: a tube-shaped structure that channels sound from the pinna to the middle ear.

The Middle Ear

• Tympanic membrane (eardrum): forms the membrane between the outer ear and the inner ear.
• Ossicles (malleus, incus, stapes): transfer sound energy from the outside air to the fluid in the inner ear.
• Oval window: the membrane between the middle ear and inner ear.

The Inner Ear

• Semicircular canals: part of the vestibular system.
• Cochlea: responsible for auditory transduction.

Auditory Transduction

• Basilar membrane moves as sounds travel through the cochlea.
• The movement of the basilar membrane is sensed by hair cells attached to the organ of Corti.
• Inner hair cells: actual auditory receptors.
• Outer hair cells: amplify sound.

Central Auditory Pathways

• Spiral ganglion neurons connect the hair cells of the cochlea with the brain.

The Primary Auditory Cortex (A1)

• Organised in columns that respond to single frequencies.
• Other neurons within A1 respond to differences in sound intensity.

Auditory Perception

Pitch Perception

• Pitch can vary due to factors other than frequency, such as intensity and context of a stimulus.
• Most frequencies are systematically encoded by the auditory system through tonotopic organisation.

Loudness Perception

• The decibel level of a sound and its perceived loudness are related but not the same.
• Decibels describe the physical qualities of the sound stimulus, while loudness is the human perception of that stimulus.
• Our ability to detect loudness varies with the frequency of a sound.

Localisation of Sound

• Horizontal plane: differences in the arrival time of sounds to each ear and differences in the intensities of sounds in each ear.
• Vertical plane: the pinna.

Hearing Disorders

• A person is considered legally deaf when speech sounds of 82 dB or less cannot be heard.
• Age-related hearing loss can result from various factors, such as poor circulation in the inner ear.
• Hearing loss can result from cumulative exposure to loud noise, made worse by the introduction of earbud devices.