Loudness and Pitch Flashcards
What is a pure tone?
When changes in air pressure form a perfect sinusoidal wave
What are the characteristics of sound?
Amplitude - size of variation is air pressure (difference between peak and trough) - related to perception of loudness
Frequency - number of cycles per second (1 Hetz = 1 cycle) Related to pitch
Characteristics of complex sounds
Natural sounds consist of fundamental frequency (lowest component) superimposed by additional waveforms with higher frequencies (harmonics)
What are the components of the outer ear?
Pinnae = virile external parts of the ear Auditory canals = 3cm tube like structure, protects the middle ear Tympanic membrane (eardrum) = cone shaped membrane separating outer and middle ear. Sound waves induce difference in pressure either side of the membrane causing vibration. - large amplitude sound = larger vibration, higher frequency = faster vibration
What are the components of the middle ear?
Small cavity that contains ossicles = 3 smallest bones:
- malleus (hammer)
- Incus (anvil)
- Stapes (stirrup)
Bones amplify the vibrations of tympanic membrane and transmit them to inner ear at oval window
What are the components of the inner ear?
cochlea - snail-like liquid filled organ.
Vibration of the oval window displaces fluid in cochlea, resulting in change in pressure which propagates up +down spiral structure
Cochlea consist of 3 parallel canals - vestibular, middle and tympanic
Auditory transduction triggered by motor of basilar membrane which separates middle and tympanic (lower) canals
What is auditory transduction?
Motion of basilar membrane translated into neural signals by structures in organ of corti - along surface
The voltage is generated with specialised + sensitive hair cells contained within organ of corti are bent - produces impulses in auditory nerve cells and sent to brain
Overstimulation by loud sounds can damage hair cells and lead to hearing loss
Loudness measurement?
Measured in logarithmic scale in decibels - change in 20 decibels corresponds to ten-fold increase in amplitude.
What is a rate code
Sound amplitude is coded in the firing rate of auditory nerve fibres. As sounds intensity increases the firing rate increases.
Some fibres have high spontaneous rates and saturate rapidly whilst others have low
- this allows us to discriminate loudness across a range of sound levels
Loudness and amplitude not directly proportional - loudness also depends on frequency, as auditory systems not equally sensitive to all sound frequencies
Pitch measurement
lower frequency - 20Hz
higher frequencies - 20,000Hz
Higher frequency = higher pitch
Place code
Sound of different frequencies cause vibrations in specific areas along basilar membranes
- low frequency towards apex
- high frequency near base
Hair cells along basilar membrane connected to different auditory nerve fibres therefore sounds of different frequencies stimulate different nerve fibres
Timing code
Frequency signalled by WHEN auditory nerve cells respond
When basilar membrane moved and hair cells bent triggering auditory nerves - specific directions of hair cells needed to fire nerve cells = called phase locking (happens until 400Hz)
(Hair cells bent from changes in air pressure with sound waves)
High frequency is rapid forward and backward movement of hair cells
Why does same note sound different on different instruments?
Pitch is determined by fundamental frequency of sound (lowest component)
Number, frequency ratios and relative amplitudes of harmonics detects quality of timbre of sound
Missing fundamental illusion
Continue to perceive pitch consistently when sounds includes fundamental frequency and excludes.
- suggests pitch not only determined in cochlea - brain infers missing fundamental from harmonics
