Perception - Loudness and Pitch Flashcards
What is the perceptual definition of sound?
Sound if the experience we have when we hear
What is the physical definition of sound?
Sound is pressure changes in air or other medium caused by the vibration of an object
What is a pure tone?
Simplest type
Occurs when changes in air pressure form perfect sinusoidal wave
What are the characteristics of pure tones?
Amplitude
Frequency
What is amplitude?
How much difference between increases and decreases in pressure
Size of variation in air pressure
Difference between peak and trough
What is amplitude related to?
Perception of loudness
What is frequency?
How many cycles of decreases and increases in pressure travel past a certain point in a second
Number of cycles per second
What is frequency measured in?
Hertz (1 Hz = 1 cycle)
What is frequency related to?
Perception of pitch
What are complex sounds?
Made up of lots of different frequencies of sound mixed together
All sound waves can be described as some combination of sine waves
What do natural sounds consist of?
Fundamental frequency superimposed by additional waveforms with higher frequencies (the harmonics)
What is a fundamental frequency?
Lowest, most dominant frequency
What is the ear divided into?
Three sub-divisions = outer, middle and inner ear
What are the features of the outer ear?
Pinnae
Auditory canal
Tympanic membrane (ear drum)
What is the pinnae?
Visible external parts of the ear
What is the function of the pinnae?
Capture sound waves and direct down ear (auditory) canals
What is the auditory canal?
~3cm tube-like structure
What is the function of the auditory canal?
Protects middle ear
What is the tympanic membrane (ear drum)?
Cone-shaped membrane separating outer and middle ear
What is the function of the tympanic membrane (ear drum)?
Sound waves induce difference in pressure either side of tympanic membrane, causing it to vibrate
Larger amplitude sounds result in larger vibrations
Higher frequency sounds result in faster vibrations
What are the features of the middle ear?
Small cavity (~2 cubic cm) containing ossicles (smallest bones in the human body)
What are the ossicles?
Malleus (hammer)
Incus (anvil)
Stapes (stirrup)
What is the function of the ossicles?
Bones amplify vibrations of tympanic membrane and transmit to inner ear at oval window
What are the features of the inner ear?
Main structure is cochlea (snail-like liquid-filled organ)
Basilar membrane
What are the features of the cochlea?
Three parallel canals - vestibular, middle, tympanic
What is the basilar membrane?
Separates middle and tympanic canals
What is the function of the inner ear?
Vibration of oval window displaces fluid in cochlea, resulting in change of pressure which moves up and down spiral structure
Auditory transduction triggered by motion of basilar membrane
Pressure waves in cochlea causing basilar membrane to move
What is auditory transduction?
Motion of basilar membrane translated into neural signals by structures in Organ of Corti, which extends along its surface
Voltage generated when specialised hair cells contained within Organ of Corti bent
Produces impulses in auditory nerve cells which are sent to brain
Hair cells extremely sensitive - overstimulation by loud sounds can damage hair cells and lead to hearing loss
How are sound levels measured to describe differences in amplitude?
In decibels
Logarithmic scale
Change of 20dB corresponds to ten-fold increase in amplitude
What is rate code?
Sound amplitude coded in the firing range of auditory nerve fibres
Responses increase with increasing sound intensity
How does a high amplitude affect the rate code?
High amplitude
High firing rate
High rate code
How does the rate code allow us to discriminate loudness across a range of sound levels?
Some fibres have high spontaneous rates and saturate rapidly while others have low spontaneous rates and saturate slowly
Are amplitude and loudness directly proportional?
No
For sound to be perceived as twice as loud, amplitude needs to be increased by factor of ~3.16 (10dB)
Does loudness depend on frequency?
Yes
Auditory systems not equally sensitive to all sound frequency
If the frequency is higher, is the pitch higher or lower?
Higher
What is the lowest and highest frequencies that a human can hear?
Lowest = 20Hz (below can feel the sound)
Highest = 20,000Hz (20kHz)
What are the two ways to code frequency?
Place code
Timing code
What is the place code?
Different auditory nerve fibres respond to different frequencies
Sound frequencies causes vibration in specific areas along the basilar membrane
Where do low frequencies cause vibrations in the basilar membrane?
Near apex
Where do high frequencies cause vibrations in the basilar membrane?
Near base
What is the timing code?
Not only which auditory nerve fibres respond but when they respond
Hair cells respond in little packets of firing, not all the time
Auditory nerve responses synchronised to changes in pressure - property called phase-locking
Occurs up to frequencies of about 400Hz (lower frequencies)
What has been suggested by experiments using electrical stimulation via cochlea implants?
Both place and timing of stimulation affect perception of pitch
Why does the same note sound different on different instruments?
Pitch determined primarily by fundamental frequency of sound
Instruments have same fundamental frequencies but different harmonics
Number, frequency ratios and relative amplitudes of the harmonics dictate the quality or timbre of sound
What is the missing fundamental illusion?
Continue to perceive pitch consistent with missing fundamental frequency
Pitch isn’t simply determined by cochlea, brain infers missing fundamental from harmonics
