PSGY1010 Audition 1 Loudness and Pitch PDF

PSGY1010 Cognitive Psychology 1 Audition I: Loudness and Pitch Dr Chung Kai Li [email protected]  Today’s lecture Learning objectives: ▪ Describe the stimulus for hearing ▪ Outline the basic structure and operation of the human auditory system ▪ Understand the perception of (a)...

PSGY1010 Cognitive Psychology 1 Audition I: Loudness and Pitch Dr Chung Kai Li [email protected]  Today’s lecture Learning objectives: ▪ Describe the stimulus for hearing ▪ Outline the basic structure and operation of the human auditory system ▪ Understand the perception of (a) loudness and (b) pitch, including how they relate to auditory input and how this input is coded by the auditory system 2  The sound stimulus If a tree falls in the forest and no one is there to hear it, would there be a sound? What is sound? ▪ Perceptual definition ▪ sound is the experience we have when we hear ▪ Physical definition ▪ sound is pressure changes in the air or other medium caused by the vibration of an object A pure tone occurs when changes in air pressure form a perfect sinusoidal wave 3  The sound stimulus Visualising sound with a Rubens’ tube 4  Characteristics of sound – pure tones Amplitude ▪ Size of the variation in air pressure (i.e. difference between peak and trough) ▪ Related to perception of loudness Frequency ▪ Number of cycles per second (1 Hertz = 1 cycle/s) ▪ Related to perception of pitch 5  Characteristics of sound – complex sounds ▪ Most sounds encountered in the world are more complex than pure tones ▪ All sound waves can be described as some combination of sine waves ▪ Natural sounds often consist of a fundamental frequency, superimposed by additional waveforms with higher frequencies (the harmonics) 6  Overview of the ear ▪ The human ear is divided into 3 sub-divisions: outer, middle and inner 7  Outer ear Pinnae ▪ visible external part of ear Auditory canal ▪ ~3cm tube-like structure ▪ protects middle ear Tympanic membrane (eardrum) ▪ Cone-shaped membrane separating the outer and middle ear ▪ Sound waves induce a 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 8  Middle ear ▪ The middle ear is a small cavity (~2 cubic centimetres) that contains the ossicles, the three smallest bones in the human body: ▪ Malleus (hammer) ▪ Incus (anvil) ▪ Stapes (stirrup) ▪ The bones amplify the vibrations of the tympanic membrane and transmit them to the inner ear at the oval window 9  Inner ear ▪ The main structure of the inner ear is the cochlea, a snail-like liquid-filled organ ▪ Vibration of the oval window displaces fluid in the cochlea, resulting in a change in pressure which propagates up and down the spiral structure ▪ The cochlea consists of three parallel canals (vestibular, middle & tympanic), visible in the cross-section shown here → ▪ Auditory transduction is triggered by motion of the basilar membrane, which separates the middle and tympanic canals 10  Auditory transduction ▪ Motion of the basilar membrane are translated into neural signals by structures in the organ of Corti, which extends along its surface ▪ A voltage is generated when specialised hair cells contained within the organ of Corti are bent ▪ This produces impulses in auditory nerve cells which are sent to the brain ▪ Hair cells are extremely sensitive ▪ Overstimulation by loud sounds can damage hair cells and lead to hearing loss 11 A ‘dancing’ hair cell  Loudness ▪ Our perception of loudness is related to the amplitude of sound waves ▪ The range of amplitudes we encounter is extremely large ▪ e.g. the amplitude of a very loud sound at a rock concert might be up to 1,000,000 times that of a sound you could barely hear ▪ In order to describe differences in amplitude, sound levels are measured on a logarithmic scale in units called decibels (dB) ▪ A change of 20dB corresponds to a ten-fold increase in amplitude 13  Loudness ▪ Rate code: Sound amplitude is coded in in the firing rate of auditory nerve fibers Auditory nerve rate-intensity functions ▪ Responses increase with increasing sound intensity ▪ Note, some fibers have high spontaneous rates and saturate rapidly, while others have low spontaneous rates and saturate slowly ▪ This enables us to discriminate loudness across a range of sound levels 14  Loudness ▪ Loudness depends on amplitude (but the two are not directly proportional) ▪ For a sound to be perceived as twice as loud, its amplitude needs to be increased by a factor of approximately 3.16 (10dB) ▪ Loudness depends on frequency ▪ Our auditory systems are not equally sensitive to all sound frequencies ▪ The red curves here indicate the number of decibels required to create the same perception of loudness at different frequencies 15  Pitch ▪ Humans are sensitive to a wide range of sound frequencies ▪ the lowest frequency humans can hear is 20Hz (below that we feel the sound) Frequency sweep stimulus (note you are unlikely to perceive the full range under non-optimal listening ▪ the highest frequency humans can hear is conditions) around 20,000Hz (20kHz) ▪ The green line indicates typical thresholds for detecting sounds of different frequencies ▪ Note the U-shape curve, indicative of poorer sensitivity for particularly low and high frequencies 16  Pitch Place code ▪ Sound frequencies cause vibration in specific areas along the basilar membrane ▪ low frequencies - near apex ▪ high frequencies - near base Simulation of basilar membrane deflections (blue) in response to frequency sweep stimulus (green) 17  Pitch Timing code ▪ Frequency is not only signalled by which auditory nerve fibers respond, but also when they respond ▪ auditory nerve responses are synchronised to changes in pressure ▪ this property is called phase- locking and occurs up to frequencies of about 4000Hz ▪ Experiments using electrical stimulation via cochlea implants suggest that both the place and timing of stimulation affect the perception of pitch 18  Pitch ▪ Why does the same note sound different when played on different instruments? Demo: ▪ pitch is typically determined by the fundamental frequency of a sound ▪ the number, frequency ratios and relative amplitudes of the harmonics dictates the quality or timbre of the sound In the demo the notes played by the various instruments all have the same fundamental frequency (~128 Hz), but different harmonics 19  Pitch ▪ If the fundamental frequency determines the perceived pitch of a sound, what happens if we remove it? ▪ The missing fundamental illusion ▪ counterintuitively, we continue to perceive a pitch consistent with the (missing) fundamental frequency ▪ suggests that pitch isn’t simply determined in the cochlea - the brain infers the missing fundamental from the harmonics 20  Summary Learning objective: Describe the stimulus for hearing ▪ Sound is pressure changes in a medium (typically air) caused by the vibration of an object ▪ Pure tones are sounds where pressure changes follow a sine wave and can described by their amplitude and frequency ▪ Complex sounds are made up of two or more waveforms with different frequencies ▪ Naturally produced sounds are typically made up of a fundamental frequency and several harmonics 21  Summary Learning objective: Outline the basic structure and operation of the human auditory system Outer ear ▪ The pinnae are the visible external part of the ear which funnel sound into the ear canal ▪ The tympanic membrane (ear drum) is a membrane separating outer and middle ear that vibrates in response to sound Middle ear ▪ The ossicles (malleus, incus & stapes) amplify the vibrations and transmit the to the oval window of the cochlea Inner ear ▪ Displacement of fluid up and down the cochlea produces vibration along the basilar- membrane ▪ This is converted into electrical signals via hair cells in the organ of Corti and sent to auditory nerve 22  Summary Learning objective: Understand the perception of LOUDNESS (how it relates to auditory input and this input is coded by the auditory system) ▪ Perception of loudness is related to the amplitude of sound waves ▪ the range of sound amplitudes encountered is very large ▪ measured on a logarithmic scale in units called decibels (dB) ▪ Sound amplitude is coded in in the firing rate of auditory nerve fibers (rate code) ▪ larger amplitude sounds result in higher firing rates ▪ Perceived loudness not directly proportional to amplitude ▪ For a sound to double in loudness, it needs to be more than doubled in amplitude ▪ Sounds with the same amplitude but different frequencies will differ in 23 loudness  Summary Learning objective: Understand the perception of PITCH (how it relates to auditory input and this input is coded by the auditory system) ▪ Perception of pitch is related to the frequency of sound waves ▪ Sound frequency is coded by the auditory system in two ways: ▪ Place code – sounds of a given frequencies cause vibration in a specific areas along the basilar membrane (low = near apex, high = near base) ▪ Timing code – auditory nerve responses are phase-locked to pressure changes in sounds up to around 4000Hz ▪ Perceived pitch of complex sounds is typically determined by the fundamental (lowest) frequency ▪ However, phenomena like the missing fundamental illusion demonstrate that pitch perception is not entirely determined by the cochlea (top-down processes) 24 Thank you Any questions?