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Chapter 11: Sound and Auditory Perception

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Psychology 2115A/B
Christine Tsang

Chapter 11 Review: Auditory Processing By the end of this section, you should know about:  Sound waves and the sound stimulus  Loudness and pitch  The physiology of the auditory system  Pitch processing in the brain Why is Sound Important?  Object identification o Helps us identify things in our environment around us  Communication o For humans, communication usually occurs via spoken language  Object location o We use sound to figure out where objects are located in space o If we combine that with visual process, we are able to quickly hone in on the source that is getting our attention Sound as a Pressure Wave  Sound is essentially a wave  Can think of a sound stimulus as changes in air pressure  Tuning fork vibrates causing vibrations in the air molecules around it to vibrate o 2 components:  Compression: increase in density of molecules in the wave – increase in density causes increase in pressure of a wave  Retraction: decline in the density of a wave and, by extension, a decline in the pressure of the air molecules themselves  Sound is a wave of mechanical energy  Setting air molecules in motion and the motion of those molecules creates a pressure wave around them  When we think of a sound source like your voice, you can feel a bit of vibration in your chest/threat when you talk – this vibration causes molecules in your mouth to vibrate which causes the physical sound that emanated from your head Sound as a Longitudinal Wave  Thinking of sounds as traveling longitudinal waves moving outward from the source  The molecules themselves are coming together and going apart, creating different density points in the wave but that movement of those molecules creates a wave in and of itself via compression and retraction Deconstruction of a Sound Wave  Pure tones: simplest form of sound wave and defined by sine waves o In the real world, we don‟t hear pure tones – no real world pure sound, can only create it artificially o Pure tone = 440 Hz  Amplitude: how much compression and contraction is occurring in the wave and how much pressure exists in the waveform o Reflects loudness perception o When you turn up the volume of a speaker, you are increasing the amplitude, or the pressure of the sound waves around you, which causes a louder perception of the sounds around you  Frequency: the number of times/second that the pressure changes repeat – how many times we can fit a cycle in a second of time o Frequency reflects the psychological perception of pitch – how many times you see the cycle over and over again in some period of time (seconds) o The more you change the frequency, the higher the pitch is perceived The Dimensions of Sound  Physical definition: sound is pressure changes in the air or other medium  Perceptual/psychological definition: sound is the experience we have when we hear Intensity  Another world for talking about amplitude changes in a sound wave  Correlated with our perception of loudness  We measure amplitude or intensity using scale to scale down the range we are capable of perceiving  Decibels are related to our perception of loudness Amplitude vs. Loudness  Amplitude is measured using the decibel scale (dB)  Human perception of amplitude encompasses a very large range o We compress it into manageable numbers:  0 dB at threshold  140 dB = jet plane at takeoff Frequency vs. Pitch  Number of cycles per second in pressure change  Frequency is measured in Hertz (Hz)  Human perception of frequency ranges form 20-20,000 Hz  Pure tones vs. complex tones Complex Tones  Fundamental frequency o What we normally hear every day o Made up of a whole bunch of sine waves added together and all the sine waves have a different frequency o Sound waves, then, contain many frequencies of sound o Missing fundamental  Has to do with the fundamental frequency, which doesn‟t have to actually be there for us to perceive it  As long as some key requirements are still there we will still hear the fundamental frequencies of the sound even though some of the frequencies may be missing  Somewhat of an illusion  Harmonics o The multiple frequencies contained in complex tones o What you end up hearing is the fundamental frequency – you hear the others, but they don‟t change the pitch, only the quality of the sounds  Additive synthesis o We are adding together sine waves o Can do this to create complex tones o Our brains are doing the opposite – trying to subtract out all the other factors from the sound  The missing fundamental phenomenon is interesting because 20 Hz never actually stimulates any receptors, and yet that is the one that we hear  What happens is that we consider all the other frequencies about it as harmonics  Theses different kinds of harmonics change the colour of the sounds (the timbre) The Range of Hearing  Audibility curve o Threshold of hearing in free field vs. frequency  Free field: the minimal amount of sound stimulus you need in order to detect the presence of that stimulus  How loud a particular frequency or pitch has to be in order for you to detect its presence o Range = 20-20,000 Hz o Highest sensitivity (lowest thresholds) for 2,000-4,000 Hz The Auditory System  Unlike vision which is mostly cortical, the auditory system is less so (much more divided)  Can be divided into 3 big structures: outer, middle, and inner ear Outer Ear  Pinna: the fleshy part of your ear that sticks out o Pinna along with auditory canal comprises the outer ear o Primary purpose: to get as much of the sounds that are coming in and channel them inside the ear/auditory canal  Ear canal (auditory canal) o Purpose: to amplify sound – to make it as intense and clear as possible  Tympanic membrane (ear drum) o Everything before tympanic membrane is the outer ear – sheet of skin that covers the hole  All components of outer ear meant to amplify the sound system coming into your ear as clear as possible so you can process it properly The Middle Ear  Contains the smallest bones in your body, and is therefore rather small all together  3 ossicles o Malleus (hammer) o Incus (anvil) o Stapes (stirrup)  Oval window o Middle ear ends at the oval window o Vibration in the oval window starts the vibration of the cochlea, which transduces sound waves into a neural signal that the brain will interpret as a hearing experience The Inner Ear  Cochlea o The spiral shaped part of the inner ear o Divided into 2 halves:  Scala vestibuli (upper half): where the oval window begins  Scala tympani (bottom half): where the round window ends  Organ of Corti o Contains all the hair cells which are the receptors for hearing o Contains fluid filled tubes – pushes the hair cells around o Stimulated by movement of these chambers of the cochlea o Contains the receptor cells of the auditory system o Cilia: hair cells that pop up above the membrane o The bending of the cilia is what transmits the vibrations down the auditory nerve fibres which travels to the cortex to be processed by the brain o Fluid filled chambers (scala tympani) causes the techorial membrane to move back and forth on top of the cilia o The bending of the stereocilia causes the receptor to send a signal to the auditory fibre itself o We talk about sound as mechanical because it is the bending of the cilia in one direction or another than becomes the neural signal o The cilia bend because of the in and out movement of the stapes o 2 types of hair cells: inner (1 row) and outer (3 rows)  Inner hair cells do the bulk of the work in terms of transducing the energy  Outer hair cells work for the inner hair cells o Hair cells bend in 2 directions – one direction causes it to depolarize, the other causes it to hyperpolarize o The bending of these hair cells that cause the depolarization of the neuron – motion of the hair cells causes depolarization How do we Perceive Pitch?  Transduction signals frequency  There are two ways nerve fibres signal frequency (two theories): o Which fibres a
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