The Nature of Sound Sound, like light, comes in waves Sound is vibration Features of sound include: –Pitch –Hertz –Decibels
Pitch A sound’s highness or lowness Dependent on the frequency of the sound wave Is measured as hertz (Hz) High pitched sounds Low pitched sounds
Hertz (Hz) A measure of the number of sound wave peaks per second; measures “frequency” Determines the pitch of the sound Human hearing goes from 20 Hz to 20,000 Hz
Frequency of Sound Waves The frequency of a sound wave is measured as the number of cycles per second (Hertz) –20,000 HzHighest Frequency we can hear – 4,186 HzHighest note on a piano – 1,000 HzHighest pitch of human voice – 100 HzLowest pitch of human voice – 27 HzLowest note on a piano
Decibel (dB) A measure of the height of the sound wave Determines the loudness of the sound Sometimes called amplitude Loud sounds Soft sounds
Above are examples of Frequency & Amplitude/decibels Timbre – distinctive quality of a sound determined by the complexity of the wave and its different combinations of frequencies. (Figure C is more complex than Figures A or B) Who hits the higher pitch? Christina or Mariah?
Hearing: Sound Waves Auditory perception occurs when sound waves interact with the structures of the ear Audition (sense of hearing) results in sound waves being collected in the outer ear, amplified in the middle ear and converted to neural messages in the inner ear.
Anatomy of Ear Purpose of the structures in the ear: –Measure the frequency (pitch) of sound waves –Measure the amplitude (loudness) of sound waves
Parts of the Ear – Sound Waves enter through the Pinna Pinna
Auditory Canal Sound Waves enter through the pinna then travel through the auditory canal. The opening through which sound waves travel as they move into the ear for processing Ends at the tympanic membrane (eardrum)
Oval Window The point on the surface of the cochlea which receives the sound vibration from the ossicles As the oval window vibrates, the fluid in the cochlea vibrates moving hair cells along the basilar membrane.
Hair cells along the Basilar Membrane move as the fluid vibrates
Anatomy of the Cochlea Another View Outer earMiddle earInner ear A sound causes the basilar membrane to wave up and down. Basilar membrane Hair cells Tectorial membrane Round window Eardrum Oval window Cochlea, partially uncoiled Stirrup Anvil Hammer Sound waves Auditory canal
Hair Cells The receptor cells for hearing in the cochlea that change sound vibrations into neural impulses. When they move they trigger action potential in the base of the hair cell (transduction). Similar to the rods and cones within the eye except hair cells are sensitive to vibrations rather than light. If these are damaged (due to prolonged loud noises) then you have nerve deafness which cannot be helped by a hearing aid.
Auditory Nerve The nerve that carries sound information from the ears to the thalamus then to the auditory cortex in the temporal lobes of the brain The auditory nerve is stimulated by the hair cells in the basilar membrane of the cochlea.
Major Divisions of the Ear Outer Ear—acts as a funnel to direct sound waves towards inner structures Middle Ear—consists of three small bones (or ossicles) that amplify the sound Inner Ear—contains the structures that actually transduce sound into neural response
Transduction of Sounds Sound waves are captured by the Pinna and sent down the ear canal where they stimulate the eardrum. The eardrum’s vibrations are amplified by the ossicles (hammer, anvil, stirrup). These vibrate the oval window on the cochlea which in turn vibrates the fluid around the basilar membrane. The fluid bends the hair cells on the basilar membrane triggering action potential in the base of the hair cells. This message is transmitted to the auditory nerve which carries the info to the thalamus and then to the auditory cortex of the temporal lobe. Review using this Nobel Prize site on HearingNobel Prize site on Hearing
How Can I Remember This? Please – Pinna Eat – Ear Canal Everything – Eardrum Offered – Ossicles On – Oval Window Cuisine – Cochlea Buffet – Basilar Membrane Helpful – Hair Cells Attendants – Auditory Nerve Take – Thalamus Away – Auditory Cortex Trash – Temporal Lobe
Distinguishing Pitch Frequency theory—basilar membrane vibrates at the same frequency as the sound wave/oval window –The higher the frequency wave the faster the firing of hair cells –Theory used to explain how you hear low frequencies Place theory—different frequencies cause larger vibrations at different locations along the basilar membrane –Different pitches stimulate different areas on the basilar membrane –The brain receives these messages and interprets them as different pitches. –Theory used to explain how you hear high frequencies. Use both theories when you listen to sounds with both high and low frequencies. See this website to see how it works.website
Frequency Theory The Basilar Membrane vibrates according to the same Frequency of the sound wave Basilar membrane Distal end Proximal end Oval window Direction of traveling wave
Place Theory: Different Frequencies stimulate different areas of the Basilar Membrane
Coding and Auditory Masking The way in which waves travel down the Basilar Membrane causes some sounds to interfere with (or mask) our ability to hear other sounds Low frequency sounds provide better masking than high frequency sounds
Auditory Masking Low frequency sounds effectively mask high frequency sounds High frequency sounds cannot effectively mask low frequency sounds Piccolo, soft Bassoon, loud Piccolo, loud Bassoon, soft Distance along basilar membrane (a) Distance along basilar membrane (b) Effect of bassoon on basilar membrane Vibration amplitude of basilar membrane Vibration amplitude of basilar membrane Effect of piccolo on basilar membrane