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SOUND & THE EAR Sound and the Ear 1. Sound Waves A. Frequency: Pitch, Pure Tone. B. Intensity C. Complex Waves and Harmonic Frequencies 2. The Ear A. The Outer Ear B. The Middle Ear C. The Inner Ear i. The Cochlear Membrane ii. Sound Transduction iii. Hearing Loss Anthony J Greene 2 Sound Waves 1. Frequency • Wavelength - distance between peaks or compressions • Hertz - cycles (1 compression & 1 rarefaction) per second - the major determinant of pitch Anthony J Greene 3 Sound Waves • Pure Tones - simple waves • Harmonics - complex waves consisting of combinations of pure tones (Fourier analysis) - the quality of tone or its timbre (i.e. the difference between a given note on a trumpet and the same note on a violin) is given by the harmonics Anthony J Greene 4 Sound Waves • Pitch and fundamental frequency - in pure tones the pitch is the fundamental frequency - with harmonics added the fundamental frequency is the dominant pure tone Anthony J Greene 5 Sound Waves 2. Intensity • Amplitude is measured in Decibels (dB)the height of the peak, or the amount of compression - determines volume • Loudness is the psychological aspect of sound related to perceived intensity or magnitude Anthony J Greene 6 Sound Waves • Humans can hear across a wide range of sound intensities – Ratio between faintest and loudest sounds is more than one to one million – In order to describe differences in amplitude, sound levels are measured on a logarithmic scale, in units called decibels (dB) – Relatively small decibel changes can correspond to large physical changes (e.g., increase of 6 dB corresponds to a doubling of the amount of pressure) Anthony J Greene 7 Sound Waves Anthony J Greene 8 Direction of Sound Air Molecules Speaker Compression Rarefaction Anthony J Greene 9 Anthony J Greene 10 Anthony J Greene 11 Harmonics & Fourier Analysis Anthony J Greene 12 Harmonic Frequencies 1f • Strings or pipes (trombone, flute organ) all have resonant frequencies. • They may vibrate at that frequency or some multiple of it • All instruments and voices carry some harmonics and dampen others 2f 1 octave 3f 4f 2 octaves 8f 3 octaves Anthony J Greene Length of string or pipe 13 Harmonic Frequencies 1f + 3f + 5f + 7f Anthony J Greene + 9f + … 14 Harmonics & Fourier Analysis Anthony J Greene 15 Harmonics & Fourier Analysis Complex sounds can be described by Fourier analysis A mathematical theorem by which any sound can be divided into a set of sine waves. Combining these sine waves will reproduce the original sound. The fundamental frequency is the pitch, and the harmonic frequencies are the timbre. Results can be summarized by a spectrum Anthony J Greene 16 Harmonics & Fourier Analysis Anthony J Greene 17 The Ear Outer Ear Middle Inner Ear Ear Anthony J Greene 18 Outer Ear • Pinna - the fleshy part of the ear • Channels sound into the auditory canal which carries the sound to the eardrum • tympanic membrane - vibrates in response to vibrations in the air Anthony J Greene 19 Middle Ear • Ossicles - the three smallest bones in the human body - malleus (hammer) incus (anvil ), stapes (stirrup ) - transmit sound to the inner ear • Eustachian tubes - connects to throat and allows air to enter the middle ear - equalizes the pressure on both sides of the eardrum Conduction Deafness Anthony J Greene 20 Inner Ear Anthony J Greene 21 Inner Ear 1. 2. • • Semi-Circular Canals The Cochlea Oval Window the connection point from the stirrup to the inner ear Round Window Anthony J Greene 22 Inner Ear 1. 2. • • Semi-Circular Canals The Cochlea Oval Window the connection point from the stirrup to the inner ear Round Window Anthony J Greene 23 Anthony J Greene 24 The Cochlea • Vestibular canal - wave travels from the oval window towards the end of the cochlea • Tympanic canal - wave travels from the end of the cochlea to the round window • Reissner's Membrane - separates the vestibular canal from the Cochlear Duct • Basilar membrane - vibrates in response to the wave traveling around it - varies in thickness so some areas vibrate best to high pitches and some areas to low pitches • Cochlear duct -the third section of the cochlea which contains the Organ of Corti • Organ of Corti - the place where physical energy is converted to nerve energy Anthony J Greene 25 The Cochlea Anthony J Greene 26 The Cochlea Anthony J Greene 27 The Cochlea Anthony J Greene 28 Sound Transduction • A traveling wave is set up in the vestibular canal • The wave causes the Basilar membrane to vibrate - each section is maximally stimulated by a different pitch - serves to sort out differing frequencies • In the Organ of Corti hair cells vibrate in response to the vibrations of the Basilar membrane • Hair cells transduce the energy into a neural impulse Anthony J Greene 29 Bassilar Membrane Anthony J Greene 30 Basilar Membrane Anthony J Greene 31 Exposure to Loud Noise Anthony J Greene 32 Summary Anthony J Greene 33