Hearing Sound Music 170: The Ear Tamara Smyth, trsmyth@ucsd.edu Department of Music, University of California, San Diego (UCSD) November 17, 2016 Sound interpretation in the auditory system is done by 1. the ear and (peripheral auditory organs) 2. the brain (auditory nervous system). Hearing is accomplished by the ear which converts sound waves to electrical impulses that are interpreted by the brain. acoustic pressure is transformed into mechanical vibrations on the basilar membrane. see animation vibrations is represented by a series of pulses that is transmitted by the auditory nerve. 1 Music 170: The Ear 2 The Human Auditory System The Outer Ear The human auditory system (ear) consists of 1. the outer ear 2. the middle ear 3. the inner ear The outer ear consists of the pinna, the auditory canal, and the tympanic membrane (eardrum). The pinna collects sound and funnels to the ear canal; acts as a filter helping us localize sounds. The auditory canal acts as an acoustic tube closed at one end; boosts hearing sensitivity in the range 2000-5000 Hz. The tympanic membrane terminates the auditory canal; vibrates in response to the produced sound. Music 170: The Ear 3 Music 170: The Ear 4
Sound in the Outer Ear The Middle Ear The intensity of the sound in the auditory canal is dependent on direction of sound reaching listener; shoulders: can reflect sound toward the pinnae; head: can cast an acoustical shadow; pinna: guiding filtering effect increases with increasing frequency (though at high frequency rises and falls). Effect of the pinna, head, and shoulders is greater when the sound s wavelength is comparable to their size: shoulders and head (6-8 wide) might influence a musical pitch of A6 (wavelength 7.7 ) the pinnae (2 wide) might influence a musical pitch of A8 (wavelength 1.9 ) Music 170: The Ear 5 The middle ear consists of the eardrum, to which three small bones, called the ossicles, are attached. The eardrum changes pressure variations of incoming sound waves (through the ear canal) into mechanical vibrations which are then transmitted via the ossicles to the inner ear. Since the eardrum seals the middle and outer ear, the Eustachian tube, which connects the middle ear to the oral cavity, is needed to equalize these two pressures. Music 170: The Ear 6 The ossicles in the Middle Ear The Inner Ear The ossicles transform small changes in pressure (sound wave) to a much greater pressure on the oval window using: 1. a lever action 2. a smaller oval window Lever: small force acts through a larger distance resulting in a larger force acting through a smaller distance. The cochlea transforms pressure variations into properly coded neural impulses. As a first approximation, consider an unraveled cochlea and its two main sections: 1. the scala vestibule (top) 2. the scala tympani (bottom) separated by the basilar membrane. d2 F2 d1 F1 lever action in the ossicles provides a factor of about 1.5 in force multiplication Since the area of the oval window is smaller than that of the eardrum (about 1/20 th the size), and pressure = force/area, greater pressure is transmitted to the inner ear (by a factor of 20 1.5 = 30). Music 170: The Ear 7 The oval and round windows are at the larger end of the cylinder, and a small hole (helicotrema) is at the smaller end to connect the two sections. Music 170: The Ear 8
Basilar Membrane Organ of Corti The basilar membrane stops just short of the end of the cochlea to allow the fluid to transmit pressure waves around the end of the membrane. When the stapes vibrates against the oval window, pressure waves are transmitted rapidly down the scala tympani, introducing ripples in the basilar membrane. higher frequencies create higher amplitude ripples in the region near the oval window (where the basilar membrane is narrow and stiff). lower frequencies create higher amplitude ripples in the region at the far end (where the membrane is slack). The mechanical vibrations of the basilar membrane are converted to electrical impulses in the auditory nerve via the organ of Corti. the organ of Corti contains several rows of tiny hair cells called (stereocilia); stereocilia bend when the basilar membrane responds to sound, generating nerve impulses; nerve impulses travel to the brain at a rate dependent on the intensity and frequency of the sound. Music 170: The Ear 9 Music 170: The Ear 10 Auditory System Summary Critical Bands Sound waves propagate through the ear canal and excite the eardrum. The stapes, the inner most ossicle of the ear, vibrates against the oval window and causes pressure variations in the cochlea, which in turn excite mechanical vibrations in the basilar membrane. The vibrations of the basilar membrane cause the stereocilia of the organ of corti to transmit electrical impulses to the brain via the auditory nerve. Each auditory nerve fiber responds over a certain range of frequencies (and sound pressures) but has a characteristic frequency at which it is most sensitive. See video summary here. When a tone disturbs the basilar membrane (BM), it does so over a certain length on either side of the point of maximum displacement. Two pure tones with frequencies 523 Hz and 1046 Hz (an octave apart) respectively, will each have a frequency response curve on the basilar membrane (i.e. an amplitude envelope). The nerve endings in the BM are excited over this narrow region (approx. 1.2 mm), corresponding to a range of frequencies, called the critical band. The critical band varies with frequency: 90 Hz wide for a 200-Hz sound, 900 Hz wide for a 5000-Hz sound. Music 170: The Ear 11 Music 170: The Ear 12
Loudness Level and Frequency Equal Loudness Curves Two sounds having the same intensity may not have equal loudness. Loudness is a subjective quality that depends on sound pressure level; frequency; spectrum; duration and amplitude envelope; environmental conditions etc. The phon is a unit of loudness for pure tones: compensates for the effect of frequency; sounds having the same number of phons sound equally loud; number of phons is the db SPL at 1000 Hz: Dependency of loudness on frequency is illustrated by Fletcher and Munson s equal loudness curves: shows the required SPL in db for different frequencies to sound; each curve is the specified number of phons. phon SPL in db at a frequency of 1000 Hz. Music 170: The Ear 13 Music 170: The Ear 14 What Equal Loudness Curves Show Musical Considerations Curves illustrate: 1. insensitivity of the ear to low-frequency tones: low-frequency tones must be at a higher SPL to sound equally loud as higher-frequency tones; e.g. a 10-phon 50-Hz tone has to be 40 db higher than one at 2000 Hz to sound equally loud. 2. insensitivity of the ear to very-high-frequency tones: e.g. a 10-phon 10-kHz tone has to be 10 db higher than one at 1000 Hz to sound equally loud. 3. sensitivity of the ear to 2-4 khz tones: how to you account for the dip at 2-4 khz? why another dip at around 12 khz? 4. loudness variation with frequency is dependent on the sound level: range and dip more significant in lower curves curve is flatter at higher sound levels There is a greater difference in the auditory system s sentivity at different frequencies for soft sounds (lower curves have a bigger dip). For loud sounds, there is less variation with frequency. Because of this, it is useful to make some considerations when making music: for a musical instrument to be heard easily, its frequency should lie between 500-4000 Hz; low frequency instruments won t be perceived as being loud; when playing fortissimo (ff), low frequencies contribute more than when playing pianissimo (pp). Music 170: The Ear 15 Music 170: The Ear 16
Loudness and Critical Band Recall, a pure tone at a particular frequency activates a region along the basilar membrane (BM): if a second tone with frequency near that of the first is played, it will activate a region on the BM that is already active and will likely be perceived as less loud; if the second sinwave has a frequency sufficiently different from that of the first, a new region on the BM will be activated, making the sound seem louder. Loudness can be used as a means of measuring CB. Music 170: The Ear 17