Chapter 10: Perception of sound

If a tree falls in the woods and there is no one around, does it make a sound? 3 requirements for sound1. Vibrating body: something to create mobile pressure changes

Vibrating bodies

Vibrating bodies create pressure changes capable of propagating from the source. It’s the pressure change that serves as the auditory signal

3 requirements for sound

2. An elastic medium: A substance capable of propagating pressure changes. Usually this is air (but not always).

3 requirements for sound

3. Receptive organ: something to translate physical pressure changes into a perceptual experience – usually ears.Difference between physical energy and sound (perceptual experience)

Sound pressure wave

Physical properties and perceptual experienceWavelength = cycles per second; Hz

Range of human frequency perception

Note: peak sensitivity around 3.5KHz; full range roughly 20-20,000Hz; drops from top with age.

Sound pressure wave

Amplitude: height of wave; measured in dB

Sound pressure wave

Overtones: no pressure wave occurs in isolation – overtones are other frequencies that occur along fundamental frequency (frequency that accounts for pitch perception) that affect “character” of sound perception: timbre. For most musical instrument overtones are harmonics (multiples of fundamental). Note: on graphs instruments are not playing exactly the same fundamental

Behavior of sound waves

While sound pressure waves are reflected and absorbed variously by different surfaces, like sound waves; they also can travel around, and through surfaces, unlike light waves; which can make them much more difficult to completely block out, hence the ability to hear something even when it is not seen.

Echoes: reflected sound - different environments have different echo characteristics or acoustics, hence the sound quality of the environment varies. Generally speaking the harder surfaces tend to reflect more sound, while more porous surfaces tend to absorb more sound.

Speed of sound: the speed with which a pressure wave travels through the medium is determined by the density of the molecules in the medium -- the denser the medium the faster the propagation. Air is the most typical medium for sound, and in air the speed of sound is 340 meters per second. But sound waves actually travel faster through water, ground, and even steel.

Impedance: the degree to which the medium resists the propagation of the sound wave. Denser mediums tend to propagate sound waves faster, but they also tend to reduce the amplitude of the wave more quickly, thus reducing the perceivable distance of the wave.

Receptive organ: Ear

• Ear: 3 major parts; outer, middle, inner

Receptive organ: Ear

1) The outer ear: structures a) Pinna: fleshy, cartilaginous, structure which extrudes from head Pinna is important for helping to funnel sound further into ear, and as gross sound

localizer. b) Auditory canal: tube structure which directs sound inward to middle ear. canal has resonance frequency of around 3,000 hz, which means that it tends to

vibrate along with frequencies of 3,000 and therefore amplify those sounds. Interestingly enough, there are only modest number of speech sound which are in the range of 3,000 hz, most are more in the range of 1,000-2,000 hz, however, high pitch screams are around this frequency typically.

Middle ear

Tympanic membrane: eardrum (sometimes included with outer ear) thin oval shaped membrane which vibrates in response to incoming wave. Tympanic membrane is highly sensitive, but can often absorb punctures and continue functioning. Main job is to vibrate ossicles.

Middle ear

a) ossicles (malleus, incus, stapes). the tiny bones of the middle ear which vibrate in response to vibrating of tympanic membrane. Major purpose is to amplify the sound wave to help reduce affects of increased impedance of cochlear fluid.Impedance matching device: about 4dB recovered from hinge design of ossicles, about 23dB from “funneling” from tympanic membrane to oval window

b) oval window: connected to stapes, vibrates in response to stapes and propagates sound wave to inner ear.

Acoustic reflex: loud, low sounds trigger stiffing of inner ear muscles restricting movement. Not effective for high pitches.

Inner ear

• Composed of semi-circular canals (vestibular sense – body posture, balance, etc) and cochlea. Cochlea is main structure for auditory info processing

Cochlea

Three main structures:1) Vestibular canal: topmost section

of cochlea2) Tympanic canal: bottom most

section of cochlea3) Cochlear duct: middle canal of

cochlea, filled with different type of fluid than tympanic and vestibular canals. Mixing of fluids can impair hearing.

Also: Round window: small elastic structure covering a small opening between tympanic canal and middle ear. This structure helps to equalize pressure from propagated wave started at oval window.

Cochlea

Basilar membrane: membrane separating tympanic canal from cochlear duct. Organ of Corti: auditory receptor organ which rests on basilar membrane inside cochlear duct. Is to ear what retina is to eye. Tectorial membrane: the membrane that extends up from Riessners membrane (the diagonal membrane which separates the vestibular canal from the cochlear duct) and arches over and contacts some of the Organ of Corti hair cells.

Cochlea

Organ of Corti hair cells: there are two types: inner and outer. Inner cells are less in number (4,5000) and are situated near where the tectorial membrane attaches to Riessner's membrane. Inner are not directly connected to tectorial membrane.Outer cells are greater in number (15,500), situated more centrally on Organ of Corti, and are connected to tectorial membrane. However, outers have very limited connections to auditory nerve (95% of auditory nerve connected to IHC)

Action in CochleaWave enters from the piston-like action of stapes moving in and out of oval window. Wave throughout cochlear fluid and displaces basiliar membrane in cochlear duct. The waving motion of basilar membrane causes tectorial membrane to displace in opposite direction of basilar membrane and get "pulled and tugged" by connections to outer hair cells. This "pulling and tugging" action amplifies the movement of fluid in cochlear duct which causes displacement of inner hair cells, which have many direct connections to auditory nerve.

Theories of pitch perception: Temporal theory

This theory (also called frequency theory) states that the entire basilar membrane vibrates in consonance with the frequency of the wave entering the cochlea. This idea was subsequently proved incorrect as it was found that the differences in the width and thickness along the length of basilar membrane made it physically impossible for it to vibrate as frequency theory predicts. However it was found that individual auditory nerve fibers could match low frequency vibrations, and could volley to match frequencies up to about 4,000 hz.

Theories of pitch perception: Place theory

First proposed by Herman von Helmholz, who noted that the basilar membrane was narrow at the base and wider at the apex. Helmholtz believed that this meant that the basilar membrane was composed of separate fibers which resonated at different frequencies along the basilar membrane, like a piano keyboard.Place theory found support in studies by Von Bekesey, who constructed a replica of the basilar membrane to study the behavior of the waves inside the cochlea. Bekesey found that different frequency waves peaked out at different places along the basilar membrane with high frequencies nearer the base, and low frequencies nearer the apex. However, Bekesey also found that localizing the place of maximal stimulation was much more precise for high rather than low frequencies.

Duplicity theory

A combination of frequency and place operate to explain the range of human pitch perception -- and varying sensitivities to pitch.

20 to 500 -- frequency coding only500 to 4,000 -- frequency and place coding4,000 to 20,000 -- place coding only

Note that it is frequencies from around 1,000 - 3,000 for which humans have greatest sensitivity and in which comprises most of human speech.

Auditory nerve

Made up of about 30,000 individual fibers mostly emanating from IHC. Nerve fibers differ in spontaneous activity (baseline firing rate) depending on where they make contact with IHC

IHC

Hi spon. activity

Med spon. activity

Lo spon. activity

OHC

Frequency tuned auditory nerve fibers

Suppose we present different frequencies at minimal dB level to individual nerve fibers.Frequency preference corresponds to location on basilar membraneBut what about loudness perception?

Loudness perception: Where does fiber connect to IHC?

B graph shows two fibers from same location on basilar membrane (therefore same frequency preference). When preferred frequency is presented at different dB levels to each with different spontaneous activity levels. Higher responds to lower intensity (lower threshold) but has lower saturation point. Lower (darker line) responds “later” but saturates later as well.

Auditory processing beyond cochlea

At left and right cochlear nuclei auditory processing is monaural; but past (superior olives; inferior colliculi etc.) processing becomes binaural. Thus, “two-eared” cues for sound localization can be exploited.

Sound localization: Cue 1 – interaural time differences

• Inter-aural time differences: the difference in arrival time of sound wave at two ears. Sound arrives at nearer ear first (when not perfectly at mid-line). Probably coded by binaural cells with variable time delays (delay lines) built into inputs from nearer ear (a). It appears that time differences are more effective cue for lower frequencies, while amplitude differences are more effective for higher frequencies.

L earR ear

Binaural cell

Direct lineDelay line

Sound localization: Cue 1 – interaural intensity differences

The difference in loudness at the two ears created by shadowing effects of head and pinnas, as well as differing distances of sound producing source from two ears. Shadowing effect is far less for lower frequencies, which are often large to go around head unblocked.

Auditory cortex

• Tonotopic organization with magnification of mid-range frequencies. Beginning of processing for more meaningful and categorical (speech vs. dog bark) aspects of audition.