Audition

Sound Any vibrating material which can be heard.

Three aspect of sound Sound production Sound transmission Sound analysis

Physical definition of sound Sound is a stimulus that has the capability for producing and audible sensation. Any object having the properties of inertia and elasticity may be set into vibration hence may produce sound. Vibration is the property of an object that makes sound production audible.

Vibrations (sinusoids), irrespective of the source, can be analyzed Sinusoidal vibrations are composed of an add- mixture of sine waves. This add-mixture of sinusoids can be decomposed to a set of given sine waves. The decomposition of sinusoidal vibrations creates a Fourier series. The process is called Fourier analysis

Three properties characterize sinusoids. Frequency. Starting phase. Amplitude.

Physical attributes of the above 3 measures Amplitude is a measure of displacement. Frequency is a measure of how often, per one unit of time, the object moves back and forth. Staring phases is the position of the object at the instant in time it begins to vibrate.

Psychological attributes of physical vibrations Displacement changes are sensed as loudness. Frequency changes are sensed as changes in pitch. Phase shifts between the two ears is perceived as location of the sound in space.

The dynamic range of hearing is so large that it is almost incomprehensible. Imagine of situation in which a given person can just detect a sound and that sound is measured. Call this 1 unit of power. Now imagine the unit of power necessary to detect a sound just under the point of damage. This value would be 1,000,000,000,000,000 (10 15 ) power larger than the first measure.

The decibel decibel (dB) = 10 log (P 1 \P 2 ) 2 = 20 log p 1 \p 2 Note that the decibel is a ratio of two pressures.

Conventions The ratio difference between two pressures of dyne/cm 2 was the smallest amount of pressure for the average young adult to detect a sound over the frequency range from 1000 – 4,000 Hz sinusoid.

The decibel expressed relative to the ratio of pressures. When the decibel is express relative to dyne/cm 2, it is expressed in terms of sound pressure level (SPL).

Acoustic (Auditory) Perception When differential pressures of air or water are applied to the eardrum, every thing being equal, one is said to hear.

Mechanical movement is the base of hearing The ear drum and the 3 bone osicles constitute a lever system.

Inside surface of the eardrum (timpani) and the middle ear bones

The ossicles of the middle ear, malleus, incus and stapes.

Note the size and position of the ear drum to the size of point 11 and in the previous slide The total area of the ear drum ranges between 0.5 and 0.9 cm 2. The area of the stapes footplate ranges from 2.65 – 3.75mm. Thus the eardrum is 15.6 – 24.3 larger in area than the stapes foot plate. A 1mm displacement of the eardrum results in a 15 – 24 mm displacement of the stapes.

The lever The 3 middle ear bones act as a large lever

A drawing of the inner ear depicting associated parts and relationship of other structures not related to hearing

The stapes sends pressure waves into the inner champers of the cochlea. The scala tympani and the scala vestibuli are water (perilymph) filed chambers. These two canals meet at the apex of the coiled cochlea, called the helicotrema. A third tube, scala media filled with endolymph, is wedged between the scala tympani and the scala vestibuli.

Water is non-compressible, there must be an escape rout for the applied pressure. The round window is the escape membrane that deforms into the middle ear space to compensate for the activity generated at the oval window

The oval window presses into the scala vestibuli sending a pressure wave through the system, to be relived by the round window. The ceiling of the membrane separating the scala vestibuli and the scala media is the basilar membrane. On this membrane rides the Organ of Corti and the tectorial membrane (see below)

The pressure wave caused by the movement of the stapes causes the basilar membrane to vibrate. Note the figure caption depicting the difference between a ribbon movement and the basilar membrane movement consequent to being fixed along the two sides of the membrane

Response of the basilar membrane to activity of the stapes.

Envelope of maximum movement of basilar membrane at different frequencies.

Von Bekesy Theory of audition The essence of the theory state that the stapes causes a traveling wave to be pushed into the cochlea. This traveling wave presses up against the basilar membrane to excite the hair cells the make up the Organ or Corti. The place theory of excitation posits that the wave will maximally stimulate that part of the basilar membrane that codes for that given frequency of auditory ability.

The acoustic stimulation is transformed into neural information by the hair cells of the Organ of Corti.

Scanning electron microscope slice of the three turns of the cochlea.

Sagital cuts through the Cochlea

Inner ear showing the Organ of Corti

Transmission microscopic slide of the Organ of Corti

Scanning electron microscope of the three outer hair cells, Organ of Corti

The bending (shearing) of the hair cells transduces acoustic information into neural information of frequency (pitch), magnitude (loudness) and timber (sound identification)

Ascending pathway of auditory sound is more complex than vision in all mammals

At every level, neurons have tuning curves which show the spontaneous rate of firing (SR) compared to the best frequency of firing.

The cortical area for audition for monkeys and man are within the banks (hidden from view) of the Sylvian fissure. In the dog and cat the auditory area is on the lateral temporal pole

The effects of loud noise on the basilar membrane of the chinchilla Note the clear space and the lack of hair cells