• If a tree falls in the forest and no one is there to hear it, is there a sound?
• What is it that makes sounds high pitched or low pitched?
• How do sound vibrations inside the ear lead to the perception of different pitches?
• How are sounds represented in the auditory cortex?
The Sound Stimulus
Pressure waves and perceptual experience
• Two definitions of “sound”
– Physical definition - sound is pressure changes in the air or other medium.
– Perceptual definition - sound is the experience we have when we hear
• Loud speakers produce sound by:
– The diaphragm of the speaker moves out, pushing air molecules together called
– The diaphragm also moves in, pulling the air molecules apart called rarefication.
– The cycle of this process creates alternating high- and low-pressure regions that travel
through the air.
• Pure tone - created by a sine wave
– Amplitude - difference in pressure between high and low peaks of wave
• Perception of amplitude is loudness
• Decibel (dB) is used as the measure of loudness
• Number of dB = 20 logarithm(p/p )o
• The decibel scale relates the amplitude of the stimulus with the psychological
experience of loudness.
• p is the sound pressure of the stimulus, and p is the standard sound pressure
which is usually set at 20 micropascals. The standard pressure is close to the
pressure of a 1,000 Hz tone at threshold in a free field. Note that adding sound
pressure level (SPL) to a dB measure indicates that the standard pressure of 20
micropascals was used.
• Frequency - number of cycles within a given time period
– Measured in Hertz (Hz) - 1 Hz is one cycle per second
– Perception of pitch is related to frequency.
– Tone height is the increase in pitch that happens when frequency is increased.
• Both pure and some complex tones are periodic tones.
Complex Periodic Sounds
• Fundamental frequency is the repetition rate and is called the first harmonic.
• Periodic complex tones consist of a number of pure tones called harmonics.
– Additional harmonics are multiples of the fundamental frequency.
• Additive synthesis - process of adding harmonics to create complex sounds • Frequency spectrum - display of harmonics of a complex sound
• Attack of tones - buildup of sound at the beginning of a tone
• Decay of tones - decrease in sound at end of tone
• Aperiodic sounds, which do have repeating sounds waves, are also widely heard in our
environment, but have not been investigated as extensively as periodic sounds.
• Timbre - all other perceptual aspects of a sound besides loudness, pitch, and duration
– It is closely related to the harmonics, attack and decay of a tone.
• Effect of missing fundamental frequency
– Removal of the first harmonic results in a sound with the same perceived pitch, but with
a different timbre.
• This is called periodicity pitch.
Musical Scale and Frequency
• Letters in the musical scale repeat.
• Notes with the same letter name (separated by octaves) have fundamental frequencies that are
multiples of each other.
– These notes have the same tone chroma.
– We perceive such notes as similar to one another.
Range of hearing
• Human hearing range - 20 to 20,000 Hz
• Audibility curve - shows the threshold of hearing in relation to frequency
– Changes on this curve show that humans are most sensitive to 2,000 to 4,000 Hz.
• Auditory response area - falls between the audibility curve and and the threshold for feeling
– It shows the range of response for human audition.
• Equal loudness curves - determined by using a standard 1,000 Hz tone
– Two dB levels are used - 40 and 80
– Participants match the perceived loudness of all other tones to the 1,000 Hz standard.
– Resulting curves show that tones sound
• Almost equal loudness at 80 dB.
• Softer at 40 dB for high and low frequencies than the rest of the tones in the
• Outer ear - pinna and auditory canal
– Pinna helps with sound location.
– Auditory canal - tube-like 3 cm long structure
• It protects the tympanic membrane at the end of the canal.
• The resonant frequency of the canal amplifies frequencies between 1,000 and
• Two cubic centimeter cavity separating inner from outer ear
• It contains the three ossicles
– Malleus - moves due to the vibration of the tympanic membrane
– Incus - transmits vibrations of malleus
– Stapes - transmit vibrations of incus to the inner ear via the oval window of the cochlea Function of Ossicles
• Outer and inner ear are filled with air.
• Inner ear is filled with fluid that is much denser than air.
• Pressure changes in air transmit poorly into the denser medium.
• Ossicles act to amplify the vibration for better transmission to the fluid.
• Middle ear muscles dampen the ossicles’ vibrations to protect the inner ear from potentially
• Main structure is the cochlea
– Fluid-filled snail-like structure (35 mm long) set into vibration by the stapes
– Divided into the scala vestibuli and scala tympani by the cochlear partition
– Cochlear partition extends from the base (stapes end) to the apex (far end)
– Organ of Corti contained by the cochlear partition
Organ of Corti
• Key structures
– Basilar membrane vibrates in response to sound and supports the organ of Corti
– Inner and outer hair cells are the receptors for hearing
– Tectorial membrane extends over the hair cells
• Transduction takes place by:
– Cilia bend in response to movement of organ of Corti and the tectorial membrane
– Movement in one direction opens ion channels
– Movement in the other direction closes the channels
• This causes bursts of electrical signals.
Neural signals for frequency
• There are two ways nerve fibers signal frequency:
– Which fibers are responding
• Specific groups of hair cells on basilar membrane activate a specific set of nerve
– How fibers are firing
• Rate or pattern of firing of nerve impulses
Bekesys’ Place Theory of Hearing
• Frequency of sound is indicated by the place on the organ of Corti that has the highest firing
• Békésy determined this in two ways:
– Direct observation of the basilar membrane in cadav