Sensory Systems: Auditory

What do we hear?

• Sound is a compression wave:

When speaker is stationary, the air is uniformly dense

Speaker Air Molecules

What do we hear?

• Sound is a compression wave:

Speaker

When the speaker moves, it compresses the air in front of it.

What do we hear?

• Sound is a compression wave:

The speaker moves back leaving an area with less air behind - called rarefaction

CompressionRarefaction

What do we hear?

• Sound is a compression wave:

Speaker

The speaker moves forward again starting the next wave

Compression

Rarefaction

What do we hear?

• Sound is a compression wave - it only “looks” like a wave if we plot air pressure against time

Air

Pre

ssur

e

Time

Properties of a Sound Wave

• 1. Amplitude: difference in air pressure between compression and rarefaction (Sound Pressure Level)

Properties of a Sound Wave

• 1. Amplitude: difference in air pressure between compression and rarefaction (Sound Pressure Level)

– What is the perception that goes along with the sensation of sound amplitude?

Properties of a Sound Wave

• 1. Amplitude: difference in air pressure between compression and rarefaction (Sound Pressure Level)

– What is the perception that goes along with the sensation of sound amplitude?

LOUDNESS

Properties of a Sound Wave

• 2. Frequency: how many regions of compression (or rarefaction) pass by a given point per second (expressed in Hertz)

Properties of a Sound Wave

• 2. Frequency: how many regions of compression (or rarefaction) pass by a given point per second (expressed in Hertz)– What is the perception that goes along with

the sensation of frequency?

Properties of a Sound Wave

• 2. Frequency: how many regions of compression (or rarefaction) pass by a given point per second (expressed in Hertz)– What is the perception that goes along with

the sensation of frequency?

PITCH

Sensing Vibrations

Sensing Vibrations

• Outer ear transmits and modifies sound (critical for sound localization)

Sensing Vibrations

• Middle ear turns compression waves into mechanical motion

oval window

stapes

Sensing Vibrations

• Middle ear turns compression waves into mechanical motion

Ear Drum

Oval window

Sensing Vibrations

• Middle ear turns compression waves into mechanical motion

Ear Drum

Oval window

Compression Wave

Sensing Vibrations

• The cochlea, in the inner ear, is a curled up tube filled with fluid.

Auditory Nerve to Brain

Sensing Vibrations

• Inside the cochlea is the basilar membrane

• Movement of the oval window causes ripples on the basilar membrane

Sensing Vibrations

• Basilar membrane measures the amplitude and frequency of sound waves

– amplitude (loudness)

–frequency (pitch)

Sensing Vibrations

• Basilar membrane measures the amplitude and frequency of sound waves

– amplitude (loudness) - magnitude of displacement of the basilar membrane

–frequency (pitch)

Sensing Vibrations

• Basilar membrane measures the amplitude and frequency of sound waves

– amplitude (loudness) - magnitude of displacement of the basilar membrane

–frequency (pitch) - frequency and location of displacements of the basilar membrane

Sensing Vibrations

• Basilar membrane measures the amplitude and frequency of sound waves

–frequency (pitch) - frequency and location of displacements of the basilar membrane

Sensing Vibrations

• Bundles of “hair cells” are embedded in basilar membrane

Sensing Vibrations

• When hair cells sway back and forth, they let ions inside

• This flow of charges is converted to action potentials and sent along the auditory pathway

The Auditory Pathway

• The auditory pathway is complex and involves several “stations” along the way to the auditory cortex in the brain

• Lots of processing must be done in real-time on auditory signals!

How Can You Localize Sound?

• Ponder this:– Imagine digging two trenches in the sand beside a lake so

that water can flow into them. Now imagine hanging a piece of cloth in the water in each trench. Your job is to determine the number and location and type of every fish, duck, person, boat, etc. simply by examining the motion of the cloth. That’s what your auditory system does!

- Al Bregman

How do we Stay Balanced?

The Vestibular System

Vestibular System (Balance)

Vestibular System (Balance)

Vestibular System (Balance)

Vestibular System (Balance)

Head accelerates this way

Cupula getspushed

Fluid goes this way

Vestibular System (Balance)

Head accelerates this way

Cupula getspushed

Fluid goes this way

Vestibular System (Balance)

• movement of the cupula is detected by hair cells

• hair cells in the vestibular system are more sensitive than hair cells on the basilar membrane!

Vestibular, Visual, and Proprioceptive Systems Work Together

• Try standing on one foot with your eyes closed!

Fun Facts about The Vestibular System

• Seasickness arises when the vestibular system and the visual system send conflicting information

Fun Facts about The Vestibular System

• Seasickness arises when the vestibular system and the visual system send conflicting information

• People can be knocked down by moving walls!

Fun Facts about The Vestibular System

• Seasickness arises when the vestibular system and the visual system send conflicting information

• People can be knocked down by moving walls!

• Alcohol causes the spins by (among other things) changing the density of the fluid in the semicircular canals

Hearing

• Detection

• Loudness

• Localization

• Music

• Speech

Detection and Loudness

• Sound level is measured in decibels (dB) - a measure of the amplitude of air pressure fluctuations

Detection and Loudness

• Sound level is measured in decibels (dB) - a measure of the amplitude of air pressure fluctuations

• dB is a log scale - 1 dB difference = 10 times the actual air pressure

Detection and Loudness

• Sound level is measured in decibels (dB) - a measure of the amplitude of air pressure fluctuations

• dB is a log scale - 1 dB difference = 10 times the actual air pressure

• We have a dynamic range that is a factor of 7.5 million!

Detection and Loudness

• minimum sound level necessary to be heard is the detection threshold

Detection and Loudness

• detection threshold depends on frequency of sound:

• very high and very low frequencies must have more energy (higher dB) to be heard

• greatest sensitivity (lowest detection threshold) is between 1000 hz to 5000hz

Detection and Loudness

• Detection can be compromised by a masking sound

• even masking sounds that are not simultaneous with the target can cause masking (forward and backward masking)