Sensory Systems: Auditory

What do we hear?
• Sound is a compression wave:
Speaker Air Molecules
When speaker is stationary, the air is uniformly dense

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:
Rarefaction
Compression
The speaker moves back leaving an area with less air behind called rarefaction

What do we hear?
• Sound is a compression wave:
Speaker
Compression
The speaker moves forward again starting the next wave
Rarefaction

What do we hear?
• Sound is a compression wave - it only “looks” like a wave if we plot air pressure against time
Period - amount of time for one cycle
Frequency = number of cycles per second
(1/Period)
Air Pressure 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?

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?

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
Oval window
Ear Drum

Sensing Vibrations
• Middle ear turns compression waves into mechanical motion
Oval window
Ear Drum
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

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 charges 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 realtime on auditory signals!

How Can You
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

• Detection
• Loudness
• Localization
• Scene Analysis
• 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 - small increases in dB mean large increases in sound energy

Detection and Loudness
• Sound level is measured in decibels (dB) a measure of the amplitude of air pressure fluctuations
• dB is a log scale - small increases in dB mean large increases in sound energy
• 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
• 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)

Detection and Loudness
• Loudness is the subjective impression of sound level (and not identical to it!)

Detection and Loudness
• For example, tones of different frequencies that are judged to be equally loud have different SPLs
(dB)

Detection and Loudness
• Hearing loss due to exposure to high-intensity sounds
(greater than 100 dB) can last many hours

Detection and Loudness
• Incidence of noise-related hearing loss is increasing dramatically
• iPods and other “earbud” music players are thought to be partly responsible
• How loud is an iPod?
– maximum volume is approximate but is somewhere between 100 dB (hearing damage in about 2 hours) to 115 dB (hearing damage in about 15 minutes)
• Consequences: difficulty understanding speech, tinnitus, deafness
• Your perception of loudness adapts so it’s hard to tell how loud your iPod is -
LOCK THE VOLUME ON YOUR iPOD!

Localization
• recall the lake analogy: task is to localize the positions of the boats on a lake using the pattern of ripples at two points on the shore

Localization
• All you have is a pair of instruments (basilar membranes) that measure air pressure fluctuations over time

Localization
• There are several clues you could use:

Localization
Left Ear
Right Ear
Compression
Waves

•
1
Localization
There are several clues you could use: arrival time - sound arrives first at ear closest to source

Localization
Left Ear
Right Ear Compression
Waves

•
1.
2.
Localization
There are several clues you could use: arrival time phase lag (waves are out of sync) - wave at ear farthest from sound source lags wave at ear nearest to source

Localization
Left Ear
Right Ear Compression
Waves

•
1.
2.
3.
Localization
There are several clues you could use: arrival time phase lag (waves are out of sync) sound shadow (intensity difference)- sound is louder at ear closer to sound source

•
Localization
What are some problems or limitations?

Localization
• Low frequency sounds aren’t attenuated by head shadow
Sound is the same
SPL at both ears
Left Ear
Right Ear Compression
Waves

•
Localization
High frequency sounds have ambiguous phase lag
Left Ear
Right Ear
Left Ear
Right Ear