Psych 1XX3 – Audition Notes – Mar 18, 2010
Auditory Mechanisms of Different Species:
Auditory mechanisms vary across different species according to specific needs.
One way that the hearing abilities of various species differ is the range of
frequencies that can be detected.
Blowing a dog whistle: you know that blowing the whistle doesn’t produce any
audible sound to your own ears, but you will certainly have a dog‘s attention! The
dog whistle produces a sound at a high frequency that is beyond the range of
human ears but well within the range of the dog’s auditory system.
Sound Frequency Perception in Vertebrates:
Humans can perceive sounds that lie anywhere between 20 and 20,000 Hz, a
respectable auditory range. Relatively speaking, whales, dolphins and dogs have a
wider hearing range, while frogs and birds have a much narrower range of
frequencies that they can detect.
At the lower frequency detection extreme are fish, while at the higher frequency
detection extreme are bats and rodents.
Environmental Impacts on Auditory Structure:
Audible frequency range is determined in part by the evolution of the structures of
the auditory system.
One key structure is the basilar membrane which contains the hearing receptors;
sounds of different frequencies are processed along different areas of the basilar
The Basilar Membrane:
The basilar membrane varies in length across species; it is shortest in amphibians
and reptiles, longer in birds, and longest in mammals.
A longer basilar membrane allows processing of a wider range of frequencies.
And so, mammals can discriminate the widest range of frequencies while most
other species cannot discriminate frequencies over 10000 Hz.
The Stimulus: Sound Waves
Like light, sound travels in waves, although sound waves travel much slower and
require some medium to travel through.
Sound waves are initiated by either a vibrating object, like our vocal cords or a
guitar string, a sudden burst of air, like a clap, or by forcing air past a small
cavity, like a pipe organ.
This causes the air molecules surrounding the source of the sound to move which
causes a chain reaction of moving air particles.
Responding to Changes in Air Pressure:
This chain reaction is much like the ripples you observe when you throw a stone
into a pond. The point where the stone hits the pond produces waves that travel
away in all directions, much like the alternating bands of more and less condensed
air particles that travel away from the source of a sound. The Eardrum Responds to Air Pressure Changes:
These alternating bands of more and less compressed air molecules interact with
the eardrum to begin auditory processing.
A band of compressed air molecules causes your eardrum to get pushed slightly
inwards, whereas a band of less dense air particles causes the eardrum to move
The changes in air pressure over time that make up a sound wave can be graphed
as a sine wave, as shown here below.
In our survey of the neurophysiology of vision, we examined three physical
characteristics of a wave: amplitude, wavelength, and purity.
In audition, the same three physical characteristics, when applied to sound waves,
translate into the three psychological properties of loudness, pitch, and timbre.
Amplitude: Measure of Loudness
Variations in the amplitude or height of a sound wave affect the perception of
Since waves of greater amplitude correspond to vibrations of greater intensity,
higher waves correspond to louder sounds.
Humans are sensitive to a very wide range of different sound amplitudes, and
because of this, loudness is measured using a logarithmic scale of decibels (dB).
In this scale, the perceived loudness of a sound doubles for every 10 dB increase.
A normal conversation takes place at around 60 dB, a whisper at around 17 dB,
and sitting in the front row at a rock concert means you get to hear the music at
around 120 dB. Frequency: Measure of Pitch
Sound waves also vary in the distance between successive peaks; this is called the
wavelength or frequency of the sound and this property affects the perception of
Pitch is measured in Hertz (Hz), which represents the number of cycles per
second, or the number of times in a second that a sound wave makes one full
cycle from one peak to the next.
If many wave peaks are condensed into one second, then this sound will be of a
high frequency, and result in the perception of a high pitched sound.
We learned that what we call the visible spectrum of light is only a small portion
of the total spectrum of light waves; similarly, the audible zone of frequencies that
humans can detect represents only a portion of the possible frequencies that can
Timbre: Measure of Complexity/Purity
The third physical property of sound is purity, which affects our perception of
Most of the sounds we hear everyday are complex sounds that are composed of
multiple sound waves that vary in frequency.
Timbre refers to the complexity of a sound.
When you pluck a guitar string, it vibrates as a whole which is the fundamental
tone, but it also vibrates at shorter segments along the string, called the overtones.
The final sound you hear is a mixture of the fundamental tone and all the
overtones, and this combination is timbre. So a piccolo and a Bassoon may both
play the same note, but because each instrument produces a unique combination
of the fundamental frequency and overtones, they still sound different to us, even
though each instrument is producing the same frequency and amplitude.
The ear can be divided into the external, middle, and inner ear and each area
conducts sound in a different way.
Incoming changes in air pressure are channelled through the external ear, onto the
middle ear, and amplified so that it can be detected as changes in fluid pressure by
the inner ear.
These changes in fluid pressure are then finally converted to auditory neural
The External Ear:
Let's begin this journey by examining the external ear, which is made up of the
pinna, the ear canal, and the eardrum.
The pinna is what you probably think of when referring to your ears; it is the
folded cone that collects sound waves in the environment and directs them along
the ear canal.
Since the ear canal narrows as it moves towards the eardrum, it functions to
amplify the incoming sound waves, much like a horn.
The eardrum is a thin membrane vibrating at the frequency of the incoming
sound wave and forms the back wall of the ear canal.
See image on next page The Middle Ear:
The middle ear begins on the other side of the eardrum, which connects to the
ossicles, the three smallest bones in the body.
These ossicles are named after their appearance and consist of the hammer,
anvil, and stirrup.
The amplification of the vibrating waves continues here in the middle ear.
The vibrating ossicles are about 20 times larger than the area of the oval window
to which they connect to create a lever system that amplifies the vibrations even
This additional amplification is necessary because the changes in air pressure
originally detected by the external ear are about to be converted to waves in the
fluid-filled inner ear. (See image below.) The Inner Ear:
The vibrating oval window connects to the cochlea of the inner ear.
The cochlea is a fluid-filled tube, about 35 mm long, coiled like a snail shell. The
cochlea contains the neural tissue that is necessary to transfer the changes in fluid
to neural impulses of audition.
The oval window is actually a small opening in the side of the cochlea, and when
the oval window is made to vibrate, it causes the fluid inside the cochlea to
The round window, located at the other end of the cochlea, accommodates for the
movement of the fluid by bulging in and out accordingly.
Inside the cochlea is a flexible membrane, called the basilar membrane that runs
the length of the cochlea like a carpet.
When the basilar membrane is pushed downwards, the fluid inside the cochlea
causes the round window to bulge out, and when the basilar membrane is forced
upwards, the round window bulges inwards. (See image below.)
Although the cochlea itself gets narrower towards the end, the basilar membrane
actually gets wider towards the end.
Because the length of the basilar membrane varies in both flexibility and width,
sounds of different frequencies cause different regions of the membrane to
Higher frequency sounds cause the end nearest the oval window to vibrate
whereas lower frequency sounds cause the end nearest the round window to
vibrate. (See image below.) Hair Cells:
The basilar membrane houses the auditory receptors, which are called hair cells.
As the membrane moves in response to the waves in the fluid, the hair cells also
move, and this movement is finally converted to neural impulses that the brain
can understand. (See image below.)
Auditory Pathway: From Receptors to Auditory Cortex
When activated, the hair cells along the basilar membrane release a
neurotransmitter. The hair cells form synapses with bipolar cells, whose axons
make up the cochlear nerve, a branch of the main auditory nerve.
Although the outer hair cells outnumber the inner hair cells by about 4 t