Transcript document

Sound Waves,
Hearing, and the Human Ear
• the frequency of a wave is the number of waves per unit of time
• usually measured in Hz (1 wave per second)
• humans can detect sound waves with frequencies between about
20 to 20 000 Hz, although this changes the older you get
• sounds with frequencies below what we can hear (<20 Hz) are called
infrasound, and those with frequencies above what we can hear (>20 000 Hz)
are called ultrasound
• bats can generate and detect ultrasonic sound waves of greater than
100 000 Hz
• dogs can detect frequencies between 50 and 45 000 Hz
• cats can detect frequencies between 45 and 80 000 Hz
• dolphins can detect frequencies of up to 200 000 Hz
• elephants can detect infrasonic frequencies as low as 5Hz
• the pitch of a sound is the ear’s response to its frequency
• high-frequency sounds have a higher pitch than low-frequency sounds
•The amount of energy which is transported past a given area of the medium
per unit of time is known as the intensity of the sound wave.
Intensity = Energy/ (time * area) = Power/ Area
• intensity is measure in Watts/m2
• because of the large range of intensities humans can hear, intensity is usually
measured using a logarithmic scale called the decibel scale (dB)
• sound “loudness” is more subjective and varies from person to person
Intensity
# of Times
Level
Greater Than TOH
Source
Intensity
Threshold of Hearing (TOH)
1*10-12 W/m2
0 dB
100
Rustling Leaves
1*10-11 W/m2
10 dB
101
Whisper
1*10-10 W/m2
20 dB
102
Normal Conversation
1*10-6 W/m2
60 dB
106
Busy Street Traffic
1*10-5 W/m2
70 dB
107
Vacuum Cleaner
1*10-4 W/m2
80 dB
108
Large Orchestra
6.3*10-3 W/m2
98 dB
109.8
Walkman at Maximum Level
1*10-2 W/m2
100 dB
1010
Front Rows of Rock Concert
1*10-1 W/m2
110 dB
1011
Threshold of Pain
1*101 W/m2
130 dB
1013
Military Jet Takeoff
1*102 W/m2
140 dB
1014
Instant Perforation of Eardrum
1*104 W/m2
160 dB
1016
• A mosquito's buzz is often rated with a
decibel rating of 40 dB. Normal
conversation is often rated at 60 dB. How
many times more intense is normal
conversation compared to a mosquito's
buzz?
It is a 20 dB difference which is equal to 102 so 100 times more intense.
Ultrasound and Medicine
• ultrasound can be used to visualize internal organs and to diagnose medical
conditions
• in ultrasound, very high frequency sound waves are passed into the body
• when the waves strike an object, they bounce back (like an echo)
• by measuring the echo waves, doctors can look at the shape, size etc. of
organs or objects inside the body (e.g. fetus, tumour)
2-D Ultrasound
3-D Ultrasound
Testicular ultrasound
A – blood clot
B - tumor
Okay, so how do we actually
hear stuff?
• The human ear consists of 3 sections: the outer, middle, and inner ear
• The outer ear extends from the pinna (ear flap/auricle) to the approximately 2 cm
long ear canal
• The ear flap (not just for earrings!) provides protection for the ear and helps
to channel sound waves from the environment to the middle ear
•Sound waves are channeled from outer
ear to the ear drum.
•when sound waves enter the ear, they
cause the eardrum to undergo
compression and rarefaction, so it
vibrates with the same frequency as
the sound wave
•The ear drum separates the outer ear
from the middle ear.
•The ear drum (also referred to as the
tympanic membrane) consists of three
small bones: the hammer, the anvil and
the stirrup.
•the hammer is connected to the ear
drum, so movement of the eardrum is
passed from the hammer, to the anvil, to
the stirrup, which is attached to the
inner ear and the vibrations can be
passed through to the oval window.
•When you hear a guitar vibrate the following
occurs in your ear:
1. The ear drum vibrates with the same
frequency of the sound
2. The three bones in the ear amplify this
sound when vibrations are passed from the
hammer, anvil and the stirrup.
3. The stirrup causes vibration from the middle
ear to the oval window.
4. The vibration of the oval window causes
pressure waves in the cochlea which is
filled with fluid.
5. The motion of this fluid stimulates the hair
cells.
• the fluid passes over the hair cells and when the frequency
of the wave matches the frequency of the particular hair cell, it
sends a nerve impulse down the auditory nerve to the brain
Animated Ear
Ear Drums
• the
cochlea is snail-shaped, and is lined with about 20 000
hair-like nerve cells
Normal
Hair Cells
Noise-Damaged
Hair Cells
• hair cells can be irreversibly damaged by loud noise (see above)
• each hair cell is sensitive to particular frequencies in vibration
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Fluid pressure is
created by the
pressure waves
from the sound.
This causes the
basilar membrane
to vibrate causing
hair cells to be
stimulated.
This causes nerve
impulses to be
sent to the auditory
nerves and then is
sent to the brain.
•Contains hair cells
Narrow and stiff
Vibrates mostly with high
frequency
Wide and flexible vibrates with low
frequency
• since the pressure wave is going from the large area of the eardrum to
the smaller area of the stirrup (P=F/A), the stirrup can vibrate with about
15x as much force as the eardrum (why we can hear very faint noises)
movement of inner ear
• the Eustachian tube connects the middle ear with the mouth(pharynx) so pressure
can be equalized in the ear.
• when you have a cold or infection, the Eustachian tube can become blocked,
leading to earaches and infections.
Hearing Loss
There are many possible causes of hearing loss:
1. conductive hearing loss where sound waves cannot travel
from the outer to middle ear (e.g. ear infection – pus build-up,
ruptured eardrum, malformation of middle ear, damage to bones
of middle ear, foreign body in ear)
2. Sensorineural hearing loss: damage to the inner ear or
the nerves that carry signals from the ear to the brain (e.g.
diseases, injury, drugs, genetic syndromes)
3. brain damage to auditory cortex (e.g. stroke)
Hearing aids can either amplify sounds or directly stimulate the cochlea
Normal guinea pig hair cells
Hair cells of guinea pig exposed to
120 dB noise (similar to rock concert)
TURN DOWN YOUR IPODS!!
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Semicircular canals
Control balance and motion
At 90 degrees to one another
Filled with fluid and contain sensory hair
cells
When you move your head the fluid puts
pressure on the hairs.
Sensory neurons detect the pressure of
the hairs.
Impulses are then sent to the brain to
detect the position of your head.