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

Source IntensityIntensity

Level# of Times

Greater Than TOH

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 Ultrasound3-D Ultrasound

Testicular ultrasoundA – 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 sound2.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.

Animated Ear

• 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

Ear Drums

• the cochlea is snail-shaped, and is lined with about 20 000 hair-like nerve cells

• each hair cell is sensitive to particular frequencies in vibration

NormalHair Cells

Noise-DamagedHair Cells

• hair cells can be irreversibly damaged by loud noise (see above)

• 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 stiffVibrates 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:

3. brain damage to auditory cortex (e.g. stroke)

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)

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!!

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.