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 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 (> Hz) are called ultrasound

bats can generate and detect ultrasonic sound waves of greater than Hz dogs can detect frequencies between 50 and Hz cats can detect frequencies between 45 and Hz dolphins can detect frequencies of up to 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/m 2 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

SourceIntensity Level # of Times Greater Than TOH Threshold of Hearing (TOH)1* W/m 2 0 dB10 0 Rustling Leaves1* W/m 2 10 dB10 1 Whisper1* W/m 2 20 dB10 2 Normal Conversation1*10 -6 W/m 2 60 dB10 6 Busy Street Traffic1*10 -5 W/m 2 70 dB10 7 Vacuum Cleaner1*10 -4 W/m 2 80 dB10 8 Large Orchestra6.3*10 -3 W/m 2 98 dB Walkman at Maximum Level1*10 -2 W/m dB10 Front Rows of Rock Concert1*10 -1 W/m dB10 11 Threshold of Pain1*10 1 W/m dB10 13 Military Jet Takeoff1*10 2 W/m dB10 14 Instant Perforation of Eardrum1*10 4 W/m dB10 16

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 10 2 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 clotB - 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.

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 hair-like nerve cells each hair cell is sensitive to particular frequencies in vibration Normal Hair Cells Noise-Damaged Hair 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 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: 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.