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Communication Topic 12: Processing Sound

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1 Communication Topic 12: Processing Sound
Biology in Focus, HSC Course Glenda Childrawi, Margaret Robson and Stephanie Hollis

2 DOT Point(s) outline the path of a sound wave through the external, middle and inner ear and identify the energy transformations that occur describe the relationship between the distribution of hair cells in the organ of Corti and the detection of sounds of different frequencies outline the role of the sound shadow cast by the head in the location of sound

3 Introduction In this section we are going to trace the path of a sound wave through the ear to the organ of Corti where it is processed. I suggest you draw a flowchart to outline the path of sound in the next few slides. There’s 9 steps to you may not get it all on a single line. You could do it vertically like this.

4 The Path of Sound Sound is transmitted as a wave through air in the auditory canal to the outer layer of the tympanic membrane. auditory canal (Eardrum) tympanic membrane

5 The Path of Sound Vibrations from the tympanic membrane are conveyed through this air-filled chamber via the movement of the interconnecting ear ossicles to the oval window of the inner ear. (Ossicles) malleus incus stapes

6 The Path of Sound The stapes vibrates the oval window setting up a pressure wave in the perilymph of the upper canal of the Cochlea. This causes Reissner’s membrane to move. oval window cochlea

7 The Path of Sound The movement of Reissner’s membrane transfers the kinetic energy to the endolymph of the middle ear canal. This vibrates the basilar membrane, stimulating the hair cells of the organ of Corti. Hair cells (Organ of Corti)

8 The Path of Sound The hair cells send messages along nerve fibres to the brain where they are interpreted. The pressure waves continue to the round window at the end of the lower canal. Auditory Nerves Brain

9 Energy Transformations
Sound waves pass in air along the auditory canal. Sound energy is converted to mechanical (kinetic) as the vibration is set up in the tympanic membrane. The mechanical energy is transmitted through the three ear ossicles to the oval window.

10 Energy Transformations
As it passes into the perilymph, as a pressure wave, the mechanical energy is transferred via Reissner’s membrane to the endolymph to the organ of Corti. Mechanical energy is now converted to electrochemical energy as information is transmitted, as nerve impulses, from the hair cells by the auditory nerve to the brain.

11 Energy Transformations
Lets review what our flow-charts should look like shall we…

12 The Organ of Corti The organ of Corti rests on top of the basilar membrane. It is composed of supporting cells and about hearing receptor cells called cochlea hair cells. Unlike other cells of the body, there is a finite number and they are not replaced as they die.

13 The Organ of Corti There is one row of inner hair cells and three rows of outer hair cells. These are sandwiched between the tectorial and basilar membranes of the cochlea.

14 The Organ of Corti The fibres of the cochlea nerve are coiled around the bases of the hair cells. The ‘hairs’ of the hair cells (cilia) protrude into the potassium (K+) rich endolymph and the longest of them are embedded in the overlying gel of the tectorial membrane.

15 The Organ of Corti Activation of the hair cells occurs at points of vigorous vibration of the basilar membrane. Hair cells nearest the oval window (base) are activated by the highest pitched sounds while those furthest away at the narrow end of the cochlea are stimulated by low frequency sounds.

16 Measurement of Sound Sound is measured by its relative intensity. The unit of measurement is the decibel (dB). A sound, 10 time as powerful as another is said to be 10 decibels more intense. A decibel level of zero (0dB) represents the faintest sound audible to the average person. The usual level for conversation is about 60dB. Sound becomes physically painful to humans above 130dB.

17 Dangers Noise is probably the most common occupational hazard facing people today. Even leisure activities produce harmful noise. Many everyday sounds may not be recognised as hazardous as they do not intrude our comfort zone.

18 Dangers A bulldozer, idling at 85dB, can cause permanent damage to hearing in one workday. Standard volume music, transmitted directly vie earphones, reach 100dB and can cause permanent damage after only 15 minutes per day.

19 Dangers dB Source 10 Normal breating 20 Whisper 50 Rainfall 60 Conversation 70 Washing machine 80 Busy city traffic 90 Hair dryer 100 Mp3 player 110 Rock concert 120 Ambulance 130 Jet plane from 30 metres away 140 Fireworks 160 Shotgun 180 Space rocket at liftoff Avoid noise which causes discomfort. Above 120dB, noise can be felt on the eardrum as a tickling sensation. Beyond 130dB the sensation turns to pain.

20 Sound Shadows The phenomenon caused by the obstruction or absorption of a sound wave by an object is its path is called a sonic or sound shadow. This is perceived as a reduction in amplitude or volume. The effect will be greatest when the sound source, the absorbing object and the person hearing the sound are all aligned.

21 Sound Shadows The sonic shadow is the region which does not receive the direct sound as the head is blocking the vibration As humans are binaural (have two ears), the head creates a sonic shadow for the ear further away from the sound source.

22 Sound Shadows Animals use the sonic shadow to determine the direction of the source of the sound. The difference in loudness and time of arrival of the sound at each ear can be interpreted by the brain to determine location.

23 Sound Shadows Many humans will turn their head when trying to determine the source of a sound. Turning the head increases the difference in time of arrival at each ear and increases the ability to determine location.

24 Sound Shadows The effect has been shown to be less important than the split-second time difference in each if the two ears receiving the same sound. Visually impaired people use the sonic shadow effect, together with echolocation and other cues for orientation.

25 Hearing Aids Some people are born with an inability to hear. Others may lose their sense of hearing suddenly or gradually. The causes of hearing loss are generally grouped into such categories as heredity, diseases, accidents, prescription drugs, aging and acoustic trauma. Some day your partner may tell you that they have selective hearing loss…. ;)

26 Hearing Aids The most avoidable of these causes is acoustic trauma or the chronic exposure to sound. Noise-induced hearing loss (NIHL) may result from a one-time exposure to loud sound or from repeated exposure to sounds of various amplitudes over an extended period of time.

27 Hearing Aids The tympanic membrane may rupture and/or the ear ossicles could fracture or be displaced by short exposure to a loud noise such as an explosion.

28 Hearing Aids Short term hearing loss is caused by fatigue of the hair cells of the cochlea. ‘Ringing’ in the ears is called tinnitus. It happens when delicate cells inside your ear that send sound messages to your brain are injured or over-stimulated. Usually your ears ring for a brief time after you’ve been exposed to loud noise.

29 Hearing Aids Continual exposure to excessive noise (over 85dB) causes damage to the cilia of the hair cells. The hair cells die and are not replaced. If enough of these cells are damaged, permanent hearing loss results. The high frequency receptors are more often damaged by loud sound.

30 Activity/Homework -Handout Table 6.3 Comparison of hearing aid with cochlear implant -Handout DOT Point 6.9 Hearing Aids and Cochlear Implants -Students to complete DOT Point 6.9 and evaluate a hearing aid and a cochlear implant

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