 What do we know about outer space?  What does the bell jar experiment tell us about sound?  Correct! Sound needs a medium to travel though....When air is removed from the jar, the sound no longer has a medium for the wave to travel though.

 We know that sound is a “pressure disturbance wave.”  When sound travels through air, it disturbs particles of air  These “disturbed particles” exert forces on particles next to them which then exert forces on particles next to them  These series of disturbances act to transfer energy through the medium  Speed of sound = how fast the disturbance is transferred from particle to particle

 The speed of any wave depends on the property of the medium  As long at the medium remains the same, the speed (velocity) of the wave will not change  Two properties affect wave speed: inertial and elastic

 Elastic= material that tends to keep its shape (steel vs. Elastic band  steel has high elasticity).  Particles have strong attraction for one another.  In general, the speed of a wave is greater in solids>liquids>gases.  Inertial = resistance to change. This is a material’s tendency to be slow to change its state of motion  The greater the density of particles, the less responsive they will be to particle interactions and the slower the wave will move  E.g. Air is more dense than helium (a sound wave travels three times faster in helium)

 Speed of sound is faster in higher temperatures. Why?

I

 Intensity refers to the “loudness” of sound  Intensity is measured in Watts (energy) per m2  Scientists decided that 1 W/m2 is too large to measure the small but wide range in intensity levels that the human ear is able to hear  As a result, special units were designed to measure sound intensity  These are called the “bel” and “decibel”

 B = log (I/I 0 )  I = intensity of interest (measured in W/m 2 )  I 0 = W/m 2 (assumed to be threshold of human hearing)  dB = 10 log (I/I 0 )  Sound intensity of 0 dB is the threshold of human hearing. This corresponds to an intensity of W/m 2  An increase of 10 dB represents an increase in sound intensity by one order of magnitude  0 dB = W/m 2  10 dB = W/m 2  20 dB = W/m 2

 The minimum sound intensity that humans are able to hear is dependant on the frequency of the sound wave  A frequency of 4000 Hz is the lowest threshold for those who have excellent hearing ability  The human ear is most sensitive to frequencies between 1000 Hz and 5000 Hz.

 What happens to the intensity of sound as you get further away (greater distance) from the source?

 Sound intensities over 90 dB (or even lower for prolonged, repeated periods of exposure) can cause permanent damage to the ear

 What is happening biologically when hearing loss occurs?  When noise is too loud, it begins to kill the nerve endings in the inner ear. Prolonged exposure to loud noise destroys nerve endings. As the number of nerve endings decreases, so does your hearing. There is no way to restore life to dead nerve endings; the damage is permanent. The longer you are exposed to a loud noise, the more damaging it may be. Also, the closer you are to the source of intense noise, the more damaging it is.

 Hearing protection (earmuffs and earplugs) must be worn if you work in an excessively noisy environment  You should wear hearing protection when using power tools, noisy yard equipment, firearms, riding a motorcycle or snowmobile  Hearing protectors can not protect adequately against intense sounds which travel through the skeleton into the bones in the middle ear (i.e. Vibrating power tools, some heavy machinery)

 How do hearing aids work?  Microphone picks up sound  Sends sound to amplifier that makes it louder  Makes some pitches of sound louder than others, depending on the hearing loss  The sound is converted into electrical impulses  Impulses are taken to receiver where it is converted back into sound energy and presented to the ear