CHARACTERISTICS OF SOUND - Speed

Is there sound in space? 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.

Is Sound a longitudinal wave or a transverse wave?

What do we know about the speed of sound? 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 sound varies in different types of media 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

Properties affecting speed of wave 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)

Velocity of Sound in Various Gases

Speed of Sound at Different Temperatures Speed of

sound is faster in higher temperatures. Why?

Characteristics of Sound - Intensity -

I

What is intensity of sound? 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”

Equation for Intensity of Sound B = log (I/I0) I = intensity of interest (measured in W/m2) I0 = 10-12 W/m2 (assumed to be threshold of human

hearing) dB = 10 log (I/I0) Sound intensity of 0 dB is the threshold of human

hearing. This corresponds to an intensity of 10-12 W/m2 An increase of 10 dB represents an increase in sound

intensity by one order of magnitude

0 dB = 10-12 W/m2 10 dB = 10-11 W/m2

20 dB = 10-10 W/m2

Frequency of Sound and Hearing 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.

Intensity of Sound

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

Hearing Damage Sound

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

Hearing Damage 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

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)

Hearing Aids 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