Unit 10Waves & Wave Properties

Sound Waves

• Waves caused by disturbing a medium – results in “noise”, music, vibrations you can’t hear by other animals can.

• Mechanical Waves– Need a medium (solid, liquid, gas) to travel

• Longitudinal Waves– Vibrations are parallel to wave motion.

Sound Waves

Sound Waves• Sound waves need a medium to travel• If the medium changes, the speed of

sound changes.• Average Speed of Sound Air: 340m/s

• Sound moves the fastest through solids and the slowest through gases.

solids liquids gases

• Sound moves the fastest through hot air and the slowest through cold air.

hot air cold air

Sound Waves• Notice the frequency and wavelength DO NOT

change the speed of sound.• Changing the frequency changes the wavelength.• Changing the wavelength changes the frequency.

• A high frequency means it is “high pitch” (alto singer)

• A low frequency means a “low pitch” (baritone singer)

• There are sounds people cannot hear. – “Ultrasonic” sounds are pitches higher than human hearing.– higher than 20,000 Hz

– “Infrasonic” sounds are pitches lower than human hearing.– lower than 20Hz

f λ f λ

• You can change the loudness of a sound two ways.• Change the amplitude (amplify)

• You can amplify a wave by turning “up” the volume, OR by adding to the wave again and again ... And again and again…– Turning “up” the volume is increasing the “decibel level” of the source.

• Everything is vibrating (every object vibrates, “natural frequency”) –

you can amplify the wave if you match its natural frequency.

• Change the intensity– Intensity can be changed by your

distance from the source. – The closer you are to the source, the more intense the sound, aka “louder”

Sound Waves

“If a tree falls in the forest and no one is around to hear it … does it still make a sound?”

– Yes! A “sound wave” is a sound wave whether or not you can hear it.

– Sound waves are a vibration. A disturbance of molecules!

– There are many sounds people cannot hear – but they are still sounds!

– If a tree falls it “disturbs” the molecules in the air and the ground, therefore creating a wave! It makes a sound!

“If a tree falls in the forest and no one is around to hear it … does it still make a sound?”

– Normal hearing frequencies range from 20 Hz – 20,000Hz

– Frequencies higher than 20,000 Hz (Ultrasonic) and lower than 20Hz (Infrasonic) are out of normal human range.

– The older you get – the smaller your range of audible sounds gets! Old people have a lot of trouble hearing high frequencies (high pitch voices) • Which is why Grandma can’t understand you

little sister as well as she can understand you!

“If a tree falls in the forest and no one is around to hear it … does it still make a sound?”

Old People Hearing Sounds - Frequency

Check out these old teachers listening to sound waves as they go higher and higher in frequency.

The teachers will put their hands down when they can no longer hear the sound.

Who will put their hand down first?(Note: The camera actually doesn’t pick up the sound after a certain frequency, so don’t worry if you can’t hear it in the video …

in class we did the same experiment)

Jump to the time:1:30

for the action!

For HopeHSBlueWave.org website use:

Video can be found in “Video” page – title:

“Old People Hearing Sounds”

People Hearing Sounds Intensity

Both Intensity and Frequency

Frequency (Hz)

Infrasonic and Ultrasonic Waves• Dog whistles are high pitched so that dogs can hear them, but

not humans.• Dolphins and bats use “sonar” aka: ultrasonic waves to

communicate and “see” barriers.

– We have imitated them and use ultrasonic waves for ultrasounds!

• Whales, elephants, hippos, and other animals use infrasound to communicate over distances—whales can do this over the distance of hundreds of miles!

• Infrasonic waves can be used to predict natural disasters such as tornados, volcanic eruptions, etc.

Doppler Effect – Sound WavesWhy does each observer hear a different sound?

This is the Doppler Effect!Each observer hears a different sound because as the motorcycle moves, it “catches up” with it’s own sound waves. Observer B hears a high frequency because the waves are closer together (small wavelength). Observer A hears a low frequency because the waves are farther apart!

The Doppler effect makes sources of sound appear to be higher and lower pitch based on their relative motion.

Moving towards the observer – higher frequency

Moving away from the observer – lower frequency

High frequency means high pitch! Low frequency means low pitch!

Doppler Effect – Sound Waves

Por ejemplo

You can actually close your eyes and tell when the car has gone by you.There is a very distinct “high – to – low” pitch (frequency) change as the car just passes the camera.

Don’t confuse this with “loudness” in the moment right before the car passes you and the moment right after the car passes you it would be the same distance from you. But the distinct difference in sound you can hear is the difference in frequency, high and low pitch due to the car moving towards you or away from you.

For HopeHSBlueWave.org website use:

Video can be found in “Video” page – title: “Doppler Effect 002” and “Doppler Effect 003”

Doppler Effect – Sound Waves

http://www.youtube.com/watch?v=Tn35SB1_NYIDoppler Effect – Sound Waves

http://youtu.be/h4OnBYrbCjY(1-2min)

FREQUENCY IS NOT LOUDNESS!

It’s not FLIPPING louder if it is more frequent!

… and remember…

Superposition Principal – Sound Video of “Beats”

• http://www.acoustics.salford.ac.uk/feschools/waves/super3.htm

Reflection – SoundSound waves reflect off of boundary (object) if they are the

same size or smaller than the boundary.

Ultrasonic waves are high frequency and low wavelength. Since they have a small wavelength, they can reflect off more barriers, so the dolphin or ultrasound can “see” more objects. For Sound Waves - reflection only happens when the sound wave is the same size or SMALLER than the boundary! Big waves would miss things!

Reflection – SoundEcholocation

• Bats and dolphins use echolocation to “see”.• They send out ultrasonic waves and when

they are reflected off a boundary the bats and dolphins can not only “see” there is an object and it’s relative size but also if it is stationary, moving towards them or away from them!

• The Doppler Effect will change the frequency of the returning wave based on it’s motion!

Resonance and Forced Vibrations• Resonance is when the frequency of a force

applied matches the natural frequency of vibration of an object and causes an increase in the wave’s amplitude.

These are all examples of mechanical waves being amplified due to resonance.