06 Feb 5 Sound, Interference, Pitch, Beats After Class

Today: Sound Waves, Interference, Beats, Pitch ,[object Object],[object Object],Exam #1 is Thursday!!! In this room, BRING A PENCIL!!! Big eared Townsend Bat, Nevada Bureau of Land Management

Last week’s puzzler: Wind and Doppler Effect ,[object Object],[object Object],The compression / rarefaction waves are what you hear… Remember it’s the frequency that determines the pitch

Last week’s puzzler: Wind and Doppler Effect ,[object Object],[object Object]

1. Yes 2. No Does the wind affect the pitch of a factory whistle you hear on a windy day?

Author’s very good analogy, why wind does not affect pitch of sound ,[object Object],[object Object]

Let’s explore Doppler effect for sound with demos and applets http://www.falstad.com/mathphysics.html ripple Nerf Doppler Demo Very good animation http://physics-animations.com/Physics/English/waves.htm Relative movement between source and receiver DOES change the frequency and wavelength Does NOT change the wave speed Pay attention to things like frequency, wavelength, and wave speed during applets

Is the frequency of a sound wave affected by bouncing off moving surface? This effect is used by some bats in echolocation! Can determine whether insect is approaching Can detect rapidly beating wings of insects Big eared Townsend Bat, Nevada Bureau of Land Management

Is the frequency of a sound wave affected by bouncing off moving surface? This effect is used in ultrasonography! Doppler shift in sound frequency reveals blood velocity User:Ekko on wikipedia (above) also: http://www.youtube.com/watch?v=L88hNMak4Go

Sound in air: Compression / Rarefaction Waves ,[object Object],[object Object],[object Object],(a) compression (b) rarefaction (c) wave propegation of a tuning fork as seen from above From http://www.pjwstk.edu.pl/~kmarasek/3-sound.ppt What is going on when I strike this tuning fork?

[object Object],[object Object],When a sound is emitted at the front of the lecture hall, what happens to the air molecules in the sound wave?

[object Object],[object Object],When a sound is emitted at the front of the lecture hall, what happens to the air molecules in the sound wave? Waves transmit energy not matter! If air molecules are dislocated only a little, how are we able to hear the sound? Pressure on our ear drums!

Speed of sound in air about 770 miles per hour ,[object Object],[object Object],[object Object],[object Object]

Clicker Question—Speed of sound ,[object Object],[object Object],[object Object],[object Object],[object Object]

Clicker Question—Speed of sound ,[object Object],[object Object],[object Object],[object Object],[object Object],Sound waves cannot travel through a vacuum!

So, you can’t have waves in space, right? http://www.youtube.com/watch?v=3drcpE-0Inc

Clicker question In air, what is the wavelength of a 440 Hz sound wave? ,[object Object],[object Object],[object Object],[object Object],http://www.phys.unsw.edu.au/~jw/notes.html 440 Hz

Clicker question In air, what is the wavelength of a 440 Hz sound wave? ,[object Object],[object Object],[object Object],[object Object],[object Object],http://www.phys.unsw.edu.au/~jw/notes.html 440 Hz ¾ meter = 340 m/s / 440 Hz Wavelength = Wave speed frequency

There can be constructive or destructive interference Constructive Interference “ In Phase” Destructive Interference “ 180 degrees out of phase” Remember the wave table demo with Ashley on Tuesday? This interference happens with sound waves as well.

Noise “cancellation” headphones use wave interference to reduce noise ,[object Object]

Sound reflects very well…and interferes with itself ,[object Object],[object Object],Much work goes into “good acoustics” for orchestra halls…preventing “dead spots” Let’s try this out! w/ speakers and sound level meters

Sound waves of two different frequencies also interfere… “beats” ,[object Object],[object Object],http://www.ngsir.netfirms.com/englishhtm/Beats.htm

Clicker Question—Beats ,[object Object],[object Object]

Frequency of wave influences pitch perception http://www.phys.unsw.edu.au/~jw/notes.html 4186 Hz 440 Hz 261.6 Hz 27.5 Hz “ Psychoacoustics” Shepard’s tones http://www.netalive.org/tinkering/shepard-effect/ Lissajous curves & beats http://www.youtube.com/watch?v=DXpntnHxNZQ

Parting question: Is pitch perception physics? Biology? Exam #1 is Thursday!!! In this room, BRING A PENCIL!!!

06 Feb 5 Sound, Interference, Pitch, Beats After Class

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