Unit 5 – Sound

Sound What is wrong with the following clip? Sound waves are vibrations that take place in what medium? If you said “air”, you are only partially correct Sound can travel in solids, liquids, and gasses Sound waves are longitudinal, and are spherical

Sound What is pitch? Frequency corresponds to pitch – high frequency sound waves give you high pitches Demonstration – frequency generator hooked up to oscilloscope and amplifier Other than pitch, what can this show? AM vs. FM radio :: what does AM and FM stand for?

Sound Human range of hearing – 20Hz to 20,000Hz Infrasonic – below 20 Hz – ex) elephants, whales Ultrasonic – above 20,000Hz – ex) dog whistle Demonstrate with WAVEPORT Older people can’t hear extreme frequencies because their eardrums become less flexible!

Sound is a longitudinal wave Just like the compression wave with a slinky, sound has compressions and rarefactions Compressions – dense regions of the medium (typically air) Rarefactions – sparsely populated regions

Homework Read pages 381 – 384 Answer “Check yourself” on page 384 Questions to be completed for tomorrow: Pg 395-396, #1-5

Speed of Sound What is the rule for knowing how far away a lightning strike is? Can you figure out how far away a strike is if you hear the thunder 10 seconds later? V = d /t ; v=340m/s, t =10s Distance = 3400m, ~2 miles!

Reflection of Sound Echo – when a sound wave reaches a new medium *some energy is reflected (echo), while some is transmitted through the medium The time delay of an echo is the extra distance divided by the speed of sound FACT – the human ear cannot distinguish an echo from the original sound if the delay is less than 1/10th a second FIND OUT – how far away must a wall be in order to hear the echo?

Reflection of Sound Echoes are very useful in nature SONAR – SOund Navigation And Ranging Submarines most popularly use this, but so do fishermen! Whales, dolphins, and bats use it to “see” their surroundings

Natural frequency If you hit a stop sign pole, or if you hit a chain link fence, they make different sounds This is because they vibrate differently Based on an object’s elasticity and shape, each object has it’s own natural frequency An object’s natural frequency can be a combination of frequencies

Natural Frequency Resonance – increasing of amplitude that happens when an object is given vibrations at its natural frequency Ex) kicking your legs on a swing – timing has to be perfect Ex) shattering a wine glass with sound Ex) a bridge collapse when a large group of people walk in step with the natural frequency

Interference When waves combine, just like the slinky demo Demonstrate with stereo speakers (note – it is not as effective inside due to reflection off of walls) Constructive interference – loud sounds Destructive interference – soft or no sounds Ex) noise cancelling head sets

Homework Read 386 – 393 Problems – p396, #9-13, 24-29

Doppler Effect What does a car engine sound like as it races by? Doppler Effect explained Doppler Effect visualized Doppler effect – change in observed frequency when the observer is in motion relative to the source

Sound Intensity vs Loudness Sound intensity – the power per unit area that a sound wave makes Loudness – subjective way of qualifying the intensity of sound  our ears our sensitive to specific frequencies, so though loudness and sound intensity are related, they aren’t the same

Sound Intensity vs. Loudness Sound intensity is measured in Watts per meters squared (W/m^2) Loudness is measured in decibels (dB) Decibels are logarithmic – this means every time sound intensity is 10 times larger, the dB level is only 10 greater (difference between multiplication and addition)

Qualities of sound The 3 - Pitch, Volume, Timbre Pitch – the frequency of a sound wave Volume – the amplitude of a sound wave Timbre – the quality that allows people to distinguish different types of sounds (i.e. saxophone from flute) Common descriptors – “pure vs. rich”; “warm vs. cold”, “ dull vs. brilliant”

Sound Almost all musical instruments are a combination of some harmonics Demonstrate on waveport combinations of waves

Beats Beats are produced when 2 waves intefere at different frequencies Fbeat = f1 – f2 Demonstrate on waveport If I had a frequency of 500 and 503, what would the period between beats be?

Air columns Given any air column: If the end is closed, there will be a node If the end is open, there will be an anti-node Open ended air column (both ends open) This is the shortest possible wave: It’s ½ of a wavelength

Air Columns Closed air columns (one end closed) ¼ of a wavelength fits for the fundamental frequency Overtones – higher frequencies that resonant in the given air column This happens as long as the end conditions are met (node for closed end, anti-node for open)