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Sound• Sound is a longitudinal wave•Sound, like all waves is generated by something vibrating•What is its medium?
Sound
• Without a medium, there is no sound
Sound• Sound waves behave as all mechanical waves
do• Human ears are capable of hearing 20-
20,000Hz roughly• All sorts of things can create sound
How the Human Ear Works
Sound
• You lose your hearing as you age, it’s called presbycusis, what? I can’t hear you! Speak up!
• You can also damage your ears by being exposed to loud sounds.
• Whenever you have “ringing” in your ears, you have sustained permanent hearing loss!
I Can’t Hear You
• Animals hear things differently than us!
How big is a sound wave?
• Sound travels at roughly 346 m/s (761 mph) in air
• We can hear frequencies between 20-20,000Hz (this is a generally accepted value)
• How long are the longest and shortest wavelengths we can hear? Do the math!
• Longest 17.3 meters Shortest =1.7cm
Directional Hearing• Humans can tell what way a sound came from
by the difference in time between when it gets from one ear to the next
• We can’t tell if a sound is directly in front or directly in back of us (with no reflection)
• We also can’t tell were a sound is coming from if the wavelength is much longer than the distance between our ears
• That’s why subwoofers don’t need to be placed where the sound is coming from
Constructive and destructive interference
• Interference applet• When setting up a sound stage, engineers
have to worry about “dead spots” where the sound is destructive interference is taking place
Beats
• Beats-Periodic variation in the amplitude of a wave that is due to the destructive interference due to multiple sources with slightly different frequencies.
• Beats applet (visual) in phase/out of phase (under waves-> general -> beats)
• Second applet go to setup and then beats
Resonance
• Natural frequency-frequency at which something naturally vibrates
• Resonance- The effect of when a vibrating object forces another object with the same natural frequency to vibrate
• Multi-pendulum• Tacoma Narrow bridge
Real life• So you say, Anderson, when am I going to use
this stuff in real life?• Anderson says when you become a construction
worker!
Doppler Effect
• Doppler Effect- observed change in frequency when there is relative motion between the source of sound and the observer
• Applet 1• Applet 2
• Doppler Rocket
Sound barrier
• When an object exceeds the speed of sound for that medium, it creates a sonic boom
• The sonic boom happens anytime an object goes faster than the speed of sound.
• Applet 1• Youtube video
Decibel Scale• Decibels (dB)- A measure of
the Intensity of a sound wave, not necessarily the perceived loudness.
• Decibel scale is an exponential scale
• Each 10 decibels makes the noise 10 times loud. For example, a 80 dB sound it 1000 times as intense as a 50 dB sound
Rueben's Tube
• Youtube Video