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