Sound as a Wave

What happened to the Tacoma Narrows Bridge?

Wind blowing through a canyon resonates the natural frequency of the bridge.The wind was a periodic force acting on the bridge and increased the amplitude

This is an example of constructive interference

What is constructive interference?

When two waves interact increasing the amplitude

*wave on a string animation

What is destructive interference

When two waves interact and decrease amplitude, or sometimes cancel out

What is sound?

Longitudinal wave through a medium that vibrates eardrum

    Our medium is air, unless we are underwater

    How does that sound differently?

Properties of a sound wave

Frequency: determines pitch, measured in Hz

(pitch: is highness or lowness of sound)

Amplitude: determines loudness, measured in decibels (dB)

*sound animation

*wave interference animation

Sound quiz!

Which is the: highest, loudest, lowest, quietest?

Velocity of sound wave

In air at normal temperature, the velocity of a sound wave is 340 meters per second

The speed of light is 300 million meters per second (300,000,000 m/s)

Supersonic: anything faster than 340 m/s

Subsonic: anything slower than 340 m/s



Changing velocity of sound

Change velocity of sound by changing density of medium

(the medium is the stuff through which a wave flows)

ex. Whales communicating across ocean



Destructive interference and beats

Beats: repeating change in amplitude caused by two waves of similar frequency interfering with each other

Good sounding sounds

Consonance: when sounds interact in a way that sounds nice

Dissonance: when sounds interact and it sounds bad

How to change pitch

1)                      Change frequency

2)                      Object making noise moves



Doppler effect

The heard change in pitch as moving object makes noise

*Police siren animation

*Doppler effect animation

How do we hear frequency?

All animals have different ranges of frequency they can hear

Ex 1: humans canít hear dog whistles

Ex 2: giraffes communicate at a lower frequency that we can hear

What frequencies can people hear?

Letís make a histogram