Sound and Standing Waves

Basics of Sound• Sound waves travel in longitudinal waves

• A crest of a sound wave is a compression, where the most particles exist

• A trough of a sound wave is a rarefaction, where the least particles exist

Sound Creation

• Sounds are made by vibrating an object

• This object vibrates at a specific frequency which produces a sound– Also known as pitch

• The loudness of a sound is based on the amplitude of the vibration (measured in decibels or dB)

Examples of Sound Waves

• 300Hz 100Hz

• 400Hz 80Hz

• 500Hz 60Hz

• 800Hz 40Hz

Resonance

• Occurs when an object is driven at its natural frequency

• At this frequency, the loudness or amplitude of the wave is increased greatly

• Example: a child on a swing story• https://www.youtube.com/watch?

v=BE827gwnnk4

Standing Waves

• A standing wave is a phenomena that occurs when the reflected wave and the incident wave interfere to create stationary nodes

Harmonics• Each harmonic

represents a standing wave that occurs when the object is driven at its natural frequency

• Tube examples…

Sound Interference

• Same as other wave interference– Crest + crest = increase amplitude– Crest + trough = decrease amplitude– Trough + trough = increase amplitude

• When the amplitude is increased, the sound gets louder

Sound Interference (cont.)

• Two sound waves add together to give a new wave

• If their frequencies are unequal, a new beat frequency is created

What interference and beats sound like…

• 400Hz and 401Hz

• 400Hz and 403Hz

• 400Hz and 405Hz

• 400Hz and 411Hz

• 400Hz and 450Hz

So we’re on the topic of human hearing…

• We can hear from 20 Hz to 20,000 Hz

• Depending on which study you look at, dogs can hear from about 50 Hz to about 50,000 or even 100,000 Hz– Dog whistles operate between 30-50 kHz

How we hear?• Sounds enter through the outer ear

– The part we can see• Then hits the eardrum• This vibrates the bones which

transmit the motion through fluid to the cocclea

• The cocclea has tiny hairs that then detect the motion and send the info to the brain

• When you swallow or yawn, this stretches muscles that replenish the air inside your ear

Other types of noise

• White noise– Sound with equal amplitude at all frequencies

• Pink Noise– Sound that, in essence, sounds like white noise to

humans because of how we hear (lemme try and explain, but we’ll see how it goes)

• Brown Noise– Same as pink, only there is a decrease in power as

there is an increase in frequency