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