waves and sound notes · 2017. 11. 27. · sound waves sound is longitudinal and it is mechanical...
TRANSCRIPT
Sound Waves
Sound is longitudinal AND it is mechanical
Sound needs a medium to travel. No sound in a vacuum.
Produced by vibrations in matter, such as vocal chords, strings, or air columns
Sound Waves
Frequency of vibrations determines the pitch
Amplitude is related to the energy and the loudness
Pressure wave travels, but air particles only move side to side
Speed of Sound
Depends on the medium(elasticity, density, temperature)
Sound through air@ 20C = 343 m/s (warmer air increases speed slightly)
𝑣𝑠𝑜𝑢𝑛𝑑𝑖𝑛 𝑎𝑖𝑟
<𝑣 𝑠𝑜𝑢𝑛𝑑𝑖𝑛 𝑙𝑖𝑞𝑢𝑖𝑑 <
𝑣 𝑠𝑜𝑢𝑛𝑑𝑖𝑛 𝑠𝑜𝑙𝑖𝑑
Speed of Sound
Light (3x108 m/s) travels much faster than sound (343 m/s) through air
This is why there’s a delay with fireworks or lightning far away
Frequency of Sound
Average human can hear between 20 Hz 20,000 Hz
Below 20 Hz is infrasonic
Above 20,000 Hz is ultrasonic
range of frequencies test
Examples: dog whistle, dolphins, ultrasounds, the mosquito ring tone on your phone
Amplitude of Sound(Loudness measured in decibels)
Example 1
You see a lightning strike and then hear the thunder 7.5 seconds later. How far away was the lightning strike?
Example 2
In a concert hall, the orchestra tunes to the oboe playing a frequency of 440 Hz. What is the period of this sound wave?
Example 3
In a concert hall, the orchestra plays middle C at 256 Hz.
What is the wavelength of middle C at 256 Hz?
If you are sitting 75 m away, how much time does it take the sound to get to you?
Example 4
A submarine sends out a sonar signal of 7000. Hz to detect an unknown object. If the speed of sound in water is 1500. m/s, and the reflected wave comes back 3.0 seconds later.
What is the wavelength of the sound wave?
How far away is the unknown object from the submarine?
Interference of Sound
Overlapping crests of a wave will result in…
...an increased amplitude.
This is constructive interference.
Overlapping a crest and a trough results in...
...a decrease in amplitude.
This is destructive interference.
Beat Frequency
Beats - the periodic variation in loudness of two sounds played together
Beat frequency = 𝑓2 − 𝑓1
What is the beat frequency when a 256 Hz and a 260 Hz tuning fork are sounded together?
Resonance
Resonance – when a driving frequency matches another object’s natural frequency and causes an increase in amplitude
• Pushing a friend on a swing
• Resonance boxes
• Speaker shattering wine glass
• Tacoma Narrows Bridge
• Low bass notes rattling car windows
Standing Waves
continuous constructive and destructive interference of waves at certain frequencies
Nodes—where total destructive interference occurs (no movement)
Antinodes—where constructive interference occurs (maximum displacement)
Doppler Effect
If the object or observer are moving towards each other, the observer will hear a higher frequency.
If the object or observer are moving away from each other, the observer will hear a lower frequency.
Sonic Boom
If an object moves faster than the speed of sound in air (343 m/s), the object is supersonic
The object continuously drags a shock wave of pressure behind it that is the sonic boom
Sound videos
Resonance and beat frequency
Example
Doctors can broadcast focused sound waves and cause kidney stones to shatter while inside the body. This is an example of
a) reflection
b) interference
c) beats
d) resonance
Example
The natural frequency of a wine glass is 337 Hz. In order to shatter the wine glass, a loudspeaker would have to play
a) A frequency that is greater than 337 Hz
b) A frequency that is less than 337 Hz
c) A frequency that is exactly 337 Hz
Example
The natural frequency of a wine glass is 337 Hz. In order to shatter the wine glass, a loudspeaker would have to play
a) A frequency that is greater than 337 Hz
b) A frequency that is less than 337 Hz
c) A frequency that is exactly 337 Hz
Example
One violin is playing a frequency of 200 Hz and another violin is playing 205 Hz. The beat frequency that you will hear is
a) 202.5 Hz
b) 405 Hz
c) 5 Hz
d) 2.5 Hz
Example
One violin is playing a frequency of 200 Hz and another violin is playing 205 Hz. The beat frequency that you will hear is
a) 202.5 Hz
b) 405 Hz
c) 5 Hz
d) 2.5 Hz
Example
An F-18 jet is flying at 686 m/s. This plane is
a) ultrasonic
b) infrasonic
c) supersonic
d) subsonic
Example
An F-18 jet is flying at 686 m/s. This plane is
a) ultrasonic
b) infrasonic
c) supersonic
d) subsonic
Example
A police car is driving away from you while the siren sounds at 500 Hz. You will hear a sound wave with a
a) Velocity greater than 343 m/s
b) Velocity less than 343 m/s
c) Frequency higher than 500 Hz
d) Lower than 500 Hz
Example
A police car is driving away from you while the siren sounds at 500 Hz. You will hear a sound wave with a
a) Velocity greater than 343 m/s
b) Velocity less than 343 m/s
c) Frequency higher than 500 Hz
d) Lower than 500 Hz
Example
The following diagram is of a standing wave. If L is 6 meters, then the wavelength of this wave is
a) 2 m
b) 4 m
c) 6 m
d) 12 m
Example
The following diagram is of a standing wave. If L is 6 meters, then the wavelength of this wave is
a) 2 m
b) 4 m
c) 6 m
d) 12 m
1.5 waves fit inside 6 meters, so 1 wavelength is 4 meters.
Example
The following diagram is of a standing wave. This standing wave has
a) 3 nodes and 4 antinodes
b) 6 nodes and 4 antinodes
c) 3 antinodes and 4 nodes
d) 6 antinodes and 4 nodes
Example
The following diagram is of a standing wave. This standing wave has
a) 3 nodes and 4 antinodes
b) 6 nodes and 4 antinodes
c) 3 antinodes and 4 nodes
d) 6 antinodes and 4 nodes
Example
A car’s horn produces a sound wave of constant frequency. As the car speeds up going away from a stationary spectator, the sound wave detected by the spectator
a) decreases in amplitude and decreases in frequency
b) decreases in amplitude and increases in frequency
c) increases in amplitude and decreases in frequency
d) increases in amplitude and increases in frequency
Example
A car’s horn produces a sound wave of constant frequency. As the car speeds up going away from a stationary spectator, the sound wave detected by the spectator
a) decreases in amplitude and decreases in frequency
b) decreases in amplitude and increases in frequency
c) increases in amplitude and decreases in frequency
d) increases in amplitude and increases in frequency