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Chapter 17Mechanical Waves & Sound
How does a disturbance produce waves?
Procedure Fill a clear plastic container with water. Observe the surface of the water by looking down at
an angle to the container. Use the pipet to release a drop of water from a height of 3 cm above the surface of the water.
Repeat Step 2 with a drop released from each of these heights: 10, 20, 50, 60, 70, 90 cm. Create a table to record your observations after each drop. These observations will be QUALITATIVE (or
descriptive, and not mumeric)
Analysis Questions
Which drop produced the highest wave?
Write a general statement (or conclusion) about how the distance a drop falls affects the wave produced in the container.
Using your knowledge of energy, conservation of energy, and energy transfer, explain why the distance a drop falls affects the height of the wave produced. Thoroughly explain your answer.
Mechanical Waves17.1 Notes
Inquiry Activity
How does a disturbance produce waves?
P 499 Complete activity, answer questions as a group Submit one paper per group with all
observations recorded and questions answered!
What are mechanical waves?Mechanical Wave
A disturbance in matter that carries energy from one place to another
Require a medium (or matter) in order to carry energy
All waves carry energy!
What is a Medium?
The material through which a wave travels
Can be a solid, a liquid or a gas
Space is NOT a medium Why?
How are mechanical waves created?
A source of energy causes a vibration to travel through a medium
Types of Mechanical Waves
Transverse Medium moves
perpendicularly (or at right angles to the direction the wave travels)
Longitudinal (Compressional)
Medium moves parallel to the direction the wave travels
Transverse Waves DemosRope (with ribbon attached)
Student line (arms over shoulders)
Parts of a Transverse Wave
Crest Highest point of the
wave
Trough Lowest point of the
wave
Compressional Wave DemosSlinkys!
Hip Bump
Parts of a Compressional Wave
Compression Area where the
particles in a medium are spaced close together
Rarefaction An area where the
particles in a medium are spread out
Waves transfer ENERGY!Waves DO NOT transfer MATTER
Waves ONLY transfer ENERGY
Example: THE HUMAN WAVE
Surface Waves
A wave that travels along a surface that separates two media (or two types of matter)
An object resting on a surface wave will move up and down, and back and forth These two motions result in a circular motion
for the object
Wave Animation
http://njscuba.net/biology/misc_waves_weather.html
Breaking Waves
Exit Exercise
With your group, make a Venn Diagram that compares and contrast Transverse and Compressional Waves.
Properties of Mechanical Waves17.2
Periodic Motion
Any motion that repeats at regular time intervals
Period The time required for one cycle, a complete
motion that returns to its starting point
Wavelength
Distance between a point on one wave and the same point on the next cycle of the wave
Between adjacent Crests (or troughs), or compressions (or rarefactions)
One Wavelength
One complete wave cycle
Frequency
A periodic motion has a frequency
Frequency The number of complete cycles in a given
time For waves, this is the number of wave cycles
that pass a point in a given time Measured in cycles per second, or Hertz (Hz)
Frequency & WavelengthAs frequency increases, what happens to
wavelength? Use the slinky at your table to determine the
answer to this question, then respond using SocrativeHINT: You can make either TRANSVERSE,
or COMPRESSIONAL waves with your slinky
Frequency Formula
Frequency = 1
*Remember, period is the amount of time it takes for a wave to complete one full cycle
period
Socrative Graph #1
Socrative Graph #2
t in seconds
Socrative Questions…
Two calculating frequency/ period questions
Wave Speed
REMEMBER v = d/t
Think of one wavelength as DISTANCE
Think of period as TIME
Wave Speed = wavelength / period
OR Wave Speed = wavelength x frequency
Wave Speed Example
One end of a rope is vibrated to produce a wave with a wavelength of 0.25 meters. The frequency of the wave is 3.0 Hertz. What is the speed of the wave?
FORMULA Speed = Wavelength x Frequency
Socrative Practice
Wave Speed Questions
Amplitude
The maximum displacement of the medium from its rest position
The more energy a wave has, the greater its amplitude
Behavior of Waves17.3
Reflection
Occurs when a wave bounces off a surface that it cannot pass through
Does not change wave speed or frequency, but does change wave direction
Refraction
Bending of a wave as it enters a new medium
Occurs because one side of the wave moves more slowly than the other side
Refraction Example
Pencil in water demonstration
Diffraction
Bending of a wave as it moves around an obstacle or passes through a narrow opening
A wave diffracts more if its wavelength is large compared to the size of an opening or obstacle
Diffraction Animation
Human moving with arms out
http://www.acoustics.salford.ac.uk/feschools/waves/diffract3.htm
Interference
Occurs when two or more waves overlap and combine together
In interference, waves DO NOT bounce off one another, but rather move PAST each other
Types of Interference
Constructive Occurs when two or more
waves combine to produce a wave with a larger displacement
This occurs when two crests meet, or when two troughs meet
Wave amplitudes are added together, producing a larger wave during the time the waves overlap
Destructive Occurs when 2 or more
waves combine to produce a wave with a smaller displacement
This occurs when a crest meets a trough
Produces a waves with reduced amplitude
Wave Superposition
When two waves interfere, the resulting displacement of the medium at any location is the sum of the displacements of the individual waves at that same location.
Animations
Constructive Interference
Destructive Interference
Standing Waves
A wave that appears to stay in one place and does not appear to move through the medium
Only certain points on the wave are stationary, not the entire wave
Happens only at certain frequencies
Standing Waves
Nodes A point on a standing
wave that has no displacement from the rest position
Complete destructive interference between incoming and reflected waves
Antinodes A point where a crest
or a trough appears midway between 2 nodes
Animation
Standing Waves
Rope/Slinky Example
Chapter 1 Midterm Review You water three sunflower plants with salt
water. Each plant receives a different concentration of salt solutions. A fourth plant receives pure water. After a two week period, the height is measured.
Identify the independent variable and the dependent variable in the experiment above.
Chapter 1 Review #2
One tank of gold fish is fed the normal amount of food once a day, a second tank is fed twice a day, and a third tank four times a day during a six week study. The fish’s weight is recorded daily
Identify the independent variable and the dependent variable in the experiment above.
17.4 Sound & Hearing
Sound Waves
Longitudinal (or Compressional) Waves
Have compressions & rarefactions Cause matter to vibrate in a direction that is
parallel to the direction the wave is moving
Sound Speed
In dry air at 20 degrees Celsius, the speed of sound is 342 m/s
Can you think of an example when you’ve experienced a sound delay?
In general, sound travels fastest in solids and slowest in gases Why?
Sound Intensity
The amount of energy that is transported past a given area of the medium per unit of time
Sound intensity refers to how much energy the sound waves is transporting
Can be measured in decibels (dB)
Sound Loudness
Subjective human response to sound Depends on sound intensity
Sound Frequency
Depends on how fast a sound is vibrating
Most people hear sounds between 2o Hz and 20,000 Hz
Infrasound Infra means “below”
Sound frequencies that are below what humans can hear
Ultrasound Ultra means “above”
Sound frequencies that are above what humans can hear
Used in sonar and ultrasound technologies
Upper-range Frequencieshttp://www.audiocheck.net/audiotests_fr
equencychecklow.php
http://www.audiocheck.net/audiotests_frequencycheckhigh.php
Ultrasound
Sounds are bounced off parts of the body and then the reflections are used to create an image of the body part
Sonar
Stands for sound navigation and ranging
Sounds are bounced off an object and then the time that the sound waves takes to return to the object is measured Uses echoes
Echo
Echoes occur when sound waves reflect off of objects they cannot pass through
Echolocation
Doppler Effect
Occurs when a source that is producing waves (like an ambulance’s siren produces sound waves) is moving with respect to any observers (like a person on the side walk watching an ambulance drive by)
There is an apparent upward shift in wave frequency for observers when the sound source is moving towards them, and an apparent downward shift in frequency for observers when the sound source is moving away from them
Doppler Effect (Continued)The Doppler effect can be observed for
any type of wave - water wave, sound wave, light wave, etc.
How We Hear Step 1: The outer ear collects sound (acoustic) energy and directs it
through the ear canal to the eardrum
Step 2: The incoming waves of sound energy cause the eardrum to vibrate,
Step 3: The vibration of the eardrum causes three smaller bones (known as the hammer, anvil and stirrup) to vibrate as well
Step 4: Sound energy is transferred to the middle ear, which amplifies the sound
Step 5: Sound travels through the inner ear, eventually causing thousands of tiny sensory hair cells to vibrate
Step 6: The motion of the cells triggers chemical-electrical signals that are transmitted through to the brain along the auditory nerve pathway. The brain can then translate the impulses of energy into recognizable sound patterns.
