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Part 1 – Wave Characteristics
What is a Wave?
• A disturbance that carries energy through matter or space.
What is a Wave?
2 Categories of Waves
• Mechanical Waves– Require a medium (material) to travel
through– Examples: water waves, sound waves
• Electromagnetic Waves (covered in AP 2)– Do not require a medium– Example: light travels through empty
space
Wave Description
• Crest• Trough• Amplitude– Intensity of the wave.– How much energy is transferred.
• Wavelength– The distance from one point on
a wave to the same point on the next wave.
Wave Description
• Frequency• How frequently a wave pulse occurs.
– Unit• Hertz (Hz)• 1 Hz = 1/seconds or s-1
– Equation
𝒇=¿𝒘𝒂𝒗𝒆𝒔𝒕𝒊𝒎𝒆
Wave Description
• Period• The time (in seconds) that it takes for one
wave to pass.
– Equation
• T = Period (in seconds)
𝑻=𝒕𝒊𝒎𝒆
¿𝒘𝒂𝒗𝒆𝒔
Wave Description
• Frequency and Period are Inversely Related
𝑻=𝟏𝒇
𝒇=𝟏𝑻
Types of Waves
• Transverse Waves–Matter moves perpendicular to the
direction of wave movement.
Types of Waves
• Longitudinal Waves–Matter moves parallel to the direction of
wave movement.
Other Types of Waves!
• Surface (water) Waves
Other Types of Waves!
• Raleigh Waves– Seismic (earthquake) waves.
Wave Motion
• Waves transfer energy!• Matter has no net movement.
Wave Motion
• Waves transfer energy!• Matter has no net movement.
Wave Speed
• Equation
• Examples1. A wave with a wavelength of 2 meters
has a frequency of 50 Hertz. What is the velocity of the wave?• Answer: 100 m/s
𝒗= 𝒇 𝝀v = wave velocity (in m/s)f = frequency (in Hertz)λ = wavelength (in meters)
Wave Speed
• Examples2. Ocean waves are washing up on shore
with a velocity of 5 m/s. If they each have a length of 1.5 meters, what is the frequency of the ocean waves?• Answer: 3.33 Hz
What is Sound?
• A sound wave is a mechanical, longitudinal wave produced by a vibration of matter.
What is Sound?
• A sound wave is a mechanical, longitudinal wave produced by a vibration of matter.
What is Sound?
• Pitch– Determined by the frequency of the
sound wave.– Humans can hear frequencies from
20Hz – 20,000Hz– (dogs can hear up to
40,000 Hz)
What is Sound?
• Amplitude– The intensity of a sound wave– How “loud” it is.– “Loudness” is measured
in Decibels (dB)
Sound in Air
• Speakers (and other objects) produce sound by vibrating and compressing air.
Sound in Air
• Compressions– Higher pressure.
• Rarefactions– Lower pressure.
The Speed of Sound in Air• 340 m/s or 761 mph• The speed of sound varies.• It is affected by the medium and
temperature.–More dense medium = faster–Warmer = faster
Wave Interactions
–When two (or more) waves are travelling through the same medium they will interact with each other.
– The net displacement is the sum of the two waves.
Wave Interactions
• Constructive Interference– The two waves are in-phase.– The Amplitudes add
together.
Wave Interactions
• Destructive Interference–Waves are out-of-phase.– Amplitudes “cancel out”
Wave Interactions
• Example:– Shown to the right are four different
pairs of wave pulses that move toward and interact with each other. Rank, from most to least, the height of the peak that results when the two pulses coincide.• Answer:
A,B,D,C
Standing Waves
• Standing waves in 1 dimension are formed when two identical waves travelling in opposite directions overlap.
• In practice, there is usually one wave which is reflecting back and forth between the two ends of a fixed length of medium. If the length is a suitable multiple of the wavelength, then the wave overlaps itself in a regular fashion and a standing wave is set up.
Standing Waves
Standing Waves• At some points (nodes) destructive interference
means that there is no net vibration of the medium, and at others (antinodes) the vibration is double the effect of a single wave.
• As a result, energy is not transferred by a standing wave, as it is with travelling waves, but is stored in the antinodes.
Natural Frequency
• The special sound an object has when it vibrates.
• Every object has a specific frequency.
Harmonics
• A harmonic is a whole number multiple of the natural frequency.
Resonance
• The frequency of forced vibrations in an object matches its natural frequency, causing it to resonate.
• This creates a huge increase in amplitude.
Boundary Conditions• At a fixed end, a wave is
reflected out of phase. Therefore, the on coming and reflected wave will always interfere destructively, producing a node.
• At a free end, a wave is reflected in phase. Therefore the oncoming and reflected waves will interfere constructively, producing an antinode.
Boundary Conditions• In the case of stringed musical instruments, the
strings are under tension and fixed at both ends.
• Wind instruments rely on a standing wave being set up in a column of air. The ends of the pipe may be open (free) or closed (fixed), although both ends cannot be closed!
Boundary ConditionsFor two close end boundaries, you end up with even number of quarter wavelengths.
Boundary Conditions
For two open end boundaries, you also end up with even number of quarter wavelengths.
Boundary Conditions
For one open end boundaries, you end up with odd number of quarter wavelengths.
Standing Waves• At some points (nodes) destructive interference
means that there is no net vibration of the medium, and at others (antinodes) the vibration is double the effect of a single wave.
• As a result, energy is not transferred by a standing wave, as it is with travelling waves, but is stored in the antinodes.
Sound Behaviors
• Sound Wave Interference– Sound waves can interfere with one
another and cause beats.– If they are equal amplitude, they will
completely cancel each other out at destruction portions
The Doppler Effect
• A change in the frequency of a wave caused by an objects motion.
The Doppler Effect
• Shock Wave– Created by an object moving faster than
the speed of a wave in a medium.
The Doppler Effect
• Sonic Boom– A loud sound created by a shock wave
when something is travelling faster than the speed of sound.
The Doppler Effect
• Weather Radar
The Doppler Effect
• Astronomers use Red Shift and Blue Shift to understand the Universe!
