 # Waves & Sound.

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Waves & Sound

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 Wavelength
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 Unit Equation 𝒇= # 𝒘𝒂𝒗𝒆𝒔 𝒕𝒊𝒎𝒆
How frequently a wave pulse occurs. Unit Hertz (Hz) 1 Hz = 1/seconds or s-1 Equation 𝒇= # 𝒘𝒂𝒗𝒆𝒔 𝒕𝒊𝒎𝒆

Wave Description Period Equation 𝑻= 𝒕𝒊𝒎𝒆 # 𝒘𝒂𝒗𝒆𝒔
The time (in seconds) that it takes for one wave to pass. Equation T = Period (in seconds) 𝑻= 𝒕𝒊𝒎𝒆 # 𝒘𝒂𝒗𝒆𝒔

Wave Description 𝑻= 𝟏 𝒇 𝒇= 𝟏 𝑻
Frequency and Period are Inversely Related 𝑻= 𝟏 𝒇 𝒇= 𝟏 𝑻

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 Examples 𝒗=𝒇𝝀
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 = wave velocity (in m/s) f = frequency (in Hertz) λ = wavelength (in meters) 𝒗=𝒇𝝀

Wave Speed Examples 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

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 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

Resonance The frequency of forced vibrations in an object matches its natural frequency, causing it to resonate. This creates a huge increase in amplitude.

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.

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.

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 Conditions For 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.

Part 2 - SOUND

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”

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 Pitch Infrasonic Ultrasonic
Sound waves with frequencies below 20 Hz. Ultrasonic Sound waves with frequencies above 20,000 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 Rarefactions Higher pressure.
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

Sound Behaviors Reflection Sound “bouncing” off of an object.
The angle of incidence equals the angle of reflection.

Sound Behaviors Refraction
Bending of a wave caused by a change in medium.

Sound Behaviors Diffraction The bending of a wave around an obstacle.

Sound Behaviors Sound Wave Interference
Sound waves can interfere with one another and cause beats.

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.