Mechanical Waves and Sound Chapter 17 Physical Science

Mechanical Waves 17-1

17-1 Learning Targets Compare and contrast the three types of mechanical waves

Mechanical Waves A disturbance in matter that carries energy from one place to another Require a medium Medium- material through which a wave travels Solid, liquid, or gas All waves except electromagnetic waves

Mechanical Waves Created when a source of energy causes a vibration to travel though a medium 3 types: Longitudinal, transvers, and surface

Wave moves horizontally from left to right Transverse Waves Wave moves horizontally from left to right Medium moves at right angles compared to the direction of the wave

Crest- highest point of transverse wave Trough-lowest point of transverse wave

Transverse wave

Longitudinal Wave Vibration of the medium is parallel to the direction the wave travels Like a spring Made of alternating compressions and rarefactions Sound Waves P waves produced by earthquakes

Longitudinal Wave

Compression-particles in medium spaced closed together Rarefaction- particles in medium are spread out

Surface Waves A wave that travels along a surface separating two media Objects move in circular pattern due to combination of movement ( from transverse and longitudinal like wave)

https://www.acs.psu.edu/drussell/demos/waves/wavemotion.html

Properties of Waves 17-2

17-2 Learning Targets Determine how frequency, wavelength, and speed are related. Determine how amplitude and energy are related.

Periodic motion - motion that repeat at regular time intervals Pendulums Period- time required for one cycle complete motion that returns to its starting point

Frequency Number of full wavelengths that pass a point in a given amount of time Measures how rapidly vibrations occur in the medium or source of wave Symbol is f SI unit for frequency is Hertz (Hz) – vibrations per second

Wavelength Distance between two neighboring crests or troughs in transverse wave Distance between two neighboring compressions or rarefactions in a longitudinal wave λ (lambda)

Wavelength is indirectly related to frequency As Frequency increases, wavelength decreases

v= fλ Period T= 1/f Wave Speed (m/s) (waves/ s) (m/wave) Wave velocity (v)= frequency (f) x wavelength(λ) (m/s) (waves/ s) (m/wave) v= fλ Period T= 1/f

v= fλ Wave Speed Problem V= 0.1 Hz x 15.0 m V= 1.50 m/s The average wavelength in a series of ocean waves is 15.0 m. A wave crest arrives at the shore on average every 10s, so the frequency is 0.100 Hz. What is the average speed of the waves? v= fλ V= 0.1 Hz x 15.0 m V= 1.50 m/s

rearrange to find λ = v/ f λ= 340 m/s 220 Hz λ= 1.7 m The speed of sound in air is about 340 m/s. What is the wavelength of a sound wave with a frequency of 220 Hz? v= fλ rearrange to find λ = v/ f λ= 340 m/s 220 Hz λ= 1.7 m

What is the period of a 5.2 Hz wave? T= 1/f T= 1/ 5.2 Hz T= 0.19 s

Amplitude The maximum displacement of the medium from its rest position Transverse wave = vertical distance between the line of origin and crest or trough Longitudinal wave= difference in pressure between maximum compression and resting state

Amplitude and energy is direct relationship The more energy a wave has, the greater the amplitude

https://www.youtube.com/watch?v=CVsdXKO9xlk http://onlinetonegenerator.com/hearingtest.html

Behavior of Waves 17-3

17-3 Learning Targets Describe reflection Describe refraction Describe factors that affect diffraction Describe two types of interference Describe what will make standing waves

Reflection Occurs when a wave bounces off a surface that is cannot pass through Does not change speed or frequency of wave Wave may be flipped upside down Transverse wave – if hits fixed boundary the reflected wave is upside down compared to original wave

Refraction Bending of light waves as they pass from one medium to another Due to changing speed of wave (one side of wave moves more slowly than other side) Objects may appear to be bent if light waves

Diffraction Bending of a wave as it moves around an obstacle or through a narrow opening Longer wavelengths are better at bending Think of radio waves

Diffraction

Wave Interference Mechanical waves are not matter, but displacement of matter. Two wave can occupy the same space at the same time Known as principle of superposition Superposition principle is the method of summing the displacement of waves When two or more waves travel through a medium, the resultant wave is the sum of the displacements of the individual waves at each point

Constructive interference 2 or more waves combine to make a resulting wave bigger than the original waves New wave has greater amplitude Crests align with crests

Destructive interference 2 or more waves combine so the resulting wave is smaller than the largest original wave Complete destructive interference is when two pulses completely cancel each other Crests align with troughs

Standing wave A wave that appears to stay in one place Combination of two waves moving in opposite directions Each having the same amplitude and frequency Interference of reflected wave on self Plucked strings

Standing wave forms only if half wavelength or a multiple of half a wavelength fits exactly into the length of a vibrating cord Or if frequency is doubled or triples

Standing Waves Node- point on standing wave that has no displacement Antinode- point where crest or trough occurs midway between two nodes

https://www.youtube.com/watch?v=_S7-PDF6Vzc

Sound and Hearing 17-4

What affects the speed of sound? 1- Medium Density sounds moves best through more dense materials Material sounds travel fastest through solids and slowest through gases 2-Temperature increase in temperature= increase in sound speed

Intensity Rate at which a wave’s energy fowls through a given area

Decibels (dB) Unit used to measure the sound intensity or loudness Sound greater than 120dB can cause pain

Loudness Loudness is just human perception Determined by intensity –amplitude

Hair dryer, vacuum cleaner dB Loudness of Sound Faintest sound heard 15 Whisper 50 Normal conversation 75 Hair dryer, vacuum cleaner 80 Noisy restaurant 100 Lawn mower 120 Threshold of pain 150 Jett taking off

Pitch Highness of lowness of sound Determined by frequency The greater the vibrations the higher the pitch Human hearing range is 20 -20,000 Hz

Infrasound- slow vibrations, frequency lower than 20 Hz Below human hearing range Ultrasound- sound wave with frequency above 20,000 Hz Above human hearing range Used for Sonar

Doppler Effect Change in pitch and frequency as the source of sound moves Frequency and pitch increase as source of sound approaches and decrease as source moves away Occurs for light, sound and other types of waves