Mechanical Waves and Sound Chapter 17 Physical Science

Mechanical Waves and Sound Chapter 17 Physical Science

Mechanical Waves 17 -1

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

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

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

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

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

Transverse wave

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

Longitudinal Wave

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

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

l https: //www. acs. psu. edu/drussell/demos/w aves/wavemotion. html

Properties of Waves 17 -2

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

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

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

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

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

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

Wave Speed Problem l The average wavelength in a series of ocean waves is 15. 0 m. A wave crest arrives at the shore on average every 10 s, so the frequency is 0. 100 Hz. What is the average speed of the waves? lv= fλ l. V= 0. 1 Hz x 15. 0 m l. V= 1. 50 m/s

l 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? l v= fλ l rearrange to find λ = v/ f l λ= 340 m/s 220 Hz l λ= 1. 7 m

What is the period of a 5. 2 Hz wave? l T= 1/f l T= 1/ 5. 2 Hz l T= 0. 19 s

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

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

l https: //www. youtube. com/watch? v=CVsd. X KO 9 xlk l http: //onlinetonegenerator. com/hearingtest. html

Behavior of Waves 17 -3

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

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

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

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

Diffraction

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

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

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

l https: //www. youtube. com/watch? v=_S 7 PDF 6 Vzc

Sound and Hearing 17 -4

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

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

Decibels (d. B) l Unit used to measure the sound intensity or loudness l Sound greater than 120 d. B can cause pain

Loudness l Loudness is just human perception l Determined by intensity –amplitude

d. B Loudness of Sound 0 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 l Highness of lowness of sound l Determined by frequency l The greater the vibrations the higher the pitch l Human hearing range is 20 -20, 000 Hz

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

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