Bell Ringer What causes sound Bell Ringer Explain

Bell Ringer • What causes sound?

Bell Ringer • Explain one station from yesterday. • How did length affect pitch? • How did sound travel through different materials?

Waves & Sound

Vocabulary • Period: Time taken for one complete cycle – Variable: T – Unit: seconds • Frequency: Cycles per second – Variable: f – Unit: Hertz (Hz) • 1 Hz = 1/second • Period and frequency are inverses of each other T = 1/f and f = 1/T

Vocabulary • Crest: High point of wave • Trough: Low point of wave • Amplitude: Distance from midpoint to crest – Variable: A – Units: meters • Wavelength: Distance from crest to crest – Variable: l – Units: meters

Vocabulary

Catch a Wave Lab

Bell Ringer • Draw a wave and label its crest, trough, amplitude and wavelength.

Types of Waves • Transverse: Motion of medium is perpendicular to direction wave travels • Longitudinal: Medium moves in the same direction as the wave travels

Types of Waves

Types of Waves • Transverse: Motion of medium is perpendicular to direction wave travels • Longitudinal: Medium moves in the same direction as the wave travels

The Origin of Sound • Produced by the vibration of material objects • Pitch: Our impression of the frequency of a sound – A high pitched sound has a high frequency • Young people hear pitches with frequencies ranging from 20 -20, 000 Hz – Infrasonic: Sound waves with frequencies below 20 Hz – Ultrasonic: Sound waves with frequencies above 20, 000 Hz

Sound in Air • Sound travels in the form of longitudinal waves • A pulse vibrates through the air as a series of compressions and rarefactions – When molecules compress, they leave areas of low pressure behind them – Other molecules will move into these regions

Sound Transmission • Sounds can travel through solids, liquids, and gases – Sound travels faster and more efficiently through solids than liquids or gases • The speed of sound is different in different materials – At room temperature (20 o. C), the speed of sound is 340 m/s • Sound cannot travel in a vacuum – No molecules to compress or expand

Bell Ringer • What is the relationship between pitch and frequency?

Wave Speed • Speed depends on the medium (material) through which it travels wave speed = wavelength x frequency v= . lf

Loudness • The intensity of a sound is related to the amplitude – Measured by instruments such as an oscilloscope – Unit: decibel (d. B) • Loudness is how our brain senses the sound (a. k. a. “volume”)

Source of Sound Level (d. B) Normal Breathing 10 Close Whisper 20 Library 40 Normal Speech 60 Busy Street Traffic 70 Subway Train 100 Loud Rock Music 115 Threshold of Pain 120 Jet Engine at 30 m 140

Interference • More than one wave can exist in the same place at the same time • Wave effects may be increased, decreased, or cancelled

Interference

Interference • Constructive Interference: Crest of one wave overlaps the crest of another – Results in increased amplitude

Interference • Destructive Interference: Crest of one wave overlaps the trough of another – Results in cancellation of amplitude

Interference and Beats • A receiver may hear two sound waves at the same time: – In phase: Compressions and rarefactions overlap each other – Out of phase: Compressions and rarefactions of each wave are offset • If the crest of one wave overlaps the trough of another, they will cancel • Beats: The periodic variation in the loudness of a sound – Produced from two sounds of slightly different frequencies

Interference and Beats

Interference and Beats

Bell Ringer • If a 400 Hz sound wave has a wavelength of 3. 71 m as it travels through water. What is the speed of sound in water? • How does this compare with the speed of sound in air?

Standing Waves • Created when a wave reflects on itself • Nodes: Parts of a standing wave which remain stationary • Antinodes: Positions on a standing wave with the largest amplitudes Node Antinodes

Standing Waves Harmonics – To determine the harmonic of a standing wave, count the antinodes. 1 st Harmonic: 2 nd Harmonic: 3 rd Harmonic: 4 th Harmonic:

Doppler Effect • The apparent change in frequency due to the motion of the source or receiver of the wave • Applies to both sound and light

Doppler Effect

Bow Waves • Wave shape produced when an object moves faster than the speed of the wave surrounding it – Two-dimensional

Shock Waves • Similar to a bow wave, except threedimensional – Cone shaped

Shock Waves • An object exceeding the speed of sound will produce a shock wave • When the compressed air at the edges of the shock wave reach the ground, a sonic boom is heard – The object does not have to initially make any sound in order to produce a sonic boom – i. e. ) The cracking of a whip is actually a mini sonic boom!

Forced Vibrations • Sounds will be more intense if additional material is made to vibrate – i. e. A guitar would not be audible if the sound was not transmitted through its wooden body

Natural Frequency • Frequency at which the smallest amount of energy is required to produce forced vibrations • Any object made of elastic material will vibrate at its own frequency when disturbed – Natural frequency depends on the elasticity and the shape of the object

Resonance • When the frequency of a forced vibration matches the object’s natural frequency – Dramatically increases the amplitude of the sound wave • i. e. ) Pumping a swing

Resonance