Physics – Waves and Sound - Quick Guides | Physics

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Study GuidePhysics–Waves and Sound1.SoundHow Sound Waves Are ProducedSound is created when an objectvibrates. When an object vibrates, it moves back and forth. As itmoves:•Itpushes air molecules together→ this is called acompression•Itpulls air molecules apart→ this is called ararefactionOne compression plus one rarefaction makesone complete longitudinal sound wave.In sound waves:•Air molecules moveback and forth•Their motion isparallel to the direction the wave travels•Energy is passed from one molecule to the nextThis is how sound travels from the source to your ears.1.Intensity and PitchPitch (How High or Low a Sound Is)Pitch depends onfrequency,which means how many vibrations happen each second.•More vibrations per second →Higher pitch (high note)•Fewer vibrations per second →Lower pitch (low note)

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Study Guide1.Intensity (How Strong a Sound Is)Intensity tells us how much sound energy passes through an area each second.•SI Unit:Watts per square meter (W/m²)But what we actually notice isloudness, which is how our ears interpret intensity.Important note:•Sounds with the same intensity but different pitch may seem different in loudness.•This is because our ears respond differently to different frequencies.2.Threshold of HearingThe faintest sound a normal human can hear is called thethreshold of hearing.3.Decibels and Sound LevelSound intensity is usually measured indecibels (dB)using:Where:•β = intensity level in decibels•I = sound intensity•I₀= threshold intensityExamples:•Normal conversation →~60 dB•Power saw →~110 dB2.Doppler EffectTheDoppler effecthappens when there is relative motion between a sound source and a listener.

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Study GuideExample: Ambulance siren•As it approaches → Pitch soundshigher•After it passes → Pitch soundslowerWhy this happens:•When source moves toward you → Waves get closer → Frequency increases•When source moves away → Waves spread out → Frequency decreases3.Tuning Forks and Natural FrequencyAtuning forkis useful for studying sound because it producesone pure frequency.Its natural frequency depends on:•Size•Shape•MaterialIf you place a vibrating tuning fork on a table:

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Study Guide•The table vibrates too•Larger vibrating area → Sound becomes louder (more intense)This is calledforced vibration.Many instruments use this idea:Example → Piano sounding board increases sound loudness.4.Resonance and Sympathetic VibrationIf two tuning forks have thesame frequency:•One vibrating fork can make the other vibrate.•This is called:○Resonance○Sympathetic vibrationResonance happens when:•Two objectshave the same natural frequency, OR•One frequency is a multiple of the other.

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