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Physics – Waves and Sound

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    Study Guide Physics – Waves and Sound 1. Sound How Sound Waves Are Produced Sound is created when an object vibrates . When an object vibrates, it moves back and forth. As it moves: • It pushes air molecules together → this is called a compression • It pulls air molecules apart → this is called a rarefaction One compression plus one rarefaction makes one complete longitudinal sound wave . In sound waves: • Air molecules move back and forth • Their motion is parallel to the direction the wave travels • Energy is passed from one molecule to the next This is how sound travels from the source to your ears. 1. Intensity and Pitch Pitch (How High or Low a Sound Is) Pitch depends on frequency , 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 Guide 1. 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 is loudness , 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 Hearing The faintest sound a normal human can hear is called the threshold of hearing . 3. Decibels and Sound Level Sound intensity is usually measured in decibels (dB) using: Where: • β = intensity level in decibels • I = sound intensity • I ₀ = threshold intensity Examples: • Normal conversation → ~60 dB • Power saw → ~110 dB 2. Doppler Effect The Doppler effect happens when there is relative motion between a sound source and a listener.

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    Study Guide Example: Ambulance siren • As it approaches → Pitch sounds higher • After it passes → Pitch sounds lower Why this happens: • When source moves toward you → Waves get closer → Frequency increases • When source moves away → Waves spread out → Frequency decreases 3. Tuning Forks and Natural Frequency A tuning fork is useful for studying sound because it produces one pure frequency . Its natural frequency depends on: • Size • Shape • Material If 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 called forced vibration . Many instruments use this idea: Example → Piano sounding board increases sound loudness. 4. Resonance and Sympathetic Vibration If two tuning forks have the same frequency : • One vibrating fork can make the other vibrate. • This is called: ○ Resonance ○ Sympathetic vibration Resonance happens when: • Two objects have the same natural frequency, OR • One frequency is a multiple of the other.

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