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Understanding Sound: Longitudinal Waves, Velocity, Reflection, and Intensity

This lecture covers key concepts of sound, including its nature as a longitudinal pressure wave that requires a medium for propagation. Explore the velocity of sound in various materials, the impact of temperature and density on sound speed, and the principles of sound reflection and the formation of standing waves. Learn about sound intensity and its measurement in decibels, including practical examples and questions. This comprehensive overview of sound is essential for grasping the fundamentals of acoustics in physics.

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Understanding Sound: Longitudinal Waves, Velocity, Reflection, and Intensity

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  1. PHYS16 – Lecture 40 Ch. 17 Sound

  2. Ch. 17 Sound • Sound • Longitudinal Pressure Waves • Sound Velocity • Sound Reflection and Standing Waves • Intensity • Beats • Doppler

  3. Sound Waves = Longitudinal Wave • Sound is a longitudinal pressure wave • Needs medium to propagate • Propagation is parallel to direction of motion

  4. Ruben’s Tube Demo http://en.wikipedia.org/wiki/Rubens'_tube

  5. Sound Velocity • 343 m/s in air at STP • Changes with: • Tension of material • Density We need 3D relationship, ρ instead of μ and (Y or B) instead of T

  6. Discussion: Sound Velocity • If density affects the velocity of sound then why does vsolid>vliquid>vgas? • If temperature of air were to change would that affect the velocity of sound? • For a sound in water, would temperature affect the velocity of sound?

  7. Sound Reflection • Sound reflects when medium changes • Tensional changes between solids and fluids • Density changes within solids and fluids • Reflections behave the Law of Reflection • Reflections are similar to waves on string

  8. Standing Waves with Sound • Need two “ends” where sound reflects • Reflections constructively and destructively interfere to setup up standing waves

  9. Standing Waves with Sound

  10. Sound Intensity • Intensity is the power per unit area in a wave http://hyperphysics.phy-astr.gsu.edu/hbase/acoustic/imgaco/isc2.gif

  11. Sound Level • Sound Level (β) - a relative intensity in decibels (dB) • 0 dB = smallest sound that can be heard • 50 dB = background noise • 70 dB = traffic • 110 dB = rock concert

  12. Example Question • If a Rock Concert is 110 dB, what is the intensity level in W/m2? A) 0.1 W/m2 B) 10 W/m2 C) 1011 W /m2 D) 1098 W/m2

  13. Example Question • A bell is rung in a room and can be assumed to be a point source. If you sit three times the distance your friend is sitting from the bell, what is the decibel drop? A) 0.1 dB B) 1 dB C) 10 dB D) 100 dB

  14. Main Points - Sound • Sound– a longitudinal pressure wave that moves through a medium • Velocity – 343 m/s in air • Reflection – occurs when velocity changes • Resonance and Standing Waves – depends on open/closed ends

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