Lesson 3.3 • Competency 3.3Complete Lesson
Back to UnitSound Waves, Intensity & Doppler Effect
Speed of sound in gases, temperature dependence v ∝ √T, sound intensity and decibels, human ear response, and the Doppler frequency shift for moving sources and observers.
Marking a lesson complete represents finishing your study of the topic; it does not denote tested mastery.
Learning Objectives
- Calculate speed of sound across varying temperatures and compute decibel levels.
- Determine apparent frequencies when a sound source or observer moves relative to the medium.
Core Theory & Concepts
Sound travels as a longitudinal mechanical pressure wave. Its propagation speed depends on the adiabatic bulk elasticity and density of the medium. Relative motion between the source and listener produces the Doppler frequency shift, fundamental to radar speed detection and astrophysics.
Doppler Shift Formula
Observed frequency f' = f · [(v ± v_o) / (v ∓ v_s)], where upper signs correspond to approaching motion.
f' = f · (v ± v_o) / (v ∓ v_s)
Common Misconceptions
Misconception
Believing the speed of sound increases when a moving car accelerates toward you.
Physical Clarification:The speed of sound in air is determined exclusively by the properties of the air (temperature, humidity, molar mass), NOT by the motion of the emitting source.
Syllabus Topic Outline
- 1Newton-Laplace formula for sound speed in gases (v = √(γRT / M))
- 2Sound intensity level in decibels β = 10 log₁₀(I / I₀)
- 3Doppler frequency equation for moving source and moving observer