Sound
Easy Overview
Have you ever seen a speaker vibrate and wondered how sound actually travels? Sound is just vibrations moving through air (or any medium) as waves. This chapter explores how waves behave — reflection, interference, resonance, and that cool effect where a siren sounds different as it passes you (Doppler effect). It's physics you can actually hear.
Waves — What Are They?
A wave is a disturbance that carries energy without carrying matter. Drop a stone in water — ripples move out, but the water stays. Sound waves are longitudinal — air molecules compress and rarefy along the direction of travel. Light waves are transverse. Key wave properties: wavelength λ, frequency f, velocity v = fλ. Frequency doesn't change when a wave enters a different medium — wavelength and speed do.
Speed of Sound — How Fast Does It Travel?
Sound travels at about 343 m/s in air at 20°C. It's faster in liquids (about 1500 m/s in water) and even faster in solids (5000 m/s in steel). Why? Molecules are closer together, so vibrations transmit faster. Temperature affects speed too: v ∝ √T. That's why sound travels faster on a hot day. Formula: v = √(γP/ρ) for gases, where γ is the adiabatic index.
Reflection and Echoes
Sound reflects off surfaces like light off a mirror. That's how echo works — you hear the reflected sound after a delay. Minimum distance for a distinct echo? About 17 meters (sound travels to the wall and back in 0.1s, the minimum time your ear can distinguish). Reverberation is multiple reflections that persist — that's why empty rooms sound echoey but furnished rooms don't. Carpets and curtains absorb sound.
Resonance and Standing Waves
When you push a swing at just the right moment, it goes higher — that's resonance. For sound, resonance happens when the frequency of a driving force matches the natural frequency of an object. A tuning fork makes a column of air resonate at its natural frequency. Standing waves are formed by two identical waves traveling in opposite directions. Nodes are points of zero displacement, antinodes are points of maximum displacement.
Doppler Effect — The Changing Pitch
You know how an ambulance siren sounds higher when it comes toward you and lower when it moves away? That's the Doppler effect. When the source moves toward you, waves get compressed — shorter wavelength, higher frequency. When it moves away, waves spread out — longer wavelength, lower frequency. The formula is f' = f(v ± v₀)/(v ∓ v_s). Top sign: source moving toward observer. It applies to light too — that's how we know distant galaxies are moving away (redshift).
Key Points
- •Sound is a longitudinal wave — particles vibrate parallel to wave direction
- •v = fλ, speed depends on medium properties
- •Speed of sound in air at 20°C ≈ 343 m/s
- •Echo: reflected sound heard after ≥ 0.1s delay
- •Resonance = matching natural frequency → maximum amplitude
- •Doppler effect: frequency shift when source/observer moves
- •Beats = interference of two waves with slightly different frequencies
Practice Questions
- An ambulance with siren frequency 700 Hz approaches you at 30 m/s. What frequency do you hear? (v = 340 m/s)
- What is resonance? Give one example from everyday life.
- A stone is dropped into a well and the sound is heard after 3 seconds. If the well is 40 m deep, find the speed of sound.
- Derive the formula for the speed of sound in a gas.