Effects of Sound Waves

Sound waves are longitudinal waves made of compressions (crowded air) and rarefactions (spread-out air). They transfer energy through a medium (air, liquid, or solid) but not through a vacuum.

Loudness and Pitch

• Loudness depends on amplitude. Bigger amplitude means a louder sound. Like tapping a drum softly versus hitting it hard.
• Pitch depends on frequency. Higher frequency means a higher pitch (a whistle). Lower frequency means a lower pitch (a bass drum).

Wave speed in a given medium does not depend on amplitude or frequency. It follows $v = f\,\lambda$.

Reflection: Echoes

When sound reflects from a surface, you may hear an echo. Hard, smooth surfaces reflect well; soft surfaces absorb sound.

Distance by echo timing uses:

$$\text{distance to object} = \tfrac{1}{2}\, v\, t$$

The half is used because the sound travels to the object and back.

Refraction and Diffraction

• Refraction: Sound bends when its speed changes (e.g., warm air to cool air). At night, cooler air near the ground can bend sound back down, making distant noises clearer.
• Diffraction: Sound spreads around obstacles and through gaps. Lower-pitch (longer wavelength) sounds diffract more, which is why bass is heard around corners.

Ultrasound (f > 20 kHz)

• Non-destructive testing: finds cracks in metal by reflection.
• Medical scanning: images soft tissues safely.
• Sonar: measures depth using $d = \tfrac{1}{2} v t$.

Absorption and Safety

• Soft materials (curtains, foam) absorb sound, reducing echoes and noise.
• Very loud sounds can damage hearing; protect ears near machinery or concerts.

Common Misconceptions

• Loudness is set by amplitude, not frequency.
• Pitch is set by frequency, not amplitude.
• Sound needs a medium; it cannot travel in space.
• In one medium, changing frequency changes $\lambda$, not $v$: $v = f\lambda$.