What is Sound?
Sound is a longitudinal wave — particles vibrate back and forth in the same direction as the wave travels, creating regions of:
- Compression: particles are pushed together (high pressure)
- Rarefaction: particles are spread apart (low pressure)
Sound requires a medium to travel (particles to compress and rarefy). It cannot travel through a vacuum.
Speed of Sound
Sound travels at different speeds in different materials:
| Medium | Speed of sound (approx.) |
|---|---|
| Air (20°C) | 340 m/s |
| Water | 1,500 m/s |
| Steel | 5,000 m/s |
Sound travels faster in denser materials because particles are closer together and collide more quickly.
Pitch
Pitch is how high or low a sound is. It depends on frequency:
- Higher frequency → higher pitch (e.g. a whistle)
- Lower frequency → lower pitch (e.g. a bass drum)
Human hearing range: 20 Hz – 20,000 Hz
Sounds above 20,000 Hz = ultrasound (used in medical scans, sonar) Sounds below 20 Hz = infrasound (elephants, earthquakes)
Loudness
Loudness (volume) depends on amplitude:
- Larger amplitude → louder sound
- Smaller amplitude → quieter sound
Loudness is measured in decibels (dB).
Long exposure to sounds above 85 dB can cause permanent hearing damage. Always protect your ears at concerts or when using loud machinery.
Echoes
An echo is a reflected sound wave. Sound reflects off hard, flat surfaces. The time delay between the original sound and the echo can be used to calculate distance:
(Divide by 2 because the sound travels there and back.)
Real world: Ultrasound baby scan at the hospital
A hospital ultrasound scanner sends pulses of sound at frequencies above 20,000 Hz into the body. The pulses reflect off internal tissues and organs (including a developing baby) at different depths. The scanner measures the time for each echo to return, builds up a 2D image — the same principle as the distance formula: distance = (speed × time) / 2.
Real world: Bats navigating in the dark (echolocation)
Bats emit ultrasonic squeaks at 20,000–100,000 Hz and listen for echoes bouncing off insects and walls. The shorter the echo delay, the closer the object. A bat can detect a moth 5 m away in total darkness — a biological sonar system using the same physics as submarine SONAR.
Real world: Concert hearing damage
A live rock concert typically produces sound levels of 110–120 dB near the speakers. Exposure to 110 dB for just 2 minutes can begin to damage the tiny hair cells in the inner ear. Unlike broken bones, these cells do not regenerate — which is why musicians and concert crew wear earplugs that reduce amplitude without blocking the music entirely.
Practice Problems
A dog whistle has a frequency of 25,000 Hz. Can humans hear it? Explain.
Show answer
A sound echo returns 0.4 s after it was made. The speed of sound is 340 m/s. How far away is the reflecting wall?
Show answer
Two musical notes have the same frequency but one is louder. What is different about their waves?