Sound is a longitudinal wave that needs a medium. Frequency, wavelength, speed, pitch and loudness, echoes, and uses of ultrasound.
Key ideas
Sound travels as compressions and rarefactions; it cannot travel through a vacuum.
Frequency is the number of waves per second (hertz); higher frequency means higher pitch.
Amplitude decides loudness; the quality (timbre) lets us tell instruments apart.
Sound travels fastest in solids, slower in liquids and slowest in gases, and faster in warmer air.
Humans hear about 20 Hz to 20 000 Hz. Below is infrasound; above is ultrasound.
An echo is heard only if the reflected sound comes back at least 0.1 s later (about 17 m away in air).
Remember
Wave speed
v = fλ
Time period
T = 1 ÷ f
Echo or SONAR
distance = v × t ÷ 2
Speed of sound in air
about 344 m/s at 22 °C
Watch out
Forgetting to halve the time (or distance) for an echo: the sound goes there and back.
Linking loudness to frequency (it depends on amplitude).
Mixing up wavelength and frequency in v = fλ.
Worked example
To hear a distinct echo, the reflected sound must arrive at least 0.1 s after the original. What is the minimum distance of the reflecting surface if sound travels at 346 m/s?
Working: sound travels 346 × 0.1 = 34.6 m there and back; ÷ 2