Sound Waves: Speed

Sound waves are vibrations that travel through a medium (air, liquid, or solid). They transfer energy without moving the medium along the path. Air particles simply vibrate back and forth as the wave passes.

What is wave speed?

• Wave speed is how fast the disturbance travels through the medium.
• In air at room temperature, sound travels about 330–350 m/s (often taken as 343 m/s).
• Sound needs a medium. It cannot travel in a vacuum.
• In general: fastest in solids, slower in liquids, slowest in gases.

Equations you need

Wave speed links to frequency and wavelength:

$$v = f\,\lambda$$

You can also find speed from distance and time:

$$v = \frac{s}{t}$$

For an echo from a wall (there and back):

$$v = \frac{2d}{t}$$

What affects speed?

• Medium: different materials give different speeds.
• Temperature (in air): warmer air → higher speed.
• Amplitude (loudness) does not change speed.
• Frequency changes do not change speed in the same medium; instead, wavelength adjusts so that $v$ stays the same. If $f$ increases, $\lambda$ decreases.

Measuring the speed of sound in air

• Echo method: stand a known distance $d$ from a large flat wall, make a sharp sound, measure time $t$ to hear the echo, then use $v = 2d/t$.
• Two microphones: place two microphones a known distance apart, record the time delay of the same sound arriving, then use $v = s/t$.

Common misconceptions

• “Louder sound travels faster.” False: amplitude does not affect speed.
• “Higher pitch means faster sound.” False: in one medium, $v$ is roughly constant.
• “Air moves with the sound.” False: air particles vibrate; they do not travel with the wave.