Sound travels fastest in solids, slower in liquids and slowest in gases, and it cannot travel through a vacuum. The reason is that sound is a wave of vibrating particles, so how close and how strongly linked the particles are decides how quickly the vibration is passed on.
This skill appears in electromagnetic spectrum and sound as a comparison question, a timing calculation or a short explanation using particle ideas.
Why does the medium change the speed?
A sound wave is longitudinal. Particles vibrate back and forth along the direction the wave travels, forming compressions and rarefactions. Each particle only passes the disturbance to its neighbour.
In a gas the particles are far apart, so each must move a long way before it meets the next. In a liquid they are closer. In a solid they are closest and held in position by strong forces, so the disturbance moves on almost at once.
These are round, textbook values for comparison only. Your question will give the values it wants you to use.
| Medium | Approximate speed of sound |
|---|---|
| Air | 330 m/s |
| Water | 1500 m/s |
| Steel | 5000 m/s |
| Vacuum | no sound |
What stays the same at a boundary?
The source sets the frequency. When the wave enters a new medium the frequency does not change, but the speed does, so the wavelength changes. A faster medium gives a longer wavelength for the same frequency, using wavelength = speed ÷ frequency.
Worked example
A sound of frequency 440 Hz travels 1650 m through air, then the same distance through water. (Invented example data, using the round speeds above.) Find the time taken in each and the wavelength in each.
Step 1, times, using time = distance ÷ speed: in air, 1650 ÷ 330 = 5.0 s. In water, 1650 ÷ 1500 = 1.1 s.
Step 2, wavelengths, using wavelength = speed ÷ frequency: in air, 330 ÷ 440 = 0.75 m. In water, 1500 ÷ 440 = 3.4 m (to 2 significant figures).
Step 3, check: 330 × 5.0 = 1650 m, 1500 × 1.1 = 1650 m, 440 × 0.75 = 330 m/s and 440 × 3.4 = 1496 m/s, close to 1500 m/s given the rounding.
Step 4, interpret: sound crosses the same distance about 4.5 times faster in water, and its wavelength is about 4.5 times longer, while the frequency stays 440 Hz.
The mistake to watch for
A common slip is to think sound is fastest in gases because gases “spread out easily”. A student writes “sound is faster in air because the particles move freely”.
Mistaken answer: Sound travels fastest in air because the particles can move around.
Free movement does not help, because the disturbance has to be passed from particle to particle.
The correction is that close, strongly linked particles pass a vibration on faster, so solids carry sound fastest and gases carry it slowest. Say “closer particles, quicker transfer”.
Check yourself
1. Put air, steel and water in order of increasing speed of sound and give the reason.
Show answer
Air, then water, then steel. Particles are furthest apart in a gas and closest in a solid, so a solid passes the vibration on fastest.
2. A worker taps a steel rail. Another worker 2000 m away hears the sound through the rail and, later, through the air. Use 5000 m/s and 330 m/s. Find each time.
Show answer
Steel: 2000 ÷ 5000 = 0.40 s. Air: 2000 ÷ 330 = 6.06, so 6.1 s (to 2 significant figures). Check: 330 × 6.06 = 1999.8 m. The sound through the rail arrives first.
3. Why do astronauts outside a spacecraft need radios to talk to each other?
Show answer
Space is close to a vacuum, so there are no particles to pass sound on. Radio waves are electromagnetic and do not need a medium.
Where this leads next
The same speed idea supports the calculation in using echo timing with a return journey. If the wave equation needs revision, return to wave behaviour.
Explaining “why” in particle language is a place where students lose marks even when the numbers are correct. A teacher can listen to your explanation and tighten it, as in online one-to-one Physics tuition.