Thread Academy

Sound Needs a Medium

Learning that sound travels through solids, liquids and gases — but not through space.

  • Define and explain Sound Needs a Medium in your own words
  • Use key terms such as vibration accurately
  • Apply what you have learned to new examples and questions
  • Avoid the common mistakes learners make with this topic

This lesson focuses on Sound Needs a Medium: learning that sound travels through solids, liquids and gases — but not through space.

Definition: Sound Needs a Medium

Learning that sound travels through solids, liquids and gases — but not through space.

Key ideas

Sound needs particles to travel through

Sound moves as compressions and rarefactions passing from particle to particle through a material. It travels fastest through solids (about 5000 m/s in steel), slower in water (about 1500 m/s) and slowest in air (about 343 m/s). In the vacuum of space there are no particles, so sound cannot travel at all.

Sounds are made by vibrations

Pluck a guitar string and it vibrates, pushing the air back and forth — that is what your ear detects as sound. The vibrating object can be a string, a drum skin, vocal cords or a loudspeaker cone. No vibration, no sound — it really is that simple.

Key term — vibration: A rapid back-and-forth movement — the source of every sound.

Worked example: Sound Needs a Medium

Why can two astronauts on a spacewalk not talk to each other directly?

Space is a vacuum with no particles, so sound cannot travel between them — they use radios instead.

Answer: Space is a vacuum with no particles, so sound cannot travel between them — they use radios instead.

Common mistakes
  • Thinking sound travels in space Sound needs a medium — with no particles in a vacuum, astronauts must use radios.
  • Forgetting the echo travels there and back Echo timing covers double the distance, so you must halve speed × time.

Practice

Which is faster: sound in air or sound in steel? Give the approximate speeds.
Think about how tightly packed the particles are.

Steel is faster: about 5000 m/s in steel versus about 343 m/s in air.

A sound has a frequency of 440 Hz. Is this describing the pitch or the loudness?
Hz is the unit of...

The pitch — hertz measures frequency, which determines pitch.

A ship's sonar echo returns after 4 s. The speed of sound in water is 1500 m/s. How deep is the water?
Halve the there-and-back distance.

Distance = 1500 × 4 ÷ 2 = 3000 m.

What is vibrating when you hear a drum?
What did the stick hit?

The drum skin (membrane) vibrates and pushes the air back and forth.

Quick check

Sound Needs a Medium — quick check

Which of these best defines "vibration"?

A rapid back-and-forth movement — the source of every sound.

Tightening a guitar string raises the note. Why?

A tighter string vibrates faster — higher frequency means a higher pitch.
Key takeaways
  • Sound Needs a Medium: learning that sound travels through solids, liquids and gases — but not through space.
  • Sound needs particles to travel through: Sound moves as compressions and rarefactions passing from particle to particle through a material.
  • amplitude: The size of a vibration; a bigger amplitude means a louder sound.
  • Watch out for: thinking sound travels in space
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