In which of the following media is the speed of sound the least?

Analyze the question: We are asked which medium has the lowest speed of sound among air, brass, water and wood.

Concept: Sound is a mechanical wave that travels by vibrations of particles. Its speed depends mainly on the elasticity (stiffness) of the medium and its density:

v = √(E/ρ)

• In solids, particles are packed closely and bound strongly, so vibrations pass quickly.
• In liquids, particles are less tightly bound, so sound is slower than in solids.
• In gases, particles are far apart, so vibrations pass on slowly and sound is slowest.

So the general order is: solids > liquids > gases.

Typical speeds:

• Air (at about 20°C): ~340 m/s
• Water: ~1,500 m/s
• Wood: ~3,000–4,000 m/s (varies)
• Brass: ~4,000 m/s

Solution: Air has the smallest speed, about 340 m/s, compared with the others, which are all over 1,000 m/s. Air is a gas, with widely spaced particles and low elasticity, so sound travels slowest in it.

Answer: A. Air

Exam tip: For "speed of sound" questions, remember S > L > G (Solid > Liquid > Gas). If the question asks for the least, pick the gas. If it asks for the greatest, pick the solid. Also remember that sound cannot travel through a vacuum.