AimMent

For Class 8, 9 & 10

Foundation - a strong base in Physics, Chemistry, Maths & Biology

Concept-first questions with clear model answers across all four subjects, aligned to NCERT. Start early, build the habit, and walk into your board exams, NEET and JEE already ahead.

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SubjectAllPhysics 19Chemistry 19Biology 19Mathematics 27
ChapterAllAtoms and MoleculesForce and Laws of MotionGravitationHeron's FormulaImprovement in Food ResourcesIs Matter Around Us PureLinear Equations in Two VariablesMatter in Our SurroundingsMotionNatural ResourcesNumber SystemsPolynomialsSoundStatisticsStructure of the AtomSurface Areas and VolumesThe Fundamental Unit of LifeTissuesWork and Energy

Showing 1 question for Class 9. Tap a card to reveal the answer.

PhysicsSoundmedium

What is an echo, and what is the minimum distance needed to hear one clearly?

Reveal answer ↓

What it is

An echo is sound bouncing back off a surface and reaching you a moment later.

Answer

An echo is the sound heard again after it reflects off a hard surface and returns to the listener. To hear a distinct echo the reflected sound must reach the ear at least 0.1 second after the original, which needs the reflecting surface to be at least about 17.2 m away (since sound travels about 344 m/s in air).

distance = (speed x time) / 2 = (344 x 0.1) / 2 = 17.2 m

  • •Echo = reflected sound heard again
  • •Ear needs a 0.1 s gap to separate the two sounds
  • •Minimum distance about 17.2 m (speed 344 m/s)

Why learn this

It's the principle behind SONAR, ultrasound scans and measuring ocean depth.

💡 Memory trick

Need a 0.1 s gap -> surface at least about 17 m away, or the echo merges with the sound.