Anonymous
Anonymous asked in Science & MathematicsPhysics · 7 years ago

# Can sound be amplified without adding energy?

Recently I went to a cave, when ever I produced a sound at a particular frequency the sound gets amplified drastically but sounds of other frequency remains silent. Why does this happen?? Does resonance amplifies sound drastically?

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• 7 years ago

Q1. Sound can be amplified without adding energy, e.g. using a funnel-shaped earpiece (see link). This concentrates the sound energy into a smaller region - so less air moves but with a bigger amplitude; no energy is added.

Q2. Resonance doesn't add energy either. At non resonant frequencies, the sound is dispersed, by multiple random reflections which tend to results in a random mixture of destructive and constructive interference.

But at the resonant frequency, the reflections reinforce (constructive interference) in certain regions (antinodes). If we are at an antinode the amount of energy transferred to our ears is increased.

Q3 Resonance amplifies sound by various amount. At some regions (antinodes) the sound is amplified - but at the the expense of the reducing the sound level at nodes. The level of amplification depends on the quality of the resonance and the position of the listener.

• 7 years ago

And, just to add to Steve's answer above ... you can move closer to the source! ;)

Sound waves typically follow an inverse square law ... if you double your distance from the source, you quarter the amplitude. Three times the distance and the amplitude is one-ninth what it was. This is because the waves spread out in a spherical pattern so the energy passing through a given area reduces the farther you are from it.