The sound that lingers

The idea

Reverberation is the persistence of sound after its source stops, caused by multiple reflections arriving less than 0.05 s apart; soft, porous surfaces reduce it.

A big hall

Priya claps once in a big empty hall. The clap seems to hang in the air for a while after her hands stop.

What do you think keeps the sound going after the clap?

• Reflections from the walls arriving one after another
• The clap is still being made
• The air keeps vibrating on its own

Add soft surfaces

Switch on sound-absorbing panels, curtains and soft chairs and compare how long the sound lasts after the source stops. The graph is a picture, not a measurement.

Sound that stays

In a large hall, sound reflects many times from the walls. The reflections arrive less than 0.05 s apart, so the sound persists after the source stops. This is reverberation. Soft, porous surfaces reduce it.

Reverberation is the persistence of sound after its source stops, caused by multiple reflections that arrive less than 0.05 s apart. Soft, porous surfaces reduce reverberation.

Notes

Reverberation is sound persisting after the source stops, from multiple reflections less than 0.05 s apart. Soft, porous surfaces reduce it.

Check yourself

What is reverberation?

Reverberation occurs when reflections from surfaces arrive with a time difference of...

Which of these reduces reverberation in a hall?

Why are sound-absorbing panels and curtains used in large halls?

• A sound heard again after one reflection from a distant cliff. That is an echo. Reverberation is the persistence of sound caused by multiple reflections.
• A sound that is absorbed by a curtain. Absorbing surfaces reduce reverberation. Reverberation is the persistence of sound after the source stops.
• Sound persisting after the source stops because of multiple reflections — correct. Yes!
• Less than 0.05 s — correct. Yes!
• More than 0.1 s. Reflections that arrive at least 0.1 s apart can be heard as separate sounds. Reverberation occurs for differences less than 0.05 s.
• 1 s. Reverberation occurs when reflections arrive with a time difference less than 0.05 s.
• Bare, hard walls. Soft, porous surfaces reduce reverberation. Bare hard walls reflect more.
• Sound-absorbing panels, upholstered chairs and curtains — correct. Yes!
• A larger number of reflections. More multiple reflections make the sound persist longer. Soft, porous surfaces reduce reverberation.
• To make the hall echo more. Soft, porous surfaces reduce reverberations, not increase them.
• To stop sound from travelling in air. Soft surfaces reduce reverberation. They do not stop sound travelling in air.
• To reduce unwanted reverberation, which could garble the sound — correct. Yes!