When a singer, instrument, or speaker makes sound in a room, the sound does not simply travel straight to your ears. It spreads outward, reflects from walls and ceilings, is partly absorbed by materials, and scatters from objects. These repeated reflections create reverberation, the lingering sound you hear after the source stops.
Understanding reverberation helps explain why music sounds rich in a concert hall but muddy in a gym or empty classroom.
Reverberation depends on room size, surface materials, and how much furniture or audience seating is present. Hard surfaces like concrete, glass, and tile reflect more sound, while curtains, carpet, and people absorb more sound energy. Designers use the reverberation time, often called RT60, to measure how long it takes sound to fade by 60 decibels.
A good room balances direct sound, early reflections, and late reverberation so speech stays clear and music feels full.
Key Facts
- Sound speed in air at room temperature is about v = 343 m/s.
- Reverberation time RT60 is the time for sound level to drop by 60 dB after the source stops.
- Sabine formula: RT60 = 0.161 V / A, where V is room volume in m^3 and A is total absorption in m^2 sabins.
- Echo delay time: t = 2d / v for a sound reflecting off a wall distance d away and returning to the listener.
- Absorption coefficient alpha ranges from 0 to 1, where 0 means nearly total reflection and 1 means nearly total absorption.
- Total absorption is A = sum(alpha_i S_i), where S_i is the area of each surface.
Vocabulary
- Reverberation
- The persistence of sound in a room caused by many reflections arriving after the direct sound.
- Direct sound
- The sound that travels straight from the source to the listener without reflecting.
- Early reflections
- The first reflected sounds that arrive shortly after the direct sound and can add loudness and spaciousness.
- Absorption coefficient
- A number from 0 to 1 that describes what fraction of incident sound energy a material absorbs.
- Diffusion
- The scattering of sound in many directions so reflections are spread out rather than focused.
Common Mistakes to Avoid
- Confusing echo with reverberation. An echo is a distinct delayed repeat, while reverberation is a dense blend of many reflections.
- Ignoring room volume when predicting reverberation. Larger rooms usually have longer reverberation times because sound travels farther before losing energy.
- Treating all surfaces as equally reflective. Carpet, curtains, people, and acoustic panels absorb much more sound than glass, tile, or painted concrete.
- Using distance to only one wall to judge room acoustics. Reverberation comes from reflections off many surfaces, including the ceiling, floor, side walls, and objects.
Practice Questions
- 1 A wall is 8.0 m from a listener. If sound reflects from the wall and returns to the listener, what is the delay time? Use v = 343 m/s.
- 2 A small recital room has volume V = 600 m^3 and total absorption A = 120 m^2 sabins. Use RT60 = 0.161 V / A to find the reverberation time.
- 3 A music room sounds harsh and unclear because it has tile floors, bare walls, and a flat ceiling. Explain two changes that would reduce unwanted reverberation and how each change affects sound reflections.