When you speak in a room, only part of what a listener hears travels directly from your mouth. The rest arrives after bouncing off walls, floor, ceiling and furniture. Those reflections give every room its sound – the intimacy of a living room, the boom of a gym, the bloom of a cathedral.
Direct sound, early reflections and reverb
- Direct sound arrives first, by the shortest path.
- Early reflections arrive within roughly 50 ms. The brain fuses them with the direct sound; they add fullness and help us judge the size of a space.
- Reverberation is the dense, decaying wash of later reflections.
An echo is a single reflection that arrives late enough (more than about 50 ms, or roughly 17 m of extra path) to be heard separately. Flutter echo is a rapid buzzy repetition between two parallel hard surfaces – clap your hands in an empty corridor to hear it.
Reverberation time (RT60)
Acousticians describe reverberation with RT60: the time for sound to decay by 60 dB after the source stops. In 1898 Wallace Sabine discovered that RT60 is proportional to the room's volume and inversely proportional to its total absorption:
RT60 ≈ 0.161 × V / A, with V in m³ and A in square-metre sabins (area × absorption coefficient).
Typical targets: recording booths below 0.3 s, home studios and classrooms 0.4–0.8 s, living rooms around 0.5 s, concert halls 1.8–2.2 s. Estimate your own room with the RT60 calculator and read Sabine's story in the library.
Absorption, diffusion and isolation
These are often confused:
- Absorption turns sound energy into heat inside porous materials – mineral wool, foam, curtains, carpets, upholstered furniture. It reduces reverberation inside a room.
- Diffusion scatters reflections in many directions (bookshelves, irregular surfaces, purpose-built diffusers), keeping a room lively without harsh echoes.
- Isolation (soundproofing) stops sound travelling between rooms. It needs mass, airtightness and decoupling – foam panels do almost nothing for it.
Bass and room modes
Low frequencies have long wavelengths – 100 Hz is 3.4 m – so they behave differently. Between parallel walls they form standing waves (room modes): at some frequencies and positions bass booms, at others it almost vanishes. Thin panels barely touch these frequencies; corner bass traps with thick, deep absorption work best. Moving speakers and your listening position by even 30 cm can make a large difference. The wavelength calculator shows why.
Quick wins for a better-sounding room
- Add a rug and curtains to bare floors and windows.
- Fill bookshelves – they absorb and diffuse.
- Treat the first reflection points on side walls and ceiling between your speakers and your ears.
- Avoid placing speakers or a desk exactly in the middle of the room.
- Put thick absorption in corners for bass.
- Measure: clap and listen, or use the spectrogram to see how quickly sound dies away.
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