Acoustics glossary

Sound that keeps circling a room for a moment after the source stops, bouncing off the walls, ceiling and floor. Too much reverberation blurs speech and makes a room sound like a bathroom or a hall. Acoustic panels shorten it by absorbing part of the reflected sound.

The time it takes a sound to fade by 60 dB – practically to silence – once the source stops. A furnished room usually measures about 0.5 s, an empty one often over 1 s. Rough targets: recording 0.3 s, home cinema 0.35 s, work and calls 0.4 s, living room 0.5 s.

The simplest way to estimate reverberation time: RT60 = 0.161 · V / A, where V is the room's volume in m³ and A is the total absorption of its surfaces in m² (area times absorption coefficient). It gives a sensible estimate but does not replace a measurement, especially in small rooms.

Improving how the inside of a room sounds: shortening reverberation, removing echo and smoothing the bass with panels, bass traps and diffusers. It does not change how much sound passes through the walls – that is the job of soundproofing.

Reducing the sound that travels between rooms: neighbours' voices, footsteps, traffic. It takes mass (heavy partitions), airtightness and decoupled wall layers. Acoustic panels are light and porous, so they do not soundproof.

A figure in decibels that says how much a wall, floor, window or door weakens the sound passing through it. The higher it is, the better the insulation. An acoustic panel hung on a wall barely changes the wall's Rw.

The share of sound energy a surface absorbs rather than reflects, in a given frequency band: 0 means total reflection, 1 total absorption. A bare wall is about 0.02–0.05; a 5 cm stone wool panel is close to 1 in the speech range.

A single number describing a material's absorption, worked out under ISO 11654 from its results in the bands from 250 to 4000 Hz. It makes products easy to compare but hides what happens in the low frequencies – so it is worth looking at the values at 125 and 250 Hz as well.

A classification under ISO 11654 based on αw: class A is αw from 0.90 to 1.00, B 0.80–0.85, C 0.60–0.75, D 0.30–0.55, E 0.15–0.25. Class A means the highest absorption.

The American counterpart of αw: the average of the absorption coefficients at 250, 500, 1000 and 2000 Hz, rounded to 0.05. You will find it on product sheets from US and other makers. Like αw, it says little about low frequencies.

The number of air vibrations per second, measured in hertz. Roughly: low frequencies from 20 to about 250 Hz (bass, boom), mids from 250 to 2000 Hz (most of the energy of speech), highs above 2000 Hz (hissing consonants, rustles). The lower the sound, the longer the wave – 125 Hz is about 2.7 m.

A unit of sound level on a logarithmic scale. An increase of 10 dB sounds roughly twice as loud, and 3 dB is a doubling of energy. A 60 dB drop, used to define RT60, means fading practically to silence.

The reflections that reach the listener or microphone first after the direct sound, usually off the side walls, the desk and the ceiling. They colour voices and blur dialogue. Panels on the first reflection points make the biggest difference.

A simple way to find first reflection points: you sit in the listening spot while someone slides a mirror along the wall at your ear height. Where you can see the speaker or microphone in the mirror is the reflection point – and the centre of a panel.

Rapid, metallic repeats of a sound bouncing between two parallel, smooth walls. You hear it when you clap in the middle of an empty room. A panel on one of those walls is enough to reduce it clearly.

Low-frequency resonances between a room's parallel walls. At some frequencies the bass adds up and booms, at others it almost vanishes, and it sounds different in different parts of the room. They are strongest in the corners, which is why bass traps go there.

A thick absorber that reaches lower than an ordinary panel and goes in the corners, where bass energy collects. Ours has 15 cm of stone wool in three layers. Mostly needed for music, home cinema and recording when the bass booms.

An element that scatters sound in many directions instead of absorbing it – for example an uneven surface of blocks or battens of different depths. It makes a room sound spacious without distinct reflections. Bookcases and slats scatter sound to some extent too.

The space between a panel and the wall. Porous material absorbs best where the air moves fastest, some distance away from the wall – so a gap behind the panel further improves the absorption of low frequencies.

A fibrous material made from molten rock, the same one used to insulate buildings. Its porous structure absorbs sound very well and it is non-combustible (class A1). In our panels it is enclosed in a frame under a veil and fabric.

The top class in the European classification of building materials' reaction to fire (Euroclasses, EN 13501-1): a non-combustible material that does not contribute to a fire. Stone wool has it; ordinary polyurethane foam is combustible.

A porous foam, usually polyurethane, typically 2–5 cm thick and often shaped into pyramids or wedges. Cheap and light, it mostly absorbs high frequencies. Ordinary polyurethane foam is combustible and yellows and crumbles over time.

A panel of wooden battens set with gaps on felt. It works acoustically when there is a thick layer of wool or air behind the felt; slats on thin felt alone absorb mostly high frequencies.