The words on every report, explained.
Nothing on an Echo report should require looking anything up separately — but if something does, this is the page it sends you to.
Reverberation time (RT60)
How long sound takes to die away in a room, measured as the time for it to drop by 60 decibels after the source stops — in practice, how long an echo or a rattle of noise lingers after someone stops talking. A high RT60 is what makes a room sound "boomy" or hard to follow.
Speech Transmission Index (STI)
A score from 0 to 1 for how intelligible speech will be in a room, accounting for reverberation and background noise together — not just whether sound decays quickly, but whether words are actually understandable once it has. A high RT60 and a poor STI often go together, but not always: a room can ring for a long time and still be reasonably clear, or decay fast and still be hard to follow if background noise is high enough.
Absorption coefficient
A number between 0 and 1 for how much sound a material soaks up rather than reflects — 0 means it reflects everything (glass, painted concrete), 1 means it absorbs almost all of it (thick mineral-fibre ceiling tile). Multiply a surface's area by its coefficient and you get its contribution to the room's total absorption.
Sabine / Eyring equations
The two related statistical methods for predicting reverberation time from a room's volume and total absorption. Sabine is simpler and slightly overestimates reverberation in very absorbent rooms; Eyring corrects for that. Echo uses whichever is the better fit for a given room rather than always defaulting to one.
Diffusion
How evenly sound scatters around a room rather than bouncing back the way it came. Two rooms can have identical reverberation times and sound completely different — one smooth, one full of flutter and slap — because one diffuses sound well and the other doesn't. Reverberation time alone doesn't capture this, which is part of why it's a prediction, not the whole story.
Flutter echo
A rapid, distinct series of repeated echoes between two hard, parallel, reflective surfaces — a handclap between two bare walls that comes back as a buzz rather than a single slap. A localised problem that a single room-average reverberation figure can miss entirely.
Background noise (dB(A))
The steady noise level in a room with no one talking — traffic through a window, HVAC, a fridge compressor — measured in A-weighted decibels, a scale adjusted to roughly match how sensitive human hearing is at different frequencies. High background noise degrades speech clarity even in a room with otherwise excellent reverberation.
C50 (speech clarity)
The ratio of "early" sound energy (the first 50 milliseconds, which the ear still uses to understand a word) to "late" energy (everything after, which just adds to the reverberant blur), expressed in decibels. A related but distinct measure to STI — some standards, like DIN 18041's Class B, specify a minimum C50 directly.
Comb filtering
A colouring of sound caused when a direct sound and a closely-delayed reflection of itself combine and partially cancel at specific frequencies — the metallic, slightly hollow quality of a voice near a single large hard reflective surface close by, like a glass table or a whiteboard.
Treatable surface area
The area of a wall, floor or ceiling that can actually take an acoustic treatment — total surface area minus doors, windows, and anything else nothing can be fixed to. Echo models the room as a plain rectangular volume and does not deduct openings, so the areas on a report are gross — deduct doors and glazing, and allow for a lower-class absorber needing more area, before ordering anything.
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