The boom has a formula

A booming room gets described as "noisy", but noise has nothing to do with it: an empty room booms at any voice level. The boom is a ratio between the air volume and the absorption of the surfaces — how much sound energy the space holds, against how much it swallows at every reflection. Sabine's formula says it in one line: the reverberation time is proportional to the volume divided by the absorption.

This calculator estimates that time for a regular room: from the volume and the surfaces it derives the seconds a sound takes to die out, compares them with the target of your use in a dimensionless index, and translates any gap into square metres of missing absorption — the shopping list, not just the verdict.

The reference — a 25 m² living room with a 2.7 m ceiling, average furnishing — rings for 0.70 seconds against a 0.6 target: index 1.16, within the ±20% tolerance.

Sabine's formula: a volume over an absorption

The numerator is the volume: 25 × 2.7 = 67.5 m³ of air in which the sound lives. The denominator is the equivalent absorption: the total surfaces of the box (floor, ceiling and walls: 104 m²) multiplied by the average coefficient with which they absorb — 0.15 for a furnished room — that is, 15.6 m² of acoustic open window: as if fifteen square metres of wall reflected nothing at all.

The time that comes out, 0.161 × 67.5 / 15.6 = 0.70 s, is the T60: the seconds a sound takes to drop by 60 decibels, the tail you hear clapping your hands in an empty room.

The structure is a hyperbola, and it is the best news on the page: doubling the absorption exactly halves the tail — and as with all hyperbolas, the first square metres are worth more than the last.

How to read the index

The index is the estimated time divided by the target, and the 0.8-1.2 band is the centre: the ear does not distinguish deviations of ±20%, and most standards treat them as compliance. Here interventions are made for taste, not necessity.

Between 1.2 and 2 the room is reverberant: the tail covers the consonants and listening effort grows with the crowd — it is the zone where treatment pays most, because the hyperbola works in your favour. Above 2 the room is out of control: the tail of one syllable covers the next, and you design (absorbent ceiling, baffles), you do not decorate.

Below 0.8 the room is drier than the target: for speech it is rarely a problem, for music it is a real defect — a "dead" room mutes the instrument. The remedy, in this one case, is removing absorption.

The absorption scale

The table keeps the reference room fixed and moves only the average coefficient — from bare room to treated room. It is the acoustic life of a home, from handover to the completed move-in.

The first row is the freshly handed-over home: 2.09 seconds, three and a half times the target — which is why empty flats all sound boomy and moving in "fixes the acoustics" without anyone having designed it. Between 0.05 and 0.10 the tail halves: the first furniture does more than any later panel.

The last two rows tell the opposite risk: below the target the room goes dead — the child's bedroom full of fabrics where the voice "dies" is as much a defect as the booming living room.

The target depends on the use

There is no "right" reverberation time in absolute terms: there is the right one for the use. Roughly 0.4-0.6 s for classrooms, offices and meeting rooms (speech intelligibility commands), 0.8-1.2 for multipurpose halls and restaurants, 1.5-2.5 for symphonic and sacred music — the organ needs the tail that would destroy a lesson.

The calculator is parametric precisely for this: you enter the target of your own standard or use, and the index adapts. School regulations in many countries set binding values for classrooms; offices and canteens have their own technical references; concert halls are designed on values agreed with the artistic director.

The surprising corollary: the same room is reverberant for a lesson and dead for a violin — the index moves from 1.74 to 0.46 by changing only the target, without touching a panel. Before treating a room, deciding what it will be used for is half the design.

From missing square metres to panels

The most operational output is the missing absorption: the m² of "acoustic open window" to add to hit the target. The reference asks for 2.5: with sound-absorbing panels at coefficient 0.9 you need about 2.8 m² of panel — three one-square-metre panels, not a renovation.

The conversion works for every material: the missing m² divided by the material's coefficient give the m² to install. A heavy curtain (0.4-0.5) needs twice as much as a technical panel; a thick rug (0.3) three times; a full, irregular bookcase also works as a diffuser.

This is where the count stops being theory: acoustic treatment quotes are compared at equal offered sabin m² — pretty but thin panels at coefficient 0.4 must cost proportionally less than technical panels at 0.9.

Frequencies: where Sabine simplifies

The absorption coefficient is not a single number: it is a curve that changes with frequency. Thin porous materials — curtains, rugs, 2-3 cm panels — absorb the mid-high frequencies well and the lows almost not at all; absorbing 100 Hz takes serious thickness, air gaps or dedicated resonators.

This count uses an average coefficient designed for the speech band (roughly 500-2,000 Hz), which is the right one for offices, classrooms and living rooms. The room that "booms with bass" — the rehearsal room, the home theatre — has a problem this number only partly sees, and which is treated with different tools.

The practical rule: if the problem is understanding words, the page's count suffices; if the problem is a droning bass, you need a per-band analysis.

Echo, first reflections and reverb: what the number does not distinguish

The T60 measures how long the sound lives, not where it comes back from. Two defects can coexist with a correct T60: flutter echo between two smooth parallel walls (the metallic "trill" when you clap) and first reflections that are too strong at the listening points — the typical problem of desks against bare walls and video workstations.

Both are solved by placement more than quantity: the same panel that barely lowers the T60 on the ceiling changes a recording when placed at the reflection point between mouth and microphone. The quantity says how many m²; the reflection map says where.

It is the same logic as the airflow: the right number badly distributed leaves dead rooms — the quantity count is necessary, not sufficient.

Mistakes that distort the count

The first is the coefficient by eye: between 0.10 and 0.20 there is a factor of two on the time — the honest estimate starts from the dominant surfaces (a living room is almost all plaster and glass, and plaster sits at 0.03-0.05). The second is forgetting what already absorbs: sofas, books, beds and people are real absorption the walls do not show.

The third is geometry: the formula assumes a regular box with distributed absorption — mezzanines, open bays and extreme proportions put it in difficulty, and the form's perimeter constraint (at least 4·√S) only protects against measurement errors, not strange geometries.

The last is one of role: this is an informative parametric estimate, fit for deciding whether and how much to treat. Where the value must be certified — classrooms, canteens, public halls with regulatory requirements — the number that counts is the on-site acoustic measurement, and the design must be signed by an acoustician. Reverberation is, after all, the third number of a room's comfort, after the U-value of the walls and the air changes — and like the other two, you calculate it first and measure it after.

The scale of the target

The first table keeps the reference room fixed — same volume, same absorption, same 0.70 s time — and changes only the use, that is, the target against which the time is judged.

The time does not move by a millisecond along the table, yet the verdict crosses every branch: the same room is nearly out of control for a lesson and dead for a concert. The missing-m² column shows the cost of strictness: the classroom target asks eleven metres of absorption where the office one asks two and a half.

That is why the question "how does this room sound?" is incomplete: the right question is "how does it sound for what I have to do in it?" — and multipurpose halls, which must answer two opposite uses, end up with curtains and movable panels precisely to travel along this table.

The scale of the height

The second table keeps floor, perimeter and materials fixed and raises only the ceiling — from flat to loft to industrial hall.

A rising ceiling brings volume faster than it brings walls: the volume grows with the whole floor plan, the surfaces only with the perimeter — which is why lofts, canteens and gyms boom "by default" even with the same materials as a flat. From 2.4 to 8 metres the index rises 55% without changing a single material.

The design corollary: in tall volumes the ceiling is the surface to treat — it is as large as the plan, it is free, and each of its square metres works twice (it absorbs and it breaks the vertical path of the sound). Sound-absorbing ceilings and hanging baffles exist for this row of the table.

People absorb

The audience is walking absorption: a seated person is worth around 0.4-0.5 sabin m² — clothes and body are porous material in every sense. The four dinner guests of the reference bring about 1.8 m² of absorption: half an acoustic correction arrives by itself with the guests, which is why the room that boomed empty sounds acceptable full.

Designers know it and design halls at partial occupancy: the auditorium tuned for a full house would sound cavernous at rehearsals; the upholstered seats of theatres exist for this too — they absorb almost as much as a spectator, and keep the hall stable between empty and full.

For this page's count the translation is simple: if the room is used mostly full (classroom, training room), the people's absorption can be counted as margin; if occupancy varies, the honest count is done on the worst case — the half-empty room.

Restaurants and open spaces: the noise spiral

In rooms where many people talk at once, reverberation triggers a spiral with a name — the Lombard effect: the sound tail raises the background noise, speakers raise their voices to beat it, the noise rises again, and the room ends up "shouty" regardless of how loudly the customers actually shout.

It is the built-in defect of design restaurants — glass, concrete, hard wood: coefficients of 0.03-0.10 on every surface — and of open spaces carved out of industrial halls. The spiral only breaks on the absorption side: lowering the reverberation time lowers the background, and with a lower background everyone speaks more quietly on their own.

The page's count gives the size of the intervention; the sector's experience adds the where: on the ceiling above the tables, on the back walls, never at the expense of the surfaces needed for cleaning and hygiene.

The map of materials

The average coefficient is built from the real surfaces, and the range between materials is enormous: plaster and concrete 0.03-0.05, glass 0.03-0.10, glued parquet 0.05-0.10, wood over an air gap 0.10-0.30, carpet 0.20-0.40, heavy draped curtain 0.40-0.50, technical porous panel 0.70-0.95.

The useful reading is by dominance: in a living room the plaster governs the average, and moving 10% of the surfaces from 0.05 to 0.90 shifts the average coefficient visibly. The second reading is value for money in sabin m²: the curtain you already need for the light is often the cheapest absorption in the house.

The usual warning: these are speech-band values — the same material drops a lot in the bass, and that is where catalogue numbers must be read per band.

Small rooms and home studios: where Sabine stops

Below 25-30 m² and at low frequencies the sound stops behaving as a "diffuse field" and room modes start to dominate: resonances tied to the room dimensions, which inflate some notes and cancel others. The average T60 remains useful for speech, but the problem of the home studio or listening room is not the tail — it is the map of peaks and dips below 200-300 Hz.

There the tools change: bass traps in the corners (where the modes pile up), speaker and listening-point placement on the right fractions of the room, and measurement with a microphone — not the estimate. The rule of thirds for the speakers and the sofa off the centre line do more than many panels.

This page's count remains the first step even there: it brings the average time into the correct zone, and hands the measurement a room that is already reasonable instead of a building site.

Frequently asked questions

What is the reverberation time (T60)?

The seconds a sound takes to drop by 60 decibels after the source stops: the "tail" you hear clapping in an empty room. It is estimated with Sabine's formula: 0.161 times the room volume divided by the equivalent absorption of the surfaces.

How is the equivalent absorption calculated?

Total surfaces times average coefficient: the page reference (a 104 m² box times 0.15 for average furnishing) gives 15.6 m² — as if fifteen square metres of wall were an open window reflecting nothing.

What is the right reverberation time?

It depends on the use: roughly 0.4-0.6 s where speech commands (classrooms, offices, meetings), 0.8-1.2 for multipurpose halls and restaurants, 1.5-2.5 for symphonic and sacred music. The calculator is parametric: enter your own standard's target and the index adapts.

What does the index against the target mean?

The estimated time divided by the target: between 0.8 and 1.2 you are within tolerance (the ear does not distinguish ±20%), between 1.2 and 2 the room is reverberant and treatment pays a lot, above 2 you design with an acoustician, below 0.8 the room is drier than needed.

Why does an empty home boom while a furnished one does not?

Because moving in is absorption: from 0.05 (empty and tiled) to 0.15 (furnished) the time drops from 2.09 to 0.70 seconds in the reference room. The first furniture does more than any later panel: the formula is a hyperbola and the first m² are worth more.

How many panels does it take to fix a room?

The "missing absorption" output divided by the panel's coefficient: the reference asks 2.5 sabin m², which with 0.9 panels is about 2.8 m² — three one-square-metre panels. With heavy curtains (0.4-0.5) it would take twice as much.

Does the neighbours' noise have to do with reverberation?

No: they are two different problems. Reverberation is sound bouncing INSIDE the room; sound insulation is sound crossing walls and floors. Absorbing panels treat the first and almost nothing of the second — the sector's most common purchasing mistake.

Why is the same room fine for speech and bad for music?

Because the target changes with the use: the reference room has an index of 1.74 against a classroom target (0.4 s) and 0.46 against a chamber-music target (1.5 s) — reverberant for the lesson, dead for the violin, without touching a panel.

Does the count work for warehouses and open spaces?

The formula holds, but above 300 m³ the volume becomes the multiplier (reverberation grows with the volume, absorption only with the surfaces) and the ceiling is almost always the first surface to treat. The verdict flags it automatically in large volumes.

What average coefficient should I enter?

An honest estimate by bands: 0.05 empty hard room, 0.10 half empty, 0.15 normally furnished, 0.30 soft furnishing with curtains and rugs, 0.50 treated. The dominant surfaces command: a living room is almost all plaster (0.03-0.05) and glass, and furniture raises the average.

Does Sabine work for bass too?

Only in part: the coefficient here is an average over the speech band (500-2,000 Hz). Thin materials absorb low frequencies poorly, and the room that "booms with bass" (rehearsal rooms, home theatres) needs thickness, air gaps or resonators — and a per-band analysis.

Does this replace an acoustic measurement?

No: it is an informative parametric estimate on a regular geometry, fit for deciding whether and how much to treat. Where the value is a regulatory requirement (classrooms, canteens, public halls) the on-site measurement rules and the design must be signed by an acoustician.

Updated July 2026