# Acoustic Panel Calculator: Panels for a Target Reverberation Time

> Sabine's formula estimates a room's reverberation time as RT = 0.161 V / A, where V is the room volume in m³ and A is the total sound absorption in m² sabins (each surface area times its absorption coefficient). Adding acoustic panels raises A and shortens RT, so the panels needed follow from the absorption a target RT requires.

## What it calculates

For a rectangular room, the calculator uses Sabine's formula to give:

- the **room volume** and surface area;
- the **current reverberation time**, estimated from an average absorption coefficient for
  the existing surfaces;
- the **absorption needed** to reach the target reverberation time, and the **extra
  absorption** beyond what the room already has;
- the **number of acoustic panels** of the size and coefficient entered;
- the **reverberation time achieved** with that many panels.

Reverberation time is the time for sound in a room to decay by 60 dB after the source stops,
the quantity measured in finished rooms under **ISO 3382-2**.

## Inputs

**Room length**, **width** and **height** are in metres. Height is to the ceiling, or to
the soffit if there is no ceiling.

**Target reverberation time** is in seconds. It is **your design input, taken from the
project's acoustic brief**. The calculator does not choose a target, and the 0.6 s it
starts with is only the value used in the worked example.

**Panel width** and **height** are in metres.

**Panel absorption coefficient or NRC** comes from the panel data sheet, for the panel as
it will be mounted. Absorption coefficients are measured under **ISO 354** or **ASTM C423**;
NRC, defined in ASTM C423, is the average of the coefficients at 250, 500, 1000 and 2000 Hz,
rounded to the nearest 0.05.

The **assumptions** panel holds the average absorption coefficient of the existing room
surfaces.

## Method

1. Volume V = length × width × height; surface area S = 2 × (L × W + L × H + W × H).
2. Existing absorption A = S × average coefficient of the existing surfaces.
3. Current RT = 0.161 × V ÷ A (Sabine, metric units).
4. Absorption needed = 0.161 × V ÷ target RT; extra absorption = needed − existing, or zero
   if the room already meets the target.
5. Each panel covers a surface, so it adds (panel coefficient − room coefficient) × panel
   area.
6. Panels = extra absorption ÷ absorption per panel, rounded up.
7. RT with panels = 0.161 × V ÷ (existing absorption + panels × absorption per panel).

## Assumptions

| Assumption | Default | Notes |
| --- | --- | --- |
| Average coefficient of existing surfaces | 0.1 | Hard walls, hard floor and glass sit low; carpet, an absorbent ceiling and soft furniture raise it |
| Sabine constant | 0.161 s/m | Metric form of the formula |
| Coefficient limit | 1.0 | Laboratory coefficients can exceed 1 at some frequencies; the calculator caps entries at 1 |

The defaults for the inputs are a 6 × 4 × 3 m room, a target of 0.6 s, and 1.2 × 0.6 m
panels with a coefficient of 0.8. They are example values, not recommendations.

## Limitations

- **Sabine assumes a diffuse sound field.** It treats sound as spread evenly through the
  room. Long, narrow, very low or very irregular rooms, and rooms with all the absorption
  on one surface, do not behave that way.
- **It averages across frequency.** One coefficient stands for all frequencies. Real
  reverberation time differs from band to band, and thin panels absorb little at low
  frequencies; check the data sheet band by band where low frequencies matter.
- **It overstates RT in very absorbent rooms.** Where the average coefficient is high,
  Sabine gives a longer reverberation time than the room will have, so the panel count is
  on the high side. The calculator warns above an average of 0.3.
- **Position is not considered.** Panels on parallel walls that face each other, or on the
  ceiling, behave differently from panels bunched in one place. Placement is part of the
  acoustic design.
- **Air absorption, people and furniture** are not modelled separately; fold them into the
  average coefficient if you include them.
- **It says nothing about sound insulation.** Panels inside a room do not stop sound
  passing to the next room; that depends on the partition, the ceiling void and flanking
  paths.

## Worked example

A meeting room 6 × 4 × 3 m with an average existing coefficient of 0.1, a target of 0.6 s
from the acoustic brief, and 1.2 × 0.6 m panels with a coefficient of 0.8:

| Step | Calculation | Result |
| --- | --- | --- |
| Volume | 6 × 4 × 3 | 72 m³ |
| Surface area | 2 × (24 + 18 + 12) | 108 m² |
| Existing absorption | 108 × 0.1 | 10.8 m² sabins |
| Current RT | 0.161 × 72 / 10.8 | 1.07 s |
| Absorption needed | 0.161 × 72 / 0.6 | 19.32 m² sabins |
| Extra absorption | 19.32 − 10.8 | 8.52 m² sabins |
| Absorption per panel | (0.8 − 0.1) × 0.72 | 0.504 m² sabins |
| Panels | 8.52 / 0.504, rounded up | 17 (12.24 m² of panel) |
| RT with panels | 0.161 × 72 / (10.8 + 17 × 0.504) | 0.6 s |

The room needs **17 panels, 12.24 m² in all**, which brings the estimated reverberation time
from 1.07 s to 0.6 s.

## How engineers use it

At design stage, an acoustic consultant or designer sets a reverberation time target for
each room type in the acoustic brief, then calculates RT from the volume and the absorption
of every proposed finish, using data sheet coefficients band by band. Panels, ceiling and
floor finishes are adjusted until each band meets the target. A single-number Sabine check
like this one is used earlier, to see whether a room is likely to need treatment and
roughly how much panel area to allow on the layout.

## When a professional design must replace it

Use the calculator to get a first view of how much absorption a room needs, or to check a
panel quantity you have been given. Replace it with an acoustic design by a consultant
wherever speech intelligibility or privacy matters, such as boardrooms, video conference
rooms, training rooms and auditoria, wherever the room is large or irregular, and wherever
the target has to be verified by measurement under ISO 3382-2 after completion.

## Frequently asked questions

### How do I calculate how many acoustic panels a room needs?

Work out the absorption the target reverberation time needs with Sabine's formula, A = 0.161 V / RT, and subtract the absorption the room already has. Divide the shortfall by the absorption one panel adds, which is its area times the difference between its coefficient and that of the surface it covers, and round up.

### What reverberation time should I aim for?

The target comes from the project's acoustic brief, set for the room's use by the designer or acoustic consultant. This calculator does not choose it. The 0.6 s it starts with is only an example value for the worked example, not a recommendation for any room.

### What is Sabine's formula?

Sabine's formula, RT = 0.161 V / A in metric units, estimates the time for sound in a room to decay by 60 dB after the source stops. V is the room volume in cubic metres and A is the total absorption in square metre sabins, the sum of each surface's area times its absorption coefficient.

### Should I use the NRC or an absorption coefficient for the panel?

Either, from the panel data sheet, as long as you know what it represents. NRC is a single average over the speech frequencies; an absorption coefficient at one frequency band lets you check that band. Use the value for the panel as it will be mounted, because the air gap behind a panel changes its absorption.

### How accurate is the Sabine estimate?

It is a design-stage estimate. Sabine assumes sound is spread evenly through the room and the calculation uses one average coefficient across frequency. It tends to overstate reverberation time in rooms with a lot of absorption, and it cannot account for where panels are placed. The finished room can be measured under ISO 3382-2.

## Sources

- [ISO 3382-2](https://www.iso.org/standards.html) — Acoustics — measurement of room acoustic parameters, reverberation time in ordinary rooms
- [ISO 354](https://www.iso.org/standards.html) — Acoustics — measurement of sound absorption in a reverberation room
- [ASTM C423](https://www.astm.org/products-services/standards-and-publications.html) — Sound absorption and sound absorption coefficients by the reverberation room method

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Source: https://hagerstone.com/calculators/acoustic-panel-calculator