**Sound absorption ****classes **

**explained **

**Sound absorption classes** are often used to **specify** sound absorption **materials **to be installed in buildings.

But have you ever wondered **what they are** exactly, **what they represent** and **how they are established**/calculated?

If you want to know, this short article has been created for **you**, with some **graphics** to make you understand the **calculation** **process**.

Enjoy the read!

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**What are sound absorption classes ? **

The **classification** is a **method**, among others, to **categorise** the **sound absorption performance** of absorbers, particularly for **construction materials**.

* ***Note**

#### You can find the details of this classification in **ISO 11654** (2023) – Sound absorbers for use in buildings – Rating of sound absorption.

**ISO 11654**(2023) – Sound absorbers for use in buildings – Rating of sound absorption

It is **simpler** and** easier** for **non-acousticians** to grasp since it **doesn’t involve numbers**.

There are five classes ranging from **Class E** (**lowest** performance) to **Class A** (**highest** performance).

**How are sound absorption classes established ? **

Establishing the **sound absorption class** of a material involves **two main steps**:

- Converting the
**frequency-dependent**sound absorption performance to a**single-number value**. - Assigning the
**single-number**performance to the**corresponding class**.

**From frequency-dependent performance to single number ****value**

Sound absorption data is typically measured in third-octave bands.

(if you need to understand about **frequencies** read this article, and **frequency bands** read this article)

The **initial conversion** involves **transitioning** from the **sound absorption coefficient** measured in third-octave bands to the **Practical Sound Absorption Coefficient** (** α_{p}**) in octave bands (as introduced in this article).

From there, we calculate the ** Weighted Sound Absorption Coefficient **(

*α**), which is a single number determined as detailed below.*

_{w}We begin with two curves:

- the
**Practical Sound Absorption Coefficient***(*) values calculated (in*α*_{p}**yellow**below). - A curve known as the
, with specific values for each octave band between 250 Hz and 4000 Hz (in*reference curve***black**below).

The second step involves calculating the ** “unfavourable deviations”** for each octave band.

* ***Note 1:**

#### An **unfavorable deviation** occurs when, for a given octave, the **measured** value is

**unfavorable deviation**

**less**

#### than the value of the **reference curve**.

#### And vice versa, a ‘*favourable’ deviation* occurs when, for a given octave, the **measured** value is

*favourable’ deviation*

**more**

#### than the value of the **reference curve**.

* ***Note 2:**

#### In the calculations, we only consider

**unfavourable deviations**.

**unfavourable deviations**

Another important point to note is if a measured value **exceeds** the reference value by **more than 0.25** (even after the shifting process), a * shape indicator *is added to the weighted sound absorption performance as follows:

**L**, like*α*(L), if excess occurs at_{w}**250**Hz**M**, like*α*(M), if excess occurs at_{w}**500**and**1000**Hz**H**, like*α*(H), if excess occurs at_{w}**2000**and**4000**Hz

**From single number performance to sound absorption class**

Once the **single number** is calculated, we** associate** it with a **class** using the table below.

The figure below presents the **reference curves** corresponding to **each class** (you have probably seen this type of graph before).