What is an acoustic material ?

An acoustic material is a material that will allow :

1. Either to improve the reverberation time of the room (RT60): this is called acoustic treatment.
2. Either to prevent the propagation of noise from one room to another: this is known as sound insulation

In conclusion, an acoustic material is a material allowing a better noise management.

The characterization of an acoustic material

A material is characterized in 2 ways from an acoustic point of view:

1. By its alpha sabine: between 0 and 1 An alpha sabine close to 1 means that the material is totally absorbent, and at 0 it is not absorbent at all.
2. By its DL_alpha: which will be expressed in decibels (db)

So you have understood it, alpha sabine will be evoked in an acoustic treatment context. The DL_alpha will be used in a context of sound insulation.

A better understanding of the characterization of sound-absorbing materials: acoustic performance

Frequencies (hertz)

As we explain in our tutorial on acoustic processing, an alpha sabine alone doesn’t mean much. You have to look at 2 very important parameters:

1. The frequencies on which this general alpha sabine performs well
2. The test methods

As a reminder, in many situations voice frequencies (500 to 2000 hertz) will be the most important to treat. Thus, as we say in our tutorial on the quality of an acoustic product, it is necessary to look at the acoustic behavior of the product in question over the entire frequency spectrum.

Similarly, make sure to compare curves with the same indicator. They are in the same form, but do not appear to have the same performance. Therefore, do not confuse the curves:

1. In alpha sabine: the maximum coefficient will necessarily be close to 1
2. In area of equivalent absorption: the maximum coefficient can go up to more than 4

In fact, it is, roughly speaking, a conversion. An alpha sabine coefficient of 1 on a particular frequency will have an equivalent absorption area equivalence with a higher number (for example, 3). In this particular case, you may think that the product expressed in equivalent absorption area is 3 times better on that frequency than the one expressed in alpha sabine. Be aware that this is not the case at all.

As you can see on the test report on the right, the frequency spectrum ranges from 0 to 10000 hertz because these are the frequencies audible to a human. It should be noted that these audible frequencies go even further. Indeed, in reality it stops rather between 15000 and 18000 hertz depending on various parameters.

As you will have understood, depending on age and the “ear”, that is to say whether or not it has suffered trauma that reduces its performance, each individual will not hear exactly the same frequencies. This is why some ultrasounds that emit particular frequencies are not audible to everyone.

As far as testing methods are concerned, it is possible to optimize performance “on paper” through certain procedures. Some manufacturers play with these parameters. It is therefore important to look at them carefully. This is what we explain in our article on acoustic panels.

The ratio of materials to frequencies and sound absorption

In reality most materials are absorbent. This is why the same room will resonate less when it is furnished. However, not all materials have the same acoustic performance.

This is the reason why most furniture is not qualified as acoustic. It should even be noted that a table will be 20 to 40 times less absorbent than an acoustic panel. The latter are specifically designed for this use and use materials thought for this purpose.

Similarly, the larger the room, the worse the acoustics will be. The more acoustic panels will be indispensable to obtain a respectable acoustics. It will be the same for rooms with an audience.

Decibels (db)

We have already dealt with this topic in another article on noise at work.

Remember that to measure it, you need a sound level meter. In the same way, beyond 80/85 db in a room, it is advisable to act. In a work context, beyond this threshold, the employer is even obliged to take measures to protect the health of his employees.

The ratio of materials to decibels

In this case, the furniture will have no impact on your acoustics.

The structure of the walls and ceilings should then be reviewed with acoustic materials that we will discuss shortly. Only sound curtains can improve the situation under certain conditions.

Sound insulation coefficient, noise attenuation, acoustic absorption and alpha w sabine coefficient

Properties and acoustic coefficient of the materials: alpha W and sabine coefficient

As previously mentioned, a distinction must be made between sound insulation and acoustic treatment. When talking about acoustic treatment, we will use the following indicators:

• The alpha W
• The sabine coefficient

To learn more, you can read our tutorial on acoustic performance.

Sound Coefficient of Materials: DL_alpha and Acoustic Attenuation

On the other hand, since we are talking about phonic coefficient, we will use the following terms:

• The DL_alpha
• Acoustic attenuation (in db)

Noise absorbing materials for acoustic treatment: list

Different materials are often used for acoustic treatment. They each have their advantages and disadvantages. Here are the main ones:

1. Acoustic foams
2. Mineral Wools
3. Polyester fibre wool (wadding) / PET
4. Absorbent cotton

Acoustic foams

Originating from the petrochemical industry, acoustic foams are an acoustic material that has been used for many years. Their big interest lies in their price (even if including the glue, if one wants good results, this argument is questionable). Nevertheless, they have many defects (fire resistance, acoustic performance, aesthetics and installation).

In summary :

• Inexpensive
• Made from petroleum
• Fasteners that do not respect the walls (leave marks when you have to remove them) and may well raise the bill
• Very low absorption quality, needs to put a maximum of it to have a correct result
• Unsightly
• Only a few are fire resistant
• Performance does not stand the test of time

For more information, you can read our article on acoustic foams.

Mineral wools: glass wool and rock wool

Mineral wools are certainly the most widely used material for acoustic treatments. This family of material is composed of 3 materials :

1. Glass wool
2. Rock wool
3. The slag wool

These are the first two that are used for acoustic treatment.

In summary :

• A good quality/price ratio
• Easy to cut
• Polluting production: these wools are accused of destroying ecosystems (many citizens’ associations around the world have been created to combat them).
• Require important precautions for their use
• Irritating
• Little hindsight on their carcinogenicity
• Not known to last over time

For more information, you can read our article on mineral wools.

Polyester fiber wool (wadding) / PET (polyethylene terephthalate)

Polyester fiber wool is a material obtained, in part, by recycling plastic bottles. It is also an efficient material from an efficient point of view. It has various advantages :

• Attractive price/performance ratio
• Non-irritating product
• Good durability

Despite the fact that this material is obtained through recycling, it cannot be considered as totally environmentally friendly. This is why:

• Plastic is a derivative of petroleum, a fossil fuel.
• The recycling process is not well mastered in many countries, so PET is often sent back to Asia to be recycled (in particular to make an acoustic product).
• This material can only be recycled once and therefore ends up in the landfill very quickly.
• Rarely contains only recycled fiber (rather about 2/3)

Feel free to read our tutorial on PET acoustic panels.

Cotton Wool

Wool cotton is also an interesting acoustic material. Here is what we can say about it:

• A little more expensive
• Harder to cut
• More ecological
• Non-irritating product
• Good durability

Soundproofing materials for sound insulation list

Most of these materials are also used in a context of sound insulation (mainly mineral wools). However, there are also other materials that are used, to date, only in sound insulation:

1. Straw
2. Hemp
3. Cellulose wadding
4. Wood fibers

Performance and properties of acoustic and phonic materials

It is important to specify that the implementation of acoustic and phonic materials will have an impact on their performance will influence their effectiveness.

For an acoustic panel, the whole of its composition will be particularly important. If non-acoustic fabrics or reverberant materials are used to shape your panel, the performance of your insulation can be totally annihilated. The positioning and number of panels will also play a role. Also, depending on your use (home, office, music room, …), you will not have the same needs.

For sound insulation, it will be the same. The choice of the partition and the quantity of insulating materials will have an impact on performance. The implementation of the work is also not an easy thing for everyone.

Calling on a professional is therefore a guarantee of quality and results. This is true for both acoustic treatment and soundproofing.