Sound absorption (part 2)

Sound absorption applications

In the last article we defined sound absorption and the coefficient to measure it. This time we will explore its most popular applications.

Fig. 1. Dodecahedral source in anechoic room.

Acoustic treatment

The absorption coefficient shows us what percentage of sound is absorbed by a material. The higher the coefficient, the higher the absorption. As we see in the table, the coefficient varies with frequency. Some materials absorbe more high frequencies than low frequencies and viceversa. Some materials don't absorbe almost any sound at all, like the poured concrete.

Fig. 2. Absorption coefficients.

This means that, if we are in an empty room with concrete on the floor, walls and ceiling, almost all of the sound energy will bounce back and forth. It's like throwing a lot of ping pong balls in all directions. All these reflections will create a very long reverberation time. This reverb time will depend on the room size too, the bigger the room, the longer the reverb time.

A place like this could or could not be right for acoustical applications. This depends on the purpose of the room. For example, if we play a flute in a room like that, we may like the sound we get as the reverb can enhance the sound of acoustic instruments. On the other hand, if we set a loudspeaker or an audio system in that kind of room, we won't hear very clear. This is because that amount of reverb can "blur" the original sound. A possible solution would be to treat the room with more absorbent material.

There are many comercial products for this, like the acoustic foams or panels. The big advantage of these materials is that they have very high absorption coefficients and thus, they can help us to control the reverb time in that room. The amount of material and its distribution in the room is a little more complicated subject. It's recommended to ask an acoustic consultant for that.

To make this concept clearer, here it is a little app that estimates the reverberation time (RT60) for different materials and room sizes.

Fig. 3. Studio with acoustic panels to control reverb time.

Noise Reduction Coefficient (NRC)

It's common that many acoustic products don't show the absorption coefficient tables. Instead, the manufacturer may give us a single number called Noise Reduction Coefficient (NRC). For example, a typical acoustic foam could have an NRC = 0.9. The NRC is an average of the absorption coefficients at 250, 500, 1000 and 2000 Hz. The NRC is used for simplicity, as it allows us to know how good a material is for sound absorption. In other words, the NRC makes simpler the comparison between different materials. Anyway, to know the absorption coefficient per band allows us to make better decisions when treating high, mid or low frequencies in a room.

As we can see, the sound absorption plays a very important role for room treatment. There are other important considerations too, like sound diffusion or soundproofing. We will talk about that in next articles. See you!