Sound absorption (part 1)

What is sound absorption?

In other articles we talked about the acoustic characteristics of some rooms and venues. We learned the difference between reverb and echo. Now, the reverb time (RT) depends on the amount of absorption in a room and its dimensions. That is, if the room has large carpets and curtains it will have less reverberation than a room with tiles and concrete. This is because the curtains and the carpet are materials that absorb more sound than a tile or concrete.

Fig. 1. Anechoic chamber

Sound absorption definition

Sound absorption is the conversion of acoustic energy into heath. Energy cannot be created nor destroyed, but it's transformed from one form to another. For this reason, when a sound wave arrives to a surface, it's transformed into heat. This amount of heat is very low though. We would need a full football stadium with people screaming to produce enough energy to heat up a cup of coffee!

Sound absorption is not the only thing that occurs when a sound wave reaches a different medium. Generally speaking, there are three phenomena:

  • Reflection. Part of the energy "bounces" back.
  • Absorption. Part of the energy will be absorbed by the material.
  • Transmission. Part of the energy will pass through the material. 

Fig. 2. Sound wave traveling. (Everest, A. Master Handbook of Acoustics)

For example, in the last image, we have a sound wave traveling from source S. While traveling in the air, there is some sound absorption (E). When it reaches a wall or obstacle, part of the energy will be reflected (A) and another part will be absorbed and transmitted (F, H).


Measuring absorption

Each material can absorb more or less sound energy. We can measure this through the absorption coefficient. This is a number that shows us what percentage of energy is absorbed by a material. It doesn't have measure units and it's represented with the letter α. A coefficient of 1 means that 100% of energy is absorbed and none is reflected back. A coefficient of 0.25 means that 25% of the energy is absorbed and the other 75% is reflected. In general, porous materials have higher absorption coefficients than rigid materials. For example, a curtain has bigger absorption coefficient than concrete.

It's interesting to note that the absorption coefficient is not constant at all frequencies. This is why is common to find tables where we can see the coefficient of different materials at different frequencies.

Fig. 3. Absorption coefficients

In the table we can see some absorption coefficients. For example, a brick is a little better to absorb high frequencies than low frequencies. On the other hand, an ordinary window glass absorbs more low frequencies than high frequencies. This knowledge is very useful when designing the acoustic treatment of a room. There are very complete tables with common construction materials available.

Also, we can see that there are different materials with similar acoustic behavior. So it is a mistake to believe in a "magic recipe" for fast and easy acoustic solutions. Most of the times we need a combination of materials, with both high and low amounts of absorption, depending on the design and needs for that particular application.



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