The acoustic diffusers

What is sound diffusion?

When a sound wave travels and finds a surface, different phenomena occur. Sound absorption converts part of that energy into heat. Part of that energy is transmitted into another room and another part of the sound energy gets reflected as echo or reverberation. But there's another interesting form of reflection: the diffusion.

To understand diffusion and diffusers, we need to first understand that the sound waves follow the Law of Reflection. This law says that the incident angle of a sound wave is the same as the reflected angle.

Fig 1. Law of Reflection

In the picture, we see that if the wave gets on the surface with a θi angle, it will be reflected with a θr angle. Both angles will be equal if the surface is flat. This behavior is similar to a pool ball when it bounces on the edge. The sound waves will reflect in this way as long as the reflecting surface is larger than their wavelength. This kind of reflection is named specular reflection.

Wavelength

Wavelength is represented by the letter lambda (λ). It indicates how long the wave is. The higher the frequency, the shorter the wave. Audible frequencies, which can go from 20 Hz up to 20 kHz have wavelengths as large as 17 meters and as small as 2 cms (less than 1 inch).

As low frequencies have large wavelengths (more than 10 feet), it's difficult to see the Law of Reflection in those cases. With high frequencies, it's easier to "see" this behavior as they are smaller. Actually, you can think of mid and high frequencies as "sound rays". In the field of acoustics, this behavior is called Ray Theory.

Fig. 2. Different wavelengths

Diffusion

When the reflective surface is irregular, the Law of Reflection applies in almost every little spot of the surface, as seen in the picture.

Fig. 3. Sound diffusion

The result of this scattering is called diffusion. A specular reflection follows the Law of Reflection in a more evident way, you will hear that as echos. A diffuse reflection, on the other hand, generates a field of sound in all directions, you will hear this as reverberation.

Fig. 4. Specular reflection vs. Diffuse reflection

To diffuse or not to diffuse

The most important point is to know if it's better to use diffusion or not. For the most common situations, it's recommended to have a good amount of diffusion in the room. The more diffusion we have, the better the sound distribution and dispersion around the place. In other words, the sound will be more homogeneous in the room.

For example, in the case of a recording studio, the diffusion helps to make the sound that the microphones sense as more balanced. In this case, diffusion helps to avoid undesirable coloration and comb filters due to echos.

In the case of a mixing or mastering studio, diffusion helps to avoid early reflections on the sweet spot. These reflections can create image problems in the mix. Also, diffusion can create the sensation of a larger space even in a small room.

In the case of a concert hall or auditorium, diffusion is very important for the sound to reach more zones in the venue.


Acoustic diffusers

But, how can we create diffusion? We can build or buy systems designed just for that. In this regard, there is a great variety of designs. For example, the QRD (quadratic residue diffuser) systems, which are made of wells of different depths and dimensions. The size of these elements will depend on the frequencies that we want to scatter; larger dimensions, lower frequencies.

Fig. 5. QRD diffuser

Convex surfaces can also work as diffusers, but concave surfaces do not (i.e. church domes). An example of a "natural" diffuser would be a wall made of stone. The irregular surface of the stone has a diffusing effect on high frequencies.

Finally, we can also use a bookshelf as a good diffuser! If we gather books of different sizes and widths and we place them in a "random" way, we can achieve better results.

As we can see, the general idea is to avoid flat surfaces. This kind of surfaces will generate rapid echoes (specular reflections) that will have a very noticeable effect on the sound. Diffusion is very important for acoustic treatment in a TV room or home theater as well. We can use the resources available to us to improve our listening experience by using sound diffusion almost everywhere.



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