Acoustics V: The Definitive Guide to Types of Acoustic Treatment

In previous posts, we established the foundation of acoustics: understanding what sound is, how its waves behave, and the crucial relationship between velocity and pressure. Now it's time to open the toolbox.

When we talk about "treating a room," we're not talking about a single solution, but a set of different tools, each with a specific function. Using the wrong tool for the problem is not only ineffective but can even worsen your room's acoustics.

In this guide, we will dissect the main types of acoustic treatment, explaining how they work, what they are used for, what they are not used for, and why.

Summary:

• Absorption (Porous Panels): Converts sound energy into heat. It is the ideal tool for controlling echoes, excessive reverberation, and mid- to high-frequency reflections.

• Diffusion: Scatters sound energy in multiple directions and intensities. It maintains the room's naturalness without as many problems as direct reflections.

• Bass Treatment (Bass Traps): Specialized resonant tools for controlling low-frequency standing waves.

1. The Most Common Tool: Absorption (Porous Panels)

These are the panels that most people associate with acoustic treatment. They are made of fibrous or porous materials like rockwool, fiberglass, or acoustic foam.

How Does It Work? They are velocity absorbers. As we saw in the previous post, they work through friction. When the sound wave forces air particles to move through the panel's fibers, the kinetic (movement) energy is converted into a tiny amount of heat, effectively "killing" the sound.

What Is It Effectively Used For?

• Controlling Flutter Echo: That rapid metallic repetition that occurs between parallel walls.

• Reducing Reverberation Time (RT60): "Drying out" a room's sound, making it less "echoey" and more controlled.

• Treating First Reflection Points: Placed on the side walls, ceiling, and front wall (attack wall) to absorb the first reflections from the monitors, ensuring direct hearing and an accurate stereo image.
  

What Is It NOT Used For?

• Soundproofing: A foam panel on the wall will not stop the sound of your amplifier from leaking to your neighbor. Acoustic treatment controls the sound inside the room. Soundproofing blocks the transmission of sound out of the room, which requires mass, decoupling, and sealing.

• Bass Absorption (if the panel is thin): Panels that are 5-10cm thick are practically invisible to the long waves of low frequencies. The sound wave of a 50Hz bass note is almost 7 meters long! It passes through a thin panel as if it weren't there.

2. The Tool of "Naturalness": Diffusion

Diffusers are panels with irregular surfaces, featuring blocks or wells of different depths, usually made of wood or dense plastic.

How Does It Work? Unlike absorption, diffusion does not remove sound energy; it scatters it. When a sound wave hits the irregular surface, it is reflected in countless directions and with different intensities. The idea is to try to even out the intensity of the reflected frequencies.

What Is It Effectively Used For?

• Treating Late Reflections: They are ideal for the back wall of a control room. They break up the wave that reflects back to the listener, preventing a direct "slapback" echo without killing the room's energy, or for controlling residual reflections elsewhere in the room.

• Creating a Sense of Spaciousness: By scattering the sound, diffusers make small rooms sound larger and more natural.

• Maintaining the "Life" of the Room: Excellent for recording rooms where you want a controlled sound, but not one that is completely "dry" or "dead."
  

What Is It NOT Used For?

• Solving Bass Problems: Standard diffusers have no effect on the long waves of low frequencies. Bass treatment requires specific tools.

• Treating First Reflection Points: In most control room designs, the goal at the first reflection points is to eliminate the reflection, not to scatter it.

3. The Main Challenge: Bass Treatment

As low frequencies (especially below 100 Hz) are the biggest problem in small rooms, they require dedicated tools and a clear understanding of their limitations. "Bass Trap" is a general term, but the mechanisms behind them are very different.

Type 1: Porous (Velocity Absorption)

• How They Work: These are very thick panels (usually 15 to 30cm or more) of porous material (rockwool/fiberglass), positioned in the corners of the room. As we've seen, the corner is an ideal location for a porous material to capture bass energy.

• What They Are Effectively Used For: Broadband bass absorption. They are the generic solution for most home studios, as they competently control a wide range of mid-bass and bass frequencies (say, from 100 Hz upwards). They are crucial for "cleaning up" the region that gets muddy and clutters the mix.

• The Crucial Limitation: As physics proves, the effectiveness of porous absorbers drops dramatically at very low frequencies (sub-bass, <100 Hz). To efficiently absorb a 60 Hz wave, a porous panel would need to have an impractical thickness for most environments. Therefore, they are a general control tool, but not the most effective solution for specific sub-bass resonances.
  

Type 2: Resonant (Pressure Absorption)

• How They Work: Here we get into the heavy artillery of bass treatment. These are "tuned" devices, like Helmholtz Resonators or Membrane Absorbers. They use the pressure of the sound wave to resonate at a target frequency, converting the acoustic energy into mechanical vibration and, subsequently, into heat.

• What They Are Effectively Used For: Narrowband bass absorption. They are surgical tools, designed and built to solve a specific problem of one or two resonant frequencies ("room modes") that are causing exaggerated peaks or nulls in the room.

• The Superiority for Sub-Bass: When the problem is a specific sub-bass frequency (a bass note that "disappears" or another that "booms" in the room), resonators are, without a doubt, the most effective tool. They act directly on the pressure peak of the wave, where a porous absorber would have very little effect, offering much more efficient absorption in a much smaller space.

Conclusion

There is no "master key" for acoustic treatment. A listening environment with accurate sound is always the result of an intelligent combination of these three tools:

• Absorption to tame echo and first reflections.

• Diffusion to maintain naturalness and spaciousness.

• Bass Traps, especially those that treat pressure, to control the 'foundation' of your acoustics.
  

That's it! Until next time.