Acoustics IV: Velocity vs. Pressure — The Definitive Guide to Bass Treatment

In the previous post, we unraveled the "language" of sound waves, exploring concepts like frequency and wavelength. We discussed how the long waves of low frequencies (bass) are particularly problematic in small rooms, generating the dreaded standing waves.

But the solution to this problem lies in a fundamental concept of acoustic physics: the relationship between particle velocity and sound pressure.

Understanding this dynamic is the secret to not only understanding how sound behaves, but also where and how to treat it effectively.

Quick Technical Summary:

• Sound Pressure: The force exerted by air particles. It is maximum at rigid surfaces (walls, corners).

• Particle Velocity: The actual movement of air particles. It is maximum away from surfaces, in the center of the environment.

• Inverse Relationship: Where pressure is high, velocity is low, and vice-versa. This relationship is the key to positioning bass absorbers.

1. What is Sound Pressure?

Sound pressure (or acoustic pressure) is the variation in air pressure caused by the propagation of a sound wave. In simple terms, it is the "force" that the wave exerts on the medium. It is the property that our ears and microphones measure.

Ideal Location: In an enclosed environment with standing waves, sound pressure is maximum at rigid surfaces (walls, ceiling, floor) and, especially, in the corners. At these points, the air particles cannot move, causing pressure to build up. These points of maximum pressure are called nodes.

2. What is Particle Velocity?

Particle velocity is not the speed of sound (which, as we've seen, is 343 m/s in air). It refers to the speed at which the particles of the medium themselves move back and forth around their resting point, in a "to-and-fro" motion. It is the "movement" aspect of the wave.

Ideal Location: Particle velocity is maximum where the particles have the greatest freedom of movement. In a room, this occurs in the center of the environment, away from surfaces. These points of maximum velocity are called antinodes.

3. The Inverse Relationship

The relationship between pressure and velocity is inversely proportional. They are never maximal in the same place.

• At a wall or corner (node): Sound pressure is maximum because the air particles cannot move, but the particle velocity is almost zero.

• In the center of the room (antinode): Particle velocity is maximum as the particles have complete freedom of movement, but the sound pressure is minimal.

4. Practical Application: Where Knowledge Meets the Studio

Understanding this relationship tells us where to place acoustic treatment panels for low frequencies.

Pressure Absorbers (Resonators):

• Mechanism: These absorbers (like Helmholtz resonators or membrane panels) are designed to vibrate at a specific frequency when struck by a sound wave. What makes the membrane vibrate is the air pressure.

• Ideal Positioning: Since they operate by converting pressure energy, they are most effective where the pressure is maximum—that is, on the walls and in the corners of the room.

Velocity Absorbers (Porous):

• Mechanism: These absorbers (rockwool, foam) work by converting sound energy into heat through the friction of air particles. They are most effective where the particle velocity is maximum.

• Ideal Positioning:

  • For mid and high frequencies, the maximum velocity occurs very close to the wall, so thin panels fixed directly to it are effective. However, as I will mention later, the action of porous panels is quite effective when positioned close to the sound source.

  • For low frequencies, the point of maximum velocity is meters away from the wall. Therefore, porous panels are not the best option for long wavelengths. Although the corner is a pressure node, it is the point where the air movement (velocity) from multiple surfaces meets, making it an interesting location for a porous material to capture energy more efficiently.

5. The "Attack Wall" Strategy

An effective attack wall is an intelligent system that uses the right type of absorber in the right place.

• Velocity Absorption: The direct sound from the monitors hits the wall, creating early high-frequency reflections that can harm the stereo image. Porous panels installed at these first reflection points act as velocity absorbers to "kill" these unwanted reflections.

• Pressure Treatment in the Corners: At the same time, low-frequency problems accumulate in the corners of the room. Here, pressure absorbers (like resonators) or thick porous absorbers in the corners are used to tame the pressure nodes, resulting in a more accurate bass response. But this requires seeking out the high-pressure locations for the best positioning of the resonators.

The attack wall strategy is proof that effective acoustic treatment is not just about spending money, but about the direct application of physics.

Conclusion: From Theory to Practical Solution

The acoustic treatment of low frequencies is not about guesswork. It is about applied physics. The knowledge of the inverse relationship between pressure and velocity tells us, with scientific precision, that the most effective points to absorb bass energy are the walls and, especially, the corners of the room with pressure absorbers.

With this foundation, in the next post, we will dive into practical solutions and explore the different types of absorbing, diffusing, and resonating materials so that you can finally build a listening environment that is predictable and reliable.

Until next time!