In the last blog, we discussed how bass trapping is such a complex task that requires the sound 'pressure' to be absorbed, yet, how some monopolists are making a fool out of consumers by simply selling porous velocity absorbers as bass traps! However, there is a secret that has been guarded by top-of-the-world engineers based on mass-spring mechanics, and surprisingly, very few people have been able to discover it.
This one physical phenomenon allows us to take advantage of porous sound absorbers by using their limited rigidity along with particular physical properties which can contribute in making them act like a semi-rigid pseudo-membrane, effectively, extending their range of absorption in the very low end!
In this article, we will learn the concept behind this mechanism, and soon will be followed by part 2, explaining the application and installation of such systems! Subscribe to stay notified.
Let's explore the intriguing behavior of Rockwool's Mineral Fiber, when used in front of a large air gap, allowing it to act as a membrane with a spring action toward the low-frequency sound.
When high-density Rockwool (next blog on density, stay tuned :))sd is positioned in front of a large air gap, it exhibits a fascinating behavior known as diaphragmatic or membrane absorption. This unique phenomenon occurs due to the specific properties of Rockwool and the presence of the air gap, allowing the material to effectively absorb low-frequency sound energy.
To better grasp the mechanics of this process, let's delve into it with more comprehensive details:
1. Mass-Spring-Mass System: Visualize the high-density Rockwool as part of a mass-spring-mass system. In this analogy, the Rockwool represents the first mass, the air gap acts as the spring, and the room air on the other side of the air gap represents the second mass.
2. Interaction with Sound Waves: When a low-frequency sound wave encounters the high-density Rockwool, it transfers kinetic energy to the fibers of the material. The fibers, due to the density of Rockwool, possess significant mass.
3. Compression and Expansion: In response to the sound wave, the Rockwool fibers undergo compression and expansion. This movement resembles the back-and-forth motion of a spring. The cyclic compression and expansion of the fibers occur as the sound wave passes through the material.
4. Energy Transfer: As the Rockwool fibers compress and expand, they absorb energy from the low-frequency sound wave. This energy is then converted into heat due to the friction generated by the movement of the fibers. Essentially, the Rockwool fibers act as energy absorbers, transforming sound energy into thermal energy.
5. Resonance and Absorption: The air gap plays a crucial role in this process. Its size and depth determine the resonant frequency at which the Rockwool fibers vibrate most effectively. By adjusting the dimensions of the air gap, specific low-frequency ranges can be targeted for absorption. This resonant frequency is where the Rockwool absorbs the most sound energy. It's important to note that resonance occurs when the natural frequency of the Rockwool fibers aligns with the frequency of the incident sound wave, resulting in enhanced absorption.
6. Spring-Like Action: The air gap acts as a spring, exerting force on the Rockwool fibers. This force opposes the movement of the fibers, providing a restoring force akin to a spring. As a result, the Rockwool fibers vibrate back and forth in response to the sound wave, effectively transforming the sound energy into mechanical energy within the material.
7. Conversion of Sound Energy: As the Rockwool fibers vibrate and transfer energy into the air gap, the sound energy is converted into heat through frictional losses within the material. These frictional losses occur due to the movement and interaction between the fibers. The conversion of sound energy into heat significantly reduces the low-frequency sound levels within the room, leading to improved acoustic conditions.
It's important to consider several factors for successful diaphragmatic absorption, including the density of the Rockwool, the dimensions of the air gap, and the overall acoustic characteristics of the room. Achieving optimal low-frequency absorption performance may require careful consideration and experimentation to ensure the resonant frequency aligns with the target low-frequency range.
In summary, when high-density Rockwool is placed in front of a large air gap, it can act as a diaphragm with spring-like behavior towards low-frequency sound. Through compression and expansion of its fibers, the Rockwool absorbs sound energy and converts it into heat. This diaphragmatic absorption offers an effective means to address low-frequency sound issues in small rooms.
In part 2, we will discuss the specifics of how installation of such mass-spring systems can be carried out in specific details. Enjoy!
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