Acoustic Foam? Is it a thing?

Acoustic foam is hailed as a savior in the world of room acoustics, promising to tame echoes and reverberation. But let's face it: there's more to this hard foam than meets the ear. In this article, we'll delve into the depths of physics to uncover the secrets behind its limitations, focusing on the elusive low frequencies. So, fasten your seatbelts and prepare for a journey into the intriguing world of acoustic foam.

Many people who have been reading our blogs and have been connected with us for a while are already aware of how thickness is the king in the world of velocity absorbers. (If you don't, I suggest you read this blog first) Well, if you're assuming that thick foam might perform well in lower frequencies, the short answer - NO! Why? That's the hot topic! To comprehend the limitations of hard foam, it is essential to first grasp the concept of porous absorbers. These specialized materials are designed to convert sound energy into heat through friction, effectively dissipating it. Porous absorbers feature tiny interconnected voids welcoming sound waves into their labyrinth of air passages. As the waves wander through this maze, they lose their energy through friction and dissipate by converting into heat energy as they navigate through the material's intricate viscous passages. This principle forms the foundation for effective sound absorption.

For nerds: It is important to note, that the above explanation is based on ideal laws of fluid mechanics. The real-world scenario is not so idealistic. The sound gets dissipated into heat to a certain extent only. At the other edge of the absorber, the remaining sound gets transmitted to the space behind, and if there’s a wall, that’s likely to be the case with most of us, the sound will reflect back, going through the absorber again, but ultimately transmitting through once again, come back into the room. The amplitude of the attenuated frequencies will be significantly low, resulting in less acoustic distortion. This was basically all you need to know about how a porous velocity absorber works. This is a more elaborated form of information that was once given to me by a veteran in my early days of engineering, and another veteran told me that I got a piece of advice worth thousands of dollars and can do wonderful work which not many people are able to do if I can read between these lines. Well, if you’re smart enough… :) It's a topic of discussion for another day, rest assured, it is on our bucket list, and soon we'll be writing an entire post dedicated to the usage of velocity absorbers with maximized efficiency and taking advantage of other physical phenomena with clever tricks! Ah, foam! Acoustic foam can indeed offer benefits in non-professional listening spaces such as offices, residential areas, or casual environments where critical sound control and precise bass management are not of utmost importance. It can serve as a cost-effective solution for addressing issues like flutter echo, which occurs when sound waves bounce back and forth between parallel surfaces. In such settings, where the primary goal is to improve basic acoustics without extensive sound control requirements, foam can be the hero we need in these spaces, saving us money and valuable real estate.

However, when it comes to professional sound spaces that demand meticulous sound control, foam reveals its limitations. Especially when dealing with low-frequency sound attenuation, foam falls flat! Let's explore its vulnerabilities, shall we? 1. High Density and Gas Flow Resistivity:

Hard Acoustic Foam often exhibit high density, resulting in high gas flow resistivity. Low-frequency sound waves, with longer wavelengths, require materials with lower gas flow resistivity to effectively dissipate their energy. Foam's high density impedes the passage of low-frequency waves, causing them to reflect rather than absorb, limiting its effectiveness in attenuating low-frequency sound.

Sadly enough, the 'high-density' is the marketing point for many hard foam sellers. It’s very amateur for an organization that poses itself as an Acoustics Expert to showcase dense materials as good absorbers out of ignorance or maybe to intentionally mislead the consumer world. I don’t know where this came from, but I am observing the myth of high-density absorbers being better for low-frequency absorption being spread a lot. Well, here’s an article for you explaining the role of densities and how should you choose the correct one for your purpose!

Well, I know it sounds counterintuitive because of the current widespread myth, but here’s some thinking exercise for you: If high-density material absorbs more sound, isn’t the wall behind the absorber much denser? HA! Well, density is not the topic of discussion today, but the next scheduled blog is on the topic of densities and GFR, so stay tuned! 2. Rigidity: Newton already told us :P: The rigidity of foam presents a significant obstacle to sound absorption, or not exactly present the “obstacle” that was needed? Since hard foam is a closed cell fiber panel, the sound waves interacting with the voids get right into it, but those incident on the surface with larger wavelengths (which potentially could get a little attenuated with surface absorption if had enough time to lose momentum a little slowly) meet the rigid structure, momentarily attaining a velocity of 0 in magnitude, eventually regain the momentum instantly, and following the newton’s laws of inertia, they ultimately reflect back!

3. Cellular Structure: A Prison of Ineffectiveness: The cellular composition of hard foam contributes to its limited sound absorption capabilities. Hard foam consists of a network of closed cells that allow the sound wave to penetrate through, but doesn’t resist the flow of sound enough to dissipate it into heat relative to the other products available in the market. As a result, a substantial portion of the sound is either transmitted through or reflected back. The not-so-viscous nature of the foam's cellular network further impedes efficient sound absorption, restricting its effectiveness in attenuating sound. In our observations, the only benefitting property that this so-called "Acoustic Foam" possess to obtain any sound attenuation is the shape - Wedges, and Pyramids. These Shapes give the sound a geometric dispersion and redirect the sound further inside the absorber resulting in more attenuation. Well, that is a topic for another day, and we soon will publish a blog on the Shapes of absorbers! The photograph below showcases an Acoustic laboratory, featuring complex fiber-based Wedges through all surfaces. Here the shape of the wedges are playing a more important role than any other factor in maximizing sound absorption.

While foam can be a suitable solution for non-professional listening spaces, it exhibits inherent limitations in sound absorption, particularly in low frequencies. Its high density and gas flow resistivity hinder the attenuation of low-frequency sound waves. Additionally, the rigidity of foam prevents the potential surface absorption of low-frequency energy, and its cellular structure contributes to inefficient sound absorption. As such, it is crucial to consider alternative solutions when dealing with critical sound control requirements in professional spaces. Nevertheless, foam can still serve as a practical and cost-effective choice for improving basic acoustics in non-professional environments, where the focus is on mitigating issues like flutter echo without necessitating precise sound control measures.