Acoustic foam is a popular product or material used to improve room acoustics. The reason for its popularity is obviously the cheap price – but it’s effective too! In this article, we will discuss the effectiveness of acoustic foams in soundproofing and room acoustics treatments.
What Does Acoustic Foam Do?
To understand the effectiveness of acoustic foam, it is important to have a basic grasp of the concept of sound absorption and reflection. Sound is a longitudinal mechanical wave that travels through solids, liquids, and air. In the case of room acoustics, the air is the most common medium used to describe sound propagation.
Sound propagates through the air according to the inverse square law, assuming there is no interference to its propagation. In layman’s terms, that means that the further you get away from a sound source (or ‘point’ in sound-engineering jargon). However, when sound encounters a hard surface during propagation, a portion of its energy is absorbed by the surface while the rest bounces back to the room as a reflected wave (aka a ‘reverberation’).
Acoustic panels are generally used to increase sound wave absorption, control reflections, and reduce reverberation and phase cancellations or comb filtering in a room. Reverberation is the effect produced when you clap in an empty hall, while comb filtering is a cancellation boost pattern caused by the overlapping of direct sound from speakers with its delayed version, which may be due to reflection or other factors.
Acoustic foam panels is one of the most commonly used materials to increase the absorption of higher frequencies. By adding more absorption to sound waves, the acoustic foam helps to control reflections and improve room acoustics.
How do acoustic foam panels work?
As we discussed acoustic panels are used to absorb the energy of sound. The absorption of the energy can be achieved mainly by a few techniques as follows.
- Refraction
The method of damping the energy of the sound by changing the propagation material
- Diffusion
The method of scattering the sound energy in many directions randomly
- Diffraction
The method of damping energy by allowing them to move within small cavities relative with wavelength of the signal
Acoustic foam is a soft material with many tiny holes and attempts to make use of all these properties.
Does acoustic foam work for Soundproofing?
The simple answer is: No, acoustic foam will not work for soundproofing a room. While the improvement of in-room acoustics focuses on the behaviour of sound within a room (which acoustic foam certainly will help with), soundproofing is all about sound transmission through walls, and the only way to achieve this well is to isolating a room from external sounds and also stopping leakages of sound from the room. The perfect theoretical way to achieve a soundproofed room is to construct a room within another room and create a vacuum within the space between the two rooms. However, practically it’s not easy. Therefore we have to use multi-layer wall-building structures to achieve the same soundproofing effect.
What type of acoustic foam is best?
When we consider which type of acoustic foam is best for room acoustics treatments, there are a few things required to understand. Let’s discuss them first and you’ll understand the capabilities of acoustic foams.
When it comes to humans, there is an audible frequency range of 20Hz to 20kHz. Based on the frequency and the velocity of sound, there will be a wavelength for each frequency. Assuming fixed temperature, humidity, and other factors, the velocity of sound is a constant, and the wavelength is inversely proportional to the frequency. This means that lower frequencies have longer wavelengths, while higher frequencies have shorter wavelengths. Since a lower frequency fluctuates with slower air particle motion, the energy loss due to its movements will be smaller, resulting in lower frequencies retaining a higher amount of energy over time. In other words, it’s more difficult to reduce lower-frequency sounds than those with higher frequencies.
The absorption coefficients of two different types of acoustic foam are presented in the table below. The absorption coefficient essentially measures how effective the material is at reducing the amount of sound reflected from its surface. The closer the absorption coefficient is to 1, the more absorptive the material is. The coefficient is defined for each octave band from 125Hz (a warm bass sound, the fundamental part of a male voice), up to 4000Hz (a high-pitched, tinnitus-like frequency).
We can see that acoustic foam is particularly good at handling the higher-end frequencies.
Several styles of acoustic foam are available. Whilst aesthetics play some role in determining the best option for you, there are also subtle differences in performance which we will discuss below.
- Egg shell
Eggshell acoustic foam panels are designed with uneven smooth level changes to add diffusion to scatter and absorb more energy.
- Flat
This is the plain acoustic foam panel. This has no diffusion and is less efficient in absorption than the egg crate design, however, some people may find them more aesthetically pleasing.
- Wedge
Wedge acoustic foam panels are similar to the egg crate design as a concept. But if you observe the smoothness of the diffusion surface, the wedge design has more sharp edges than an egg create.
Acoustic Foam Density
Density is another critical factor when answering the question ‘how effective is acoustic foam’. Acoustic foams are usually made of melamine foam, which can be found in densities of 8kg/m3 to 25kg/m3. Increasing the density of the material decreases the number of air cavities within the foam, changing the main absorption technique from diffraction to refraction and exhibiting more absorption in low-frequency absorption than a foam with less density. The thickness of the foam is also essential in absorption. The lower the frequency, the higher the wavelength, meaning a thicker acoustic panel is required to absorb it.
Since the absorption performance of acoustic foam is comparatively limited than other materials, the best practice is to get foam panels of the highest density and highest thickness.
Conclusion
To summarise, acoustic foam is one of the most popular and cheapest acoustic treatment solutions and is highly effective at improving the sound quality within a room by reducing reverberation and echoes. Higher-density panels are better at reducing the lower-end frequencies as they utilise the properties of absorption, rather than relying mainly on diffusion, which only works well for the highest frequencies.
