Acoustic foam is a common product, often recognized by its unique shape — typically that of an egg carton. Why is this funky shape used to help with noisy environments? Why not just foam pads or something more simple?

Before we jump into the why, let’s look at what acoustic foam actually does and does not do.

First and foremost, acoustic foam does not block sound. Although it's frequently associated with soundproofing(Source: CC BY 2.0 Victor), more often acoustic foam is used along with other things to create a noise free room. There are other elements that do block sound, but not foam. A good analogy to understand how acoustic foam works is waves and ripples in water.

Imagine a wave hitting a solid flat wall in a body of water — when the wave hits the solid wall it doesn’t just disappear, it bounces off. When you see this, it is obvious some, if not most, of the wave has been reflected back. If you were to pay attention, you would notice that the flatter and harder the wall, the more of the wave gets reflected back.

Let's go back to sound and apply this same thought, have you ever heard an echo? Perhaps in the bathroom, or in an empty room with hard floors or outside when you are near a rock cliff. Echoes happen when you are around hard flat surfaces for the exact same reason that waves reflect off of hard flat walls. While foam doesn’t block sound, hard flat surfaces actually do — they just also happen to reflect sound. A concrete wall will help keep sound outside the room from coming in and also keep the sounds in a room from getting out, while at the same time reflecting them so they echo. This is the reason that gymnasiums and churches tend to echo.

That foam is basically the exact opposite of a hard flat surface. Now think about what would happen when a wave hits an object with the physical properties of acoustic foam. You can imagine the wave in water again here if you want to. The wave doesn’t simply get reflected back, instead it gets bounced all over and spreads out due to the bumpy patterns in the foam. The wave is also partially absorbed, but we will talk about that more in a second.

If you have ever shopped for acoustic foam you may have also noticed it doesn’t all have the same pattern. The patterns in acoustic foam are actually designed for different kinds of noise and noise levels. Some foams are better for voices, while others may be better for music or street noise. It all comes down to the size, texture and thickness of the foam being used

Like a wave in the ocean, sound carries energy — it's in the form of a pressure wave, and that energy needs to go somewhere. In the case of acoustic foam, the energy is taken care of in two ways. The first way occurs as discussed above: the sound is spread out due to the uneven surfaces of the foam. Not only does acoustic foam have an obvious pattern, but the foam itself is made of many little bubbles that further spread the sound wave. The egg carton-like shape means that when the sound waves reflect, some them are likely to hit the foam, again causing them to spread out more. This spreading out of the sound wave is one way they are dampened — but we haven’t gotten rid of the energy, just made it diffuse.

(Source: CC BY-ND 2.0 David Poe)

The other method involves the dampening property of acoustic foam, which absorbs acoustic energy. In this case, the pressure wave of the sound is converted into mechanical movement of the foam and the particles that make up the foam. If you have ever held a balloon with loud music nearby, you would have felt the vibrations when some acoustic energy was turned into mechanical energy. For foam, the same thing happens, except that the nature of the foam acts like a shock absorber to further turn that mechanical vibration into a very small amount of heat energy.

So acoustic foam does two things — it disperses the sound so no one spot is very loud, and it converts sound energy into heat energy. If you have a room lined with acoustic foam then you will get even more sound dampening as the reflected sound is further absorbed by the foam. If you really want a sound proof room, then you should first construct it with hard flat sound blocking walls, and then line the inside of the roam with acoustic foam.