How Does Soundproofing Foam Work (& Does It Work Both Ways)?

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  • How does acoustic foam work?
  • Does soundproofing foam work both ways?
  • Learn about the physics behind acoustic panels
  • Also, check out our useful guide to acoustic foam panel placement!

Soundproofing means preventing sound waves from entering or leaving a room. It is generally a very difficult and expensive task.

Soundproofing foam may help a little bit in achieving this goal, but a lot of the frequencies acoustic foam panels target would also be absorbed or reflected by the wall, ceiling, or floor.

This applies to sound leaving and even more to sound entering the room. Therefore, regarding their soundproofing capabilities in the classic sense, foam panels are rather limited.

However, soundproofing foam does reduce the overall acoustic energy in the room and, thus, the amount of noise that bothers your neighbors or roommates.

Why are acoustic panels a great tool for your home studio? Let’s find out!

How Does Soundproofing Foam Work?

Soundproofing foam makes use of a physical concept called dissipation. In this process, acoustic energy is transformed into heat.

It occurs mainly in porous materials; chances are it’s already happening around you in textiles like carpets or curtains.

Imagine the sound waves bouncing around in the carpet pile, causing friction and consequently transforming the energy and absorbing the sound.

Of course, now there are much more effective materials than carpets, specifically designed for this purpose.

Especially mineral fiber and polyethylene foam come with excellent absorption parameters.

However, no matter how suitable the material is for the job, one parameter makes or breaks the absorber: size. Or, to be more specific: thickness.

Let’s take a closer look at the physics behind this.

The Physics Behind Acoustic Panels

When a sound wave hits a wall, there a three possible outcomes that are relevant to us:

  1. Nothing: the sound wave simply passes through the wall as if nothing happened.
  2. Absorption: the sound wave is absorbed, for example, through dissipation.
  3. Reflection: the sound wave is reflected into the room. Following the law of angle of incidence is equal to the angle of reflection.

Which outcome it is, is depending on the frequency of the sound wave and their corresponding wavelength.

As you can see, the wavelength describes the actual physical longitude it takes for a wave to complete one cycle.

The lower the frequency, the longer its wavelength.

Here is a table of some common frequencies and their corresponding wavelengths:

Frequency Wavelength

20 Hz

17 m
100 Hz 3.4 m
1 kHz 0.34 m or 34 cm
2 kHz 0.17 m or 17 cm

So what is the connection between all this wavelength talk and your acoustic panels?

To influence a soundwave, an object has to have a depth of at least 1/4 of the wavelength of that specific soundwave. 

So, no matter if you are looking at absorption or reflection: if the wall, soundproofing foam, or carpet doesn’t have the depth of 1/4 of the wavelength, it will not affect the soundwave.

That is why sounds through a wall are muffled, and you only hear the bass of the party next door.

High frequencies with short wavelengths are easily absorbed by pretty much anything, but bass frequencies with long wavelengths move through walls as if they don’t even see them.

How Do I Achieve Soundproofing?

As the introduction states, making a room soundproof is complex and expensive. With what we have covered so far, one solution seems quite obvious: thick walls.

Unfortunately, that is not a parameter that we can usually control.

There are options, though: you can put up drywall with sheeting in between or artificially thicken your walls with mass-loaded vinyl (mlv).

However, the effort and cost you put into a project like this rarely correspond with the usage of the room. 

The most professional way to achieve soundproofing in control rooms or recording studios is a room-in-room concept.

Imagine the room floating within the other room, cushioned by the air between the walls. If done correctly, no sound will enter or leave the room.

Again, for most of us, a project like this is completely out of reach. So we put together a short list of things you can do that work on a smaller scale (and with a smaller budget):

  • Door Seal Kits can make a difference for your roommates or family members
  • Try to schedule your recordings to times when there is less traffic and less noise
  • Work out times with your neighbors where you bother them less
  • If you want to work outside of those hours, invest in some quality headphones

What Can I Do With My Sound Absorbing Foam?

Acoustic panels are a great tool for creating a more professional listening environment. The process of shaping the acoustics in your room like that is called Acoustic Treatment.

Check out our article on Soundproofing vs. Acoustic Treatment for even more details!

At first glance, it seems like sound-absorbing foam can only do one thing: absorb sound (duh..). But used correctly, you can solve multiple issues in your listening environment. 

Reverberation Time

The acoustic foam panels absorb sound waves and shorten the reverberation time in your room. Usually, an RT60 of 0.25-0.4s is desired in control rooms, but opinions differ. 

Sidenote RT60: Stands for Reverberation Time 60 and describes the time it takes until the sound pressure level in the room has decreased by 60dB. This corresponds to 1/1000 of the initial sound pressure level.

Many acoustic panels will come with a table, including their absorption qualities depending on the frequency band.

The physical symbol for this value is α (alpha). So if your foam has an α of 0.6 at 1kHz, it will absorb 60% of the soundwaves in this frequency band.

In general, the absorption quality increases with frequency for the reasons we covered above.

With this information, you can dive into the mathematics behind it and calculate the amount of sound-absorbing foam you need.

One more tip: you can “trick” physics into thinking your acoustic panel is thicker than it is by mounting it at some distance to the wall.

This will increase the low-frequency performance of your sound-absorbing foam.

However, to target those really low frequencies, you will have to work with acoustic elements called bass traps.

Early Reflections

The second issue acoustic panels can solve is early reflections.

While they absorb sound and reflections at any point in transmission, we are most concerned about early reflections, which arrive at the listener between 1ms and 50ms after the initial signal.

Those sound waves can create a (false) perception of room size and coloration in the sound.

By strategically placing sound-absorbing foam on your walls and ceiling, those reflections can be eliminated or at least attenuated by an amount that makes them more or less irrelevant.

So how do you place them?

Take a mirror and slide it across the ceiling and walls. If you can see a speaker from your listening spot, you want to put one of your acoustic panels there.

Don’t forget the back wall!

Sound Absorbing Foam vs Diffusers

Acoustic panels come in many different shapes and forms. The most common are burl or pyramid foam panels.

The structure of these panels might lead to some confusion with other acoustic elements called diffusers.

While acoustic foam (no matter which form) is made to absorb, diffusers are designed to reflect sound waves in a diffuse matter.

To achieve this, wood or other reflective material blocks are placed irregularly on the acoustic element.

Sound waves arriving will then be scattered in many different directions, eliminating direct reflections and breaking the law of angle of incidence is equal to the angle of reflection. 

So why would I use diffusers if I can solve different issues with sound-absorbing foam?

We have talked about the desired reverberation time in a control room.

Now, if you are in a small bedroom with lots of absorbent materials and some acoustic elements, you might already have an RT60 that is quite low, but you are still dealing with direct reflections e.g. from your back wall.

Diffusers will get rid of those early reflections, but instead of sucking the sound energy out of the room, it will be scattered and adds to the diffuse sound field, giving your room a more natural feeling.


Are Acoustic Panels Two-Way?

The soundproofing capabilities of acoustic panels are limited, but if they have an effect, it will prevent sound from leaving the room.

They are fantastic tools to absorb sound waves on the inside. Acoustic elements that might have a small effect on outside noise coming in are bass traps.

Which Foam Type Is Best For Soundproofing?

None in the classic sense. Polyethylene and Basotect are great for acoustic treatment, though!

Does Soundproofing Work Both Ways?

If done correctly, soundproofing will prevent sound from leaving and entering the room.

Which Shape Of Acoustic Panels Is Best?

It depends. To achieve the best result, check out the absorption properties of the panels for different frequency bands and find the one that is most suitable for you.

Don’t confuse sound-absorbing foam with diffusers.

Before you go, check out our guide to the 7 Best Acoustic Foam Panels For Home Studios!