Sound Absorbing Foam Panels For Reduction Of External Noises

Acoustic Soundproofing Foam Reduces External Noises

Acoustic soundproofing foam is predominantly used as the sound treatment foam. It is intended to improve the sound produced inside the recording studio. However, sound absorbing foam contributes to sound insulation from the outside noises as well. Naturally, the foam absorbs sounds hitting it from either front or back side. Contrary to widespread opinion, depending on the source of external noise, and depending on the extent of the wall coverage, sound absorbing foam panels can be used for effective sound insulation from the outside noises.

Sources Of External Noises In A Recording Studio

The sources of external noises vary. In buildings, the main sources are vibrations of the windows and vibrations of the walls. Vibrations of the windows are caused directly by the outside noise that reaches the window panes. Vibrations of the walls are often caused by the vibrations of the frame of the building. The vibration of the frame, in turn, is caused by the vibration of the surrounding earth. The earth vibrates because of the nearby traffic, or similar sources.

How Does Acoustic Soundproofing Foam Reduce External Noise?

How can sound absorbing foam panels significantly reduce external noise in a recording studio? As we will see below, for a certain range of frequencies, soundproof foams are a nearly perfect absorber of sound. For noise concentrated in that frequency range, all you need to do is place soundproofing foam panels at the sources of the external noise. These sources could either be the windows, or the walls where most of the noise is originating. You will be placing acoustic soundproof foam on or near the walls and windows anyways for sound treatment. This will automatically reduce the level of external noises as well. Just keep in mind that, in order to reduce the external noise significantly, all sources of noise must be covered with soundproof foam. This should include all the walls, windows and doors. Sometimes this will mean covering all of the wall surface with soundproofing foam panels.

How We Investigated Noise Reduction For Most Popular Sound Deadening Foams

We performed computer simulations of sound propagation through external noise deadening foams.

In simulations that follow we will assume that the noise is coming either from the window or from the wall. We will assume that the noise waves are directed perpendicularly (in the normal direction) to the window or wall surface. This is much different than the sound that is created inside the recording studio. The sound created in the studio hits on the panels possibly from a variety of directions. In order for sound to really be coming perpendicularly from the wall, the foam must be attached directly to the wall. For instance, it could be, as is customary, glued to the wall.

The consequence here will be that the external sounds are incoming toward the bottom side of the soundproofing foam panels, perpendicular to the bottom side of the foam tile. This, again, is much different than the sound generated inside the recording studio. The sound generated inside the studio will reach the top side of the foam panels first.

What type of sound absorbing foam is best to reduce external noises? We have simulated several types of soundproofing foam panels. The foams simulated include: flat foam, regular wedge foam, pyramid foam, narrow wedge, and vertical wedge foams.

Experimentally Established Sound Absorption Coefficients For Popular Acoustic Foams

All types of acoustic foam do a nearly perfect job absorbing noise with frequencies over 800 Hz at predominantly normal incidence. We can see that in the table below.

Source: Webarchive.

Great Absorption Of External Noise Over 800 Hz Of Sound Insulation Foam

These measurements were done based on the sound absorption standard (link) which measures absorption of the sound hitting the top surface of the sound proof foam panel. This is not true for the external noise. Nevertheless, we will still be able to use the above results for frequencies above 800 Hz. We will assume that for frequencies above 800 Hz, absorption of all the commercially available panels is nearly perfect regardless whether the sound is hitting the panel from the top (as measured in the table above) or from the bottom, as in the case of external noise coming from the window or wall. So external noise with frequencies above 800 Hz should be adequately absorbed by any type of sound proof panels above.

Absorption Of External Noise With Frequencies Below 800 Hz

What about frequencies below 800 Hz? Frequencies below 800 Hz are crucial for recording studios because fundamental frequencies of human voice, as well as fundamental frequencies of many musical instruments lie in this frequency region. Likewise, the first few overtone frequencies fall below 800 Hz as well. Microphones will be very sensitive to these frequency region. Therefore, as much external noise as possible must be eliminated at these frequencies.

We see in the table above that some foams absorb as little as 10% of the noise at frequencies around 100-200 Hz. However, absorption itself does not tell the whole story. It is also important what happens to the noise that is allowed through the foam. Does that noise diffuse in different directions, or does it go straight out? The noise that is significantly diffused has a better chance of being absorbed to a significant degree in some other sound absorbing foam before it hits the microphone.

Speed Of Sound Variation In Sound Deadening Foam Below 800 Hz

For frequencies of noise below 800 Hz, an interesting phenomenon occurs. The speed of sound in the foam starts to drop as the frequency drops below 800 Hz. The speed of sound can be as low as 140 m/s at 200 Hz and as low as 90 m/s at 100 Hz. For such low speeds of sound, much lower than speed of sound in the air, strong internal reflection of sound can occur when the sound or noise is approaching the foam panel from the bottom side.

We performed a number of computer simulations to that regard, using bending of light simulation software at PHET. The simulations were designed to represent bending of light. However they apply to bending of sound waves as well as long as the speed of sound is properly chosen. This is because laws of reflection and refraction apply equally to both sound and light. The simulations we had access to correspond to sound frequency of 200 Hz in the sound absorbing foam and to speed of sound in the foam of 140 m/s. They correspond to speed of sound in the air of 330 m/s.

Due to low speed of sound for frequencies below 800 Hz, internal reflection of the sound can still cause significant absorption of sound even though the sound absorption coefficient for such frequencies is low.

Let’s look at how different shapes of sound deadening foam can cause different levels of internal reflection. Bear in mind that large internal reflection for certain foam shapes will cause sound to spend longer time, and longer paths inside the sound deadening foam, which in turn will increase the total sound and noise absorption.

We’ll look into these sound deadening foam shapes: Flat, pyramid, regular wedge, narrow wedge, vertical wedge.

Flat Acoustic Sound Absorbing Foam: Some Internal Reflection, No Diffusion

We simulate sound propagation in flat foam such as Auralex Sonoflat, pictured above. The figure below shows the simulated sound rays.

 

The figure shows a simulation of paths of five initially parallel sound rays. All five rays are coming from below in the direction straight up, normally to the bottom surface of the flat foam. The internal reflection is present but most of the sound will just go through the flat foam once and exit straight up, with no diffusion whatsoever.

Pyramid Wedge Sound Absorbing Foam: No Internal Reflections, Diffusion In Four Directions

In the figure below

 

we simulate sound propagation in a pyramid sound deadening foam. Speed of sound is 140 m/s. As you can see, the sound rays are penetrating into the foam from the bottom side, below. Also, notice that, as the sound ray hits the pyramid edge, it refracts, and leaves the foam and enters the room. The refraction will either be to the left or to the right, depending on what pyramid foam surface the ray hits on. There is no internal reflection at this speed of sound. We do get diffusion of sound, both to the left and right as pictured. In addition, in the pyramid sound absorbing foam the diffusion in the third dimension out of the screen and into the screen is possible. Altogether there are four directions in which sound will diffuse into, left, right, in and out.

Regular Wedge Sound Absorbing Foam: No Internal Reflections, Diffusion In Two Directions

The picture of the simulated sound propagation is the same as in figure above. By “regular wedge” we mean that the angle at the top vertex of the wedge is rather large, perhaps as large as 60 degrees or larger. We will contrast this with the “narrow wedge” foam which will have a top vertex angle less than 60 degrees. The noise will propagate straight up through the foam only once, and then refract on transmission into the air in the room. The sound will diffuse into two directions only, left and right.  You can see that in the figure above. There will be no diffusion in and out of the screen. Therefore, there is less diffusion with regular wedge sound absorbing foam.

Narrow Wedge Sound Proof Insulation Foam: Two Internal Reflections, Wide Diffusion In Two Directions

We simulated sound propagation through a narrow wedge sound proof insulation foam. You can see the result of the simulation in the figure below.

Notice that the sound enters the foam from below, internally reflects off the side of the wedge, and then internally reflects once again on the other side. Afterwards, sound exits close to the top vertex of the wedge, and at a relatively large angle. Because of two internal reflections, and the relatively long path through the wedge, the sound absorption will be quite good. In addition, the large angle diffusion will cause the noise from the outside propagate along the walls for a longer while. This gives more absorption of the noise along the studio walls before reaching the microphone.

Vertical Wedge Sound Proof Insulation Foam: Three Or More Internal Reflections, Complete Noise Bounce Back

Our simulation of a vertical wedge sound proof foam in figure below

 

shows a possibility of a complete internal reflection from the slanted side of the wedge, followed by the internal reflection off of the vertical side. The two internal reflections send the sound back into the foam. This forces the sound to travel a significant distance through the foam. Then, the sound reflects off the bottom, and then again reflects internally off the slanted side of the wedge. After that the sound goes straight down, that is, in the direction back to the wall. At this point the sound can either go back out through the wall, or go on a repeated internal loop through the foam. Repeated long internal loops will cause significant absorption in the foam.

We have seen (ref Jones) that the speed of sound goes down as the frequency of sound goes down from 200 Hz toward sub-100 Hz. This means that this internal reflection will play a role for sub-200 Hz frequencies. For higher frequencies, however, the complete internal reflection as depicted in figure above will turn into partial internal reflection and partial transmission. Partial transmission and partial internal reflection will make overall absorption of external noises as studied here less pronounced. This will happen for frequencies higher than 200 Hz.

Conclusion: Best Sound Absorbing Foam To Absorb External Noise, Based On Noise Absorption And Noise Diffusion

Based on our assumptions of the noise coming from the wall and impinging normally from the bottom of the sound absorbing foam, it appears, that for absorption of frequencies of 200 Hz and below, the vertical wedge foam will be best. See our review of vertical wedge acoustic foam here. For frequencies higher than 200 Hz, the internal reflection effect will be diminished. Then, the absorption will be strongest for the foam with the most volume given the same thickness. For higher frequencies, the flat foam will win out.

Another consideration is external noise diffusion. When the foam’s geometry is causing high diffusion of noise, it increases the chance of the external noise hitting on other sound absorption foams in the recording studio. This adds to overall noise absorption. The narrow wedge sound proof insulation foam has the highest noise diffusion among the foams studied in this post. Find the review of our recommended narrow wedge soundproof foam here.

As you can see, different foams will absorb different amounts of external noise. Also, they will diffuse the external noise into the room differently. Vertical wedge foam will absorb the most external noise, while the narrow wedge will diffuse the external noise the most.

When designing your recording studio, you should take these result into account, depending on the severity of the external noise.

In addition, we recommend different types of soundproofing foam in different parts of the recording studio. That way you will achieve the best absorption of sounds generated in the studio. You will reduce echo, flutter, and resonant standing wave buildup as much as possible. We go into more detail about this and present our recommendations here.