Soundproof Foam Panels Placement In Home Recording Studio, Part 1, Normal Incidence

We will discuss flat soundproof foam panels, pyramid soundproof panels, and a variety of wedge soundproof foam panels. We will find out how each of them can best contribute to the improvement of recordings in your sound recording studio by removing room-related distortions in the recordings.

By simulating a variety of angles of incidence for each of the different geometries of foam panel surfaces, we find out which foam geometry is best suited for a specific location in the room.

Reflection-based Distortions That Acoustic Soundproof Foam Can Reduce

The types of distortions that can ruin your recording discussed here include echoes, reverberation, and standing waves. Reflection of sound from the walls and other objects in the recording studio causes all of these three distortion effects.

For instance, wall-to-wall standing waves build up after several bounces of the sound from the studio walls. After several bounces, the wave begins to constructively interfere with itself. This interference creates strong standing waves at specific frequencies that are resonant in the room. These resonant frequencies become disproportionally represented in the recording. They distort the sound balance.

Echoes, as another example, are, again, caused by multiple reflections of a short sound wave from walls. Echoes cause multiple repeated pickups of the original sound signal, possibly attenuated, in the microphone.

All of these distorting effects are a consequence of sound reflection. It is therefore plausible that reducing sound reflection from walls in the recording studio will possibly reduce all of the reflection-based distortions: echoes, reverberations, and standing waves.  One of the best ways to reduce reflections is through judicious placement of soundproof foam panels.

Soundproof Foam Panel Types For Distortion Reduction Compared In This Post

Acoustic soundproof foams have been used as acoustic treatment for a long time. They have proven to be an efficient means of improving the sound of recordings both in home studios and in professional recording studios.

These are the types of acoustic soundproof foams that we will compare in this post: Pyramid Acoustic Treatment Foam, Regular Wedge Acoustic Foam, Narrow Wedge Acoustic Foam, Vertical Wedge Foam, and Flat Soundproof Acoustic Foam.

Superb Acoustic Foam Soundproofing In The Frequency Range Above 500 Hz – 800 Hz

We will focus in this post on performance and placement of various soundproof foam panels based on their absorption properties for frequencies lower than 500 Hz. These frequencies are important as the fundamental frequencies of both male and female voices. The sub-500 Hz frequencies are also fundamental frequencies of many musical instruments’ low-to-mid tones. As we can see from the table below, sound absorption in wedge-type foam and pyramid-type panels is superb for all sound frequencies above 800 Hz. This holds true for flat-type foam panel of 2 inch thickness as well.

 

Check out the above table of sound reduction coefficients. Observe that a 2 inch thick wedge acoustic damping foam will absorb over 90% of sound in normal incidence conditions for frequencies above 500 Hz. A 2 inch thick pyramid soundproof acoustic foam will absorb over 90% of sound in normal incidence conditions for frequencies above 800 Hz. A thicker, 3 or 4 inch foam will absorb over 90% at even lower frequencies.

So, as far as over 800 Hz frequencies of sound are concerned, many types of acoustic damping foams will work well, even under the normal incidence conditions. Let’s turn our attention now to sub-800 Hz frequency range.

Physical Properties Of Soundproof Foam Panels At Sub-800 Hz Frequencies: Speed Of Sound, Absorption

Check out the table above. The absorption coefficient for normal incidence is severely reduced for frequencies below 800 Hz. The absorption starts from nearly perfect 100% absorption of the sound above 800 Hz. It ends up at around 10% absorption in the 100-200 Hz frequency range. There is a great reduction in overall absorption as the sound frequency drops.

Absorption behavior in the frequency range below 800 Hz therefore becomes crucial for the distortion elimination in a sound recording studio. We will see here how the right placement of the right type of soundproof foam in the recording studio can have a big impact on noise and distortion elimination.

Speed of sound: It is interesting that acoustic foams are very peculiar when it comes to speed of sound propagating through them. Foams are made of solid frame and are filled with air, yet speed of sound in acoustic foams is lower than both speed of sound in the air and speed of sound in bulk solid frame. Furthermore, speed of sound diminishes with frequency, which is not common in solid materials, liquids or gases. Figure below shows speed of sound in acoustic soundproof foam adopted from this source.

Figure shows approximate speed of sound in foam as a function of sound frequency. For our purposes in this section we will work with a representative frequency of 240 Hz. This implies the speed of sound of 140 m/s. This is significantly lower than speed of sound in air which is approximately 340 m/s.

Refraction And Reflection Of Sound On Flat Soundproof Foam-Air Boundaries

The much lower speed of sound in soundproof foam panels has consequences. One is strong refraction of sound waves that hit the acoustic foam surface. Refraction means bending of the direction of the sound wave when the sound wave passes the air-foam boundary. The lower the speed of sound in foam, the more the sound wave will bend.

Another important behavior is sound reflection off of the air-foam boundary. In reflection, the sound bounces back into the air without entering the foam.

Both the reflection and the refraction behavior in acoustic soundproof foams can be simulated easily. We used an online simulator on PHET. This is intended for simulations of light bending upon refraction and reflection. However, the laws of refraction and reflection are similar for any type of wave. In particular, the bending angle in refraction depends on the ratio of wave speeds in air and in the acoustic foam. Likewise, the angle of reflection is the same as incident angle for reflection of both sound and light waves.

Illustration Of Sound Refraction And Reflection On An Air-Soundproof Foam Boundary

The illustration below shows a flat foam, in gray-blue. In the image, you can see refraction and reflection of a sound wave, in shades of red. The reflection happens on the boundary between air, in white color, and the flat soundproof foam. Dark red stands for high intensity of sound, while the lighter red line represents an attenuated sound wave.

 

The sound wave originates at the source on top right. The sound ray is a directed line representing the position, the direction of motion, and the trajectory of a sound wave. We assume in this picture that the sound is traveling in air with the speed of 330 m/s and in foam with the speed of 140 m/s.

Below the sound source is the flat variety acoustic soundproof foam. When the sound ray first hits the surface of the foam, it is partially reflected back into the air. This reflection obeys the law of reflection. This is represented by a fainter red line pointed back up, and still to the left. At the same time, a part of the wave is refracted into the foam, following the law of refraction. This is represented by a dark red line directed in the direction toward the bottom of the foam. Next, the sound ray hits the boundary between the bottom of the foam and the air below or the wall below. The wave is internally reflected and starts to move back up toward the room. A few more reflection/refraction occurrences happen until the entire sound wave exits the foam, either back into the room, or out through the wall.

One can clearly envision that, the longer the path of the sound ray through the foam, the more sound will be absorbed.

Investigating Reflection, Absorption, And Diffusion Of Sound For Acoustic Soundproof Foams

Given the accessible tools, we chose to investigate reflection, absorption, and diffusion of different shapes of acoustic soundproof foams. We focused on a foam that has speed of sound of 140 m/s, which, in turn, corresponds to frequency of sound of 250 Hz.

In this and in the accompanying post, we consider five different types of soundproof foam, pyramid, regular wedge, narrow wedge, vertical wedge, and flat soundproof foam. We perform computer simulations of sound waves with the intent to determine which type of foam absorbs the most sound at a given angle of incidence.

We vary the angle from 0 (normal incidence) to 15, 30, 45, 60, to 75 degrees. Then, we compare the different geometries of acoustic soundproof foams. We compare both the length of sound path inside the form, and by the diffusion of the reflected sound.

Why look at the length of the sound path inside the foam? Because, the longer the path, the more of the sound will be absorbed.

Why look at the diffusion of the reflected sound? High and random diffusion is important because it prevents the creation of prominent standing waves through back-and-forth reflections in the recording room.

Normal Incidence Results For Best Absorption And Diffusion

Based on speed of sound assumed above, we find that the best performing geometry is narrow wedge, followed by vertical wedge.

Narrow Wedge Acoustic Treatment Foam Results For Normal Incidence

Let’s look at the pattern of a sound ray entering and bouncing off a narrow wedge acoustic treatment foam in figure below.

The original sound ray impinges straight down onto the steep incline of the narrow wedge. It then splits into the refracted part, going into the wedge, to the right, and the reflected part, bouncing off the steep wedge.

The refracted part of the sound wave bounces off the side of the panel, then bounces off the bottom of the panel again, and makes a whole loop through the wedges, then another, and possibly another, before it finds its way out of the foam. The path that circles multiple times around the foam provides for significant absorption due to its length.

The reflected part of the sound wave refracts into the foam through the neighboring wedge on the left, makes one bounce off the bottom of the foam, and exits through a wedge four wedges away, at an angle. This means that the reflected wave is not reflected straight back. Instead, it contributes to diffusion of the reflected waves.

Overall, as seen in the image, the diffusion is quite significant. We see about 10 distinct directions of outgoing sound rays. This is a sign of significant diffusion of the reflected waves.

Vertical Wedge Sound Foam Results For Normal Incidence

Figure below

shows bouncing off of a sound ray off of a vertical wedge acoustic dampening foam. One of the surfaces of the wedge is vertical, thus the name “vertical wedge”. The refracted part of the sound ray is stronger than the reflected here.

The refracted part enters the foam toward the right, then bounces off the bottom of the foam, and then makes a whole loop through the wedges, another half loop bouncing off the right side, the bottom, and finally finds its way out to the left side. The length of this sound ray is significant as well, indicating good absorption under the normal incidence angle.

The diffusion of the reflected wave in this case is not very significant, but there is no ray reflected straight back.

The other acoustic dampening foam geometries, flat foam, regular wedge, and pyramid, did not perform as well in this simulation.

Conclusion: Our choice for acoustic soundproof foam with best absorption of low frequencies/best diffusion of low frequencies for normal incidence is narrow wedge type foam.

Check out our review of one of the best such narrow wedge soundproofing foams.
Our review of a runner up, vertical wedge soundproofing foam is here.

For simulations on what foam works better at incident angles other than normal incidence, see review of foam performance under oblique incidence here. Reading this part is important. You will be able to fully understand our choices for placement of different types of soundproof foam panels in different places of the recording studio.

Final Conclusion: What Soundproof Acoustic Foam To Purchase And Where To Place It

Figure below

 

shows a top view of an example recording studio. The front wall is on the left, the side walls are on top and the bottom, and the rear wall is on the right. The circle on the left represents the position of the source of sound. The thin rays originating in the circle represent the directions of sound rays originating at the source of sound such as loudspeaker. The black rays point to the positions on the walls where the wedge foam should be placed.  You should turn the vertical side of the wedge toward the source of the sound. The orange rays point to the positions of narrow wedge soundproof foams. Again, the orange lines represent the lines of the ridges of the wedges.

Positioning Soundproof Foam Panels On The Side Wall

Figure below

shows a side view of a side wall belonging to the same recording studio. Here we see the flat side of soundproof panels. We also see their recommended orientation. The black panels represent vertical wedge soundproof foam panels while the orange panels represent narrow wedge soundproof foam panels. The position of source is the same as in the top view picture, about one fifth of the way from the left (rear) wall to the right (front) wall. The panels on the side wall closest to the sound source should be the vertical wedge panels. Following them should be a layer of narrow wedge panels. This, again, is followed by the layer of vertical edge panels.

The height of the sound source is assumed to be at the middle of the height of the room in the figure. At this level, all of the ridges of both vertical edge and narrow wedge soundproof foam panels should be vertical, except for the vertical wedge panel closest to the source of sound which should be horizontal. As stated before, all vertical edge sides should be facing the source of sound.

You should turn the soundproof foam panels that are either higher or lower than the height of the sound source in the direction toward the source of sound. The side of the wedge (vertical side in case of vertical wedge panel) should be directed toward the source of sound. This means that some of the panels will be tilted as shown in the figure.

Positioning Soundproof Foam Panels On The Rear Wall

Figure below

 

shows the optimal positioning of the soundproof panels on the rear wall, behind the sound source. This is an example only. The single sound source is located in front of the rear wall, at the mid height, and at the mid width of the rear wall, approximately half of the height away from the rear wall. You should consider the typical positions of sound sources in your case. These sources include various musical instruments, loudspeakers, or vocalists.

Based on the optimal choice of soundproof foam panels, we suggest the optimal positioning of the foam panels. Optimal positioning is as follows: In the center of the rear wall you should place the vertical wedge panel, with its ridges oriented horizontally. Up, down, left and right of the central vertical wedge panel, you should place four narrow wedge panels. These should have their ridges perpendicular to the line connecting them to the center of the central panel, see the figure. This placement leaves empty space at the edges and corners of the room. There, you can place supplemental soundproofing materials. A great choice would be to use bass traps, see Project 2 Roominator Kit Review.