Sound Deadening Foam Panels With Great Diffusion: Auralex Acoustic Pyramid Foam Review

Auralex Acoustic Pyramid Foam is an all time classic when it comes to sound deadening foam panels. You will find this foam in anechoic chambers and in professional recording studios. It covers the walls of many home recording studios as well.

The purpose of this review is to fully investigate the real usefulness of Auralex Studiofoam Pyramid. You’ll find out what type of pyramid foam is best for your recording studio, and where the best placement for Auralex Acoustic Pyramid foam is. In addition, we’ll suggest how to supplement the pyramid foam with other types of sound deadening foam. You can supplement with narrow wedge sound deadening foam panels, regular wedge foam panels, and vertical wedge panels. Also, we will see that acoustic pyramid foam plays well with home recording studio package offers such as Auralex Roominators.

Questions About Auralex Studiofoam Pyramid Answered

Are you equipping a professional recording studio, or want your DIY home recording studio sound like a big, professional studio? Then you should be taking advantage of the Auralex Pyramid sound deadening foam. But you still have questions, such as:

• Is it possible to get great sound treatment with Auralex Pyramids?
• How much foam is right for my space?
• Where on the walls and ceilings should place the foam panels?
• What thickness of the pyramid sound deadening foam is best?
• How to orient the panels for best absorption?
• What are the ways get good absorption at lower frequencies, such as fundamental frequencies of human voice?
• How to maximize the available space in the recording studio?

These questions and more will be answered in this post. Keep reading!

Dimensions, Thickness, Studiofoam Sound Deadening Foam Material

Auralex Acoustic Pyramid Foam comes in two thicknesses, 2 inch and 4 inch thickness. This is important as, especially for low frequencies absorption. At low frequency, increased thickness will significantly increase the absorption. On the other hand, 4″ thick foam will take up a lot of room on the walls. Thick foam layer could significantly reduce the amount of available space in the studio.

The panel dimensions are 2 ft by 2 ft for both 2 in and 4 in Studiofoam Pyramid Foam. According to the company, the wedges in 4 in. foam are 3 inches tall while the wedges in the 2 in. foam are 1.5 inches tall. The foam is made of durable, polyurethane open cell foam material. Open cell structure is important for effective sound absorption. Even though Studiofoam Pyramid foam is termed “high density”, it is still very light and easy to handle and install.

Low And Variable Speed Of Sound In Sound Deadening Acoustic Foams

Speed of sound in open cell polyurethane foams behaves different than in other materials like solids. Speed of sound in a foam depends strongly on frequency of the sound. This differentiates it from other forms of matter such as solid, liquid, or gas. In these, speed of sound is to a large degree independent of frequency. The figure below

schematically shows the dependence of the speed of sound (c) in the foam on frequency (f). Speed of sound is measured in meters per second. Frequency is measured in Hertz (Hz). We can see that, speed of sound for all frequencies up to 4000 Hz is less than speed of sound in the air, which is around 330 m/s in normal conditions. Also, for frequencies below 1000 Hz, the speed of sound in this foam will drop dramatically with frequency. We will simulate such low speed of sound and show that it influences the absorption and reflection properties such as diffusion of the reflected waves, below. Low speed of sound allows “packing” of a larger portion of the sound wavelength into the foam, which is claimed to increase absorption as well.

Noise Reduction Coefficient, NRC, And Sound Absorption Coefficients, Experimentally Determined

In Table below, we present the results from the measurements of noise reduction coefficient, NRC, and sound absorption coefficients at a variety of sound frequencies. They were measured by Riverbank Acoustical Laboratories using standardized procedures ASTM C423-90a and E795-92. The method used is called Room Reverberation Method.

Comparison: 2″ Vs. 4″ Foam

We observe that both 2 in. and 4 in. Auralex Studiofoam Pyramid foams have excellent sound absorption coefficient. It has nearly perfect value above 500 Hz for the 4″ Pyramid Foam and above 900 Hz for the 2″ Pyramid Foam. The absorption coefficient drops down below these frequencies. Overall, the 2″ thick Auralex Studiofoam Pyramids has noise reduction coefficient, NRC, of 0.80 and the 4″ thick Pyramids has NRC of 0.95. NRC is a special average of sound absorption coefficients for discrete frequencies between 125 Hz and 4000 Hz.

Overall, we can see that, for the relevant frequencies below 500 Hz, the 4″ Pyramid Foam has up to three times higher noise absorption coefficient. This means three times more absorption at these frequencies, despite only twice the thickness. Also, as observed in the pyramids vs wedge acoustic foam comparison post that the sound absorption coefficient is highly variable in the popular 2″ pyramid foam. In that foam it varies by 50% between 0.09 and 0.13 for frequencies between 125 Hz and 200 Hz. We also observe that sound absorption coefficient is relatively constant between 0.21 and 0.28 for frequencies between 100 Hz and 160 Hz in the 4″ Auralex Studiofoam Pyramids foam.

It is important to note that these sound absorption measurements were made under specific conditions. The measurements predominantly measure absorption under normal incidence. Here the sound impinges onto the foam surface perpendicularly, under a right angle. We will look into simulations of these foams under a great variety of angles of incidence between 0 and 90 degrees.

Pyramid Foam Has Great Diffusion In 3D

Compared to most other popular sound deadening foams such as Auralex Wedge, Auralex Wedgies, the Studiofoam Pyramid foams have better diffusion in 3D. What is the significance of high diffusion? Diffusion is reflection of an incoming sound wave which comes in a single direction, into many outgoing directions. In reviews of other foam geometries we have seen that foams differ widely in their diffusion. However, when the incoming sound direction is perpendicular to the ridge of the wedge in Studiofoam Wedge type foams, the sound will only reflect in directions inside the plane defined by that wedge and the incoming wave direction. Thus the diffusion will be only two dimensional, thus not so high.

Diffusion in 3D with the pyramid foam means that parts of the incoming wave will reflect in directions that do not all lie in the same plane. That is a much more significant diffusion.

High diffusion is important in preventing formation of powerful standing waves which distort the recordings strongly at discrete frequencies.

Analysis Of Reflection “To The Side” In 3D

In the figure below

we illustrate how an incoming wave in a normal incidence can reflect in two different directions depending on where exactly the incoming wave hits the side of the pyramid. As a comparison we present the same incoming wave impinging on a wedge soundproofing foam on the right.

Here we see that the incoming wave refracts, bounces off the bottom of the foam, and exits through the other side of the wedge. It therefore travels back into the room exactly in the opposite direction from where it came from. In pyramid foam, however, that same normally impinging incoming wave will again refract through the side of the pyramid, bounce off the bottom of the pyramid, but then, instead of hitting the other, opposite, side of the pyramid, it hits the neighboring side of the pyramid on the way out. It then refracts out back into the room under an angle that is not normal to the base anymore. That is a perfect example of enhanced 3D diffusion that just does not happen in wedge foam.

Surely, it depends on where exactly the normally impinging wave hits the side of the pyramid. If the incoming wave hits the side of the pyramid close to the middle of the base, then the reflected wave will still be normal, that is, perpendicular to the base, on its way out. That is because that part of the wave will not go out through the neighboring side of the pyramid. Instead, it will go out through the opposite side of the pyramid. However, at least a part of the wave will hit the neighboring side of the pyramid. That part will exit through the neighboring side, and be reflected off the normal, incoming direction.

For More Info…

We have compared diffusion in pyramid foam vs wedge foam in more detail in this post: Sound Proof Foam Panels: Pyramid Acoustic Foam Vs Wedge Acoustic Foam.

This 3D diffusion in normal incidence will be important when we discuss below where exactly the pyramid foam is best used when turning a room into a sound recording studio.

Absorption And Reflection Off A Pyramid Foam Under Different Angles Of Incidence

We will present simulations of reflection and absorption of sound off of Auralex Studiofoam Pyramid under the following conditions, as an example of what is happening. You can see more information about what software we used in our simulations here: Sound Proof Foam Panels: Pyramid Acoustic Foam Vs Wedge Acoustic Foam.

We will assume, for the sake of giving example, that the speed of sound in the foam is 140 m/s and that the speed of sound in the air is 330 m/s. This will give us correct bending of sound rays in simulations. In reality, speed of sound in foam varies with frequency. For low frequencies it could be as low as 80 m/s and as high as 200 m/s. Further, we will assume that the wave enters on one side of the pyramid and exits on the other, that is, we will consider only two-dimensional reflection and diffusion. The 3D diffusion adds to the total diffusion. The effect of 3D diffusion has been considered above.

Normal Incidence

Figure below

shows a typical pattern of refraction of a normally incident foam onto a pyramid shaped foam. The assumption about the speed of sound is given above. There are multiple reflections, including total internal reflections, and multiple refractions. A good portion of wave energy is reflected straight back, in the direction straight up in the figure. This makes sense as the symmetry and the law reflection demands that the angle of reflection is the same as the angle of incidence.

Again, the image does not include 3D effects, so in reality, more of the wave energy is diffused “sideways”, into the 3rd dimension, not pictured in the image. But still, a lot of sound energy will be reflected straight back. This is the reason why, despite good 3D diffusion, we recommend to use the 4″ Auralex Studiofoam Pyramid foam where the sound waves will likely impact on the wall under the 90 degree angle. This way, the portion of the sound reflected straight back will be minimized. The strong, unwanted standing waves at discrete frequencies will not be present as much as they would be with the 2″ pyramid foam.

15 Degree Angle Of Incidence

Figure below

shows the situation where the angle of incidence of a sound wave is 15 degrees. The incoming wave is starting at the top right corner of the image. As we follow the path of the wave, we see that wave first refracts into the foam on the side of the pyramid. Then the wave reflects internally and totally off the bottom of the foam. After that it makes its way to the left side of the pyramid 2 pyramids away. Finally it refracts back into the air.

Overall, that path follows the law of reflection. This means that the outgoing wave is 15 degrees to the normal, just like the incoming wave was. Needless to say, to such path, another possible path should be added. In that other path the wave refracts through the neighboring side, making its way into the 3rd dimension, out of the plane of the image. This contributes to more sound diffusion.

Figure below

shows a similar 15 degree angle of incidence impact, but emphasizes the path of the part of the wave that initially reflects off of the side of the pyramid. That wave makes its way into the foam by refracting on the side of the neighboring pyramid after initial reflection. The propagation angle at this moment is very flat, or horizontal. This ensures a long path nearly horizontally through the foam. When that part of the sound wave eventually exits the foam and makes its way back into the room, it will be significantly attenuated, or absorbed. In addition to possible reflection/refraction paths presented, again, there will be additional paths out of the plane of the image which will certainly improve diffusion due to a greater variety of reflected angles.

30 Degree Angle Of Incidence

Our simulations of sound rays show similar patterns of reflection and refraction for 30 degree angle of incidence as in 15 degree angle of incidence case. We do not present images here. They are identical as the ones presented in Auralex Studiofoam Wedge Panels Soundproofing Foam Panels Review for the same, 30 degree, angle of incidence.

The major difference to reflection and absorption in Auralex Wedge panels is that the Auralex Pyramids will again reflect into the 3rd dimension. This will definitely increase diffusion. The exact increase of added diffusion on overall absorption is unclear. Assuming the amount of absorption is otherwise the same, the increased diffusion means that the sound will reflect in many different directions. It will have many more opportunities to be absorbed in sound deadening foam on other walls of the recording studio, before it will make its way to the microphone. This way, increased diffusion indirectly increases overall absorption.

45 Degree Angle Of Incidence

Our simulations show that under the 45 angle of incidence, overall, the initially refracted and initially reflected wave both end up traveling through the foam under more horizontal angles, thereby increasing the path inside the foam, and, overall, increasing absorption. So we like the Auralex Studiofoam Pyramid foam more for expected angles of incidence of 45 degrees. Figure

shows the expected refractions and reflections of an incoming wave with angle of incidence of 45 degrees. The wave is assumed not to hit the neighboring sides of the pyramid. It is assumed to stay in the plane of the image. Parts of the wave will certainly leave the plane of the image. This will increase the diffusion into the 3rd dimension in a pyramid foam. This does not happen in the regular wedge foam. We can see in the figure how the initially refracted wave makes it into the foam on the left. The wave then reflects back three pyramids down to the right. The final outgoing angle is 45 degrees to the normal. This obeys the law of reflection.

The other part of the incoming wave which initially reflects off of the pyramid side will, to a large degree, make its way into the next pyramid. It will then internally reflect down to the bottom of the foam. Then it will make another cycle bottom-to-pyramid before it exits the foam. This increases the total path through the foam and thus increases absorption.

60 Degree Angle Of Incidence

Figure below

shows what happens under the 60 degree angle of incidence of the sound wave. Due to the impact into the side of the pyramid being nearly normal incidence, a locally 0 degree angle, the wave refracts straight through. Due to the pyramid top angle, the wave internally reflects straight down, and then from the bottom of the foam, straight back up, and then retraces its way backwards.

The wave exits the foam along exactly the same path it came in from. Therefore, the initially refracted wave will bounce right back in the opposite direction, after one loop back and forth through the thickness of the foam. The initially reflected wave will, due to normal impact, reflect straight back also. Both of these paths are highly undesirable. Reflection straight back will have a huge potential to form short standing waves in the room. These will cause distortions.

The only redeeming feature of the pyramid foam as compared with the regular wedge foam is that, at least some of the wave will not follow the path back as depicted, but will escape into the 3rd dimension, and exit the pyramid on the side, out of the plane of the image. This part of the wave will not be forming standing waves described in the previous paragraph.

Overall, we do not recommend using pyramid foam for expected angles of incidence of 60 degrees.

75 Degree Angle Of Incidence

Figure below

shows what the refraction and reflection pattern that happens when the sound impinges on the pyramid sound deadening foam under the 75 degree angle of incidence. The majority of the sound energy refracts into the foam. This is due to the near-normal incidence onto the side of the pyramid. Then, the wave propagates to the other, left, side of the pyramid and totally internally reflects off of it. It then goes nearly straight down toward the bottom of the foam. There it internally reflects from the bottom of the foam or from the wall. It then follows the symmetric path back to the tip of the pyramid where it exits under the 75 degree angle, for the most part.

Of course, as always in the reflection/refraction images above, we have assumed here that the wave has not hit the neighboring side of the entrance side of the pyramid, but it has hit the opposite side to the entrance side of the pyramid. If the wave did hit the neighboring side, it would travel out of the plane of the image. This would produce a bit more diffusion into out of plane directions.

Overall, looking at the figure, the length of the path of the sound wave under the 75 degree incidence is not very long. Dispersion is not great either. We do not recommend using the pyramid foam for these angles of incidence.

Overall Assessment Of Reflection And Absorption Properties Of Pyramid Foam

As our simulations indicate, pyramid foam such as Auralex Studiofoam Pyramid will perform best in the range of angles of incidence between 15 and 30 degrees. In this range, as well as for normal incidence, it should outperform foams with wedge geometries due to its extra diffusion into the 3rd dimension. We additionally recommend the 4″ pyramid foam over the 2″ pyramid foam for extra absorption.

Placement Of Pyramid Sound Deadening Foam Panels In A Recording Studio

Based on our simulations, above, we can see Auralex Studiofoam Pyramid Foam placed on walls and ceilings in a recording studio predominantly in placement where the expected angle of sound incidence is up to 30 degrees. Even in such placements you should preferably use the thicker, 4″ thick, foam panels. This is because of overall better absorption for important frequencies of about 100 Hz – about 500 Hz. Absorption of these frequencies is up to 3x greater with the 4″ thick pyramid foam.

Fire Retardancy And Durability

Auralex Studiofoam Pyramids sound deadening foam is made of open cell polyurethane foam material that contains no melanine. Lack of melanine makes for good durability. Open cell cellular structure increases the absorption. This is due to energy transfer from sound energy to thermal energy via turbulent airflow through the open cells.

Availability, Colors

Both the 2″ and the 4″ thick Auralex Studiofoam Pyramids sound deadening foam is available in three colors: Purple, Burgundy, and Charcoal.

One of these colors should match the decor of your room and the other sound deadening foams you may place on the walls and ceilings.