Sound Proof Foam Panels For Best Recording Possible
Are you a musician wondering how to achieve best recording studio acoustics? Maybe you are recording your own podcast? You may have tried setting up a recording studio or a sound booth to record your performances. You have then probably come across the unwanted echoes, flutter, or similar noises and sound distortions. Surely you have noticed these distortions degrading or downright destroying the quality of your recordings. You looked around and realized that sound proof foam will be light weight, flexible, and will be easy to install. You have checked them online and you have asked around. Now you wonder what type of sound proof foam panels to use, and how to place these panels in the recording studio or your sound booth. Look no further. You will find actionable advice below.
Ambitious Goal: Detailed Discussion Of Sound Treatment With Soundproof Foam
We have an ambitious goal for this article to compare and contrast two of the most commonly used types of acoustic foam panels. These are pyramid acoustic foam and wedge acoustic foam. We will point out at important qualities you need to look for in both, and which sound proof foam panel works better, given the requirements. You will know what to do to remove the artifacts such as unwanted echoes, reverb, spurious resonances and the like. Your recordings will reach the quality of your live voice or the live sound of your musical instruments. We will review the best available pyramid acoustic foams and wedge acoustic foams. We will also point to a discussion of what type of sound proof foam panels you need and where to place them in the recording room to achieve the very best sound in your recordings.
Important Qualities Of Sound Proof Foam Panels: Sound Absorption Average, Directionality, Diffusion
For quality recordings, the most important qualities of sound proof foam are sound absorption average, noise reduction coefficient, sound absorption coefficient. In addition, directionality of reflected sound waves, and associated diffusion are important.
Sound Absorption Average, SAA, Of Open Cell Acoustic Foam
The most important quality of sound proof foam is the amount of sound absorption. When the sound proof panels absorb just the right amount of sound and at all relevant frequencies, then there is sufficient feedback for the performer, yet not too much of the echo in the recording. Finding balance is the key.
Sound absorption by soundproof foam panels is represented by Sound Absorption Average, SAA. SAA can reach or even exceed the value of 1 for certain frequencies, which guarantees hundred percent absorption. Such high absorption is in part due to open cell structure of the foam. Open cell acoustic foam has foam bubbles that have several open (unfilled) sides. Open cell foam structure allows the air enter the foam as the sound wave hits, thus allowing sound energy conversion into heat as the atoms of air flow past the open cell walls.
Sound Absorption Average and Noise Reduction Coefficient For Pyramid Foam And Wedge Foam
Modern standard in sound absorption measurements is SAA, Sound Absorption Average (Source). SAA has replaced the noise reduction coefficient NRC (Source).
SAA is more accurate and detailed. Both NRC coefficient and SAA coefficiant have values between 0 and 1 where 0 represent perfect reflection of all sound and 1 represent perfect absorption of all sound.
However, SAA or NRC measurement standards do not offer a complete description of absorption properties of sound proof foams such as pyramid shaped foam or wedge shaped foam. The absorption coefficients are measured exclusively around discrete frequencies and they are mostly measured for normal (perpendicular) incidence onto the soundproof foam panels (see Source). In this article we will provide a broader view which, however, will still include the SAA and NRC data.
Table Of Experimental SAA And NRC Data For Auralex Wedges And Pyramids
Comparing NRC for the most popular Auralex Wedges with 2 in. thickness with Auralex Pyramids with the same thickness, we first notice that the Wedges have NRC of 0.8 and the Pyramids have NRC of 0.7 (see table below).
|2″ Wedges||0.17||0.11||0.16||0.24||0.3||0.45||0.64||0.91||1.01||1.06||1.05||1.02||1.03||0.99||0.97||0.95||1||1.05||0.8||A||2’x4’x2″ Foam Panel|
|2″ Pyramids||0.11||0.13||0.09||0.13||0.18||0.27||0.34||0.57||0.73||0.9||0.96||1.05||1.07||1.03||0.98||0.96||0.98||1.05||0.7||A||2’x4’x2″ Foam Panel|
Major Difference At Lower Frequencies
Both foams seem to absorb sound pretty good overall. The only significant difference lies in absorption at low frequencies. At 500 Hz, the Wedges’ absorption coefficient is still 0.91 while the Pyramids’ is down to 0.57. Frequencies below 1,000 Hz, however, are the most important frequencies for most recording studios and sound booths. This is because the fundamental frequency range of a male voice is between 85 Hz and 155 Hz, and the fundamental frequency of a female voice ranges between 165 Hz and 255 Hz. (Source: msu.edu).
The reason for the wedge acoustic foam having a greater NRC than the pyramid acoustic foam is the greater volume. For the same surface area of the tile, defined as width times length of the tile, the pyramid acoustic foam has a lesser volume than the equally thick wedge acoustic foam. Since the sound absorption happens in the cells of the foam, generally this means that the greater the volume, the greater the absorption.
The Volume Difference
Consider the pyramid acoustic foam and the wedge acoustic foam that have the same base a and the same height (thickness) h, as in figure below
Both the pyramid foam and the wedge foam have the same room-facing surface area equal to A = 2*a*sqrt(h2+a2/4) However, their volumes differ. The volume of the pyramid is V = 1/3 a2 h and the volume of the wedge is V = 1/2 a2 h. Therefore the wedge has 50% more volume. The amount of absorption is proportional to the volume therefore it is natural to expect that the wedge acoustic foam would absorb more. This is apparent in the absorption coefficient table above. The table shows absorption coefficients for both 2″ Auralex Studiofoam Wedges and 2″ Auralex Studiofoam Pyramids for a variety of frequencies under the standard, mostly normal incidence conditions.
We find that both foams have perfect absorption for normal incidence for all frequencies above 1,000 Hz. For frequencies between 1,000 Hz and 200 Hz, the wedge foam has an advantage of up to 30% higher absorption coefficient. Overall, we see that 2″ Studiofoam Wedges have NRC of 0.80 while 2″ Studiofoam Pyramids has the NRC of 0.70.
A surprising result occurs, however, for the absorption measurement at 125 Hz which is important because it is the typical fundamental frequency of a male voice. At 125 Hz the absorption coefficient of 2″ Studiofoam Pyramids is 0.13, greater that the absorption coefficient of 2″ Studiofoam Wedges! How can that be?
We claim that this is due to an important difference in how sound waves at 125 Hz reflect in 3D once they enter the pyramid acoustic foam, see section on Directionality below.
Directionality Of Sound Proof Foam Panels
Second most important quality of the soundproof foam is direction of waves that are reflected from the foam, or directionality. You will not want your soundproof foam to allow the sound to reflect in highly predictable directions, such as straight back.
Such predictable reflections can support building of standing waves in the recording studio or the sound booth. Standing waves can be detrimental to a studio recording because they distort the sound by increasing the room response at certain resonant frequencies. There can be a big difference in direction of reflected waves between the pyramid foam and the wedge acoustic foam. Another important property related to directionality is diffusion. Diffusion of sound happens when a soundproof foam allows the reflected sound to be directed in multiple different directions. A greater diffusion will prevent buildup of standing waves in the recording studio.
We will treat sound as narrow sound rays for the purpose of this section. These narrow sound rays experience reflection and refraction as they hit onto the surface of the foam. We will focus our attention to the important range of 20 Hz – 500 Hz where neither the wedge foam nor the pyramid foam are absorbing the sound adequately. However, when positioned right, both such forms can absorb more.
Refraction: Breaking Of The Sound Wave Upon Transmission
For maximal absorption in the foam to occur, the sound rays must enter into the foam and preferably stay in the foam for as long as possible. The process of wave entering into the foam is called transmission. When transmission happens, the sound wave will “break”, or change its direction of propagation. This is called refraction of sound. The conceptual picture of the process of transmission of a sound wave from air to a soundproof foam is depicted in the figure below. Note the breaking of the sound wave direction as the sound ray enters the foam from air.
An important quantity that determines the breaking angle in refraction is speed of sound in air and speed of sound in foam. Speed of sound in air is independent of frequency and is about 330 m/s under normal conditions (room temperature, normal atmospheric pressure). What is interesting, however, is that, for frequencies between 20 Hz and 200 Hz, speed of sound in foam can depend on the frequency significantly. In particular, we see that speed of sound is as much as 3x slower in a foam than in air at around 100 Hz. This is important as the angle of refraction (the angle of bending of the sound ray as the sound ray enters into the foam) becomes very large.
In the figure above, the breaking of the incoming sound ray corresponds to the speed of sound in the foam of about 110 m/s.
We have used simulations assuming the speed of sound in foam equal to 140 m/s or about two times slower than speed of sound in air. We performed simulations using online simulator at PHET.
3D Directionality Difference Between The Pyramid Acoustic Foam And The Wedge Foam
Wedge Acoustic Foam
Consider normal incidence of a sound ray on a wedge acoustic foam first. Look at the figure
In the right part of the figure, normal incidence of a sound wave on the wedge acoustic foam is pictured. Sound wave (in blue) initially travels straight down, then gets bent as it crosses the boundary from air into the foam, then reflects of the bottom of the foam (assuming the foam is attached to the wall), and then takes up the symmetric path back up, hits the boundary foam-air on the other side of the wedge, refracts, and goes straight back up in the exact opposite direction where it came from. Thus it reflects back normal to the base, away from the wall.
Pyramid Acoustic Foam
Now consider a similar normal incidence onto the pyramid shaped acoustic foam. That is depicted on the left side of the figure above. The blue sound wave, or sound ray, impinges onto the right hand side surface of the pyramid. Depending on where exactly the sound wave hits the foam (specifically, close to the center of the side surface), it is still possible that it takes the same path as it would in the wedge acoustic foam. This is not pictured.
What can also happen, however, is that the ray, instead of going out on the left side panel of the wedge, it hits the front side of the pyramid first. What happens then is that, instead of the sound exiting in the exact backwards direction as in the wedge acoustic foam, it internally reflects and stays inside the pyramid. This is also known as total internal reflection. It may even internally reflect several more times after that before it exits the pyramid. A case of single internal reflection and exit toward the opposite (back) side panel of the pyramid is pictured on the left side of above figure.
This is our explanation why, even though the pyramid acoustic foam has less volume than the equally-sized wedge acoustic foam, it can still absorb more sound than the wedge foam, especially for low frequencies when the speed of sound is very low (as low as 100 m/s) and the internal reflection happens readily.
Diffusion Of Reflected Sound: Pyramid Acoustic Foam Vs Wedge Acoustic Foam
A sound ray that impinges on a surface of the acoustic foam or another substance that has the speed of sound different from the speed of sound in the air will experience reflection. Reflection of sound can either be specular or diffuse.
In specular reflection a single incoming sound ray results in a reflected ray going in a single direction as well. Specular reflection occurs in flat and smooth surfaces such as flat acoustic foam.
Diffuse reflection happens when a single ray impinging on an acoustic panel reflects in many different directions. Such diffuse reflection is helpful in a recording studio because it randomizes the waves reflected of the walls covered with acoustic foam, thereby reducing the chance of a buildup of distorting standing waves in the room. Diffuse reflection occurs in complex-shaped, or rough surfaces such as pyramid acoustic foam or wedge acoustic foam.
The set of directions of reflected sound rays is different in pyramid foam compared with the wedge foam. For example, a sound wave impinging in perpendicular direction (normally) onto the wedge foam whose ridge is in the up-down direction, can only be reflected left or right. This is due to symmetry. An example of a possible reflection is in the right part of the figure above. Notice how the wave bounces back and returns in the opposite direction where it came from. A sound wave directed normally toward the pyramid shaped foam can, however, reflect left, right, up, down, as well as in directions that are somewhere in-between. An example of such reflection of a sound wave impinging normally is in the left part of the figure above.
Similarly, there will be a more diffuse pattern of reflection off of the pyramid acoustic foam for incoming sound rays impinging in non-normal directions, compared with the wedge acoustic foam.
Overall we can say that the pyramid foam offers a more diffuse reflection pattern than the wedge foam. Pyramid acoustic foam is therefore preferable because it generally offers more diffuse reflections and is therefore less likely to allow resonant standing waves buildup.
Note On Speed Of Sound In Acoustic Foams
Speed of sound is measured to be as low as 110 m/s for in foams, as the report Jones illustrates. It turns out that speed of sound is strongly depending on the frequency of sound in polyurethane foam materials. Indeed, speed of sound approaches 100 m/s at the frequency of around 100 Hz. This has several important consequences for such important low frequencies. First, it bends the sound waves significantly due upon crossing the air-foam boundary, as illustrated above. Second, slow wave speeds allow for “packing” of more wavelengths into the relatively thin depth of the sound proof foam, therefore increasing overall absorption. This is mentioned here.
In figures on this page, we assumed frequency of sound of about 240 Hz, and we assumed speed of sound in such foam is about 140 m/s, about one half of the speed of sound in air. This is quite amazing. Such low speed of sound inside the foam can cause many internal reflections, which further amplify the absorption of sound inside the foam.
Note: Why Is NRC Sometimes Greater Than 1?
In the table above we observe that, for frequencies over 1,000 Hz, the measured absorption coefficient is often greater than one. The reason for absorption coefficient reported to be greater than one for some frequencies and some types of soundproof foam is diffraction.
What is diffraction? Diffraction is the deviation of the movement of the sound from the straight line, or from the sound ray behavior as we described it above. Even though sound is a wave, in many circumstances we can consider it to be a ray traveling through space on straight lines. This is what we did above. Often times this is a great description of the behavior of sound, and we can describe many phenomena this way. In the absorption coefficient formulas used in the experiment that measures absorption coefficient, and in the formula that gives the final value of NRC for a given frequency, diffraction is not taken into account.
Sound Bends Around The Corner
In simple terms, because of diffraction, the sound bends around the soundproof foam panel. Part of the sound is absorbed on the sides of the panel. This makes the measured absorption coefficient larger, even larger than 1. In particular, in reality, sound absorption coefficient can never be above 1, as this would imply that more than 100% of sound is absorbed. This would violate the law of conservation of energy. However, the way NRC and AAS are measured, often times, due to diffraction, the calculated absorption coefficient, NRC and AAS will be greater than 1.
Wave Diffraction At A Surfer’s Paradise
A great example of diffraction or bending of waves that approach an absorbing surface are waves hitting on the beach. Uluwatu surfing beach on the island of Bali, Indonesia, is pictured in the image below. As you can see, the breakers (in white) will bend around the corners of the beach. They will then hit the beach from many different directions. It looks as if the waves are trying to hit the beach at the right angle. Absorption of the wave energy happens even on the beaches that are not perpendicular to the original direction of the water waves. The length of the beach that absorbs the water wave energy is longer than the length visible from the perspective of waves far from the beach.
Difraction of ocean waves hitting a beach that bends. Surfing waves approach the Uluwatu beach from the west (left). They are absorbed by the west-facing part of the beach. The waves further to the north (up) turn south due to the bend in the Uluwatu beach. These waves hit the north side of the beach from the north. Effectively, both the west side of the beach and the north side of the beach contribute to wave absorption. When such effective increase of absorption due to a longer beach line is not taken into account, it appears as if more of the energy is absorbed than expected.
Advantages Of Pyramid Acoustic Foam And Wedge Acoustic Foam
So which foam to use in what circumstances? As discussed above, there are two advantages of pyramid acoustic foam: greater and more diverse diffusion of reflected waves, and higher absorption coefficient around 125 Hz compared with the wedge acoustic foam. The advantage of wedge absorption foam is greater absorption of sound in the 200 Hz to 1,000 Hz range.
Bottom line, if you are recording human voice, especially male voice, and it is important that the fundamental frequencies of it (around 100 Hz – 150 Hz) be as well absorbed as possible, then using pyramid acoustic foam makes more sense. If you are concerned with higher pitch instruments, then the wedge foam will absorb more. Another reason for using pyramid acoustic foam is to reduce the build up of standing waves. These build up readily between opposite and parallel walls in the recording studio.
However, when you know, approximately, where the sound is coming from, and you want maximal absorption for directional waves, then you can take advantage of the directionality of the wedge foam and the generally higher absorption coefficient of the wedge foam and use wedge acoustic foam.
Where To Place Pyramid Acoustic Foam Tiles and Wedge Acoustic Foam Tiles In A Recording Studio
In a separate post we describe positioning of different types of soundproof foam panels in a recording studio.
There we use computer simulation to investigate the differences in sound absorption of pyramid and wedge acoustic foam. We consider both normal sound incidence and oblique (non-normal) sound incidence. In simulations we go beyond the SAA and NRC measurement standards talked about in this post.
In the recording studio conditions, sound will indeed approach the sound proof foam at a variety of angles of incidence. Angles of incidence will depend on the location of the sound proof panels. These angles will also depend on the location of the sound source relative to the sound proof panels. We suggest to position pyramid and wedge sound proof foam panels away from the source so that the angle of incidence will not be too close to zero. For such larger angles we see that the wedge acoustic foam will absorb better.
Check out our review of the best pyramid shaped acoustic foam. You will find there details on the recommended pyramid acoustic foam which we couldn’t cover in this post.
For more information on wedge acoustic foams, check out our review of the best wedge acoustic foam.
We have also recently reviewed a special type of wedge acoustic foam that we call “vertical wedge” acoustic foam. See the review of the acoustic foam with vertical wedges here.