Acoustic Foam Buying Guide
Why Do I Need Acoustic Treatment?
Have you ever been in a listening environment that doesn’t seem to accurately represent what you are listening to? Chances are that no matter how many sets of speakers you swap out to find the right sound, the elusive "perfect" tone will escape you. No, it is not your speakers, but rather poor acoustics in your listening room. Whether you are building a project studio, critical listening room, or home theater, finding the right tonal balance for your setup is crucial to great sound.
When sound radiates from a source the waves disperse throughout the environment. Waves will reflect off of surfaces and continue to bounce around in the room, much like if you were to throw a bouncy ball off a wall really hard. When a wave encounters a change in acoustic impedance, such as hitting a solid surface, acoustic reflections occur. These reflections will occur multiple times before the wave becomes inaudible. Reflections can cause acoustic problems such as phase summation and phase cancellation. When the direct source wave combines with the reflected waves, a new complex wave is created. This complex wave will change the frequency response of the source material.
Have you ever been walking around a room when music is playing and you stand in one spot where the bass response is super loud, or in a location where you can't hear bass at all? In rooms, especially small rooms, modes occur and create standing waves. A room mode is a location in the room in which there is an attenuation of a certain frequency. Lower frequencies are usually noticed more in room modes than higher frequencies since lower frequencies have longer wavelengths. There are an infinite number of modes that can build up in a room given that modes occur on all six surfaces.
Room Design Considerations
In a project studio, much like a recording studio control room, the ideal frequency response of the room is flat. This will allow the engineer to know that what they are hearing out of the speakers is an accurate representation of their mix. Many times, in a non-ideal room, the mix that sounds perfect in the studio sounds bad on other systems, such as a car. One reason for this could be that the engineer's listening position happens to be in a room null, where they cannot hear bass. Since they cannot hear the bass they will tend to boost it in their mix.
Flat frequency responses allow the engineer to know exactly what they are hearing and that the source is true. How does one obtain a flat response? Acoustic treatment. Placement of the treatment is critical to its success. The placement of the treatment should be applied so that the first reflections are attenuated. There should be enough live and diffusive surfaces so that the room has a normal feel and is not an anechoic chamber. In most situations, acoustic treatment should be placed on the walls parallel to the listener's positions, symmetrically. These will be on the side walls at ear height. The more coverage you can place in this area the better. The next area of the room that needs to be treated are the front walls and the back wall so that the reflections that are not caught on the front wall are caught on the back. Depending on the size of the room and the budget, 25-40% of the surface area of the space should be covered in acoustic treatment. Other areas that could be treated are the corners of the room, where bass tends to build up. Since lower frequencies have longer wavelengths, the thicker the treatment around corners the better.
Similar placements can be used for home theaters. Much like a listening room, these areas are again, on the sides by the listener's ears, back and front walls, and corners treating first reflections. Depending on the speaker arrangement, the location of the panels can change. This is especially true in systems that feature more than a stereo setup, as more speakers will add to the complexity of the design.
One method that can be used to find first reflections is to grab a buddy and have them walk the walls with a mirror against the wall while you sit in the listening position. Once you can see a speaker's tweeter in the mirror mark that point as a place to put treatment. These points will be places where first reflections happen. Another factor that affects how rooms should be treated are the speaker's dispersion pattern. Placement of the speakers also plays a large role in the sound of the room, but that will be an article for another day.
Using the bouncy ball analogy, acoustic treatment placed on surfaces, is like there is a hole in the surface where the bouncy ball can escape, instead of bouncing back into the room. Acoustic treatment acts as a barrier to sound, so that when the energy from the soundwave hits the foam it has a harder time reflecting back into the room. The energy is transferred to heat. Acoustic treatment comes in a variety of shapes, sizes, and materials. For this article and corresponding video, the material of the treatment is polyurethane. Acoustic treatment must be able to handle a wide range of frequencies with wave lengths ranging from 56 feet to 0.6 inches. Furniture in a room, no matter what it is constructed from, will play a role in the sound of the room as well. That couch in back of the room acts as an absorber, while your wood desk may act as a diffusor.
There are two types of methods used for acoustic treatment. Those are absorption and re-direction, or diffusion. Absorption is typically used to make a room feel less reverberant. For example, if you move into a new house and stand in an empty room and clap, you hear a little reverb and slap back of the original clap. Once you add your couch, loveseat, and TV in the room you can hear this effect less and less. Absorption performance can only handle a select range of frequencies, which are usually not linear in their effective ranges. The most common form of acoustic absorption are porous absorbers. The soundwaves cause air particles to vibrate in the material and the resulting friction loss converts the energy to heat. The amount of frictional loss, or absorption, is directly related to how tightly packed, or dense, the material is. The tighter the material is packed, the less absorption the material has.
Acoustic foams are a staple in the recording industry. Often times in pictures of studios, you see these foam panels lining the walls. These foams are typically made of open cell polyurethane. Common shapes are square wedges and pyramids. While the material loses density with patterns, the patterns increase the surface area of the panels, which increases the absorption. Different shapes have different absorption coefficients, characteristics, and frequency responses. The thicker the material, the better the absorption characteristics.
If you want to read the theory and math behind the example room....feast below. Otherwise skip down to the Acoustic Treatment Design Section.
Ugh… The Math
Now the fun part. Calculating where the standing waves will occur involves a little math. Low frequency room modes drastically affect the acoustic performance of a small room. When a frequency found in Formula 1 is produced, a standing wave will occur. When this occurs a cancellation of this frequency will occur at the midpoint of the surfaces. Multipliers of the mode frequency (ƒ1) will create more standing waves. There will be many more modes in the room, including modes from the width and height. Other modes for the length occur at 2ƒ , 3ƒ, etc. There are three types of modes, axial, tangential, and oblique. Axial modes involve two parallel walls, tangential modes involve four walls, while oblique modes involve all six walls of a room.
Modes can be expressed as three digit numbers. As an example, 1,0,0 is the first order standing wave for the length mode. When there are two zeros in the mode mapping, the result is an axial mode. One zero means the mode mapping is a tangential mode. A mode mapping with no zeros is an oblique mode. [3]
Formula 1:
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Where C is the speed of sound (1,130 ft / s) and L is the distance between the two walls. Formula 2 gives the frequency for every mode of a rectangular room. Formula 1 is derived from Formula 2. [6]
Formula 2:
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Where C is the speed of sound, L is the length of the room in feet, W is the width of the room in feet, H is the height of the room in feet.
P, Q, & R are integers 0,1,2,3…
For the following example we will calculate the room modes for a room (the one used in the corresponding video) the size of 8' 10.5' x 11.16' (ratio of 1:1.3:1.5) In most cases making an "ideal" room is not realistic. Most of the time project studios and home theaters are at the mercy of a set room. We can see that the room in this example is less than ideal.
The first room mode for the 11.16' length dimension, using Formula 1, was found to be 50 Hz, 2ƒ=100 Hz, 3ƒ=150 Hz etc. The modes associated with the three dimensions of the room can be found in the chart below. The chart was generated by an Excel Spreadsheet "Room-Mode-Calculator-Imperial" by John H. Brandt [5]. If you do not feel like doing the math, this is an excellent resource.
Note: Modes were calculated for more frequencies, but for space purposes, only a select frequency range are shown.
ALL MODES
In small rooms standing waves will generally not affect the sound above the limit determined by Formula 3. Usually in small rooms this is around 350 to 450 Hz. [2]
Formula 3:
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Where C is the speed of sound (1,130 ft/s) and Lmin is the smallest room dimension. In the case for this example the frequency was found to be 423 Hz.
In order to see where the room modes occur, the room can be broken up into regions. Region 1 is from 0 Hz to the first mode of the longest dimension ƒ1. At this frequency the room cannot be treated. Region 2 is the boundary of the first mode up until ƒ where ƒ=3C/Lmin. This regions mode impact the room's performance. Bass treatment can work well to tame these frequencies. Region 3 spans from ƒ to 4ƒ. This region can be treated with diffraction and diffusion. The final region contains wavelengths that are usually short compared to the size of the room. Acoustic foam can help tame this region. [3] The spacing of the modes plays a large role in the sound of the room. Generally speaking modes should be between 5 Hz and 20 Hz apart. Any mode closer together than 5 Hz can cause problems. We mapped out the spacing for the example room in Chart 2. [4]
ALL MODES
To find the reverberation of a room Formula 4 is used. MFP is described as the average distance a sound wave travels in a room between reflections. [3]
Formula 4:
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Where V is volume of the room and S is total surface area of the room. In our example, the average distance traveled between reflections is 6.45 ft. If this is divided by the speed of sound (1,130 ft/s) we can find the first reflection time. In our example each reflection occurs in 5.7ms. Generally it take 5-6 reflections for a wave to become inaudible, based on this it will take roughly 34.2ms to become inaudible. Typically any early reflections (under 18ms) need to be addressed and attenuate.
Room Analysis Method
Using Dayton Audio's OmniMic we can find that the measurements of the room, from the listening position, are similar. In most cases it is suggested that the listening position in a project studio is 38% of the length (longest dimension) of the room. For a 132 in (11.16 ft) room this is at 50.88 in (4.24 ft). For all before and after measurement the OmniMic was placed at tweeter height of near field studio monitors on 42 in stands (around 53 in) in the middle of the room's width (5.25 ft) and 4.24 ft length from the back wall (38% of the length of the room).
Note: measurements would change as the listening position and speaker placement changes.
For this example the speakers are placed in an equilateral triangle from the listening position. Each speaker was placed 37 in from the side wall and 5.8 in from the back wall.
Acoustic Treatment Design
The project studio we have analyzed in this article has many issues. Using Sonic Barrier acoustic treatment we were able to reduce the early reflections of the room by placing them strategically throughout the room. Using methods described earlier in the article, we found where to place the treatment. The surface area of the wall (excluding the ceiling and floor) is 346.5 ft2. To achieve 25% coverage we will be using 86 ft2 of acoustic treatment, with a combination of 1 in and 2 in thick panels from Sonic Barrier. To accomplish this we used four boxes of each thickness.
In order to design the space, a layout of the room was created. From there, using the mirror trick described earlier, the places where treatment was needed were marked. We tried to focus treatment on the walls and corners. While it is usually beneficial to have "cloud treatment", where treatment is applied to the ceiling, this example did not do that in an attempt to focus on the first wall reflections. The room has a ceiling fan in the middle of the room, thus hindering the ability to do cloud treatment. The room has hardwood floors, a window on the back wall, and on the front wall a mirror sliding door leading to a closet. Other design considerations taken into account are aesthetics and ergonomics. While having the "perfect" sounding room is good, ease of use, comfort, and looks are important as well. A studio is supposed to be a creative environment as such ambience is important. Neglecting any of these other considerations could stifle creativity, and the purpose of the space.
There are many types of acoustic treatment foams on the market. The treatment comes in many shapes and sizes, such as flat, pyramid, wedges, saw tooth, and beveled panels. While each has benefits, in general the more material that the panel has, the denser it is. The denser the material, the higher the absorption coefficient, especially at lower frequencies. These acoustic panels are designed for mid to higher frequencies, while each design will have their own characteristics. To tame lower frequencies bass traps are required, which are usually placed in corners of rooms where bass frequencies build up. Diffusion is also a nice addition to a room, especially on the back wall directly behind the listening position.
Room Results
The procedure for the install of the project studio was as follows. First the speakers were placed in the location to create an equilateral triangle with the listening position. The speakers that were used were the Yamaha MSP7 Studio Monitors.
Note: measurements would change as the listening position and speaker placement changes.
Figure 2: Polar Pattern of the MSP7s Courtesy of Yamaha
Empty Room
In the empty room the only equipment used were speakers in location, a computer, interface, monitor selector, a computer running OmniMic and OmniMic.
Graph 1: The RT60 curve of the empty room
The RT60 graph is a great way to see what the reverberation time of a room is. This is the time required to drop 60 dB from the initial source sound [6]. We can see that the time for the reverb to decay is about 500ms. Another useful graph in analyzing rooms is the waterfall graph. The waterfall graph shows the frequency response decay over a selected period of time.
Graph 2: Waterfall graph of the empty room
Room with Furniture
The next set up that was tested was a typical room with a futon, wood desk and music production equipment.
Graph 3 shows the RT60 curve which shows the decay of the reverb is less than 500ms, which has been shortened as compared to the empty room. The futon is a large reason for some of this damping. It is a large object that absorbs reflections, it acts as a bass trap as well, due to the thickness.
Graph 3: The RT60 graph of the room with furniture
Graph 4: Waterfall graph of the room with furniture
Treated Room
The room was then treated with 86 ft2 of Sonic Barrier acoustic treatment, placed in locations that were described above. The decay of the reverb is considerably less than that of the empty room as well as the room with furniture.
Graph 5: The RT60 curve of the treated room
Graph 6: The waterfall graph of the treated room
For this project we focused the acoustic treatment on taming more of the high frequencies. We can see from the waterfall graphs of each environment, that the high frequency flutter was tamed in the treated room.
Resources
[1] Everest, F. Alton. Sound Studio Construction on a Budget. New York: McGraw-Hill, 1997. Print.
[2] Davis, Don and Carolyn Davis. Sound System Engineering. Howard W. Sams & Co., 1994. Print.
[4] Leduc, Michel. "Listening Room Acoustics: Room Modes & Standing Waves Part 1." Audioholics, June 29, 2009, http://www.audioholics.com/room-acoustics/listening-room-acoustics-1. Accessed February 5, 2018
[5] Brandt, John H. Resources & Tools. http://www.jhbrandt.net/resources/#tools. 2016.
[6] Everest, Frederick A, and Ken C. Pohlmann. The Master Handbook of Acoustics. New York: McGraw-Hill, 2015. Print.