The Science & Development Of The Cumulus
- Walls and ceilings act as waveguides to gather sound in corners
- The Inverse Square Law: Double the distance = ¼ the energy
- High density Broadway versus low density foam
- Using air space to your advantage to increase bass absorption
Walls and ceilings act as waveguides to gather sound in corners
For years, acousticians have always employed corners as their starting point when absorbing sound energy. This is because the walls and ceilings (and floor for that matter) act as waveguides that work together to redirect sound into the corner. The ‘epicenter’ is the tri-corner where the walls and the ceiling intersect. This is without question, the eye of the hurricane. It therefore makes sense that this be the primary location for placing an absorbent panel. The Cumulus name comes from the cloud family and if you think of the ‘accumulation of sound waves’ the name certainly is well chosen.
The concept of waves accumulating or being guided also applies to PA systems and how these devices employ horns to direct sound waves. In Figure 1, you can see how a horn’s 90° walls disperse sound waves outward from the driver, directing the energy out the energy. The walls of your room do the same thing but in reverse. Sound waves are guided toward the corners as shown in figure-2. High frequencies are directional and reflect off of walls toward the corners while low frequencies travel along room surfaces, eventually finding the corners.
By positioning an acoustic panel like the Cumulus in the tri-corner, you not only capture the direct radiating field (Figure 3, red lines), but also capture the reflections from the ‘wall-to-wall’ junction and the two ‘wall-to-ceiling’ junctions. Thus the reason corners are extremely effective for acoustic panel placement.
The Inverse-Square Law: Double the distance = ¼ the energy
When designing a sound system, sound pressure levels are calculated based on a function known as the inverse-square law. What this basically means is that each time you double the distance; the sound pressure will be reduced by a factor of four (-6dB). This is because sound, for the most part, expands in a spherical manner.
This image illustrates sound emanating from a traditional horn. Notice how the acoustic energy spreads as sound travels away from the source.
Now, consider the same effect when we apply the concept to a reflected sound off a nearby wall. With acoustics, if you can capture the sound before it expands, it is easier to control. Thus another very important reason why corners are so beneficial; you essentially get more absorption for your money!
High Density Broadway Versus Low Density Foam
Another important aspect to the Cumulus is the quality of the acoustic material. Most folks do not realize that the density of the material plays a critical role in how well sound will be absorbed. In acoustics, we do not want to simply absorb high frequencies, but ultimately would like to absorb all frequencies evenly.
Technically speaking, acoustic absorbers work by converting sound energy into heat. This science is known as thermo-dynamics. The point here is that when something moves, even if ever so slightly, it requires energy and the energy will dissipate into heat.
The two most common materials used to control sound are glass wool fiber (fiberglass bat) and open-cell urethane foam. These come in various formulations and densities. Too high of a density, and the absorption suffers. Too low of a density and the product will not have any effect on bass frequencies.
The graph to the right compares high density Primacoustic 2″ Broadway panels to 2″ urethane acoustic foam. Notice that both are equally effective in the upper registers above 1000Hz. But as you go down to deeper and deeper bass, the foam product quickly looses its effectiveness below 500Hz. In fact, it has almost zero effect on some of the most troublesome frequencies of all.
For instance, urethane foam is only 30% effective at 250Hz while the Cumulus high density glass wool panel is 85% effective at that same frequency. Broadway panels deliver even absorption from about 250Hz all the way up. This results in a more natural sounding room that will sound more musical and transparent.
Using Air to Increase Absorption
A great way to increase the absorption of an acoustic panel is by creating an air space behind it. This works particularly well as you increase the panel’s density.
This graph compares a 2″ Broadway panel mounted directly to a wall surface to one that has a 2″ air cavity behind it. This results in a 20% increase of absorption at 400Hz just from moving the panel away from the wall surface by 2 inches.
The Cumulus with it’s corner location takes advantage of this well know acoustic trick. Sound waves that pass through the Cumulus are forced through a second time after reflecting off the walls and ceiling behind. More energy is absorbed each time the sound waves must pass through the high density material. A corner mounted panel like the Cumulus does the work of two panels for the price of one.
Calculating The Effect Of A Deep Air Cavity
Corner mounting the Cumulus creates a much deeper air cavity than a wall panel can achieve. The extra depth aids in the absorption of low frequencies making the Cumulus an effective mid-bass trap.
By applying the quarter-wavelength calculation, we can predict absorption attributed to the air cavity. When mounted, the 24″ sides create an air cavity behind the panel with a depth from 12 to 17 inches. It can be determined that the Cumulus will effectively attenuate frequencies between 100Hz and 500Hz while the acoustic panel surface will effectively absorb high frequency energy. This will help reduce troublesome mid-bass and be particularly helpful at eliminating standing waves from rooms with 8ft to 11ft ceilings that naturally resonate between 100Hz and 142Hz.
Putting it all together
The three absorption techniques discussed above are shown plotted individually and combined into a fourth composite chart for the Cumulus.