For this installment of “Similar, yet Different”, we will be going retro – The Barrel Vs. The Pyramid! These two shapes are the historic foundations for modern acoustic treatments. How did they come about? Why do they work? How are they different?
These two shapes have origins before written history. The pyramids of Egypt may not have been renowned for their acoustic properties, but they certainly show the historical age of this shape. The barrel is also an ancient shape, born from the pillars of historic temples and gathering places. Both of these shapes have been used in architecture for the expanse of human history, and their acoustic properties have been studied in numerous environments, in many different applications.
These shapes are often referred to as “primitive.” The barrel, in its basic form, is a truncated cylinder – basically a segment of the cylinder. The pyramidal is, in this case, a modified/offset pyramid… a pyramid without equal sides. Their differences begin with this fundamental variable – the pyramid has angled planes and the Double Duty™ is a large curved surface.
Symmetry vs. Asymmetry
Another simple difference is the fact that the Double Duty™ is symmetric and the offset pyramid is asymmetric. The barrel primarily scatters sound across the curve of the face – sending acoustic energy in a wide arc. However, the pyramid’s facets are all angled slightly differently – reflecting in different directions. This allows the pyramid to be installed in complex arrays which create more “random” reflections due to their different facet angles. The Barrel is primarily a one-dimensional diffuser, and installs either horizontally or vertically.
There is another phenomenon that helps to contribute to diffusion – and that is diffraction. Diffraction is what happens to sound when it hits a corner or edge. Unlike light, acoustic energy is the physical fluctuation of pressure changes – which gives sound the ability to travel around corners. This bending varies by the wavelength of the sound and the size of the object in encounters. Both barrels and pyramids have facets which introduce diffraction, and while both can be made in different sizes, the offset pyramid has different sized facets on each device – contributing to more randomized diffraction at different frequencies.
Inverse Square Law
Without getting too heavy into math, as sound travels it decreases in intensity. This is due to the fact that sound “spreads out” as it travels. It is produced with a finite amount of energy, so intensity drops as it covers more space. Both the barrel and pyramid increase the rate that sound “spreads out,” which diminishes the intensity of the sound – however, they both do it slightly differently, however..
The Double Duty’s™ curvature leaves the wave primarily intact, but it increases the rate of expansion across the curve. This redirection is very smooth and predictable, where the random facets of the offset pyramid break up the sound into sections which travel in different directions. At the intersection of those facets, diffraction takes the reigns and scatters sound even further.
By increasing the rate of expansion of the wave, you decrease its intensity while also breaking up the wavefront which helps to reduce echoes and flutter. Both the barrel and the pyramid are perfect for larger spaces, as those massive surfaces do a great job of controlling reflections from large wave fronts. Also, due to their simple shapes, they can be made really big, which helps!
There is one more feature which is sometimes overlooked. Because of the material of their construction, and the large volume of air behind them, these diffusers exhibit a certain amount of “bass trapping.” The Double Duty™ diffuser got it’s name due to this characteristic. It’s not just a diffuser, but also a bass trap. The Pyramidal diffuser also exhibits bass absorption, though it is slightly less.
So there it is… the battle of the classics! Sometimes, keeping it simple is the way to go!