Originally Posted by Klinkenborg
Thanks for the explanation; I understood this was the theory but was interested in the practical application - just how effective is this theoretical cancellation compared with a 'total' absorber. I am lucky enough to be able to spare the metre-thick space I would lose with an SBA so was keen to learn which system is actually superior, regardles of cost and complexity,
Disclaimer: Please note that I have not actually built one of these things and have no hands-on experience with this concept. I have a strong background in physics and engineering, and my understanding only theoretical at this point. If someone has built one of these systems and has seen different results than what is being discussed, please speak up! Data trumps theory every time.
I suspect that the SBA with a large absorber at the back of the room would create superior results. Here's why.
1. The concept of the planar wave pitch-catch relies on the idea that the wave arriving at the back of the room is an exact match to the wave that originated at the front of the room, transformed only with a delay and a slight attenuation. This ignores any frequency specific absorption that may have occurred in the walls, the equipment, the furniture, or the people in the room. Conversely, a passive absorber at the back of the room has no expectations for what the waves it receives should look like. Whatever they are, it absorbs them.
2. Think about how this planar wave is formed. We start with an array of discrete sources, each emitting an approximately spherical wave. As we get farther from the front wall those spherical wave-fronts combine to form an approximately planar wave travelling through the room. Thanks to the wavelengths involved, we can be reasonably assured that this happens before that wave-front reaches our ears. But now think about how this looks at the back of the room. We have a planar wave coming in, and it somehow needs to break into pieces to be absorbed by an array of discrete sinks. Each rear driver is properly timed to create a low pressure zone to "suck up" its fair share of that incoming pressure pulse. For me, this splitting up of the planar wave is much harder to visualize than what's happening at the front of the room. It should work. But it's certainly a much more complicated way to absorb a pressure wave than a passive absorber would be, and it surely can't happen perfectly.
An open-backed room is the theoretical idea. It can generate no reflections. In my mind, the SBA seems closest to that theoretical ideal. The DBA is an approximation of it with the primary advantage being space savings.