Imagine this paradoxical situation: you're standing in front of a loudspeaker, but you're perceiving the sound as coming from behind. To be sure, audio tricks are plentiful in this era of surround sound and 5, 6 or 7 channel systems. But this is more than just a trick—to accomplish something like this, you'd have to throw out everything you know about classical loudspeaker design.

I heard about a demonstration of this effect a short while before I had to choose the topic of my fourth-year thesis in industrial design. Though I had no earthly clue how it was accomplished, I was determined to find out, and the project presented here is the result of this research.

Classical acoustics

Traditional loudspeakers have been the same pretty much since the first design was originally proposed over 100 years ago. Two important dimensions are inherent to speaker designs, and rather than launch into long and potentially incomplete definitions, I will link back to their Wikipedia entries. These are directivity and frequency response.

Directivity is achieved in different ways and by different techniques, but fundamentally, the ability to focus sound will always be subject to the fact that loudspeakers create sound in a way that is analoguous to the way a bare lightbulb radiates light: in every direction. There are different philosophies concerning directivity, but for all practical purposes, it is difficult to achieve any amount of strict control over the propagation of sound.

The second aspect, frequency response, is fundamentally tied to the way loudspeakers work. Much like a piston, traditional loudspeakers create soundwaves by vibrating back and forth. Whatever surface does the vibrating, there will be inefficiencies due to the moving part's weight. What's more, different frequency ranges are best produced by different techniques and different transducer designs. Therefore, any good sound reproduction system will necessarily involve multiple transducers that collectively work to reproduce the entire spectrum of audible frequencies, each to varying degrees of efficiency. The result is that no system can approach perfect frequency response.

To create the effect described above, then, would require looking beyond traditional acoustics.more >>