German physicist Hermann von Helmholtz was a brilliant man but his work on sound and fluid dynamics is most well known for the Helmholtz Resonance that is the underlying principle of bass reflex speakers. He has done work far beyond that including his groundbreaking work in fields as diverse as mechanics, sensory physiology, optics, neurology, acoustics, aesthetics, and electromagnetism. Some of his work was so fundamental in nature that it is still in the process of being studied and incorporated into current technology.
Another brilliant man was Gustav Kirchhoff a German physicist who contributed to the fundamental understanding of electrical circuits, spectroscopy, and the emission of black-body radiation by heated objects.
Both these men’s work was combined into a mathematical principle known the Helmholtz-Kirchhoff Integral, which most of us mortals will find incomprehensible, but yet it has a simple and astonishing consequence:
If you know the sound pressure fluctuations on the surface of the body, you can derive them for any point within the body. Not surprisingly, the opposite holds true as well: If you know what you want the sound pressure fluctuations to be at any point within a room, you can accomplish that by exciting the walls of the room accordingly. By this one can actually originate wave fronts from the middle of a room by exciting the rooms boundaries.
For those who know me, I take a particular interest in stereo imaging and run a company that builds very cost effective loudspeakers that seem to image rather well.
As to the next step in stereo imaging, there is a technology that is currently evolving called wave field synthesis which has first been postulated by A.J. Berkhout at the Delft University of Technology back in 1988. If you can precisely excite the walls enclosing a body of space, you can create any sound within that volume that you like. Imagine walking in a room of complete silence and only when you stand in a very particular spot, can you hear whatever it is the architects of this soundscape might have wanted you to hear.
Imagine being able to walk around the virtual soundscape room full of sounds and have every sound stay in its precise location no matter where you are or how you turn your head.
The way this is accomplished is by placing a fantastically large amount of loudspeakers all around the walls of a room. Most current and technologically serious attempts other than the "Sound Igloo" shown succumb to reality by not attempting to create a three dimensional wave field which would require the entire surface of the room to be covered in speakers, but rather a two dimensional one, that will not accurately maintain the stability of a sound source’s position when moving in the up and down position. For most environments like concert halls or cinemas this is a fair simplification and a very necessary one. Even with this dimensional restriction, the number of loudspeakers required goes into the thousands.
This promise of a brave new world of audio and stereo imaging still has many challenges, apart from the massive cost, current recording technology and issues concerning real world acoustics are hurdles to overcome.
The Technical University of Berlin has set up an experimental wave field synthesis environment with 2640 speakers and 220 woofers distributed across 110 modules spanning a length of 86m The total power rating of the 1100 amplifiers involved is 57,200 watts. Significant signal processing capability is adding to the cost.
One of the first applications of this listening environment was the broadcast of a live concert recorded at Germany's Cologne Cathedral. Oliver Messiaen’s monumental organ composition "Livre du Saint Sacrement" was performed and then “holophonically” reproduced almost instantly 500km away in Berlin as part of the 5th Sound and Music Computing Conference. The work is arguably not exactly easy listening, and the extreme technical complexity in recording, transmission and reproduction was plagued by occasional outages that were sympathetically tolerated by an audience well primed to expect such difficulties. Media reports about the Messiaen transmission were favorable but not enthusiastic.
In my personal opinion the choice of music was interesting to say the least as the acoustic environment of a large church is not very precise and organ sound seems to come from all over anyways. My want would be to repeat such a transmission in an acoustically acclaimed concert hall like the Vienna Musikverein and perhaps with a full symphony. I believe that the outcome would be not only more measurable, but more pleasing to the ear as well.
Arved Deecke is founder of the Danish / Mexican Loudspeaker company KVART & BØLGE that makes audiophile quarter wave loudspeakers and sound systems at a price anyone can afford. In his free time he blogs about all things related to sound, music and audio.
