For a few months now, I’ve been acting as an acoustical consultant to Bruce Odland and Sam Auinger on their project “Sonic Vista,” that opened on October 2nd for a 5 year long installation. Bruce and Sam have been making publicly installed sound art pieces in North America and Europe for over twenty years. I first heard their work at MassMOCA in 2006, and met Bruce while he was putting the finishing (soldering) touches on “Harmony in the Age of Noise” at Tufts University in 2008. Sonic Vista is installed in Frankfurt, Germany on a train & pedestrian bridge connecting two sections of Frankfurt’s greenway, located here.
Sonic Vista is a real-time sound art installation. It gathers all the airborne noise from the industrial cityscape, filters it acoustically, and plays the resulting harmonious sound back through two giant, brightly colored spherical speakers placed directly over the heads of listeners as they walk along the pedestrian bridge. Walking along the bridge literally becomes a consciousness raising event as one enters the sonic environment of the piece. Noise from the cityscape (backhoes, earth movers and jackhammers from the nearby bank construction site, trains, dogs, humans, jets, boats, busses, and other city roar) is captured by two 4” diameter, 18 foot long (B0) and 12 foot long (F#1) tubes, each containing one microphone. The sound reaching the microphones is naturally filtered by the harmonic overtone series of acoustic modes supported by the long tubes. These microphone signals travel directly to the speakers, and are not processed with any electronic effects (such as reverb or chorus). To hear Sonic Vista, follow this link to SoundCloud. The exact position of the microphone in each tube affects the harmonic balance it detects, as shown by the following plot:
I aided in the design of the 1 meter diameter spherical speakers, first in solving the directivity problem: How can the speakers be designed so that listeners walking along the pedestrian bridge hear a uniform sound pressure level as they walk? The problem with overhead speakers is that they typically send most of their sound straight downward – so it is much louder directly under them than it is off to the side. The problem is made more challenging by the wide bandwidth — the lowest note is 60 Hz, and the highest notes are near 3000 Hz.
I’ll skip all the math and acoustics theory, and just state the solution: a 12 inch diameter woofer needs to be positioned behind a 4 inch diameter hole that all the sound must exit through. This greatly improves the radiation pattern for the highest notes. The heights and horizontal spacing of the two speakers also play an important role in setting how loud the sound will be when one is in between the two speakers. Interestingly, I was able to balance these factors and the system as built achieved a pressure that was uniform within a few dB as listeners walk along the bridge underneath the speakers. The plot below shows the loudness as a function of position along the bridge at the highest and lowest pitches, along with the 35+ dB soundfield variation that would have resulted without this hole to fix the radiation pattern. The speakers are located at 50m and 64.7m, the two loudest locations along the bridge.
The second problem arose due to the solution of the first: That the speaker radiates through a 4” diameter, 4″ long port created an undesirable resonance that I correctly predicted through the use of a computational acoustics model. This resonance was actually eliminated by simply equalizing the speakers electrically.
This front cavity does lead to the loss of almost half of the sound output, but it’s a high sensitivity, 300 Watt maximum input driver, so it is still more than capable of being loud enough. This is a small price to pay for the tremendous improvement in radiation pattern and listener experience. To hear Sonic Vista, follow this link to SoundCloud.