Audio Engineering Society
Chicago Section
Meeting Review, October 30th 2001
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10/30/01 Meeting Highlights As
the story goes, cryptography expert and US military Colonel George Fabyan,
was told that the 17th century writings of Francis Bacon contained
scientific ideas encoded in seemingly ordinary prose. Further investigation
showed that these messages, which were encoded using one of the earliest
known versions of a binary code, contained references to an �acoustic
levitation device.� This idea captivated Colonel Fabyan, who was determined
to build the device. It consisted of two cylinders with musical strings
mounted vertically on the outer walls. These were supposed to vibrate
sympathetically within an outer shell, and cause that shell to levitate due
to the �force field� created by the acoustic energy. When
a first attempt by one of Fabyan�s civil engineers to build and operate such
a device failed, he became determined to hire an acoustical expert and make
it work. Meanwhile, a leading researcher of the early 20th
century, Wallace Clement Sabine, was working at Harvard studying room
acoustics under less than ideal conditions. He often worked late into the
evening in order to get accurate measurements after the constant clatter of
horseshoes on the cobblestone streets of Boston died down. So he was quite
receptive when Fabyan offered to build him a lab amidst secluded amidst
Illinois farmland, in exchange for help in optimizing Fabyan�s acoustic
levitation device. The
main part of the Riverbank facility toured by the AES, still stands largely
undisturbed, as originally designed by Sabine. The facility consists of
several large rooms with rigid walls (to make them very reverberant), which
are used in testing of acoustical spaces and materials. Plus, it still
contains the prototype acoustic levitation device, which never did work
properly. Unfortunately, Sabine died unexpectedly just as the building was
completed, forcing Fabyan to enlist two lesser known members of the Sabine
family to carry on the operation at Riverbank. Because
of Fabyan�s military activities, the lab continued to be active in military
decoding work as well as acoustics during the World Wars, being considered
the birthplace of the CIA by some historians. However, in the modern era
acoustics research is the only activity at Riverbank, now done under the supervision
of the Illinois Institute of Technology. The rooms, some elaborately
decoupled and some coupled by openings (walls or floors) which can be fitted
with building materials under test, are used for various sound transmission
and absorption measurements. The measurement data is then compiled on
computers located in a separate master control room. The eighty year-old
space, often driven by corner loaded, calibrated speaker sources, and made
more diffuse by spinning vanes, is considered one of the most characterized
acoustic spaces in the world. Of
course a few things have changed from the early part of the century, when the
main room had to be lit by small openings in the ceiling to avoid measurement
degradation from the noisy lights of that era. For example, measurements are
probably more accurate and repeatable, with the use of a speaker instead of a
set of excited pipes, and a rotating microphone linked to a computer instead
of a crouching man with a stopwatch and a pencil! But as the years march on, some
things (such as the 100 Hz reverberation time of 12� seconds) still remain the same! Thanks again to Mark and David
at Riverbank Labs, for an enjoyable peek into the history of our field. Note
that interested readers can learn even more by reading John Kopek�s book on
Riverbank history. The book, entitled �The Sabines at Riverbank,� can be
purchased direct from the laboratory.
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