Audio Engineering Society

Chicago Section

Meeting Review, October 30th 2001


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10/30/01 Meeting Highlights
by Brad Olson

Riverbank Laboratories Tour

In October, Chicago section AES members learned how a military colonel and a pioneer of acoustics crossed paths earlier this century, spawning one of the oldest and most storied facilities specializing in the study of architectural acoustics, the Riverbank Acoustical Laboratories in Geneva, Illinois.

 

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.