Richard T. Watkins and Dr. Tim Leishman, Physics and Astronomy
A few weeks into the development of this project, several factors became apparent that we did not anticipate when I wrote the original proposal. The table size needed to be larger than first envisioned to make possible the experiments and data collection that would be conducted with this device. The six-foot diameter described in the original proposal was not large enough for a couple of reasons. First, not all the acoustic instruments, with their performers, that we had hoped to examine would fit on the surface without being off-center acoustically. For example, to place the tone holes for a cello near the acoustic center (a point which we can only theorize at this time), the table would need a diameter of at least seven feet. Since this, like most acoustic centers, is unknown, we now realize that the table needs to be much larger than six feet. The discovery of acoustical center, whether that is a fixed point or an average local point relative to the pitches being played on a given instrument, is, in fact, one of the areas of research that will become possible with this device. To get meaningful data, we need the ability to move the performer and instrument back and forth until we find the acoustical mean. Second, we want sufficient surface area beneath the performer to achieve acoustic interaction with the floor of the platform. Most acoustic studies have been engineered without this consideration. But in the real world, instruments are played in an environment that has a solid floor. They are not free-floating in space, as they would be in a purely anechoic environment. We want our studies to retain the effect the floor has on the sound being examined. Again, this means that the table must be large.
With the table having grown in size, we now had to consider ways to build it so that it would be modular in design. There was always the need to have it break into pieces so that it could be easily moved, but now it has to be much stronger because of moments of inertia the substructure would encounter with longer members. We also need to construct it in manageable pieces that can be assembled and disassembled within the anechoic chamber or the new reverberation chamber in the lower part of the building and moved easily from place to place. The plan is to eventually use it in both areas for different kinds of research. Weight was now a compelling issue that we dealt with by designing an aluminum superstructure fabricated somewhat like the wing of an aircraft. It had to be strong enough to be suspended without flexing on the wire mesh that is the floor of the anechoic chamber. If the platform twist or bends, the rotating top of the table will not be able to move. The motor we chose also had to be the smallest, lightest motor available that was powerful enough to move the now 600-pound turntable, as well as the instrument being tested. Our present design is sufficient to hold a 1200-pound piano, with its performer, safely on top and rotate them at a rate of up to 2 rpm.
The process is a simple one: The instrument and performer sit on the table under an arch of precision microphones. The table remains stationary as the instrument is played and the sound is recorded digitally through the array of microphones to discrete tracks. The table is then rotated in steps of five degrees or less, with the sound measurement being repeated until the instrument has traveled through a complete 360-degree revolution. The drive motor must start and stop the table at precise, uniform increments, or the data collected will not be accurate. The musician must maintain the same precise position for the duration of the data collection process. The instrument must be kept at the same height and must be pointed away from the body in the same fashion. Other methods, such as a rotating arch of microphones, have been tried to obtain this data, but the results have been flawed. The arch motion is problematic: to minimize audio reflections, it needs to be made so thin that the mics bounce and oscillate. The resulting data is distorted and gives an inaccurate picture of the sound. This is why the automated table is so essential.
While doing research, we looked at several commercially available rotating platform designs. However, these were primarily used for commercial display and advertising and were not designed to rotate incrementally. We also examined the possibility of outsourcing the building of the platform, but the bids received were greater than ten thousand dollars. Our design, with the motor and controller, is expected to be at or near six thousand dollars.
With the completion of the larger part of the design work in December, we decided to place the project on hold. The anechoic chamber is scheduled to undergo a renovation, and some possible changes to the electrical system could affect the way the platform should be constructed. If the chamber has 220-volt two-phase power brought in, that could change the motor we are using. Also, the physical dimensions of the chamber could change due to the thickness of the sound-absorptive materials. If the chamber is larger as a result, there will not be a change to the design; if it has a smaller floor space, the table will need to be reduced in size. Last summer I spent an average of 20 hours a week during spring and summer terms of 2004 researching materials and the best motor and controller combination. The bulk of our time was spent in detailing the process by which the platform would be assembled and developing a user-friendly modular design. We looked at several designs that ranged from a nine-piece to a five-piece top. The weight of each piece was the primary consideration. I wanted to keep each piece under 90 pounds so that a minimum number of people would be needed to assemble the platform. We met our goal with every piece except for the middle piece, which has the motor gearhead and mount in it. This piece will need to be examined further to find ways to break it down into more manageable parts. Andrew Boon, a mechanical engineering student, was assigned the task of doing the CAD drawings and the beam analysis for the platform’s framework. He has done a marvelous job, as can be seen in the CAD drawings in figure 1 below. Tooling is another issue that I have been finding solutions for a large reduction gear might need to be fabricated in-house, and that can be done with the help of our own mechanical engineering department. Other items that will be required I have located on campus or have found sources for. When we finally get the materials, we will be able to move rapidly to complete the rotating table.