The development of the HR-30 tone ring was accomplished using a combination of Steve Huber's discerning ear and some of the best scientific instrumentation available for investigating vibratory physics. Measurements were taken on 14 exquisite prewar flathead Gibson tone rings. The goal was to first determine the nature of both their internal vibrations and emitted sound, and then determine how best to design a tone ring and rim to mimic these prewar vibratory properties.

The vibration measurements were done on an apparatus of Dr. Jim Rae’s design. The vibrator and some of the measuring electronics were attached to an optical rail via a platform that can be moved up and down to allow measurements of bridges, rings, rim, or whole banjos. The vibrating probe and its electronics are mounted on fine micro-manipulators that allow the probe’s tip to be oriented perpendicular to the surface to be vibrated. One of the manipulators is used to lower the vibrating probe by just a few microns to ensure that every contact is made with nearly the same initial force.

The vibrator and the force measuring circuitry were connected to a PC running a program called Labview. That includes a hardware device for digitizing the signals from the vibrating probe as well as delivering the desired waveform to the shaker. The programs that Dr. Rae wrote use Labview’s state of the art Fourier transform routines for taking complex sound waves or internal vibrations and splitting them into the individual frequencies they contain. For all of the measurements, the parts sat on vibration isolators designed by Dr. Rae. The data from each measurement is written to an Excel readable file where many complicated analyses and plots can be done to every file or combinations of many files using Excel macros.

Several kinds of measurements were routinely made. In most, the probe was lightly pressed again a rim or ring, etc and the computer program started so that the probe is pushed up and down onto the surface of the part at frequencies dictated by the computer program. Generally uniform amplitude white noise was used at a 1 Hz resolution to deliver 8000 frequencies to the part each at the same amplitude. The frequency range of 1Hz to 8000Hz extends both below and above the frequencies where banjos make sufficient sound to be relevant.

The probe contains an accelerometer and a force gauge. It measures the force it takes to compress the banjo part to a particular acceleration rate and then the computer plots the acceleration per force at each of the 8000 frequencies. This is called the driving point admittance which essentially measures the ability of the part to accept vibrations at that point. This shows the frequencies where the part is likely to vibrate but does not quantify what the vibration levels will be far away from the point where the probe sits down. To measure the vibrations at a distance, a small low mass accelerometer is attached elsewhere on the banjo part. It measures the acceleration at each frequency in the region where it is attached.

The output of the accelerometer is divided by the force in the vibrating probe which produces the vibrations and the computer plots the vibration amplitudes per unit driving force at each frequency. This measurement shows how the part actually vibrates. Each of the main vibratory peaks represents a vibration mode, a frequency region where the part has a propensity to vibrate.

With different computer programs written by Dr. Rae, it is possible to measure the rates at which vibrations or emitted sound waves decay after they are excited. The program allows the determination of the decay rate for each and every frequency of the 8000 routinely used. Most of the measurements could also be used to quantify emitted sound by simply substituting a microphone for the accelerometer. These sound measurement programs were written so that the computer did not have to be the vibration source. Vibrations were provided either by the banjo’s strings or by a tap from an impact hammer.

In addition, the rim and tone ring were prepared so that their detailed structure could be observed either under a light microscope or a scanning electron microscope. Some theory was utilized by modeling the rim and tone ring in a finite element analysis program called ANSYS and then seeing how predicted vibration modes changes when part geometry was changed.

For all of the measurements, the parts sat on vibration isolators designed by Dr. Rae. With the final versions of the isolators, the sound emitted by tapping the ring when it was resting on the isolators was essentially identical to that obtained when the ring was hanging from a very thin string or rubber band.

Using these measurements, we established plots of several vibration and sound parameters vs. frequency for the two prewar Gibson banjos that the expert listeners chose to be the best sounding prewars. We then proceeded to engineer a tone ring design whose vibratory curves fell right between those of the selected prewars. These comparisons support our claim that the new HR-30 ring is essentially identical to the finest pre war flatheads.

We also tested the majority of tone rings currently on the market to see if their vibratory curves matched those of our two prewar standard banjos. None of their curves matched our prewar standard.