Ashlie Burton, and Dr. Lawrence Rees, Physics and Astronomy

Introduction
2ft Prosthetics has been making below-the-knee prostheses for developing countries since 2010. While these feet have been helpful, improvements should be made to the design. One of the common complaints about the PVC foot was the amount of noise created during gait. The purpose of this project was to research different sound reduction designs for the PVC prosthesis. Initially two designs were drafted, out of the two, one was built and tested.

The new prosthesis tested was very similar to the original PVC Design. The design was simply modified by gluing strips of bicycle tire inner tube in between each layer of the prosthesis during assembly.

Methodology
To test these feet, a mold was made to try to insure each foot was made with the same size and angles in the ankle joint. Five original design and four modified design prostheses were made. The nine feet were then taken to an anechoic chamber where the prosthetic testing fixture was set up. The testing fixture consists of two pneumatic pistons that strike the foot at the heel and toe.

Each foot was put on the testing fixture and recorded for approximately thirty seconds, cycled through a total of 100 steps, and then recorded for thirty more seconds. This was done to study the reliability of the design after heavy use.

After sound testing, the feet were then brought out of the anechoic chamber and tested for durability. The prosthesis were put on the same pneumonic testing fixture and cycled until failure.

Results
The reported results include two main data manipulations. First, the background noise from the testing fixture was recorded. From this, a filter was created and put over the recordings to isolate the sounds coming from the prosthesis. Second, sound clips not including the bending of prosthesis were cut from the recording. This was done so that the varying frequency of the pneumatic pistons would not affect the data. The average volume of the recordings is shown below (Figure 1).

Discussion

After reviewing the data, it shows that the tire design reduces noise. The overall average volume was -66.7 dB and -71.15 dB for the original and tire designs respectively. However, it seems that in the testing of the first foot, the microphone was mistakenly placed closer to the testing fixture, skewing the results. Without using data from original foot one, the average is -70.4, which is only slightly higher than the tire design. Despite this result, it seems that the tire design does improve the sound, but our filter cannot isolate the sounds created by the bending prosthesis. When listening to the filtered recording, the pneumatic piston is quieter but still very prominent. A better filter needs to be used to accurately represent how the original foot and new design compare.

Durability testing has not yet been completed, but both designs are currently being tested. As of now, only one prosthesis has broken. This foot was one of the original designs and it failed after only a few steps during sound testing. The soleplate of the prosthesis broke from the ankle, a failure common in poorly made prostheses. Given the failure type, it is likely that that this foot was built incorrectly. The remaining prostheses are currently being cycled until failure and we are hopeful that the new design will be as durable as the original.

Conclusion

Though initial testing shows promise, there is still much research to be done. The filter on the recordings needs to be improved as the pneumatic pistons still contribute to the majority of the noise. The feet would be more accurately tested on a quieter machine or on actual amputees. Within the next couple months durability testing will be finished along with the testing of the second design. This research will help amputees in developing countries have more comfortable low-cost prostheses.