CONVERTIBLE CELLO PROTOTYPE
May 2017
Given the constraint of designing a cello for elementary school-age children, our team spent the majority of a semester researching users and refining design concepts. With two weeks left, we had our concept - a cello which could be converted from acoustic to electric for durability and portability outside of orchestral performances. In the following weeks, I led the process of rapidly mechanically designing and building the prototype of this instrument as detailed here.
Our design consisted of an extended neck which supported the tensioned strings, with body attachments for either an acoustic or electric instrument. A significant challenge in the design was to create attachment points that were sturdy, visually unobtrusive, easy to detach/attach, and didn’t interfere with the acoustics of the bodies. To do this, we made sure that the bodies’ only points of contact with the neck were at their very top and bottom edges and at the bridge. We designed strategic cutouts in the neck and body into which the contrasting parts could nest, creating secure connections. These were then easily secured by clasps on the bodies, which snapped into hidden hooks in the neck.
Acoustically, the largest challenge a convertible cello presented was the transmission of vibrations from the strings to the acoustic body. In standard cellos, this is done through the bridge of the instrument, meaning the bridge must make contact only with the strings and the cello body. In our instrument, this was impossible to do without a body attached. Therefore, we designed a bridge that would extend through a hole in the neck, with a widened top that rested on the electric neck until a body was attached. This pushed the bridge extension toward the strings, such that the bridge’s only points of contact were with the body and strings. While this design caused slight string length variations after each body change, the instrument had only to be quickly tuned before it was playable.
Our instrument’s acoustic body imitated the dimensions of a typical 3/4 size cello. One notable difference was the lack of curved front and back faces, which we learned play a role in the structural integrity of standard cellos but are not acoustically necessary. In our case, the body was not supporting the tension of strings, and it happened that the center post and bass bar were enough to support the force of the bridge. We therefore used flat faces for the sake of future repairability, as well as ease of design and construction in our prototype. While we initially considered using just the extended neck as our electric instrument, we learned that the best sounding electric cellos include a small acoustic chamber onto which the pickups are placed for a richer and more natural range of tones, hence the inclusion of a smaller electric body.
In our prototype, we chose to use acrylic for the bodies due to its ease of construction; we laser cut the faces, heat formed the sides around molds, then bonded the two together. Acrylic was also chosen over wood because of the problems associated with wood warping, which we determined weren’t worth the acoustic advantages in a children’s practice instrument (the acrylic, in fact, produced a surprisingly rich sound). However, through the use of mechanical tuners and a neck that was separate from the bodies, warping was far less of an issue in the electric neck, which we cut from a single piece of lumber. Amplification of the electric instrument was acheived by attaching adhesive pickups to the electric body and routing the signal through a small audio amplifier.