© 2016-2018 by NINA SINATRA

ELEPHANTINE SOUND

2012

Elephantine Sound was a project completed for MIT course 3.054: Cellular Solids under Prof. Lorna Gibson. Cellular structures encompass two- or three-dimensional prismatic or polyhedral arrays. These architectures are found in a wide range of materials, from foams and catalytic substrates to trabecular bone and wood. The course was focused on mathematical, microstructural, and mechanical characterization of cellular structures.

 

This project was developed as a capstone project for this course. After visiting the Harvard Museum of Comparative Zoology, I was intrigued to learn about elephants' long-range hearing ability. The goal of this project was to fabricate a scale model of an elephant skull to understand how sound propagates through the extensive cellular chambers inside the cranium. The cranium of the Asian Elephant (Elephas maximus) is composed of honeycomb cavities that yield  superior out-of-plane compressive and shear properties, and greatly expand the animal’s acoustic abilities - namely the production and detection of infrasound.  The large cranial resonating chambers allow elephants to create and receive low-frequency sound waves, and to produce harmonics over a 10.5 octave range (as compared to 1-2 octaves in humans). As a result, elephants are able to distinguish low frequency sounds from background noise. The honeycomb structure of the elephant cranium is believed to amplify low frequency sound waves so that emitted infrasound signals can propagate through a farther distance and allow communication with other members of the clan.

 

In order to analyze the acoustic properties of the elephantine skull, I retrieved a CT scan of an Asian elephant cranium through Digimorph, an NSF-funded digital library based at the University of Texas, Austin. Using the scan, I 3D printed a 0.5-scale model of a juvenile Elephas maximus skull using a Z Corporation Spectrum printer and a polymer-gypsum powder blend. A 0.5-scale model of a juvenile Tursiops truncates dolphin skull was printed as a control; this bottlenose dolphin skull does not include the honeycomb structure used by Elephase to detect infrasound.

 

A custom LabView program connected to an accelerometer and spectrum analyzer, the latter of which was wired to both skull models. White noise was amplified toward the skull models, which were suspended in air during testing. Data revealed that the elephant skulls were able to amplify significantly more soundwaves than the non-honeycomb dolphin skulls.  This brief exploration of sound and biomimetic structure provided a fascinating insight into the connections between elephantine behavior and physiology. 

 

Porous Bone Structure

The cavities within our 3D printed elephant skulls replicated those of the Asian elephant. This specimen is part of the collection at the Harvard Museum of Comparative Zoology.