Page 52 - Spring2022

ADDITIVE MANUFACTURING FOR ACOUSTICS
plans online or drawing one’s own plans, it’s possible to use AM to build a kazoo, guitar, or flute. Recent review articles such as that by Michon et al. (2018) have high- lighted studies using AM and acoustics. An interesting use case is the fabrication of instruments where the user has access to the drawings or 3D model of an instrument. An example of this were researchers who had drawings of clarinet mouthpieces from the 1890s and fabricated them using AM (Cottrell and Howell, 2019). The authors printed multiple mouthpieces using different techniques such as FDM or stereolithography and received quali- tative feedback from professional musicians regarding quality. AM effects such as surface finish or material strength were found to vary from process to process by the evaluating musician’s response and these features affected the perceived quality of each mouthpiece.
String instruments produced using AM are rare due to material availability, limiting materials selection to plas- tics, metals, or ceramics, all of which produce a dynamic response that differs from traditional wood designs (Qian,
2019). Studies of AM ukuleles using FDM showed large differences in both measured A-weighted sound level and timbre compared with a traditional wood instru- ment. This is because the AM-constructed material has a different stiffness from the wood instrument. The two instruments compared in Qian’s study (2019) are shown in Figure 3a.
AM offers new capabilities enabled by the ability to create complex designs that are not possible using stan- dard manufacturing techniques. For example, Ritz et al. (2015) investigated the field of microtonal music, which employs more equally spaced intervals in an octave than are employed in a standard 12 semitone equal tem- perament used in most Western music. By using AM technology to create a double-helix flute, they were able to exploit geometry to create these tones. Taking a more physics-based design approach, Thacker and Giordano (2021) used fluid computational approaches to design improved recorder instruments which were then fabri- cated with AM and their performance compared with the fluid model (Figure 3b).
Applications to Acoustic Metamaterials
Acoustic metamaterials are engineered structures that can control acoustic waves in ways that are not possible with typ- ical “bulk” materials such as steel or plastics (Haberman and Norris, 2016). Acoustic metamaterials are usually produced by constructing a material from periodically repeating “unit cells” that, when properly designed, exhibit novel acoustic behavior like band gaps (a range of frequencies where waves cannot propagate through the material). One example of this behavior can be observed in an artistic structure in Madrid that is composed of periodically arranged long metal cylinders (Martinez-Sala et al., 1995). This structure has a band gap in the audible range (around 1,600 Hz), so if you were to play a 1,600-Hz tone on one side of the structure, it would be filtered by the structure rendering it inaudible on the other side. Like this structure, acoustic metamaterials can act as a “filter” for acoustic waves.
Although research on acoustic metamaterials has been ongoing for several decades, many of these ideas have only recently taken form with the advancement of AM. The reason is that AM can create objects with intricate and complex geometries, such as the structures shown in Figure 4, a and c, that are impossible to create using traditional manufacturing techniques. Metamaterial
Figure 3. 3D-printed musical instruments. a: Printed ukulele (left) and wood instrument (right). Reproduced from Qian (2019), with the permission of the American Institute of Physics (AIP). b: A recorder that has been optimized using computer software (left) and a printed prototype (right). Reproduced from Thacker and Giordano (2021), with permission from the American Institute of Physics (AIP).
52 Acoustics Today • Spring 2022