Page 54 - Spring2022

ADDITIVE MANUFACTURING FOR ACOUSTICS
et al. (2021). In that work, waves were generated in an AM-fabricated specimen using a transducer configured
to excite waves on the surface of the material. An air- coupled transducer then measured the response of the material using the airborne acoustic wave generated from the surface motion of AM material. Using this approach, the properties of the AM-fabricated material can be extracted. This ultrasonic technique measured the acous- tic response from steels manufactured using various AM techniques and compared observations with steels made using traditional methods. The results showed differ- ent acoustic responses for the different manufacturing methods, indicating that materials created using AM fab- rication techniques should be carefully evaluated prior to use in critical components. Other recent NDE research relevant to AM fabrication has shown the possibility of using ultrasonic methods to determine the mechanical parameters and print quality in real time as the part is printed (Gillespie, 2021). Additional recent work has also shown how acoustic metamaterials can be used to enhance ultrasonic NDE measurements by creating filter- ing materials (Smith and Matlack, 2021) to better isolate the ultrasonic response or a portion of the structure of interest. This and similar approaches to ultrasonic NDE are uniquely enabled by AM.
Acoustic Transduction Materials and Devices
The ability to generate and sense sound has always been central to the study, application, and enjoyment of acous- tics. As a result, there has been considerable exploration of the structure and materials used to construct acousti- cal devices. Present-day examples range from consumer electronics with exotic geometries and transduction com- ponents to mass-manufactured microelectromechanical systems (MEMS) microphones and accelerometers in smart devices to high-precision ultrasonic measurement systems. Noting that key developments in the science and technology of acoustic transducers are often enabled by technological advances, AM technology provides an exciting opportunity to explore how to improve well- established approaches to acoustic transduction.
An obvious application example of AM technology is the ability to create highly unique geometries that cannot be created using traditional manufacturing techniques. A notable recent work by Nielsen et al. (2021) considered this case in a numerical study that considered the values
and distribution of “stiffness, mass, and damping of both the speaker diaphragm and surround” to optimize loud- speaker response.
One of the more interesting prospects of AM technology is the potential to directly print the transducer compo- nents or materials (Chen et al., 2020) sand do so in a streamlined process that could be extended to include fabrication and assembly of electrical, mechanical, and transducing components (Ambriz et al., 2017). Kierzew- ski et al. (2020) created a macroscopic embodiment of piezoelectric material that was first described by Bauer et al. (2004). The work of Kierzewski et al. (2020) leveraged the geometric freedom offered by AM paired with a multi- step assembly process to essentially replicate the response of a condenser microphone and extend it to a collection of cavities. Although not currently ideal for application, this work shows the relative ease of creating true transducing
“materials” using AM techniques that have not yet been fully leveraged for this type of technology.
Additive technology is also of interest for the direct man- ufacture of transduction materials. The most common materials in accelerometers and underwater transduc- ers are piezoelectric ceramics. There is considerable progress on various manufacturing techniques to print piezoelectric ceramics using approaches like selective laser sintering and paste extrusion followed by postpro- cessing (reviewed by Chen et al., 2020). Cui et al. (2019) have taken a very different approach by investigating novel AM techniques to create lattice structures that display piezoelectric coupling with tailored anisotropy and directional sensitivity that could ultimately be used for a wide range of applications. The opportunities that arise in being able to create active materials with confor- mal geometries, tailored piezoelectric coupling constants, and multimaterial components have a vast potential to significantly alter how vibroacoustic transducers are cre- ated in the future.
Hearing Prostheses and Hearing Aids
In the early stages of development, AM technology was collectively referred to as “rapid prototyping” due to the fact that part quality was insufficient for use in func- tional parts or products. One of the earliest examples of the transition of AM technology from rapid prototyp- ing to “rapid production” was in the field of hearing aid technology (Widmer and Dutta, 2005). Most hearing
54 Acoustics Today • Spring 2022