A moth’s ear inspires directional passive acoustic structures

Lara Díaz-García – lara.diaz-garcia@strath.ac.uk
Twitter: @laradigar23
Instagram: @laradigar

Centre for Ultrasonic Engineering, University of Strathclyde, Glasgow, Lanarkshire, G1 1RD, United Kingdom

Popular version of 2aSA1-Directional passive acoustic structures inspired by the ear of Achroia grisella, presented at the 183rd ASA Meeting.

Read the article in Proceedings of Meetings on Acoustics

When most people think of microphones, they think of the ones singers use or you would find in a karaoke machine, but they might not realize that much smaller microphones are all around us. Current smartphones have about three or four microphones that are small. The miniaturization of microphones is therefore a desire in technological development. These microphones are strategically placed to achieve directionality. Directionality means that the microphone’s goal is to discard undesirable noise coming from directions other than the speaker’s as well as to detect and transmit the sound signal. For hearing implant users this functionality is also desirable. Ideally, you want to be able to tell what direction a sound is coming from, as people with unimpaired hearing do.

But dealing with small size and directionality presents problems. People with unimpaired hearing can tell where sound is coming from by comparing the input received by each of our ears, conveniently sitting on opposite sides of our heads and therefore receiving sounds at slightly different times and with different intensities. The brain can do the math and compute what direction sound must be coming from. The problem is that, to use this trick, you need two microphones that are separated so the time of arrival and difference in intensity are not negligible, and that goes against microphone miniaturization. What to do if you want a small but directional microphone, then?

When looking for inspiration for novel solutions, scientists often look to nature, where energy efficiency and simple designs are prioritized in evolution. Insects are one such example that faces the challenge of directional hearing at small scales. The researchers have chosen to look at the lesser wax moth (fig 1), observed to have directional hearing in the 1980s. The males produce a mating call that the females can track even when one of their ears is pierced. This implies that, instead of using both ears as humans do, these moths’ directional hearing is achieved with just one ear.

Lesser wax moth specimen with scale bar. Image courtesy of Birgit E. Rhode (CC BY 4.0).

The working hypothesis is that directionality must be achieved by the asymmetrical shape and characteristics of the moth’s ear itself. To test this hypothesis, the researchers designed a model that resembles the moth’s ear and checked how it behaved when exposed to sound. The model consists of a thin elliptical membrane with two halves of different thicknesses. For it, they used a readily available commercial 3D printer that allows customization of the design and fabrication of samples in just a few hours. The samples were then placed on a turning surface and the behavior of the membrane in response to sound coming from different directions was investigated (fig 2). It was found that the membrane moves more when sound comes from one direction rather than all the others (fig 3), meaning the structure is therefore passively directional. This means it could inspire a single small directional microphone in the future.

Laboratory setup to turn the sample (in orange, center of the picture) and expose it to sound from the speaker (left of the picture). Researcher’s own picture.
Image adapted from Lara Díaz-García’s original paper. Sounds coming from 0º direction elicit a stronger movement in the membrane than other directions.

3pMUa4 – Acoustical analysis of 3D-printed snare drums

Chris Jasinski – jasinski@hartford.edu
University of Hartford
200 Bloomfield Ave
West Hartford, CT 06117

Popular version of paper 3pMUa4
Presented Wednesday afternoon, December 09, 2020
179th ASA Meeting, Acoustics Virtually Everywhere

For many years, 3D printing (or additive manufacturing) has been a growing field with applications ranging from desktop trinkets to prototypes for replacements of human organs. Now, Klapel Percussion Instruments has designed its first line of 3D-printed snare drums.

Snare drums are commonly used in drum sets, orchestras, and marching bands. They are traditionally made with wood or metal shells, metal rims, plastic (mylar) skins, and metal connective hardware including bolts, lugs, and fasteners. For the first phase of Klapel prototypes, the shell and rim are replaced with a proprietary carbon fiber composite. Future iterations intend to replace all of the hardware with 3D printing as well.  The shell and rim are produced layer by layer until the final shape is formed. Even with high quality printers, layers can be seen in the final texture of 3D-printed objects. These layers appear as horizontal lines and vertical seams where each layer starts and finishes.

Snare drums

3D-printed snare drum and detail of finished texture.

Klapel Percussion Instruments contacted the University of Hartford Acoustics Program to assess if having a 3D-printing shell and rim changes the fundamental vibrational and acoustical characteristics of the drum. To test this, undergraduate students developed a repeatable drum striking device. The machine relies on gravity and a nearly zero-friction bearing to strike a snare drum from a consistent height above the playing surface. With precise striking force, the resulting sound produced by the drum was recorded in the University of Hartford’s anechoic chamber (a laboratory designed to eliminate all sound reflections or ‘echoes’, shown in the example photo of the striking machine). The recordings were then analyzed for their frequency content.

Snare drums

Snare drum striking machine inside Paul S. Veneklasen Research Foundation Anechoic Chamber at University of Hartford.

Along with the acoustical testing, the drum shell (the largest single component of a snare drum) underwent ‘modal analysis’, where 30 points are marked on each shell and struck with a calibrated force-sensing hammer. The resulting vibration of the drum is measured with an accelerometer. The fundamental shapes (or ‘modes’) of vibration can then be visualized using processing software.

Snare drums

Vibrational mode shapes for maple drum shell [left] and 3D-printed shell [right].

Ultimately, the vibrational and acoustical analysis resulted in the same conclusions. The fundamental shapes of vibration and the primary frequency content of the snare drum is unaffected by the process of 3D printing. The most prominent audible frequencies and vibrational shapes are identical in both the maple wood shell and the carbon fiber 3D-printed shell, as seen in the visualized modes of vibration. This means that the 3D-printed drum technology is a viable alternative to more traditional manufacturing techniques for drums.

There are substantial, measurable variations that impact the more subtle characteristics of the drum at higher, less prominent frequencies, and for more complex vibration shapes. These are noticeable above 1000 Hz in the frequency analysis comparison.

Snare drums

Frequency analysis at two striking locations for maple (wood) and carbon fiber (3D-printed) drum.

Future testing, including subjective listening tests, will aim to identify how these smaller variations impact listeners and performers. The results of the future tests can help determine how acoustical metrics can predict listener impressions.