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.

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.

2pSA9  – Acoustic transients from the impact force excitation of beams and wind chimes – Peter Stepanishen

Acoustic transients from the impact force excitation of beams and wind chimes
Peter Stepanishen, steppipr@uri.edu
University of Rhode Island
Department of Ocean Engineering
Narragansett, RI 02871


Popular version of paper 2pSA9
Presented on Tuesday December 3, 2019
178th ASA Meeting, San Diego, CA
The origin of wind chimes dates back to 1100 BC in Eastern and Southern Asia where the chimes were intended to ward off evil spirits and attract benevolent spirits. Modern wind chimes typically consist of 4 to 8 aluminum tubes with varying lengths and associated resonant frequencies corresponding to a specific musical scale. In addition the wind chimes also include a wind catcher and associated wind clapper to impact the chimes as illustrated in Figure 1 below:
The present paper addresses  the underlying physics of wind chimes from the viewpoint of a structural acoustician.  The impact excitation and vibration of the structure is  addressed including the effects of the surrounding air on the vibration characteristics of the wind chime which is modeled as a sum of cylindrical pipes or beams. The directional characteristics of the transient acoustic field are then addressed.
The dominant sound producing features for each cylindrical pipe/beam are simply described as a sum of  temporally decaying modal beam vibrations with different resonant frequencies which are inversely related to the square of the length of the pipe. A simple illustration of the predominant sound producing lowest frequency modal vibration is  illustrated in the accompanying video:
The video simply illustrates the lowest modal vibration of a free free beam which is presented as a simple model of a cylindrical wind chime vis-à-vis a cylindrical shell model.  The ends of the beam/pipe undergo the maximum transverse deflection and vibration whereas two nodal points with zero deflection are also apparent for the fundamental modal vibration.  In contrast to stringed musical instruments, the higher order modal vibrations are associated with a nonharmonic series of resonant frequencies with an increasing number of nodal points as the modal number increases. Experimental results confirm the validity and usefulness of the cylindrical beam model of the wind chime pipes.
Acoustic transient radiation from a vibrating chime is addressed in the paper using a space-time superposition of ring sources along the axis of the chime. The ring sources are shown to result in a space-time varying force on the air in contact with the chime.  Furthermore, the force is simply related to the previously noted sum of  temporally decaying modal beam vibrations. The directional properties of the acoustic field are discussed and it is shown that the field exhibits nulls in the directions along and perpendicular to the axis of the chime.

2aSA9 – Acoustic black holes in airfoils – Kaushik Sampath

NRC Postdoctoral Fellow, Acoustics Division, Code 7165,
U S Naval Research Laboratory
4555 Overlook Ave SW,
Washington, DC 20375
Caleb F. Sieck – caleb.sieck@nrl.navy.mil
Matthew D. Guild – matthew.guild@nrl.navy.mil
Charles A. Rohde – charles.rohde@nrl.navy.mil
Acoustics Division, Code 7165,
U S Naval Research Laboratory
4555 Overlook Ave SW,
Washington, DC 20375
Popular version of paper 2aSA9 – “Incorporating acoustic black holes in hydrofoils”
Presented at 11.30 am on December 3, 2019
178th ASA Meeting, San Diego, California.
Most of us who have flown in an airplane can recall how bumpy it gets when there is ‘turbulence’. It is scary to watch wings bend the way they do, even though they are designed to withstand such bumps. However, as one can imagine, these vibrations are not desirable and affect the aircraft’s longevity and performance. When we slice an aircraft wing somewhere in between (as highlighted in the sketch below), we find that it has a unique shape, called an ‘airfoil’. This is the shape that makes the plane fly, and also, as a result, bears the brunt of turbulent air and those vibrations.
In 1988, Mironov pointed out that vibrations that strike on one end of a beam may never make it through if the other end is tapered gradually enough all the way down to a ‘zero thickness’. In other words, those vibrations get trapped or absorbed inside forever – also known as the ‘acoustic black hole’ effect. In reality, since it isn’t possible to make an edge have zero thickness, scientists have figured out that sticking some damping material near the edge (similar to foam or rubber on furniture feet) works almost as well.
In this study, we explore a way this effect could help reduce those airfoil vibrations. For the airplane, only the shape on the outside of the airfoil matters, not the inside. We take advantage of this fact and show that it is possible to design an airfoil with these black holes inside, without changing either the outside shape or the total weight.
Shown below are three of the designs that we tested. We fix the mass of the structure and damping that we use, and redistribute them between the three cases. The first case has uniformly spread structure and damping, to represent a ‘standard’ design. We’re basically trying to improve on this. The second case has a single black hole inside along with appropriate damping, while the third case has three.
We use some of the latest in 3d printing technology to create these complex designs. For testing them, we vibrate all three airfoils on their front edge in the same way and measure how vibrations move through the airfoil length all the way to the back edge. Shown below are the vibration levels that we measured at the rear edge over three frequency ranges. Note, lower the vibration, the better.
When compared with the uniform case, the sample with one black hole does 10-15% in the low and mid frequency ranges, and ~30% better in the high frequency range. The three-black hole case does almost similar (~1% worse in fact) for the low frequency range, but performs 50-65% better for higher frequencies. These results are promising and motivate us to expand our research in this direction.
Work sponsored by the Office of Naval Research.

3pSA – Diagnosing wind turbine condition employing a neural network to the analysis of vibroacoustic signals – Andrzej Czyzewski

Andrzej Czyzewski
Gdansk University of Technology, Multimedia Systems Department
80-233 Gdansk, Poland
e-mail: multimed.org@gmail.com

Popular version of paper 3pSA 
Presented Wednesday afternoon, December 4, 2019
178th ASA Meeting, San Diego, California

The maintenance of wind turbines sums up to approx. 20-35% of their life-cycle costs. Therefore, it is important from the economic point of view to detect damage early in the wind turbines before failures occur. For this purpose, a monitoring system was built that analyzes both acoustic signals acquired from the non-contact acoustic intensity probe, as well as from the traditional accelerometers, mounted on the internal devices in the nacelle. The signals collected in this way are used for long-term training of the neural network. The appropriately trained network automatically detects deviations, signaling them to technical service. In this way, artificial intelligence is used to automatically monitor the technical condition of wind turbines.
Existing methods are mostly based on different types of accelerometers mounted on the blades of the wind turbine or on the bearings of the electric power generator. Contactless methods we develop provide many benefits (e.g. no need to stop the wind turbine for mounting of accelerometers). The main source of acoustic signals obtained without contact is a special multi-microphone probe that we have constructed. A special feature of this solution is the ability to precisely determine the direction from which the sound is received. Thanks to this, the neural network learns non-mixed up sounds emitted by mechanisms located in various places inside the turbine. The acoustical probe is presented in Figure 1, and the device containing electronic circuits for processing acoustic signals is shown in Figure 2.

Figure 1 Acoustical probe (a) and complete acoustical vector sensor (b)

Figure 2 Device collecting vibroacoustic signals (a),

which also contains a neural network module that detects if these signals are abnormal (b).


In addition, we are also developing methods for visual surveillance of a wind farm, which by their nature belong to non-contact methods. We received encouraging results by amplifying the invisible vibrations in video. The method we applied is called the motion magnification in the video (invented by scientists from MIT). We used this approach for extracting information on the vibrations of the whole wind turbine construction. What comes out of this can be seen in the two short films pasted below, the first of which shows the original video image, and the second after applying the invisible pixel movements caused by vibrations and swaying of the wind turbine tower.

Video 1. Original video recording of a working wind turbine

Video 2. The same turbine as in Video 1 after applying the pixel movements magnification

Since image vibrations can be transformed into acoustic vibrations, we were able to propose a method for monitoring wind turbines using a kind of non-contact vibrometry based on video-audio technology.

The neural network depicted in Figure 3 is the so-called autoencoder. It learns to copy its inputs to its outputs prioritizing the most relevant aspects of the data to be copied. In this way, it extracts relevant data from complex signals, so it also becomes sensitive to unexpected changes in the acoustic and video data structure. Therefore, a properly trained network can be entrusted with the task of supervising a wind turbine, i.e. checking that everything is in order with it.

Figure 3 Autoencoder neural network architecture, reflecting the principle that the encoder on the left sends only a minimal amount of relevant data, and yet the decoder on the right can reproduce the same information that the entire network sees on its inputs.

The research was subsidized by the Polish National Centre for Research and Development within the project “STEO – System for Technical and Economic Optimization of Distributed Renewable Energy Sources”, No. POIR.01.02.00-00-0357/16.

1aSAb4 – Seismic isolation in Advanced Virgo gravitational wave detector – Valerio Boschi

Seismic isolation in Advanced Virgo gravitational wave detector

Valerio Boschi – valerio.boschi@ego-gw.it

European Gravitational Observatory
Istituto Nazionale di Fisica Nucleare
Sezione di Pisa
Largo B. Pontecorvo, 3
56127 Pisa, Italy


Popular version of paper 1aSAb4

Presented Monday morning, May 13th, 2019

177th ASA Meeting, Louisville, KY


Imagine to drop a glass of water in the ocean. Due to that the global level of all the seas on the Earth will increase by an extremely small amount. A rough estimate would lead you to this amazingly tiny displacement: 10-18 m !! This length is equivalent to the sensitivity of current gravitational wave (GW) detectors.

GWs are ripples of space-time, produced by the collapse of extremely dense astrophysical objects, like black holes or neutron stars. Those signals induce on the matter small variation of length (less than 10-18 m at 100 Hz) that can be detected only by the world most precise rulers, the interferometers.

Second generation gravitational wave interferometers like the Advanced Virgo experiment, shown in fig. 1, which is based in Cascina, Italy and the two US-based Advanced LIGO detectors, are collecting GW signals since 2015 opening the doors of the so-called multi-messenger astronomy.

In order reach the required level of sensitivity of current interferometers many disturbances need to be strongly reduced. Seismic noise if not attenuated would represent the main limitation of current detectors. In facts, even in the absence of local or remote earthquakes, ground moves by mm in the frequency region between 0.3 and 0.4 Hz. This motion, called microseism, is caused by the continuous excitation of the Earth crust produced by the sea waves.

In this conference contribution we will present an overview of the seismic isolation systems used in Advanced Virgo GW interferometer. We will concentrate on the so-called super-attenuator, the seismic isolator used for all the detector main optical components, shown in fig. 2. This complex mechanical device is able to provide more than 12 orders of magnitude of attenuation above a few Hz. We will also describe its high-performance digital control system and the control algorithms implemented with it. Thanks to the performance and reliability of this system the current duty cycle of Advanced Virgo, is almost 90 %.

Figure 1 Aerial View of Advanced Virgo (EGO/Virgo collaboration)


Figure 2 Inside view of a super-attenuator





1pSA8 – Thermoacoustics of solids – Can heat generate sound in solids? – Haitian Hao

Acoustical balance between the singers and the orchestra in the Teatro Colón of Buenos Aires

Haitian Hao – haoh@purdue.edu
Mech. Eng., Purdue Univ.
Herrick Labs,
177 S. Russell St.
West Lafayette, IN 47906

Carlo Scalo
Mech. Eng., Purdue Univ.
Herrick Labs,
177 S. Russell St.
West Lafayette, IN 47906

Mihir Sen
Aerosp. and Mech. Eng.
Univ. of Notre Dame,
Notre Dame, IN

Fabio Semperlotti
Mech. Eng.
Purdue Univ.
West Lafayette, IN

Popular version of paper 1pSA8, “Thermoacoustic instability in solid media” – Haitian Hao, Carlo Scalo, Mihir Sen, and Fabio Semperlotti
Presented Monday, May 07, 2018, 2:45pm – 3:00 PM, Greenway C
175th ASA Meeting, Minneapolis

Many centuries ago glass blowers observed that sound could be generated when blowing through a hot bulb from the cold end of a narrow tube. This phenomenon is a result of thermoacoustic oscillations: a pressure wave propagating in a compressible fluid (e.g. air) can sustain or amplify itself when being provided heat. To date, thermoacoustic engines and refrigerators have had remarkable impacts on many industrial applications.

After many centuries of thermoacoustic science in fluids, it seems natural to wonder if such a mechanism could also exist in solids. Is it reasonable to conceive thermoacoustics of solids? Can a metal bar start vibrating when provided heat?

The study of the effects of heat on the dynamics of solids has a long and distinguished history. The theory of thermoelasticity, which explains the mutual interaction between elastic and thermal waves, has been an active field of research since the 1950s. However, the classical theory of thermoelasticity does not address instability phenomena that can arise when considering the motion of a solid in the presence of a thermal gradient. In an analogous way to fluids, a solid element contracts when it cools down and expands when it is heated up. If the solid contracts less when cooled and expands more when heated, the resulting motion will grow with time. In other terms, self-sustained vibratory response of a solid could be achieved due to the application of heat. Such a phenomenon would represent the exact counterpart in solids of the well-known thermoacoustic effect in fluids.

By using theoretical models and numerical simulations, our study indicates that a small mechanical perturbation in a thin metal rod can give rise to sustained vibrations if a small segment of the rod is subject to a controlled temperature gradient. The existence of this physical phenomenon in solids is quite remarkable, so one might ask why it was not observed before despite the science of thermoacoustics have been known for centuries.

“Figure 1. The sketch of the solid-state thermoacoustic device and the plot of the self-amplifying vibratory response.”

It appears that, under the same conditions of mechanical excitation and temperature, a solid tends to be more “stable” than a fluid. The combination of smaller pressure oscillations and higher dissipative effects (due to structural damping) in solids tends to suppress the dynamic instability that is at the origin of the thermoacoustic response. Our study shows that, with a proper design of the thermoacoustic device, these adverse conditions can be overcome and a self-sustained response can be obtained. The interface conditions are also more complicated to achieve in a solid device and dictates a more elaborate design.

Nonetheless, this study shows clear theoretical evidence of the existence of the thermoacoustic oscillations in solids and suggests that applications of solid-state engines and refrigerators could be in reach within the next few years.