1pAB4 – Size Matters To Engineers, But Not To Bats

Rolf Müller – rolf.mueller@vt.edu
Bryan D. Todd

Popular version of paper 1pAB4, “Beamwidth in bat biosonar and man-made sonar”
Presented Monday, May 7, 2018, 1:30-3:50 PM, LAKESHORE B,
175th ASA Meeting, Minneapolis.

Bats and Navy engineers both use sonar systems. But do they worry about the same design features?

To find out, we have done an exhaustive review of both kinds of sonar systems, poring over the spec sheets of about two dozen engineered sonars for a variety of applications and using computer models to predict 151 functional characteristics of bat biosonar systems spanning eight different biological families. Crunching the numbers revealed profound differences between the way engineers approach sonar and the way bats do.

The most important finding from this analysis is related to a parameter called beamwidth. Beamwidth is a measure of the angle over which the emitted sonic power or receiver sensitivity is distributed. A small beamwidth implies a focused emission, where the sound energy is – ideally – concentrated with laser-like precision. But the ability to generate such a narrow beam is limited by the sonar system’s size: the larger the emitter is relative to the wavelength it uses, the finer the beam it can produce. Reviewing the design of man-made sonars indicates that beamwidth has clearly been the holy grail of sonar engineering — and in fact, the beamwidth of these systems hews closely to their theoretical minima.

bats

Some of the random emission baffles made from crumpled aluminum foil that served as a reference for the scatter seen in the bat beam width data.

But when it comes to beamwidth, tiny bats are at a significant disadvantage: even the largest bat ears are barely ten times the size of the animals’ ultrasonic wavelength, while engineered systems can exceed their wavelengths by 100 or 1000 times. Remarkably, our analysis showed that bats seem to disregard beamwidth entirely. In our data set, the bats’ beamwidth scattered widely towards larger values; the scatter was even larger than that for random cone shapes we created from crumpled aluminum foil. Clearly, the bats’ sonar systems are not optimized for beamwidth. But we know that they are incredible capable when it comes to navigating complex environments — which begs the question: what criteria are influencing their design?

We don’t know yet. But the bats’ superior performance demonstrates every night that giant sonar arrays with narrow beamwidths aren’t the only and certainly not the most efficient path to success: smaller, leaner solutions exist. And those solutions will be necessary for compact modern systems like autonomous underwater or aerial vehicles. To make sonar-based autonomy in natural environments a reality, engineers should let go of their fixation on size and look at the bats.

4bPA2 – Perception of sonic booms from supersonic aircraft of different sizes

Alexandra Loubeau – a.loubeau@nasa.gov
Structural Acoustics Branch
NASA Langley Research Center
MS 463
Hampton, VA 23681
USA

Popular version of paper 4bPA2, “Evaluation of the effect of aircraft size on indoor annoyance caused by sonic booms and rattle noise”
Presented Thursday afternoon, May 10, 2018, 2:00-2:20 PM, Greenway J
175th Meeting of the ASA, Minneapolis, MN, USA

Continuing interest in flying faster than the speed of sound has led researchers to develop new tools and technologies for future generations of supersonic aircraft.  One important breakthrough for these designs is that the sonic boom noise will be significantly reduced as compared to that of previous planes, such as the Concorde.  Currently, U.S. and international regulations prohibit civil supersonic flight over land because of people’s annoyance to the impulsive sound of sonic booms.  In order for regulators to consider lifting the ban and introducing a new rule for supersonic flight, surveys of the public’s reactions to the new sonic boom noise are required. For community overflight studies, a quiet sonic boom demonstration research aircraft will be built. A NASA design for such an aircraft is shown in Fig. 1.

(Loubeau_QueSST.jpg) - sonic booms

Figure 1. Artist rendering of a NASA design for a low-boom demonstrator aircraft, exhibiting a characteristic slender body and carefully shaped swept wings.

To keep costs down, this demonstration plane will be small and only include space for one pilot, with no passengers.  The smaller size and weight of the plane are expected to result in a sonic boom that will be slightly different from that of a full-size plane.  The most noticeable difference is that the demonstration plane’s boom will be shorter, which corresponds to less low-frequency energy.

A previous study assessed people’s reactions, in the laboratory, to simulated sonic booms from small and full-size planes.  No significant differences in annoyance were found for the booms from different size airplanes.  However, these booms were presented without including the secondary rattle sounds that would be expected in a house under the supersonic flight path.

The goal of the current study is to extend this assessment to include indoor window rattle sounds that are predicted to occur when a supersonic aircraft flies over a house.  Shown in Fig. 2, the NASA Langley indoor sonic boom simulator that was used for this test reproduces realistic sonic booms at the outside of a small structure, built to model a corner room of a house.  The sonic booms transmit to the inside of the room that is furnished to resemble a living room, which helps the subjects imagine that they are at home.  Window rattle sounds are played back through a small speaker below the window inside the room.  Thirty-two volunteers from the community rated the sonic booms on a scale ranging from “Not at all annoying” to “Extremely annoying”.  The ratings for 270 sonic boom and rattle combinations were averaged for each boom to obtain an estimate of the general public’s reactions to the sounds.

(Loubeau_IER.jpg) - sonic booms

Figure 2. Inside of NASA Langley’s indoor sonic boom simulator.

The analysis shows that aircraft size is still not significant when realistic window rattles are included in the simulated indoor sound field.  Hence a boom from a demonstration plane is predicted to result in approximately the same level of annoyance as a full-size plane’s boom, as long as they are of the same loudness level.  This further confirms the viability of plans to use the demonstrator for community studies.  While this analysis is promising, additional calculations would be needed to confirm the conclusions for a variety of house types.

1aPP1 – With two ears and a cochlear implant, each ear is tuned differently

David Landsberger – David.Landsberger@nyumc.org

New York University School of Medicine
Department of Otolaryngology – EAR-Lab
462 First Ave STE NBV 5E5
New York, NY 10016, USA
www.ear-lab.org

Popular version of 1aPP1 Electrode length, placement, and frequency allocation distort place coding for bilateral, bimodal, and single-sided deafened cochlear implant users
Presented Monday morning, May 7, 2018, 8:05-8:25 AM, Nicollet D2
175th ASA Meeting, Minneapolis, Minnesota.

Imagine listening to the world with two ears that are tuned differently from each other. A key pressed on a piano would be perceived as different notes in the left and right ear. A person talking would sound like two different people simultaneously saying the same thing, one to each ear. This is in fact the experience for many people listening with two ears where one of the two ears has a cochlear implant.

The cochlea in a normal hearing ear is arranged “tonotopically.”  That is, high frequencies are represented in the bottom (base) of the cochlea and low frequencies are represented at the top (apex) of the cochlea. The regions between the base and apex of the cochlea represent different frequencies and are ordered along the cochlea from low (in the apical region) to high (in the basal region) along the cochlea.

Cochlear implants take advantage of the tonotopic property using an array of electrodes inside the cochlea. Stimulation from an electrode placed deeper into the cochlea provides a lower pitch than an electrode placed closer to the base of the cochlea.  Cochlear implant signal processing therefore provides information about low frequencies on apical electrodes and high frequencies on basal electrodes.

However, there is a mismatch between the frequency represented by a given electrode and the frequency expected by a normal ear at the same location. For example, the deepest electrode might represent 150-200 Hz but be placed in a location that expects approximately 1000 Hz. One factor effecting this relationship is the placement of the electrodes in the cochlea.  This depends on electrode length, surgical placement, and size of the individual’s cochlea.  Another factor is the “frequency allocation” which is the mapping of which frequency ranges are represented by each electrode [1]. The result is that the world is presented pitch shifted (and warped) by a cochlear implant relative to what would be expected by a normal ear.

This distortion may or may not be an issue for traditional cochlear implant users who are bilaterally deaf and listen to the world via a single unilateral implant. For these users, although pitch may be transposed, the transposition is consistent and therefore may be easier to perceptually manage. However, it has become more common for cochlear implant users to listen to the world with two ears (i.e. a cochlear implant in each ear, or a cochlear implant in one ear with acoustic hearing in the other). In this situation, each ear will be differently transposed. This may result in a single auditory object being perceived as two independent auditory objects and may provide contralateral spectral interference. The bilateral listener with a cochlear implant will likely listen to the world with conflicting information provided to each ear.

In the following presentation, we will quantify the magnitudes of these distortions across ears. We will discuss limitations (and potential modifications) to electrode design frequency allocations to minimize this problem for cochlear implant users listening with two ears.

Audio Demos:

(Figure 1) audio files “chickenleg.wav” and “ring.wav”

“Two audio demonstartions of listening to sounds that are differently tuned in each ear. In each sample, a sound is presented normally to one ear and pitch shifted to the other ear.  The first sample consists of speech while the second sample consists of music. These samples simulate only a pitch shift and not hearing loss or the sound quality of a cochlear implant. Note: demos should be played back over headphones.”

[1] D.M. Landsberger, M. Svrakic, J.T. Roland and M. Svirsky, “The Relationship Between Insertion Angles, Default Frequency Allocations, and Spiral Ganglion Place Pitch in Cochlear Implants,” Ear Hear, vol. 36, pp. e207-13., 2015.

4aMU6 – How Strings Sound Like Metal: The Illusion of the Duck-Herders Musical Cape

Indraswari Kusumaningtyas – i.kusumaningtyas@ugm.ac.id
Gea Parikesit – gofparikesit@ugm.ac.id

Faculty of Engineering, Universitas Gadjah Mada
Jl. Grafika 2, Kampus UGM
Yogyakarta, 55281, INDONESIA

Popular version of paper 4aMU6, “Computational analysis of the Bundengan, an endangered musical instrument from Indonesia”
Presented Thursday morning, May 10, 2018, 10:00-10:15 AM, Lakeshore A
175th ASA Meeting, Minneapolis, MN

Bundengan is an endangered musical instrument from Indonesia. It has a distinctive half-dome structure, which is originally built by duck herders and used as a cape to protect themselves from adverse weather when tending their flocks. To pass their time in the fields, the duck herders play music and sing. The illusive sound of the bundengan is produced by plucking a set of strings equipped with small bamboo clips and a number of long, thin bamboo plates fitted on the resonating dome; see Figure 1. The clipped strings and the long, thin bamboo plates allow the bundengan to imitate the sound of the gongs and kendangs (cow-hide drums) in a gamelan ensemble, respectively. Hence, it is sometimes referred to as the poor-man’s gamelan. Examples of the bundengan sound can be found from: http://www.auralarchipelago.com/auralarchipelago/bundengan.

Kusumaningtyas Parikesit – Figure 1. Construction of the bundengan 300 dpi.jpeg
Figure 1. The construction of the bundengan (left). A set of strings with small bamboo clips and a number of long, thin bamboo plates are fitted on the grid (right).

Amongst the components of the bundengan, arguably the most intriguing are the strings. We use computational simulations to investigate how the clipped strings produce the gong-like sound. By building a finite element model of a bundengan string, we visualize how the string vibration changes when the number, size (hence mass), and position of the bamboo clips are varied.

We first simulate the vibration of a 20 cm string, first with no bamboo clip and then with one bamboo clip placed at 6 cm from one of its end. Compared to the string with no clip (Figure 2a), the addition of the bamboo clip alters the string vibration (Figure 2b), such that two vibrations of different frequencies emerge, each located at different sections of the string divided by the bamboo clip. A relatively high frequency vibration occurs at the longer part of the string, whereas a relatively low frequency vibration occurs at the shorter part of the string. This correlates well with our high-speed recording of the bundengan string vibration; see http://ugm.id/bundengan.

Kusumaningtyas Parikesit - Figure 2. Bundengan string without and with clip 300 dpi.jpeg
Figure 2. Contour plot of the bundengan string vibration when plucked at the centre of the string for (a) no bamboo clip, and (b) one bamboo clip located at 0.06 m.

We also simulate how the position of the bamboo clip affect the frequencies of the string vibration and, hence, the sound produced by the clipped string. Figure 3 demonstrates that, for the string with a bamboo clip, we have two strong peaks at frequencies lower and higher than the frequency of the peak when there is no clip. The magnitudes of these two peaks change as the clip is shifted away from the end of the string, changing the pitch of the sound.

Kusumaningtyas Parikesit - Figure 3. Frequency spectrum 300 dpi.jpegFigure 3. Frequency spectra of the bundengan string vibration when the location of the bamboo clip is shifted from 1 cm to 9 cm from one end of the 20 cm string. The spectrum for the string with no clip is also given (top graph).

In a bundengan string equipped with bamboo clip, the emergence of the two different-frequency vibrations at different sections of the string is the key to the production of the gong-like sound. The vibration spectra allow us to understand the tuning of the bundengan string due to the position of the bamboo clip. This can serve as a guide to design the bundengan, providing possibilities for future developments.

 

List of Figures.
Kusumaningtyas Parikesit – Figure 1. Construction of the bundengan 300 dpi.jpeg 
Kusumaningtyas Parikesit – Figure 2. Bundengan string without and with clip 300 dpi.jpeg
Kusumaningtyas Parikesit – Figure 3. Frequency spectrum 300 dpi.jpeg

1aNS3 – Low-frequency sound control by means of bio-inspired and fractal designs

Anastasiia O. Krushynska – akrushynska@gmail.com
Federico Bosia – fbosia@unito.it
Nicola M. Pugno – nicola.pugno@unitn.it
Laboratory of Bio-inspired and Graphene Nanomechanics
Department of Civil, Environmental and Mechanical Engineering
Uiversity of Trento
Via Mesiano 77
Trento, 38123, ITALY

Popular version of paper 1aNS3, “Fractal and bio-inspired labyrinthine acoustic metamaterials”
Presented Monday morning, May 7, 2018, 9:15-9:35, Nicolett 3D
175th ASA Meeting, Minneapolis

Road, rail, airports, industry, urban environments, crowds – all generate high-volume sound. When sound becomes uncomfortable or even painful to the ear, it is generally called noise. Nowadays, noise is one of the most widespread environmental problems in developed countries, negatively affecting public health and quality of life. Recent findings of the World Health Organization show that noise pollution is not only annoying for a large percentage of the population, but also causes sleep disturbance, increases the risk of cardiovascular diseases, intensifies the level of stress and hinders learning processes. Low-frequency noise is the most troublesome type and is mainly produced by road vehicles, aircraft, industrial machinery, wind turbines, compressors, air-conditioning units, etc.

The attenuation or elimination of low-frequency noise is a challenging task due to its numerous sources, its ability to bypass obstacles, and the limited efficiency of most sound barriers. The laws of acoustics tell us that if a solid wall is used to attenuate noise, sound transmission is inversely proportional to its mass per unit area and the sound frequency. This means that very heavy walls, more than ten meters thick (!), are necessary to efficiently reduce typical low-frequency noise in the frequency range between 10 and 1000 Hz.

Fortunately, modern technology can provide more innovative and efficient solutions, based on so-called acoustic metamaterials. These are engineered structures capable of effectively slowing down sound speed and reducing sound intensity thanks to enhanced internal structural losses. The latter can be induced by incorporating internal resonators, which transfer mechanical vibrational energy into heat, or by using a geometry-related mechanism, exploiting the artificial elongation of sound propagation paths by means of narrow, so-called “labyrinthine” channels. In this work, we develop labyrinthine acoustic metamaterials with long narrow channels inspired by the structure of spider webs or arranged along fractal space-filling curves. These particular designs help to extend the metamaterial functionalities as compared to simpler configurations analyzed in previous years.

What happens if a sound wave enters a straight narrow channel? Depending on the channel geometry, it can either propagate through it, or be attenuated. For narrow channels, friction effects in the vicinity of the channel walls hinder wave propagation, and can eventually lead to its total attenuation. For moderately wide channels, if the sound wavelength matches the distance between the two channel edges (i.e., it equals an integer number of half wavelengths), resonance takes place, allowing to amplify the sound transmission. Both the described effects take place at single frequencies.

But what happens if the channels are arranged in the shape of a maze or if there is a set of coiled channels? We now know that for certain configurations, other types of collective resonances can be induced – Mie resonances – that enable the achievement of total reflection at rather wide frequency ranges.

We have found out that natural spider-web designs for the channel labyrinths provide sufficient freedom for the development of metamaterials with switch on/off regimes between total transmission and total reflection that can be easily adapted for controlling low-frequency sound. In particular, we have shown that a light-weight re-configurable structure with a square cross section of 0.81 m2 is capable of totally reflecting airborne sound at frequencies of 50-100 Hz and above [1]. Moreover, by modifying the channel thickness and length, we can tune operating frequencies to desired ranges. In fact, the proposed metamaterials provide exceptional versatility for application in low-frequency sound control and noise abatement.

Incorporation of more advanced designs, e.g. coiling wave paths along space-filling curves, enables to develop more compact configurations and opens a route for creating efficient sound absorbers [2]. Space-filling curves are lines constructed by an infinite iterative process with the aim to fill in a certain area, e.g. a square or cube. Since the work of G. Peano (1890) until the 1980s, these curves were considered no more than mathematical curiosities, and only recently have they found application in fields like data science and routing systems. The use of space-filling curves for wave path labyrinths in combination with the added effect of friction in narrow channels has allowed us to achieve total reflection or to improve wave absorption of low-frequency sound. The absorption can be increased up to 100 % at selected frequencies, if a hybrid configuration with incorporated Helmholtz resonators is used [3]. This could be the next chapter to be written in the story of efficient noise abatement through innovative metamaterials.

fractal 

[1] A.O. Krushynska, F. Bosia, M. Miniaci and N. M. Pugno, “Spider web-structured labyrinthine acoustic metamaterials for low-frequency sound control,” New J. Phys., vol. 19, pp. 105001, 2017.

[2] A.O. Krushynska, F. Bosia, and N. M. Pugno, “Labyrinthine acoustic metamaterials with space-coiling channels for low-frequency sounf control,” Acta Acust.united Ac., vol. 104, pp. 200–210, 2018.

[3] A.O. Krushynska, V. Romero-García, F. Bosia, N.M. Pugno, J.P. Groby, “Extra-thin metamaterials with space-coiling designs for perfect sound absorption”, (working paper), 2018.