2aPA8 – Taming Tornadoes: Controlled Trapping and Rotation with Acoustic Vortices

Asier Marzo – amarzo@hotmail.com
Mihai Caleap
Bruce Drinkwater

Bristol University
Senate House, Tyndall Ave,
Bristol, United Kingdom

Popular version of paper 2aPA8, “Taming tornadoes: Controlling orbits inside acoustic vortex traps”
Presented Tuesday afternoon, May 24, 2016, 11:05 AM, Salon H
171st ASA Meeting Salt Lake City

Tractor beams are mysterious beams that have the ability to attract objects towards the source of the emission (Figure 1). These beams have attracted the attention of both scientists and sci-fi fans. For instance, it is quite an iconic device in Star Wars or Star Trek where it is used by big spaceships to trap and capture smaller objects.


Figure 1. A sonic tractor beam working on air.

In the scientific community, they have been studied theoretically for decades and in 2014, a tractor beam made with light was realized [1]. It used the energy of the photons bouncing on a microsphere to keep it trapped laterally and at the same time heated the back of the sphere with different light patterns to pull it towards the laser source. The sphere had a diameter of 50 micrometres, was made of glass and coated with gold.

A tractor beam made with light can only manipulate very small particles and made of specific materials. Making a tractor beam which uses mechanical waves (i.e. sound or ultrasound) would enable the trapping of a much wider range of particle sizes and allow almost any combination of particle and host fluid materials, for example drug delivery agents within the human body.

Recently, it has been proven experimentally that a Vortex beam can act as a tractor beam both in air [2] and in water [3]. A Vortex beam (such as a first order Bessel beam) is analogous to a tornado of sound which is hollow in the middle and spirals about a central axis, the particles get trapped in the calm eye of the tornado (Figure 2).

Figure-02 - Acoustic Vortices

Figure 2. Intensity iso-surface of an Acoustic Vortex. 54 ultrasonic speakers emitting at 40kHz arranged in a hemisphere (see [2] for fuller details) create an acoustic vortex that traps the particle in the middle.

The problem is, that only very small particles are stably trapped inside the vortex. As the particles get bigger, they start to spin and orbit until being ejected (Figure 3). As in a tornado, only the small particles remain within the vortex whereas the larger ones get ejected.


Figure 3. Particle behaviour depending on its size: a small particle is trapped (a), a middle particle orbits (b) and big particles gets ejected (c).

Here we show that, contrary to a tornado, we can change the direction of an acoustic vortex thousands of times per second. In our paper, we prove that by rapidly switching the direction of the acoustic vortex it is possible to produce stable trapping of particles of various sizes. Furthermore, by adjusting the proportion of time that each vortex direction is emitted, the spinning speed of the particle can be controlled (Figure 4).

Figure-04 - Acoustic Vortices

Figure 4. Taming the vortex: a) the vortex rotates all the time in the same direction and this rotation is transferred to the particle. b) the vortex switches direction and thus the angular momentum is completely or partially cancelled, providing rotational control.

The ability to levitate and controllably rotate inside acoustic vortices particles such as liquids, crystals or even living cells enables new possibilities and processes for a variety of disciplines.


  1. Shvedov, V., Davoyan, A. R., Hnatovsky, C., Engheta, N., & Krolikowski, W. (2014). A long-range polarization-controlled optical tractor beam. Nature Photonics, 8(11), 846-850.
  2. Marzo, A., Seah, S. A., Drinkwater, B. W., Sahoo, D. R., Long, B., & Subramanian, S. (2015). Holographic acoustic elements for manipulation of levitated objects. Nature communications, 6.
  3. Baresch, D., Thomas, J. L., & Marchiano, R. (2016). Observation of a single-beam gradient force acoustical trap for elastic particles: acoustical tweezers. Physical Review Letters, 116(2), 024301.

5aMU5 – Guitar String Sound Retrieved from Moving Pixels

Bożena Kostek – bokostek@audioakustyka.org
Audio Acoustics Laboratory
Faculty of Electronics
Telecommunications and Informatics
Gdansk University of Technology, Narutowicza 11/12
80-233 Gdansk, Poland

Piotr Szczuko – szczuko@sound.eti.pg.gda.pl
Józef Kotus – Joseph@sound.eti.pg.gda.pl
Maciej Szczodrak – szczodry@sound.eti.pg.gda.pl
Andrzej Czyżewski – andcz@sound.eti.pg.gda.pl
Multimedia Systems Department, Faculty of Electronics, Telecommunications and Informatics, Gdansk University of Technology, Narutowicza 11/12, 80-233 Gdansk, Poland

Popular version of paper 5aMU5, “Vibration analysis of acoustic guitar string employing high-speed video cameras”
Presented Friday morning, May 28, 2016, 9:00, Solitude Room
171st ASA Meeting, Salt Lake City

The aim of this study was to develop a method of visual recording and analyzing the vibrations of guitar strings using high-speed cameras and dedicated video processing algorithms. The recording of a plucked string reveals the way in which the deformations propagate, composing the standing and travelling wave. The paper compares the results for a few selected models of classical and acoustic guitars, and it involves processing the vibration image into to the sound recording. The sound reconstructed in this way is compared with the sound recorded synchronously with the reference measurement microphone.

The measurements were made for three different models and types of guitars (Fig. 1a,b,c). The Martin D-45 is one of the best mass produced acoustic guitars in the world. Its top plate is made from spruce, its sides and back from Indian rosewood, its neck from mahogany, and its fingerboard from ebony. The guitar shape is of the Dreadnought type. In the experiments, acoustic strings were used, metal, thickness of 0.52.

Classical guitar model: MGP 145 classic, c, ar, tailpiece, prototype model. Made in 2014 by Sergei Stańczuk in SEGA Luthier Guitar Studio in Warsaw as a prototype model of classical guitar with two tailpieces. In the experiments, acoustic strings were used, metal, thickness of 0.52.

Defil guitar is a classical instrument made in 1978 by a Polish company, designed for amateur players. In the experiments, the classic nylon strings were used, with a thickness of 0.44.

a) b)Kostek_et_al_Fig.1b - Guitar c)Kostek_et_al_Fig.1c - Guitar

Fig. 1. Guitars under research: a) Martin Dreadnought D-45 acoustic guitar, b) SEGA MGP 145 classic c) classical guitar Defil

Acoustic guitars can be tested applying the acoustic methods and recording of the emitted sound, mathematical modeling and simulation (including the finite element method) or direct vibration measurement using various vibrometric methods (laser, piezoelectric transducer, electromagnetic transducer, an analogue movement meter or digital high-speed cameras and optical displacement and deformation measurement).

Fig. 2 shows the layout of the experimental setup. Video tracking and measurement of vibration are made through the use of two identical and synchronized cameras, acquiring 1,600 frames per second video with a resolution of 2000×128 pixels, and the exposure time of 100ms. A high-class measurement microphone along with an acquisition system were used for the audio recording simultaneously with video shooting. The cameras are placed side by side and oriented towards: a foothold in the bridge, the area above the opening of the sound hole, the neck section up to fret 19 (the first camera) and a section of the neck from fret 19 to fret 6 (second camera).

Kostek_et_al_Fig.2 - Guitar

Fig. 2. Setup for the video and audio recordings of the strings vibrations.

In order to accurately measure the deformation of the strings, the video analysis algorithm was created to determine the position of the elementary section of the string visible in each column of the image. Recording, lighting, and exposure conditions were to ensure that the string was the brightest part of the image, and the result of would only be one pixel. The results of the analysis from both cameras, i.e. two vectors describing the position of the string sections were combined into a single series.

It was noticed that the string at rest acted on the bridge with the strength of its tension. Stimulation of the strings was associated with its deformation – increased stress and delivery of energy. After a substantial simplification of the analysis it was possible to perform a simple summation of the deviations of each point on the string and the conversion of the value obtained into a sound sample for each video frame.

Analysis of the averaged spectra highlights the differences between the image acquired and microphone recorded sound (Fig. 3.) Spectra were scaled so as that the amplitude of the first harmonic f = 110 Hz was equal for both recordings.

Martin and Luthier guitars (Fig. 3a, 3b) had very thick acoustic strings, which do not deflect much. Defil guitar (Fig. 3c) has soft strings for classical play that easily deform and vibrate with a large amplitude. The colors of generated sounds are different: the ratio between the harmonics is not maintained. This is due to the participation of the soundboard in the generation of sound.

a)Kostek_et_al_Fig.3a b)Kostek_et_al_Fig.3b c)Kostek_et_al_Fig.3c

Fig. 3. Comparison of the average spectra for the signals obtained from the microphone and reconstructed by an optical method: a) Martin Dreadnought D-45 acoustic guitar, b) SEGA MGP 145 classic c) classical guitar DefilCONCLUSION

A method of obtaining the string deformation characteristics from an image and acquiring sound samples from the observed vibrations was presented. Significant differences resulting from not taking into account the impact of soundboard were observed, therefore further work in this area will focus on the systematic study of differences in the spectra and modelling the participation of the guitar soundboard in the creation of sound.

This research study was supported by the grant, funded by the Polish National Science Centre, decision number DEC-2012/05/B/ST7/02151.

The authors wish to thank Chris Gorski and Sergiusz Stańczuk for providing the guitars.

1pAA6 – Listening for solutions to a speech intelligibility problem

Anthony Hoover, FASA – thoover@mchinc.com
McKay Conant Hoover, Inc.
Acoustics & Media Systems Consultants
5655 Lindero Canyon Road, Suite 325
Westlake Village, CA 91362

Popular version of paper 1pAA6, “Listening for solutions to a speech intelligibility problem”
Presented Monday afternoon, May 23, 2016, 2:45 in Salon E
171st ASA Meeting in Salt Lake City, UT

Loudspeakers for sound reinforcement systems are designed to project their sound in specific directions. Sound system designers take advantage of the “directivity” characteristics of these loudspeakers, aiming their sound uniformly throughout seating areas, while avoiding walls and ceilings and other surfaces from which undesirable reflections could reduce clarity and fidelity.

Many high-quality sound reinforcement loudspeaker systems incorporate horn loudspeakers that provide very good control, but these are relatively large and conspicuous.   In recent years, “steerable column arrays” have become available, which are tall but narrow, allowing them to better blend into the architectural design.  These are well suited to the frequency range of speech, and to some degree their sound output can be steered up or down using electronic signal processing.

Figure 1 - steerable column arrays - speech intelligibility

Figure 1. steerable column arrays

Figure 1 illustrates the steering technique, with six individual loudspeakers in a vertical array.  Each loudspeaker generates an ever-expanding sphere of sound (in this figure, simplified to show only the horizontal diameter of each sphere), propagating outward at the speed of sound, which is roughly 1 foot per millisecond.  In the “not steered” column, all of the loudspeakers are outputting their sound at the same time, with a combined wavefront spreading horizontally, as an ever-expanding cylinder of sound.  In the “steered downward” column, the electronic signal to each successively lower loudspeaker is slightly delayed; the top loudspeaker outputs its sound first, while each lower loudspeaker in turn outputs its sound just a little later, so that the sound energy is generally steered slightly downward. This steering allows for some flexibility in positioning the loudspeaker column.  However, these systems only offer some vertical control; left-to-right projection is not well controlled.

Steerable column arrays have reasonably resolved speech reinforcement issues in many large, acoustically-problematic spaces. Such arrays were appropriate selections for a large worship space, with a balcony and a huge dome, that had undergone a comprehensive renovation.  Unfortunately, in this case, problems with speech intelligibility persisted, even after multiple adjustments by reputable technicians, who had used their instrumentation to identify several sidewall surfaces that appeared to be reflecting sound and causing problematic echoes. They recommended additional sound absorptive treatment that could adversely affect visual aesthetics and negatively impact the popular classical music concerts.

Upon visiting the space as requested to investigate potential acoustical treatments, speech was difficult to understand in various areas on the main floor.  While playing a click track (imagine a “pop” every 5 seconds) through the sound system, and listening to the results around the main floor, we heard strong echoes emanating from the direction of the surfaces that had been recommended for sound-absorptive treatment.

Nearby those surfaces, additional column loudspeakers had been installed to augment coverage of the balcony seating area.  These balcony loudspeakers were time-delayed (in accordance with common practice, to accommodate the speed of sound) so that they would not produce their sound until the sound from the main loudspeakers had arrived at the balcony. With proper time delay, listeners on the balcony would hear sound from both main and balcony loudspeakers at approximately the same time, and thereby avoid what would otherwise seem like an echo from the main loudspeakers.

With more listening, it became clear that the echo was not due to reflections from the walls at all, but rather from the delayed balcony loudspeakers’ sound inadvertently spraying back to the main seating area.  These loudspeakers cannot be steered in a multifaceted manner that would both cover the balcony and avoid the main floor.

We simply turned off the balcony loudspeakers, and the echo disappeared.  More importantly, speech intelligibility improved significantly throughout the main floor. Intelligibility throughout the balcony remained acceptable, albeit not quite as good as with the balcony loudspeakers operating.

The general plan is to remove the balcony loudspeakers and relocate them to the same wall as the main loudspeakers, but steer them to cover the balcony.

Adding sound-absorptive treatment on the side walls would not have solved the problem, and would have squandered funds while impacting the visual aesthetics and classical music programming.  Listening for solutions proved to be more effective than interpreting test results from sophisticated instrumentation.

1aAA4 – Optimizing the signal to noise ratio in classrooms using passive acoustics

Peter D’Antonio – pdantonio@rpginc.com

RPG Diffusor Systems, Inc.
651 Commerce Dr
Upper Marlboro, MD 20774

Popular version of paper 1aAA4 “Optimizing the signal to noise ratio in classrooms using passive acoustics”
Presented on Monday May 23, 10:20 AM – 5:00 pm, SALON I
171st ASA Meeting, Salt Lake City

The 2012 Program of International Student Assessment (PISA) has carried out an international comparative trial of student performance in reading comprehension, calculus, and natural science. The US ranks 36th out of 64 countries testing ½ million 15 year olds, as shown in Figure 1.


Figure 1 PISA Study

What is the problem? Existing acoustical designs and products have not evolved to incorporate the current state-of-the-art and the result is schools that are failing to meet their intended goals. Learning areas are only beginning to include adjustable intensity and color lighting, shown to increase reading speeds, reduce testing errors and reduce hyperactivity; existing acoustical designs are limited to conventional absorptive-only acoustical materials, like thin fabric wrapped panels and acoustical ceiling tiles, which cannot address all of the speech intelligibility and music appreciation challenges.

What is the solution? Adopt modern products and designs for core and ancillary learning spaces which utilize binary, ternary, quaternary and other transitional hybrid surfaces, which simultaneously scatter consonant-containing high frequency early reflections and absorb mid-low frequencies to passively improve the signal to noise ratio, adopt recommendations of ANSI 12.6 to control reverberation, background noise and noise intrusion and integrate lighting that adjusts to the task at hand.

Let’s begin by considering how we hear and understand what is being said when information is being delivered via the spoken word. We often hear people say, I can hear what he or she is saying, but I cannot understand what is being said. The understanding of speech is referred to as speech intelligibility. How do we interpret speech? The ear / brain processor can fill in a substantial amount of missing information in music, but requires more detailed information for understanding speech. The speech power is delivered in the vowels (a, e, i, o, u and sometimes y) which are predominantly in the frequency range of 250Hz to 500Hz. The speech intelligibility is delivered in the consonants (b, c, d, f, g, h, j, k, l, m, n, p, q, r, s, t, v, w), which occur in the 2,000Hz to 6,000 Hz frequency range. People who suffer from noise induced hearing loss typically have a 4,000Hz notch, which causes severe degradation of speech intelligibility. I raise the question, “Why would we want to use exclusively absorption on the entire ceiling of a speech room and thin fabric wrapped panels on a significant proportion of wall areas, when these porous materials absorb these important consonant frequencies and prevents them from fusing with the direct sound making it louder and more intelligible?

Exclusive treatment of absorbing material on the ceiling of the room may excessively reduce the high-frequency consonants sound and result in the masking of high-frequency consonants by low-frequency vowel sounds, thereby reducing the signal to noise ratio (SNR).

The signal has two contributions. The direct line-of-sight sound and the early reflections arriving from the walls, ceiling, floor and people and items in the room. So the signal consists of direct sound and early reflection. Our auditory system, our ears and brain, have a unique ability called temporal fusion, which combines or fuses these two signals into one apparently louder and more intelligible signal. The goal then is to utilize these passive early reflections as efficiently as possible to increase the signal. The denominator in the SNR consists of external noise intrusion, occupant noise, HVAC noise and reverberation. These ideas are summarized in Figure 2.

Dantonio figure2

Figure 2 Signal to Noise Ratio

In Figure 3, we illustrate a concept model for an improved speech environment, whether it is a classroom, a lecture hall, a meeting/conference room, essentially any room in which information is being conveyed.

The design includes a reflective front, because the vertical and horizontal divergence of the consonants is roughly 120 degrees, so if a speaker turns away from the audience, the consonants must reflect from the front wall and ceiling overhead. The perimeter of the ceiling is absorptive to control the reverberation (noise). The center of the ceiling is diffusive to provide early reflections to increase the signal and its coverage in the room. The mid third of the walls utilize novel binary, ternary, quaternary and other transitional diffsorptive (diffusive/absorptive) panels, which scatter the information above 1 kHz (the signal) and absorb the sound below 1 kHz (the reverberation=noise). This design suggests that the current exclusive use of acoustical ceiling tile and traditional fabric wrapped panels is counterproductive in improving the SNR, speech intelligibility and coverage.

Dantonio figure3 - classrooms

Figure 3 Concept model for a classroom with a high SNR

3pID2 – The Sound of the Sacred: The State of the Art in Worship Space Acoustics

David T. Bradley – dabradley@vassar.edu
Vassar College
124 Raymond Ave
Poughkeepsie, NY 12604-0745

Erica E. Ryherd – eryherd@unl.edu
Durham School of Architectural Engineering and Construction
University of Nebraska – Lincoln
203C Peter Kiewit Institute
Omaha, NE 68182-0176

Lauren Ronsse – lronsse@colum.edu
Columbia College Chicago
33 E. Congress Parkway, Suite 601
Chicago, IL 60605

Popular version of paper 3pID2, “The state of the art in worship space acoustics”
Presented Wednesday afternoon, May 25, 2016, 1:55 in Salon D
171st ASA Meeting, Salt Lake City

From the clanging of bells to the whisper of burning incense, sound is essential to the worship experience. It follows that the acoustic environment is paramount in the sacred place – the worship space – and thoughtful design is required to achieve a worship experience full of awe and wonder. The first intentional sacred spaces were constructed over 11,000 years ago [1] and, although architectural acoustics design practices have changed immeasurably since then, the primary use of these spaces remains essentially unchanged: to provide a gathering space for communal worship.

view from ezrat nashim - worship spaceFigure 1: Temple YoungIsrael, a 550-seat orthodox synagogue in Brookline, MA designed by Gund Partnership and Cavanaugh Tocci Associates, Inc. (Photo credit: Christopher A. Storch) ceiling dome artwork - worship spaceFigure 2: Al Farooq Masjid of Atlanta (interior view of main dome), a 1500-seat Islamic mosque designed by Lee Sound Design, Inc. and Design Arts Studio and EDT Constructors, Inc. (Photo credit: Wayne Lee)

To meet this need, the four key acoustical goals that modern worship space designers must consider are to optimize reverberation time, eliminate acoustical defects, minimize ambient noise, and maximize dynamic range. These four goals are imperative in virtually all types of worship spaces around the world, despite vast dif­ferences in religious practices and beliefs. In the recent publication, Worship Space Acous­tics: 3 Decades of Design [2], the application of these goals is seen in 67 churches, synagogues, mosques, and other worship spaces designed in the past thirty years. Each space and each religion has its own id­iosyncratic acoustic challenges, from a visually translucent but acoustically transparent partition required to separate men and women in an orthodox syna­gogue in Brookline, MA (Figure 1) to the attenuation of a focused acoustic reflection from the dome of a mosque in Atlanta, GA (Figure 2). One space featured in the book, Temple Israel (Figure 3), shows the connectedness of acoustical design in worship spaces. It is part of a special project, the Tri-Faith Initiative, a 14-acre complex in Omaha, NE uniting three Abrahamic faith groups, Temple Israel, Countryside Community Church (United Church of Christ), and The American Muslim Institute. As each of these three worship spaces is constructed on the site, they all must have the four key acoustical goals considered.

figure_3 - worship spaceFigure 3: Temple Israel, a 900-seat reform synagogue in Omaha, NE designed by Acentech Incorporated and Finegold Alexander + Associates. (Photo credit: Finegold Alexander + Associates) view from the stageFigure 4: The Star Performing Arts Centre, a 5000-seat multi-use space in Singapore designed by Arup (completed as Artec) and Andrew Bromberg of Aedas.

In the past three decades, worship spaces have seen an increased need for multi-functionality, often hosting religious services with varying acoustical needs. For example, the Star Performing Arts Centre (Figure 4) in Singapore serves as the home of the New Creation Church, seats 5000 people, and supports programing ranging from traditional worship services to pop music concerts to televised national events. Some spaces must even serve more than one religion, such as the Sacred Space (Figure 5), a multi-faith house of worship constructed in the shell of an old Boston chapel that caught fire in the mid-90s, now used for meditation, private worship, and small gatherings. These varying usage requirements require careful consideration of the acoustic design, often relying on variable acoustics such as retractable sound absorption and the use of sophisticated electroacoustics systems.

interior view - worship spaceFigure 5: The Sacred Space, 150-seat multifaith house of worship in Boston, MA designed by Acentech Incorporated and Office dA. figure_6 - worship spaceFigure 6: Mean reverberation times at 500 Hz octave band center frequency for 67 worship spaces of varying seating capacity.

Although the use of each space may vary, the most important acoustic goal remains to optimize the reverberation time. This is the time necessary for sound in a space to decay to one-millionth of its original intensity. Essentially, it describes how the sound energy decays, perceived as the fading away of sound over time. Typically, reverberation time decreases as more sound absorption is added, and increases as the size of the space increases. Figure 6 shows the mean reverberation times (500 Hz) for the various seating capacities of the 67 worship spaces. Seating capacity is generally directly proportional to size of the space, and the reverberation times here show the general trend of increasing with increasing size up to about 2000 seats. For 2000 seats and beyond, the data show a marked decrease in reverberation time. For these larger spaces, there tends to be a higher proportion of sound absorbing material used in the design, typically to allow for the use of electroacoustics systems that require a large number of loudspeakers. Spaces that rely heavily on electroacoustics to achieve the desired sonic environment require non-reflective surfaces and lower reverberation times for the microphone-loudspeaker systems to work properly.

Regardless of reverberation time, the goal remains the same, to create a gathering space for worship where the sound is sacred.

  1. K. Schmidt, “Göbekli Tepe, Southeastern Turkey: A Preliminary Report on the 1995-1999 Excavations,” Paléorient, 26(1), 45-54, 2000.
  2. D. T. Bradley, E. E. Ryherd, and L. Ronsse (Eds.), Worship Spaces Acoustics: 3 Decades of Design, (New York, NY, Springer, 2016).