Orienting an Outdoor Amphitheater Surrounded by Natural Rock Formations

Joseph Morris – jmorris@resolutgroup.com
Instagram: @resolutgroup

Resolut Group
181 E 5600 S, Suite 200
Murray, UT, 84123

Popular version of 2pAAa6 – Measurement-informed orientation of an amphitheater surrounded by natural rock formations using in-situ impulse response analysis.
Presented at the 190th ASA Meeting
Read the abstract at https://eppro01.ativ.me/web/planner.php?id=ASASPRING2026

–The research described in this Acoustics Lay Language Paper may not have yet been peer reviewed–

A venue’s beauty may draw guests in, but the acoustic experience is what brings them back.

When attending a symphony, the first desired experience is to hear and appreciate music. Musical quality is important for design aspects of an outdoor amphitheater, especially when the design is impacted by large, natural rock formations. Maxwell Park in Hilldale, UT contains several striking, extraordinary features that naturally catch any eye. Although there’s beauty in this scenery, acoustic tests were needed to help enhance the listening experience, i.e., find the best stage direction. Because of these colossal rocks, the way the stage faces can strongly change what the audience hears.

During predesign, two realistic stage orientations were compared: a stage on the north side facing south (Figures 1-3), and a stage on the east side facing west (Figures 4-6). The orientation matters because rocks are reflective and if reflections arrive at the audience at different times and from different directions, they can smear the sound and make music feel less clear. To find the best option, four types of tests were performed at each orientation: an impulse response test (to see when echoes arrive), a “chirp” test (to check how different pitches carry and whether they interfere), a real-time analysis (to see how evenly sound spreads across the audience area), and a music listening test (to hear the real-world result). The client team, including several city officials and the architect, was on-site during testing and though graphs, tables, and data analyses are useful, hearing the difference in-person made the greatest impression.

Aerial view of an outdoor event layout with a marked stage area and audience zone in a desert-like terrain.

Figure 1: Northern Stage Design

Sound level meter mounted on a tripod in a desert clearing with towering red rock cliffs and sparse vegetation under a clear blue sky.

Figure 2: Northern Stage Simulation

Tripod with mounted sound level meter standing on a wide, dry dirt field surrounded by distant rocky hills under a clear blue sky.

Figure 3: Southern Audience Area

Aerial view of an outdoor stage and audience area marked in red near sparse vegetation.

Figure 4: Eastern Stage Design

Tripod-mounted microphone setup on a dirt path with desert shrubs and red rock cliffs in the background under a clear sky.

Figure 5: Eastern Stage Simulation

Wide view of a dirt plain with green shrubs and rugged rocky mountains under a clear blue sky.

Figure 6: Western Audience Area

From the north side, sound struck the nearby eastern rock face almost immediately and provided positive reinforcement for the direct sound. However, the sound reflecting off the western rock face arrived at a significant delay causing negative reinforcement which distorted the overall quality. Those repeated reflections, arriving at slightly different times, blurred the sound, made it harder to tell where the music was originating, and confused listeners. During the music listening test, the clients were so distracted that some initially assumed the sound equipment was the problem, calling it, “low quality”.

On the east side, the audience heard stronger direct sound, with only mild reflections from farther rock formations. Those reflections helped the sound feel full and supportive. During the music listening test, listeners described their experience as clear, impactful, more evenly heard across the area, and emotional to the music that was played. The same equipment and music were used for both orientations, but the natural rock formations from the eastern side had the greatest acoustic impact.

Measurements, figures, and charts help explain why one option performs better, but the shared, real-time listening experience made the difference. Based on that direct experience (supported by test results), the east-side stage orientation was the clear recommendation to, and accepted by, the client.

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-01

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-03

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.

References

  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.

MEASUREMENT SETUP AND METHOD OF VIBRATIONS RECORDING
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.

METHOD OF RECONSTRUCTING SOUND FROM IMAGE
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 SOUND
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.

ACKNOWLEDGEMENTS
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.

Dantonio1

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