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Proceedings of Meetings on Acoustics

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POMA - 160th Meeting Acoustical Society of America
Conference Location: Cancun, Mexico Conference Date: 15 - 19 November 2010
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Attenuation of low-frequency underwater sound using bubble resonance phenomena and acoustic impedance mismatching

Kevin Lee, Kevin T. Hinojosa, Mark S. Wochner, Theodore F. Argo IV, Preston S. Wilson, and Richard S. Mercier

POMA Volume 11, pp. 005001 (June 2011); (17 pages)

Online Publication Date: June 12, 2011

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Air bubbles can be a significant source of attenuation in underwater sound propagation, but such effects have not been experimentally verified for low frequencies in part due to the difficulty in creating large stable bubbles. This work is in part an extension of a previously reported study in the acoustic effects of large resonant encapsulated air bubbles in a 1D waveguide [J. Acoust. Soc. Am. 127 2015 (2010)]. Now, both bubble resonance effects and acoustic impedance mismatching effects are investigated in a large-scale tank setup. Both of these mechanisms are shown to attenuate sound at low frequencies (50 to 200 Hz) through experiments in which a sound source is surrounded by a column of freely-rising sub-resonant bubbles, a matrix of tethered resonant air balloons, and a combination of the two. Experiments with a matrix of thicker-shelled encapsulated bubbles demonstrate decreased attenuation due to weaker resonant interaction and the use of a polydipserse versus monodisperse encapsulated bubble size distribution demonstrates a flattened frequency response for fixed global void fraction. Finite-element models of both an effective medium bubble cloud and discrete stationary bubbles are discussed and compared with experiment. [Work supported by Shell.]
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43.20.Hq Velocity and attenuation of acoustic waves
43.30.Es Velocity, attenuation, refraction, and diffraction in water, Doppler effect
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Construction, calibration, and field test of a home-made, low-cost hydrophone system for cetacean acoustic research

Eduardo Romero Vivas and Braulio León López

POMA Volume 11, pp. 010001 (March 2011); (10 pages)

Online Publication Date: March 12, 2011

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Marine Mammals are reliable bioindicators of aquatic ecosystems health. Since cetacean highly relay on the use of sound for conspecifics interaction, feeding, and navigation, research in bioacoustics becomes fundamental to unravel the influence of anthropogenic activities on their environment and vocal behaviour. Unfortunately, the widespread of studies in this area are often limited for the lack of affordable equipment. This paper first describes how to build a low cost hydrophone suitable for cetacean acoustic research and then shows how to perform hydrostatic pressure tests and acoustic calibrations using easily available tools. Finally, field recordings of individuals of two dolphin species: long-beaked common dolphin (Delphinus capensis) and bottlenose dolphin (Tursiops truncatus) in La Paz Bay, Baja California Sur, Mexico using the proposed hydrophone and a professional hydrophone system [AQ-1s and ITC-1042 transducers (10 Hz - 100 kHz)] are compared.
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43.30.Xm Underwater measurement and calibration instrumentation and procedures
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The use of acoustic monitoring in the National Marine Fisheries Service marine mammal incidental take authorizations

Shane Guan

POMA Volume 11, pp. 010002 (June 2011); (10 pages)

Online Publication Date: June 12, 2011

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Under the Marine Mammal Protection Act (MMPA), the National Marine Fisheries Service (NMFS) is charged with conservation and management of marine mammals under NMFS' jurisdiction. A regulatory mechanism is in place for NMFS to issue incidental take authorizations under the MMPA to operators whose activities may adversely affect marine mammals. As a mandate under the MMPA, mitigation and monitoring measures are prescribed to minimize any impacts on marine mammals and their habitat (prey species and acoustic environment) and to assess the degree of such impacts when such incidental take authorizations are issued. However, due to the limited visual range under and above water, monitoring measures using acoustical techniques are often required to supplement visual monitoring and to assist in making mitigation decisions. This presentation provides a summary of existing acoustic monitoring measures that are currently used in NMFS' marine mammal incidental take authorizations; discusses the limitations, technical issues, practicality, and training needs involving acoustic monitoring; and highlights future needs in standardizing, improving, and expanding such techniques for more effective mitigation and monitoring measures.
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43.80.Nd Effects of noise on animals and associated behavior, protective mechanisms
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The Evolution and Effect of Hoods on Pipes

Susan Rawcliffe

POMA Volume 11, pp. 015001 (February 2011); (10 pages)

Online Publication Date: February 11, 2011

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Hooded flutes or pipes were found primarily among the prehispanic cultures of West Mexico, and consist of an awning shape around three sides of the pipe's air ducted aperture, and angling over its top. During construction, a hooded pipe's sound can be manipulated through variations of the size and shape of the aperture, the design of the over-arching hood and to some extent, the flute's body shape. Minute differences in construction can make a large difference in the timbre. A hood can limit the ability to play the partials of the tube, but it contributes a characteristically marvelous rough, reedy timbre that can be manipulated through air pressure variations. Sonograms, recordings, photos and measurements of a few prehispanic hooded pipes located in Fowler Museum, UCLA, are compared to each other and to those of a Rawcliffe hooded pipe. For contrast, the sounds and sonograms of an ancient pipe without a hood will be included. Hopefully, the results of this analysis will illustrate some sound preferences found within the prehispanic instrumentarium as well as explore possibilities for recreating ancient sounds in contemporary flutes. Time permitting, the presentation will conclude with a brief performance on the Rawcliffe pipe.
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43.66.Jh Timbre, timbre in musical acoustics
43.75.Qr Flutes and similar wind instruments
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Acoustic Performance of an Installed Real-Time Three-Dimensional Audio System

Kenneth Faller II, Stephen A. Rizzi, Noah Schiller, Randolph Cabell, Jacob Klos, William L. Chapin, and Aric R. Aumann

POMA Volume 11, pp. 015002 (March 2011); (13 pages)

Online Publication Date: March 31, 2011

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The Exterior Effects Room (EER), located at the NASA Langley Research Center, is a facility built for psychoacoustic studies of aircraft community noise. Recently, the EER was significantly upgraded to allow for simulation of aircraft flyovers in a three-dimensional audio and visual environment. The upgrade included installation of 27 satellite and 4 subwoofer loudspeakers that are driven by a real-time audio server. The audio server employs an implementation of the Vector Base Amplitude Panning (VBAP) method to position virtual sources at arbitrary azimuth and elevation angles in the EER. Real-time application of filters, time delays, and gains are required to compensate for installation effects, including those associated with the irregular room geometry, colorization due to varying loudspeaker installations, and crossover filtering. The authors previously showed [J. Acoust. Soc. Am., 127, 1969 (2010)] that color compensation and crossover filtering could be achieved for satellite and subwoofer loudspeakers. However, the resulting FIR filters were too long (32,768 taps) to implement in real-time. The focus of this work is on development of reduced-length surrogate IIR filters and on measurement of the acoustic performance of the installed real-time system.
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43.55.Jz Sound-reinforcement systems for rooms and enclosures
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Combining auditory and tactile inputs to create a sense of auditory space.

Ross Deas, Rob B. Adamson, Philip Garland, Manohar L. Bance, and Jeremy Brown

POMA Volume 11, pp. 015003 (June 2011); (9 pages)

Online Publication Date: June 05, 2011

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To localize a sound, the auditory system uses multiple cues, including binaural differences in timing and level that arise from the separation of the ears by the solid mass of the head. It has repeatedly been shown that the ability to utilize these cues is plastic and experience-based. Vibrotactile input shares many common features with auditory signals, and there is some overlap between the frequency range of the sensitivity of the ear and skin. In this study, we examine whether the auditory system is capable of combining auditory and tactile inputs to localize sounds using a multi-speaker array. To induce deficits in azimuthal localization, one ear was plugged. To examine cross-modal localization, the input level to the plugged ear was recorded via microphone, and a vibratory signal that was perceptually equal in intensity was presented to the shoulder on the same side as the plugged ear. The participant's ability to localize low-pass, band-pass, high-pass, and broadband sounds was measured. Results showed that relative to baseline (plugged) conditions, localization performance improved, suggesting that listeners can combine auditory and tactile information to create a sense of auditory space.
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43.60.Jn Source localization and parameter estimation
43.66.Qp Localization of sound sources
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Prospects of the treatment of acoustical insulation in building codes of Mexico

Mario E. Vergara Balderas

POMA Volume 11, pp. 015004 (September 2011); (13 pages)

Online Publication Date: September 30, 2011

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The acoustical insulation of dwellings in order to protect them from environmental noise is an issue not yet addressed in building regulations in Mexico, but the Federal Government, through the National Housing Commission (CONAVI),has promoted the development of a Building Code for Dwellings, which in the future could include provisions about this subject. So far, authorities have focused their attention on the problems of energy efficiency and thermal insulation of public buildings. On the subject of housing, they have proposed levels of thermal insulation that could be adopted as standards and have also promoted the study of constructive solutions appropriate to those standards. The levels of acoustical and thermal insulation that are produced by various constructive solutions used by housing developers in the metropolitan area of the city of Puebla, Mexico, are analyzed in this work in order to compare their performance with international standards. A more comprehensive regulatory framework is needed in Mexico, and the results of this research will produce recommendations on acoustical insulation capacities that the National Housing Commission could include in its code. [Project supported by funds from CONACYT and CONAVI.]
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43.50.Sr Community noise, noise zoning, by-laws, and legislation
43.55.Rg Sound transmission through walls and through ducts: theory and measurement
43.55.Ti Sound-isolating structures, values of transmission coefficients
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Noise in the Classroom

Ana Jaramillo and Michael Ermann

POMA Volume 11, pp. 015005 (December 2012); (7 pages)

Online Publication Date: December 05, 2012

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During our entire life span we spend time in the activity of learning (anything from basic survival skills to highly intellectual processes), but this activity occupies most of the time during the first years of our lives. The classroom becomes then the "official" learning space that is designed to be conducive to this activity. The learning process, regardless of the task difficulty, is not a mechanical one. It requires a mental process, concentration, attention in various degrees. Distractions can be detrimental to the learning. Most of the learning activities that occur at school settings require some kind of oral communication (teacher-student or student-student), and these activities call for appropriate room acoustics. In the presence of high levels of background noise, human beings have other resources to better understand the signal of interest. Those can be visual cues, previous knowledge of the topic, or mental ability to "fill in the blanks" in the received speech. The last two, are abilities that come with age and experience.
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43.50.Qp Effects of noise on man and society
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Modeling nonlinear acoustic waves in media with inhomogeneities in the coefficient of nonlinearity

L. Demi, Martin Verweij, and K.W.A. Van Dongen

POMA Volume 11, pp. 020001 (November 2010); (6 pages)

Online Publication Date: November 23, 2010

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The refraction and scattering of nonlinear acoustic waves play an important role in the realistic application of medical ultrasound. One cause of these effects is the tissue dependence of the nonlinear medium behavior. A method that is able to model those effects is essential for the design of transducers for novel ultrasound modalities. Starting from the Westervelt equation, nonlinear pressure wave fields can be modeled via a contrast source formulation, as has been done with the INCS method. An extension of this method will be presented that can handle inhomogeneities in the coefficient of nonlinearity. The contrast source formulation results in an integral equation, which is solved iteratively using a Neumann scheme. The convergence of this scheme has been investigatedfor relevant media (e.g., blood, brain, and liver). Further, as an example, the method has been applied to compute the 1D nonlinear acoustic wave field in an inhomogeneous medium insonified by a 1 MHz Gaussian pulse propagatingup to 100 mm. The results show that the method is able to predict the propagation and the scattering effects of nonlinear acoustic waves in media with inhomogeneities in the coefficient of nonlinearity. This motivates a similar extension ofthe 3D INCS method.
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43.25.Jh Reflection, refraction, interference, scattering, and diffraction of intense sound waves
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Effects of ultrasound on osteoblast proliferation and its mechanisms triggered by calcium transport

Amit Katiyar, Kausik Sarkar, and Randall Duncan

POMA Volume 11, pp. 020002 (March 2011); (15 pages)

Online Publication Date: March 31, 2011

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We demonstrate that the near field LIPUS stimulation can enhance osteoblast cell proliferation in monolayer culture and this enhancement can be maximized by the optimization of key ultrasound parameters as intensity and frequency and also the excitation period. We considered discrete US intensities from 1 mW/cm2 to 500 mW/cm2 and found that approximately 75 mW/cm2 was most suitable for enhanced osteoblast proliferation. US exposure at higher intensity (~500 mW/cm2 (SAPA)) proved detrimental to osteoblast cells. We varied US frequency and studied one lower (500 kHz) and one higher frequency (5 MHz) than the mostly used frequency of 1.5 MHz. We did not find the statistically significant difference in osteoblast cell proliferation for such variation. Finally we vary the duration of US stimulation for peak cellular proliferation. At optimum US intensity, we found that longer stimulation of 30 min a day was not significantly different from a shorter stimulation of 10 min a day. The results of this study suggest that US stimulations of optimum parameters can enhance cellular activity of bone cells such as increased osteoblast proliferation in monolayer culture.
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43.80.Gx Mechanisms of action of acoustic energy on biological systems: physical processes, sites of action
43.80.Jz Use of acoustic energy (with or without other forms) in studies of structure and function of biological systems
43.80.Sh Medical use of ultrasonics for tissue modification (permanent and temporary)
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Excitation thresholds for subharmonic response of ultrasound contrast microbubbles

Amit Katiyar and Kausik Sarkar

POMA Volume 11, pp. 020003 (June 2011); (15 pages)

Online Publication Date: June 05, 2011

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The excitation threshold for subharmonic generation plays an important role in accurate prediction of various subharmonic scattering related observations. The classical understanding suggests that minimum subharmonic threshold occurs near twice the resonance frequency. We numerically show that for a number of models---Newtonian (Chatterjee and Sarkar 2003) and viscoelastic (Sarkar et al 2005) interfacial rheological models, and a model due to de Jong et al (1994)--- of encapsulation as well as for free bubbles, the minimum subharmonic threshold is obtained near twice the resonance frequency. However, for other models---linear viscoelastic model (Church 1995, Hoff et al 200), model due to Marmottant et al (2005) and strain-softening interfacial rheological model (Paul et al 2010), the minimum threshold does not always occur at twice the resonance frequency. They have minimum threshold near resonance frequency depending on encapsulation parameters such as elasticity and viscosity.
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43.25.Yw Nonlinear acoustics of bubbly liquids
43.80.Qf Medical diagnosis with acoustics
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Three-Dimensional Laser Doppler Vibrometry of the Dry Human Skull

Carmen L. McKnight, Rob B. Adamson, Wael A. Alian, Manohar L. Bance, Darrel A. Doman, and Jeremy A. Brown

POMA Volume 11, pp. 020004 (June 2011); (11 pages)

Online Publication Date: June 05, 2011

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The results of two experimental studies of the vibratory response of dry human skulls at acoustic frequencies (100 Hz - 20 kHz) are presented. In both experiments the motor from a commercial bone vibrator, the Baha® Divino{trade mark, serif}, was used to excite vibrations in dry human skulls. The first experiment used stepped sine wave excitation to drive vibrations in the skull bone, and the amplitude and phase of the response were recorded with a scanning laser Doppler vibrometer (LDV) at 2200 points over the skull. Three distinct kinds of vibration were seen over the measured frequency range: 1) quasi-rigid whole-head motion at frequencies below 1000 Hz, 2) modal vibration above 1000 Hz and 3) wave propagation above 8000 Hz. The second experiment used swept sine wave excitation and measured the vibratory response at five locations with a high frequency resolution in order to identify resonant frequencies. At the high end of the frequency range, the dependency of the mode number on frequency was found to follow a square root dependency, consistent with vibrations being flexural waves. This result is supported by analytical and finite element models of a sphere with similar geometric and material properties.
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43.80.Ev Acoustical measurement methods in biological systems and media
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Time-reversal Techniques in Ultrasound-assisted Convection-enhanced Drug Delivery to the Brain: Technology Development and In Vivo Evaluation

George K. Lewis Jr., Sabrina Guarino, Gaurav Gandhi, Laurent Filinger, George K. Lewis Sr., Willam L. Olbricht, and Armen Sarvazyan

POMA Volume 11, pp. 020005 (July 2011); (26 pages)

Online Publication Date: July 10, 2011

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We describe a drug delivery method that combines Time-Reversal Acoustics (TRA) with Convection-Enhanced Delivery (CED) to improve the delivery of therapeutics to the interstitium of the brain. The Ultrasound-assisted CED approach (UCED) circumvents the blood-brain barrier by infusing compounds through a cannula that is inserted into the brain while simultaneously delivering ultrasound to improve the penetration of pharmaceuticals. CED without ultrasound-assistance has been used to treat a variety of neural disorders, including glioblastoma multiforme, a malignancy that presents a very poor prognosis for patients. We describe a novel system that is used to infuse fluids into the brain parenchyma while simultaneously exposing the tissue to safe levels of 1-MHz, low intensity, ultrasound energy. The system includes a combined infusion needle-hydrophone, a 10-channel ultralow-output impedance amplifier, a broad-band ultrasound resonator, and MatLab®-based TRA control and user-interface. TRA allows easy coupling of ultrasound therapy through the skull without complex phase-correction and array design. The smart targeting UCED system has been tested in vivo and results show it provides 1.5-mm spatial resolution for UCED and improves tracer distribution in the brain over CED alone.
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43.25.Gf Standing waves; resonance
43.80.Gx Mechanisms of action of acoustic energy on biological systems: physical processes, sites of action
43.80.Sh Medical use of ultrasonics for tissue modification (permanent and temporary)
43.80.Vj Acoustical medical instrumentation and measurement techniques
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Standing wave measurements in tubes

Stanley A. Cheyne and Walter C. McDermott

POMA Volume 11, pp. 025001 (December 2011); (6 pages)

Online Publication Date: December 06, 2011

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Sound speed measurements have been made using a standing wave technique for possible inclusion in Project Listen Up! In a previous paper [S. A. Cheyne and W. C. McDermott, "Sound speed measurements in air using a variable sound source and tubes," J. Acoust. Soc. Am. 125, 2625 (2009)], sound speed measurements in tubes were made using traditional standing wave techniques. A low‐cost variable frequency sound source was constructed and used to produce the sound waves. Resonances in the source itself made it difficult to make reliable measurements. Also, it was difficult to detect the tube resonances by simply listening to the resonances with one's ear. In this work, we have improved the quality of the apparatus to make it easier to generate and detect tube resonances.
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43.10.Sv Education in acoustics, tutorial papers of interest to acoustics educators
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The current state of acoustics education at Brigham Young University

Kent L. Gee, Scott D. Sommerfeldt, and Tracianne B. Neilsen

POMA Volume 11, pp. 025002 (November 2012); (7 pages)

Online Publication Date: November 21, 2012

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The current acoustics program at Brigham Young University reflects efforts intended to better prepare students for jobs in industry, research, and academia. In the classroom, ongoing modifications to courses are intended to provide students with a solid foundation in core acoustics principles and practices. A new advanced undergraduate course has been developed to provide students with formal training in acoustics prior to the graduate level and prepare them for research, internships, and entry-level acoustics positions. In the laboratory, graduate students not only carry out research of significance, but often serve as peer mentors to undergraduate students new to the group. This permits undergraduate students greater opportunity to participate meaningfully in research activities and has resulted in an increase in undergraduate-authored presentations and publications.
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43.10.Sv Education in acoustics, tutorial papers of interest to acoustics educators
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A directional dogbone flextensional transducer

Stephen C. Butler

POMA Volume 11, pp. 030001 (November 2010); (23 pages)

Online Publication Date: November 23, 2010

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Sonar flextensional transducers, in order to transmit energy in one direction, are combined into arrays of elements that are spaced one-quarter of a wavelength apart. The directionality (front to back ratio) is a modest 6 dB, caused by diffraction effects of the size of the projectors. Here a projector that is one-third of a wavelength in size capable of developing unidirectional beams with a front to back ratio greater than 20 dB is described. The directional class VII dogbone flextensional is a modified version of a class IV flextensional having a concave shell rather than a convex one. The piezoelectric ceramic stack is a trilaminar bar with two active sections separated by an inactive center section. Driving both sections of the stack in-phase, the shell is driven into an omnidirectional pattern. Driving each stack section180 deg out of phase causes a bending mode, resulting in a dipole pattern. Driving both sections using complex coefficients determined from the omnidirectional and dipole patterns, a cardioid directional pattern is developed with a front-back pressure ratio of 50 dB that is over an octave bandwidth wide. COMSOL acoustics module FEA code is used to predict in water electroacoustic performance and compared with experimental data.
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43.30.Yj Transducers and transducer arrays for underwater sound; transducer calibration
43.38.Ar Transducing principles, materials, and structures: general
43.38.Fx Piezoelectric and ferroelectric transducers
43.38.Hz Transducer arrays, acoustic interaction effects in arrays
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Four mass coupled oscillator guitar model

John Popp

POMA Volume 11, pp. 035001 (November 2010); (32 pages)

Online Publication Date: November 23, 2010

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Coupled oscillator models have been used for the low frequency response (50-250 Hz) of a guitar [G. Caldersmith, "Guitar as a reflex enclosure,"J. Acoust. Soc. Am. 63, 1566-1575 (1978); O.Christensen, "Quantitative models for low frequency guitar function,"in 103rd ASA Spring Meeting (1982) and J. Guitar Acoustics 6,10-25 (1982); T. D. Rossing, J. Popp, and D. Polstein, "Acoustical response of guitars," in Proceedings of Stockholm Music Acoustics Conference,Royal Swedish Academy of Music (1985), pp. 311-332]. These 2 and 3 mass models correctly predict measured resonance frequency relationships under various laboratory boundary conditions, but did not always represent the true state of a guitar in the players hands. The model presented has improved these models in three ways: (1) a fourth oscillator includes the guitar body; (2)plate stiffnesses and other fundamental parameters were measured directly and effective areas and masses were used to calculate the responses, including resonances and phases, directly; and (3) neck vibrations were calculated and measured and shown to effect the response significantly in some guitars. The calculated and measured resonances and phases agree reasonably well.
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43.75.Gh Plucked string instruments
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Optimal valve slide lengths for double and triple horns

Frederick Young

POMA Volume 11, pp. 035002 (November 2010); (8 pages)

Online Publication Date: November 23, 2010

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The optimal valve slide lengths for three valve double and triple brass horns are investigated to insure that intonation errors never exceed five cents sharp or flat over the complete range of the instrument. Without altering the normal tuning slide setting it is shown by the method of least squares that the root mean squared error is 3.25 cents using the proper valve slide lengths. It is also shown that the RMS error can be reduced to 2.6 cents by tuning the open tones 3.2 cents sharp and making suitable valves slide adjustments. The problems caused by 13 and 123 valve combinations in the low register of the F horn are solved by using pedal notes on the alto E flat side of the horn.
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43.75.Fg Brass instruments and other lip-vibrated instruments
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The natural frequencies of muted circular drums

Frederick Young

POMA Volume 11, pp. 035003 (December 2010); (7 pages)

Online Publication Date: December 10, 2010

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The natural frequencies of circular drum heads are calculated by the use of finite elements.The results for unmuted drum heads are almost identical to the well known analytical solution expressed in Bessel functions. Mutes of various sizes and shapes are used to stop a section of the head from vibrating. For example, a circular damper having one-quarter of the drum head diameter raises the resonant frequencies of the first eight modes of vibration by 11, 1.7, 3.8, 6.2, 9.2, 5.2 and 10.1 percent. For the undamped head modes two and three are identical as well as modes seven and eight. Each of the pairs have differing resonating frequencies. The changes in resonant frequencies are strongly influenced by the mute location. It seems that the frequency changes associated with muting might be used for musical purposes.
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43.75.Hi Drums
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Five valve compensating brass wind instrument

Frederick Young

POMA Volume 11, pp. 035004 (January 2011); (8 pages)

Online Publication Date: January 30, 2011

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A companion paper shows how a triple brass instrument can be designed to yield nearly perfect well tempered intonation. This cannot be applied to the larger low brass instruments because the inertia of triple valves is too great when made of brass. In May 2009 this author presented most of the known 3 and 4 valve systems including compensating ones. However well optimized they are not in tune within plus or minus 5 cents. To solve this problem an instrument with four double valves and a tritone switch valve is presented and optimized to minimize the RMS intonation error. The ratios of slide lengths to instrument lengths are given by 0.126, 0.0631, 0.198, 0.337, and 0.413. The compensating slide ratios are 0.0522, 0.0261, 0.172 and 0.139. The instrument is to be tuned 3 cents sharp on the open tones. Descending from the second open tone in semitones results in errors in cents of only 3.15, -2.97, -2.81, 2.65, 0, 0, 3.5, -2.97, -2.81, 2.65, 0 and 0. The RMS intonation error is 2.37 cents which is very acceptable in elite musical performance.
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43.75.Fg Brass instruments and other lip-vibrated instruments
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Internal visualization of finished guitars and their sound

Jesus A. Torres, Gabriel Hernandez, Alejandro Granados, and Abel Garcia

POMA Volume 11, pp. 035005 (May 2012); (5 pages)

Online Publication Date: May 21, 2012

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The internal structure of two classical guitars (using a near infrared transmittance technique) and typical sound spectrums from each were compared. Both guitars were crafted by Paracho luthiers: one with carbon fiber reinforcements, and the other one with traditional Torres design. For the experiments, a fluorescent light bulb was inserted through the soundhole, and after, it was substituted with an incandescent one. The internal structure of each guitar was photographed, using a standard digital camera and a sony camcorder with infrared cut filter (nightshot). A typical song fragment was played in each single guitar whereas the same experimental conditions were kept; meanwhile the corresponding mono record was captured. Sound spectra were averaged using selected fragments from the audio files. The images revealed the internal structure, which were useful inspecting the internal guitar designs. Spectrums of different song fragments played in the same guitar were very similar. However, the spectra did change playing the same fragments using the other guitar. The procedures here explained allowed to notice important differences in guitars, using inexpensive and easy-to-implement techniques. This infrared photography technique is a significant improvement in comparison with optical photography, furthermore it leads to explore the whole soundbox structure.
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43.75.Gh Plucked string instruments
43.75.Yy Instrumentation and measurement methods for musical acoustics
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Broadband Active Noise Control with Transfer Path Equalization

Oliver Pabst and Delf Sachau

POMA Volume 11, pp. 040001 (January 2011); (8 pages)

Online Publication Date: January 30, 2011

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Active systems for noise reduction are typically of interest for applications in which low frequency noise is a main source of disturbance and only limited amounts of installation space and payload are available. When applying active noise control methods in small aircraft cabin areas such as the crew rest compartment of a long range jet, the transfer path between sensor and actuator is strongly influenced by room acoustics and transducer behavior, thereby affecting control in a frequency selective manner, especially when using filtered reference algorithms. In this study, a concept for transfer path equalization in a feedback control approach with virtual microphones and the feedback filtered reference least mean squares algorithm are evaluated. The implementation and influence of the prediction error filter based equalizer on broadband active noise control are analyzed in simulation as well as in experiments in an acoustic mockup. The use of the equalizer improves broadband attenuation (f < 350 Hz) by 2-4 dB with a total attenuation of up to 20 dB at the error sensor.
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43.50.Hg Noise control at the ear
43.50.Ki Active noise control
43.60.Mn Adaptive processing
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Active noise barrier minimizing pressure gradient

Carl Hart and Siu-Kit Lau

POMA Volume 11, pp. 040002 (February 2011); (8 pages)

Online Publication Date: February 25, 2011

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Minimization of the sound pressure field within the shadow zone of a noise barrier is achieved by reducing the pressure gradient along a line, at the top of a barrier, via active noise control. The noise control effectiveness of a barrier is increased by this strategy, especially for specific system configurations. The proposed method was evaluated by numerical simulation. Results indicate that system orientation has little effect on minimizing the pressure gradient at the top of the barrier when the error sensors are invisible to the primary noise disturbance. Highly effective control within the shadow zone and close to the barrier is possible when the system is oriented at an angle where two or more error sensors are in line with the first diffracting edge and the primary noise disturbance. Increasing the spatial extent of the quiet zone is possible by increasing the number of control sources, where the error sensors have a line of sight with the primary noise disturbance. [Work supported by a fellowship (CRH) and grant (SKL) from the University of Nebraska - Lincoln.]
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43.50.Ki Active noise control
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Urban Noise and Transport as a Strategy of Environmental Quality

Arturo Figueroa, Martha Orozco-Medina, and Javier García

POMA Volume 11, pp. 040003 (March 2011); (6 pages)

Online Publication Date: March 12, 2011

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Urban noise has become an essential element of environmental analysis in recent years. It is a good tool for assessing environmental quality in urban environments. Residents of large cities are demanding more options of transportation to achieve energy efficiency, low emission of pollutants, and strategic routes, among others. This proposal presents the results of a diagnosis study of urban noise in a heavy traffic avenue in Guadalajara city. The project was performed at the time the infrastructure for the Bus Rapid Transit (BRT) was completed and right after the new transportation system started working on Independencia Avenue. The results of the project indicated that the operation of a fast and environmentally friendly bus reduces noise emissions significantly, and it also contributes to the improvement of acoustic quality of the avenue, benefiting both, people and the environment.
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43.50.Rq Environmental noise, measurement, analysis, statistical characteristics
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Noise propagation through open windows of finite depth into an enclosure

Caleb Sieck and Siu-Kit Lau

POMA Volume 11, pp. 040004 (February 2011); (7 pages)

Online Publication Date: February 25, 2011

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Show Abstract
Predicting the insertion loss of an opening backed with an enclosed space is important for building noise control. Recent research in sound transmission through apertures of finite depth in infinite rigid baffles has included the effects of propagating and evanescent modes within the aperture in order to extend models to higher frequencies. The present study extends the model to the case of the aperture backed by a cavity as opposed to sound radiating into half-space. The role of coupling between the aperture modes, radiation modes, and cavity modes in the transmission was investigated. The results were compared to those of previous models which neglected the depth of the aperture and finite element modeling using COMSOL Multiphysics. Comparisons show that the current model is effective at predicting the sound transmission loss through the aperture and the acoustic field within the cavity for an obliquely incident plane wave. By changing impedance conditions on the half-space side of the aperture and within the aperture, the model has been used to evaluate passive noise control techniques.
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43.50.Gf Noise control at source: redesign, application of absorptive materials and reactive elements, mufflers, noise silencers, noise barriers, and attenuators, etc.
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