Physics & Astronomy

Acoustics research at BYU is strongly cross-disciplinary in character and focuses on the following areas: active noise and vibration control, sound-structure interaction, nonlinear acoustics, audio acoustics and architectural acoustics. The research in acoustics is both experimental and computational in nature and includes simulation and measurement of physical systems, as well as signal processing. Computer facilities are readily available with a number of powerful software packages. In addition, the laboratory is equipped with state-of-the-art acoustic equipments and various anechoic and reverberation chambers that are used for experimental verification of studies.

Weekly research meetings are held are held in conjunction with the ASA student chapter.  Meetings are Thursdays at 11:00 in C261 of the Eyring Science Center.

Students and faculty at Brigham Young University have attended conventions of many professional societies including the Acoustical Society of America, the Audio Engineering Society, and the Institute of Noise Control and Engineering to present their research. Use the bookmark links below for quick access to abstracts and slide show presentations.

1. R. Chester, T.W. Leishman "Active control of free-field sound using near-field energy-based error signals"

1. K. L. Gee, M. Shepherd, et al. "Analysis of high-amplitude jet noise using nonlinearity indicators " 
2. A. Boone, J. Blotter, S. D. Sommerfeldt, T. W. Leishman, "Using a scanning laser Doppler vibrometer to measure acoustic intensity "
3. M. Green, S. D. Sommerfeldt, "Global active control of broadband noise from small axial cooling fans, Part 2 " 
4. X. Chen, T. W. Leishman, "Simultaneous equalization of a loudspeaker and a room: Insights from a one-dimensional model"
5. D. Nutter, T. W. Leishman, J. Abbott, "Energy-based field quantities in reverberation chamber measurements"

1. M. Shepherd, T. W. Leishman, U. Sentagi, "Acoustics of a Planetarium"[Slide Presentation]

2. T. W. Leishman, "Alternating Methods for Assessing the Omnidirectionality of Sources for Room Acoustics Measurements." [Slide Presentations]

3. D. C. Copley, B. Faber, S. D. Sommerfeldt, "Energy density active noise control in an earthmoving machine cab." 

1. S. Rollins, T. W. Leishman,  "Preservation of the Acoustics of the Salt Lake Tabernacle."

2. C. Duke, S. D. Sommerfeldt, "Multi-channel active noise control on an axial fan using variable loads."

3. M. Green, S. D. Sommerfeldt, "Global active control of broadband noise from small axial cooling fans."

4. T. Neilsen, M. Isakson, B. Xu, "Frequency dependence of posterior probability distributions for Biot parameters."

5. B. Xu, T. Neilsen, "Posterior probability distributions for Biot parameters from experimental data."

1. D. Nutter, M. Shepherd, T. W. Leishman, B. Shafer, "Effect of Diffusers and Low-Frequency Absorption on Modal Density of Two Reverberation Chambers."

2. M. Shepherd, X. Chen, T. W. Leishman and S.D. Sommerfeldt, “Experimental equalization of a one-dimensional sound field using energy density and a parametric equalizer,”  [Slide Presentation]

3. B. Monson, S. D. Sommerfeldt, "Optimal configurations for active control of axial fans."

4. J. Chatterley, A. Boone, J. Blotter, S. D. Sommerfeldt, T. Lago, "Sound-quality analysis of sewing machines."

5. M. C. Harris, J. D. Blotter, S. D. Sommerfeldt, "Development of a near-field acoustic holography system based on pressure and velocity measurements."

6. J. Hill, J. D. Blotter, T. W. Leishman, "Harsh environment windscreen analysis and design."

7. T. W. Leishman, G. Dix, "Additional modeling details for the suspension of a moving-coil loudspeaker."

1. S. Rollins, T. W. Leishman, “Acoustical measurement of the Salt Lake Mormon Tabernacle,” J. Acoust. Soc. Am., 115, (5), p. 2477.    [Slide Presentation]
2. H. Smith, T. W. Leishman, “Numerical evaluation of the omnidirectional behavior of regular polyhedron loudspeakers,” J. Acoust. Soc. Am., 115, (5), p. 2525.    [Slide Presentation]
3. X. Chen, S. D. Sommerfeldt, T. W. Leishman, “An adaptive equalization scheme using acoustic energy density,” J. Acoust. Soc. Am., 115, (5), p. 2612.
4. B. B. Monson, S. D. Sommerfeldt, C. Duke, “Active noise control of small axial cooling fans,” J. Acoust. Soc. Am., 115, (5), p. 2498.     [Slide Presentation]
5. L. L. Locey, B. L. Woolford, S. D. Sommerfeldt, J. D. Blotter, “Error analysis and implementation issues for energy density probe,” J. Acoust. Soc. Am., 115, (5), p. 2576.
6. M. C. Harris, J. D. Blotter, S. D. Sommerfeldt, “Free field reconstruction using energy density measurements,” J. Acoust. Soc. Am., 115, (5), p. 2576.
7. S. S. Utami, “An acoustical analysis of a room with a concave dome ceiling element,” J. Acoust. Soc. Am., 115, (5), p. 2581.

1. B. Faber, S. Sommerfeldt, “System identification and energy-based active noise control performance,” J. Acoust. Soc. Am., 114, (4), p. 2332.
2. S. Rollins, T. Leishman, G. Dix, “Experimental evaluation of the omnidirectional behavior of platonic polyhedron loudspeakers,” J. Acoust. Soc. Am., 114, (4), p. 2343.    [Slide Presentation]
3. T. Leishman, “Vibration-controlled modules for use in active segmented partitions,” J. Acoust. Soc. Am., 114, (4), p. 2385.
4. T. Leishman, “Research in the field of active sound transmission control,” J. Acoust. Soc. Am., 114, (4), p.2390.
5. S. Sommerfeldt, P. Remington, “Active control of low-speed turbofan tonal noise,” J. Acoust. Soc. Am., 114, (4), p. 2385.
6. H. Smith, T. Leishman, “Computer modeling of a large fan-shaped auditorium,” J. Acoust. Soc. Am., 114, (4), p. 2411.      [Slide Presentation]
7. L. Locey, S. Sommerfeldt, “Analysis and comparison of three energy density probe designs,” J. Acoust. Soc. Am., 114, (4), p. 2443.
8. X. Chen, T. Leishman, S. Sommerfeldt, “Equalization of a one-dimensional sound field using acoustic energy density,” J. Acoust. Soc. Am., 114, (4), p. 2460.

1. L. Locey, T. Leishman, S. Sommerfeldt, B. Anderson, “Comparison of echo criteria for a large fan-shaped auditorium,” J. Acoust. Soc. Am., 113, (4), p. 2189.
2. T. Leishman, B. Anderson, “Derivation of moving-coil loudspeaker parameters using acoustical testing techniques: Theoretical developments,” J. Acoust. Soc. Am., 113, (4), p. 2201.
3. B. Anderson, T. Leishman, “Derivation of moving-coil loudspeaker parameters using acoustical testing techniques: Experiment results,” J. Acoust. Soc. Am., 113, (4), p. 2202.

1. B. Anderson, T. Leishman, “An acoustical measurement method for the derivation of loudspeaker parameters,” presented at the 115th Convention of the Audio Eng. Soc., 2003-10, preprint number pending.

1. B. Faber, S. Sommerfeldt, “Active minimization of acoustic energy density in enclosed sound fields,” J. Acoust. Soc. Am., 112, (5), p. 2302.

1. B. Anderson, W. Strong, “The effect of inharmonic partials on pitch of piano tones,” J. Acoust. Soc. Am., 111, (5), p. 2394.
2. L. Ireland, S. Sommerfeldt, “Numerical model for multi-mode active control of turbofan noise,” J. Acoust. Soc. Am., 111, (5), p. 2452.
3. K. Gee, S. Sommerfeldt, “Multi-channel active control of axial cooling fan noise,” J. Acoust. Soc. Am., 111, (5), p. 2453.

S. Rollins, T. W. Leishman, “Acoustical measurement of the Salt Lake Mormon Tabernacle,” J. Acoust. Soc. Am., 115, (5), p. 2477.

Anacoustical survey of the Salt Lake Mormon Tabernacle has beenperformed to assess the behavior of the hall in itscurrent state. The tabernacle is a well-known historical building witha large elongated dome ceiling. This paper discusses the measurementsused to characterize the hall. Several parameters derived from omnidirectional,directional, and binaural impulse response measurements are presented. Color maps of the parameters over audience seating areas reveal their spatialvariations. These maps and the statistical properties of the parametersaid in clarifying the acoustical characteristics and anomalies of thehall.

PACS: 43.55.Gx

H. Smith, T. W. Leishman, “Numerical evaluation of the omnidirectional behavior of regular polyhedron loudspeakers,” J. Acoust. Soc. Am., 115, (5), p. 2525.

Aregular polyhedron loudspeaker (RPL) consists of a rigid enclosure inthe shape of a regular polyhedron with loudspeaker drivers mountedwithin each face. Room acoustics measurements often incorporate dodecahedron loudspeakerspresumably as omnidirectional sources of sound. This research is intendedto support experimental work to determine which of the fiveregular polyhedron loudspeaker configurations actually produces the most omnidirectional fieldover a wide frequency range. Loudspeaker models based on theboundary element method were created to assess frequency-dependent radiation patterns.These models assume that the loudspeaker drivers radiate as planecircular pistons. Far-field sound pressures were generated for several fieldpoints around the models (5-deg polar and azimuthal angle increments).Results are presented as directivity balloon plots and compared withexperimental measurements. Other numerical results are shown as variations onthe experimental setup. These involve dimensional scaling of an entireloudspeaker and dimensional scaling of driver diameters while keeping enclosuredimensions constant. They also involve generation of data at moreclosely spaced field points (1-deg polar and azimuthal angle increments).Results from a spherical enclosure with radiating pistons centered onthe angular coordinates of dodecahedron facial centers are also comparedto those produced by a dodecahedron loudspeaker model.

PACS: 43.55.Mc
 

X. Chen, S. D. Sommerfeldt, T. W. Leishman, “An adaptive equalization scheme using acoustic energy density,” J. Acoust. Soc. Am., 115, (5), p. 2612.

Thispaper presents an equalization system based on the filtered-x algorithmand acoustic energy density. Previous theoretical results have shown thatequalization filters using acoustic energy density produce greater spatial uniformityin equalization than those using acoustic pressure. The paper discussesthe implementation of the equalization system in a one-dimensional duct.An adaptive algorithm is utilized in the time domain todetermine and apply the equalization filter. Results from a traditionalfilter design and the energy density filter design are compared.

PACS: 43.60.Dh
 

B. B. Monson, S. D. Sommerfeldt, C. Duke, “Active noise control of small axial cooling fans,” J. Acoust. Soc. Am., 115, (5), p. 2498.

Anactive noise control system has been developed for the reduction of noise radiated by small axial cooling fans, such asthose found in desktop computers. This system, based on amodified version of the filtered-x LMS algorithm, uses four smallactuators surrounding the fan in a mock computer casing. Globalattenuation of the fan noise has been demonstrated using four-channelcontrol. Due to industry volume constraints, a smaller system isrequired to replace the existing one, while still maintaining similarperformance. The system is designed with a smaller fan withsmall actuators, so as not to exceed the size of the original fan. In order to maintain comparable air flow,the smaller fan must run at higher speeds, causing potentiallygreater output noise levels at higher frequencies. Global attenuation ofthese levels is desired. Implementation and experimental results of thesystem will be presented. A comparison of the two systems,and the feasibility of the replacement of the larger fanby the active control system with a smaller fan will be discussed.
PACS: 43.50.Ki  
 

L. L. Locey, B. L. Woolford, S. D. Sommerfeldt, J. D. Blotter, “Error analysis and implementation issues for energy density probe,” J. Acoust. Soc. Am., 115, (5), p. 2576.

Previousresearch has demonstrated the utility of acoustic energy density measurementsas a means to gain a greater understanding of acousticfields. Three spherical energy density probe designs are under development.The first probe design has three orthogonal pairs of surfacemounted microphones. The second probe design utilizes a similarly sizedsphere with four surface mounted microphones. The four microphones arelocated at the origin and unit vectors of a Cartesiancoordinate system, where the origin and the tips of thethree unit vectors all lie on the surface of thesphere. The third probe design consists of a similarly sizedsphere, again with four surface microphones, each placed at the vertices of a regular tetrahedron. The sensing elements of allthree probes are Panasonic electret microphones. The work presented herewill expand on previously reported work, and address bias errors,spherical scattering effects, and practical implementation issues. [Work supported by NASA.]
PACS: 43.58.Fm  
 

M. C. Harris, J. D. Blotter, S. D. Sommerfeldt, “Free field reconstruction using energy density measurements,” J. Acoust. Soc. Am., 115, (5), p. 2576.

Forwardwave propagation to the farfield using nearfield pressure measurements hasundergone extensive development. Up until now, discrete pressure measurements aretypically made using a conformal array of microphones. However, complexacoustic fields require large microphone arrays in order to accomplishaccurate field reconstruction. The use of energy density sensors hasindicated several advantages over conventional pressure microphones in active noisecontrol applications. This study examines possible benefits of energy-based measurementsin free field reconstruction. To determine and quantify the possibleadvantages of energy density measurements, an analytical model was builtto evaluate the performance. The error in acoustic fields reconstructedusing energy density sensors is compared with conventional pressure microphonereconstruction accuracy. A number of analytical studies will be presentedshowing the benefits of using energy-based measurements in free fieldreconstruction.
PACS: 43.20.Ye
 

S. S. Utami, “An acoustical analysis of a room with a concave dome ceiling element,” J. Acoust. Soc. Am., 115, (5), p. 2581.

Concavesurfaces are often considered detrimental in room acoustics, especially becauseof the impact they have on the distribution of soundenergy. This paper explores certain acoustical characteristics and anomalies foundin spaces below concave dome ceiling elements. The architectural designof the Darusshollah mosque in East Java, Indonesia is usedas a case study with specific spatial and functional concerns. Investigations of the mosque have been conducted through both a1:12 scale model and a computer model that utilizes raytracing and image source methods. Analysis techniques are discussed. Resultsare presented and compared to provide useful insights into theacoustics of such distinctive environments.
PACS: 43.55.Fw  

B. Faber, S. Sommerfeldt, “System identification and energy-based active noise control performance,” J. Acoust. Soc. Am., 114, (4), p. 2332.

Anactive noise control system, which minimizes acoustic energy density, canoften become unstable due to significant changes in the acousticalenvironment in which it operates. Typically, the control system ofinterest, which is based on a modified version of thefiltered-x LMS algorithm, is operated with a fixed set ofsystem identification filter coefficients. These coefficients are determined off-line, priorto execution of the main control algorithm. The performance of the control system depends on the quality of the systemmodel obtained through the system identification algorithm. Changes in thesystem transfer function and impulse response, due to various factors,including temperature, boundary conditions, and sensor/actuator location, have been investigated.Results will be presented, which give indications regarding what typesof changes to the system have the greatest effect onthe transfer function and impulse response, and how sensitive thecontrol system is to those effects. The effects of changingsampling rates and system identification filter lengths on the qualityof the system model have also been investigated and willbe discussed.

PACS: 43.50.Ki       

S. Rollins, T. Leishman, G. Dix, “Experimental evaluation of the omnidirectional behavior of platonic polyhedron loudspeakers,” J. Acoust. Soc. Am., 114, (4), p. 2343.

Manyarchitectural acoustics measurements require the use of an omnidirectional source.For several years, the source predominantly used for such applications has been the dodecahedron loudspeaker with small in-phase drivers mountedin each face. While other platonic polyhedron loudspeakers (PPLs) havenot been used as frequently, they also produce nearly omnidirectionalfields over limited bandwidths. Above cutoff frequencies specific to theirgeometries, all PPLs depart from ideal omnidirectional behavior, with varyingdegrees of directivity. While these cutoff frequencies are typically higherfor higher-order polyhedra, they commonly fall within the bandwidths ofstandard measurements. The five types of PPLs have been constructedand measured to gain greater insight into their omnidirectional behaviors.Their frequency responses were taken at 2664 points over asphere (5-deg polar and azimuthal angle increments) in an anechoicchamber. The measurements were then processed to produce directivity balloonplots. However, to better compare directivities and find the sourceconsistently producing the most omnidirectional field over a useful bandwidth,a frequency-dependent standard deviation formula was implemented. Average values ofthe standard deviation parameter produce figures of merit that furthercharacterize omnidirectionality. [Research supported by funding from the NSF REUprogram.]
PACS: 43.38.Ja, 43.55.Mc

 

T. Leishman, “Vibration-controlled modules for use in active segmented partitions,” J. Acoust. Soc. Am., 114, (4), p. 2385.

Activesegmented partitions (ASPs) have become an important topic in theresearch of active sound transmission control (ASTC). Many ASTC applications require lightweight partitions, high transmission loss over a broad frequencyrange, and applicability to a variety of source and receivingspace conditions. This paper provides theoretical, numerical, and experimental evaluationsof specific ASP module configurations used to satisfy these requirements.The control objective for each is to induce global vibrationcontrol of its various transmitting surfaces through direct minimization ofthe normal vibration of its principal transmitting surface. Normal-incidence transmissionloss estimates are based on equivalent circuit analysis and correspondingexperimental measurements are made using plane wave tube techniques. Thepaper explains why characteristic single diaphragm configurations are unable touse this strategy effectively. It also discusses two dual diaphragmconfigurations that use the control scheme particularly well to produceefficient global control of their transmitting surfaces and achieve veryhigh transmission loss over a broad frequency range. The designsfacilitate incorporation into full ASP arrays and satisfy other usefulcriteria.
PACS: 43.50.Ki

 

T. Leishman, “Research in the field of active sound transmission control,” J. Acoust. Soc. Am., 114, (4), p.2390.

Soundtransmission through an unperforated partition may be described as thevibro-acoustic process in which fluid-borne sound waves incident upon thepartition from the source side force it to vibrate andconsequently radiate transmitted fluid-borne sound waves into the space onthe opposite (receiving) side. Active reduction of sound transmission throughpartitions, or active sound transmission control (ASTC), denotes an activemodification to this process. It may be defined as thereduction of transmitted sound through the action of electro-mechano-acoustical devicesregulating any or all stages of the vibro-acoustic sound transmissionprocess. In certain analyses, this branch of active noise controlhas referred to situations in which electro-mechanical actuation is appliedonly to a partition through which sound is transmitting. However,since ASTC may be achieved by other means, and sincemuch research in the field has utilized other techniques, thislimitation is clearly too restrictive. This paper reviews and categorizesmany research efforts in the field, and provides a briefhistory of the efforts based on an extensive bibliography ofthe subject matter. It also identifies research areas that couldbe pursued in the future.
PACS: 43.50.Ki       

 

S. Sommerfeldt, P. Remington, “Active control of low-speed turbofan tonal noise,” J. Acoust. Soc. Am., 114, (4), p. 2385.

Activenoise control has been proposed as a technique for reducingthe tonal noise radiated from turbofan engines. The sound fieldin the duct of a turbofan engine is characterized byacoustic modes, which exhibit both a radial and a circumferentialspatial dependence. The dominant circumferential modes are determined by therelationship between the number of rotor and stator blades. Usingthese concepts, an active noise control system has been developedto measure and minimize the modes in the duct ofa turbofan engine. By using multiple source and sensor locations,it has also been shown that it is possible tocontrol multiple radial modes within the engine duct. Some ofthe issues associated with the design of the control systemwill be reviewed, and results obtained using the Active NoiseControl Fan (ANCF) at NASA Glenn Research Center will bepresented. [Work supported by NASA.]
PACS: 43.50.Ki       

 

H. Smith, T. Leishman, “Computer modeling of a large fan-shaped auditorium,” J. Acoust. Soc. Am., 114, (4), p. 2411.

Aresearch project was recently undertaken to analyze the acoustical characteristicsof a 21<th>000-seat fan-shaped auditorium. Careful geometric modeling of the hall has been a significant part of this study. Becauseof its size, shape, and other architectural features, computer modelinghas presented some interesting challenges. For example, it has beenshown experimentally that the concavely oriented rows of (moderately absorptive)seats produce significant scattering that aggregates toward the focal pointof the hall. This paper will discuss how the seatscattering and scattering from other bodies have been included inthe model. Other challenges in modeling a hall this sizewill also be discussed.
PACS: 43.55.Ka       

 

L. Locey, S. Sommerfeldt, “Analysis and comparison of three energy density probe designs,” J. Acoust. Soc. Am., 114, (4), p. 2443.

Previousresearch has demonstrated the utility of acoustic energy density measurementsas a means to gain a greater understanding of acousticfields. Three energy density probes are under development. The firstprobe has three orthogonal pairs of microphones embedded on thesurface of a sphere. The second design is a similarlysized sphere with four surface mounted microphones, equidistant from thecenter of the sphere. The four microphones are arranged tocorrespond with the origin and unit vectors of a Cartesiancoordinate system, where the origin and the tips of thethree unit vectors are on the surface of the sphere.As a result, all four microphones lie on the surface of the top hemisphere. The third design consists of asimilarly sized sphere with four surface microphones arranged in atetrahedron configuration. The author will discuss some of the errorsand limitations associated with the four microphone designs as comparedto the six microphone design. [Work supported by NASA.]
PACS: 43.58.Fm       

 

X. Chen, T. Leishman, S. Sommerfeldt, “Equalization of a one-dimensional sound field using acoustic energy density,” J. Acoust. Soc. Am., 114, (4), p. 2460.

Formany years, reproduced and reinforced sound fields have been equalizedusing acoustic pressures measured at discrete field points. In thispaper, well-known problems associated with this equalization method are investigatedusing a one-dimensional model. An optimum equalization scheme is thensuggested that is based on the spatially averaged mean-square pressureof the field. A practical method based on the energydensity of the field is subsequently discussed and compared tothe other methods. Although the energy density is evaluated ata single field point, its spatial uniformity is shown toprovide results approaching those of the optimum equalization method.
PACS: 43.38.Md
 

1. L. Locey, T. Leishman, S. Sommerfeldt, B. Anderson, “Comparison of echo criteria for a large fan-shaped auditorium,” J. Acoust. Soc. Am., 113, (4), p. 2189.

Complaintsof perceived echoes at specific locations in a 21,000 seatfan-shaped auditorium have prompted the measurement and analysis of numerousimpulse responses. The responses were first processed using the echocriterion of Niese, then using the criterion of Dietsch andKraak. This paper compares the two criteria and explores their abilities to assess whether peaks and anomalies of the measured responses were likely to produce audible echoes in the hall. The Niese criterion was found to better predict the perception of echoes produced by broad irregular decay trends. The Dietsch and Kraak criterion was able to better predict echoes produced by sufficiently strong specular reflections. Neither criterion alone was ableto fully characterize these perceptions. Results obtained for both criteriaat various seat locations will be presented and compared withsubjective findings.
PACS: 43.55.Gx       

 

2. T. Leishman, B. Anderson, “Derivation of moving-coil loudspeaker parameters using acoustical testing techniques: Theoretical developments,” J. Acoust. Soc. Am., 113, (4), p. 2201.

Moving-coilloudspeaker driver parameters are generally derived through the measurement ofelectrical impedances. Nevertheless, because these drivers are electro-mechano-acoustical transducers, theirparameters may also be determined from measurements taken in otherphysical domains. This paper presents theoretical concepts that show howthey may be determined acoustically using plane wave tube techniques. A driver is mounted in a baffle to form acomposite partition between a source tube and a receiving tube. The frequency-dependent transmission loss of the partition is determined usingupstream and downstream sound field decompositions that compensate for possiblenonanechoic receiving tube conditions. A transmission loss curve based onan equivalent circuit model of the system is then fitto the measured curve to extract specific driver parameters. Different electrical conditions are imposed at the driver terminals to modifythe transmission loss in ways that allow the determination ofadditional parameters.
PACS: 43.38.Ja       

 

3. B. Anderson, T. Leishman, “Derivation of moving-coil loudspeaker parameters using acoustical testing techniques: Experiment results,” J. Acoust. Soc. Am., 113, (4), p. 2202.

Aunique acoustical method of measuring small-signal moving-coil loudspeaker parameters hasrecently been developed. This technique involves the use of aplane wave tube to measure acoustical properties (e.g., reflection andtransmission coefficients) of a driver under test (DUT). From thisdata, small-signal parameters are derived using curve-fitting techniques. Electrical conditionsare easily controlled and automated to allow for the derivation of additional parameters. Current parameter measurement techniques require measurement ofthe electrical impedance of the DUT. This paper will discussthe acoustical measurement apparatus, experimental measurement techniques, and compare itsmeasured parameters to those derived using electrical techniques.
PACS: 43.38.Ja       

B. Anderson, T. Leishman, “An acoustical measurement method for the derivation of loudspeaker parameters,” presented at the 115th Convention of the Audio Eng. Soc., 2003-10, preprint number pending.

Because loudspeaker drivers are electro-mechano-acoustical transducers, their parameters may be measured from physical domains other than the electrical domain. A method has been developed by the authors to determine moving-coil loudspeaker parameters through the use of acoustical measurements. The technique utilizes a plane wave tube and the two-microphone transfer function technique to measure acoustical properties of a baffled driver under test (DUT). Quantities such as the reflection and transmission coefficients of the DUT are first measured.  Driver parameters are then extracted from the measurements using curve-fitting techniques and theoretical solutions to equivalent circuits of the composite system. This paper discusses the acoustical measurement apparatus, system modeling, and a comparison of acoustically measured parameters to those measured using common electrical techniques. Parameters derived from the various methods are also compared to reference parameters to establish bias errors.

1.B. Faber, S. Sommerfeldt, “Active minimization of acoustic energy density in enclosed sound fields,” J. Acoust. Soc. Am., 112, (5), p. 2302.

Minimizationof acoustic energy density has been investigated for active noise control applications for enclosed sound fields. The standard approach of minimizing the squared acoustic pressure has been shown to oftenlead to localized control of the sound field, which maybe undesirable. It has been shown that minimizing the energydensity often leads to improved global attenuation of the field,since quantities dependent on both the pressure and particle velocity are minimized. This penalizes active control solutions in which thepressure is minimized and the particle velocity is increased, asoften occurs when simply minimizing the squared pressure. Minimizing theacoustic energy density also requires multiple microphones to be integrated as part of the error sensor. However, in many casesthe number of microphones can be reduced by strategically placing the sensor in a location where one component of theparticle velocity is assumed to be negligible. Active control resultswill be shown for an enclosed sound field to comparethe reduction obtained using energy density with that obtained forminimizing squared pressure. In addition, the attenuation obtained using a full three-dimensional energy density sensor will be compared with thecontrol achieved using an error sensor with fewer microphones.
PACS: 43.50.Ki

1.B. Anderson, W. Strong, “The effect of inharmonic partials on pitch of piano tones,” J. Acoust. Soc. Am., 111, (5), p. 2394.

Piano tones have partials whose frequencies are sharp relative to harmonic values. A listening test was conducted to determine the effectof inharmonicity on pitch for piano tones in the lowest three octaves of a piano. Nine real tones from thelowest three octaves of a piano were analyzed to obtain frequencies and amplitude of their partials. Synthetic inharmonic tones were produced from these results. Synthetic harmonic tones, each with atwelfth of a semitone increase in the fundamental, were alsoproduced. A jury of 10 listeners matched each synthetic inharmonictone to one of the synthetic harmonic tones. The effectof the inharmonicity on pitch was determined from an average of the listeners' results. For the nine piano tones studied,inharmonicity increased pitch from a twelfth of a semitone tonearly a semitone.
PACS: 43.75.Cd
 

 

2.L. Ireland, S. Sommerfeldt, “Numerical model for multi-mode active control of turbofan noise,” J. Acoust. Soc. Am., 111, (5), p. 2452.

Anumerical model has been developed to investigate the possibility ofimplementing active control to minimize noise radiation from turbofan engines.Previous experimental work on the NASA Glenn Research Center ANC fan have been encouraging, but the question remains of whetherthe modal approaches investigated can be expected to work effectivelyon real engines. The engine model presented here uses anindirect boundary element method, implemented with Sysnoise, and a multi-modeNewton algorithm to simulate the active control. Experimental results obtainedusing the NASA Glenn ANC fan, which has a simplifiedcylindrical engine geometry, were used as a benchmark for themodel. The model results have been found to compare wellwith the experimental results. Recently, a more realistic, but stillaxisymmetric, engine geometry has been used. Preliminary results will bepresented which indicate that a modal ANC approach should stillbe effective for controlling turbofan noise.
PACS: 43.50.Ki       
 

 

3.K. Gee, S. Sommerfeldt, “Multi-channel active control of axial cooling fan noise,” J. Acoust. Soc. Am., 111, (5), p. 2453.

Amulti-channel active control system has been applied to the reduction of free-field tonal noise from a small axial cooling fan.The experimental apparatus consists of an aluminum enclosure which housesthe fan, an infrared detector-emitter pair which serves as areference sensor, loudspeakers, microphones, and appropriate filters and amplifiers. Theresearch fan and loudspeakers have been modeled theoretically as pointsources to derive a mathematical expression for radiated power. Theminimization of this power has served to guide the numberand location of control sources, as well as to discoverpotential error microphone locations in the extreme near-field. Experiments with these microphone positions have shown that there are predictable near-fieldlocations which consistently lead to significant reductions in the global mean-squared pressure for the first four harmonics of the bladepassage frequency. For example, a four channel configuration with theerror microphones placed appropriately results in global mean-squared pressure reductionson the order of 10 dB or more for thetargeted frequencies.
PACS: 43.50.Ki       

 

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