1134 J. Audio Eng. Soc., Vol. 55, No. 12, 2007 December

Jan Abildgaard Pedersenconference chair

Wieslaw WoszczykAES president

AES 32nd

INTERNATIONALCONFERENCE

Stanley Lipshitzkeynote speaker

AES 32nd

INTERNATIONALCONFERENCE

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Once again Denmark hosted another AES con-ference, this time on the topic “DSP forLoudspeakers.” The conference was a three-day event held at the excellent PharmakonConference Center. 125 engineers from more

than 20 countries around the world traveled to this lovelyarea of North Zealand, a short train ride northwest of Copenhagen. The conference offered a great combination oftechnical papers, workshops, technology demonstrations,informal discussions, and social events. The large number ofdelegates participating in the conference clearly showed that“DSP for Loudspeakers” is a significant area of interestwithin the audio engineering community.

Digital signal processing (DSP) offers loudspeaker systemdesigners a very powerful set of tools. It may be as simple asa delay line, which hardly needs DSP anyway. Or it may bea precise and detailed filtering with just the desired magni-tude and phase characteristics. Such filtering may be used toconstruct a single very accurate loudspeaker, or used forroom correction, or it may be used to create beam-steeringarrays or even wavefield synthesis. In addition to filtering,digital signal processing may be used for distortion compen-sation or for enhancing the subjective performance by

means of nonlinear processing. On the analysis side, DSPcan be used to monitor in real time the state of the loud-speaker and model important driver parameters such asvoice coil temperature. Throughout the last couple ofdecades, AES members have been in the forefront of thisresearch.

Meanwhile, the maturity and price/performance ratio ofessential system components such as processors, softwaredevelopment tools, AD/DA converters, and power ampli-fiers have reached a point where it makes commercial sensefor a large part of the audio industry to apply loudspeaker-specific DSP in their products. Indeed, there are alreadyclasses of products completely dependent on it.

Jan Abildgaard Pedersen, conference chair, and his com-mittee assembled a conference program that presented anoverview of the current state-of-the-art in a broad perspectiveand addressed many of the scientific disciplines involved inthis emerging field. Knud Bank Christensen, papers chair,coordinated the technical program of 21 papers in 7 sessions.

DAY 1Jan Abildgaard Pedersen opened the conference on Friday bywelcoming everyone to Denmark and expressing his plea- ➥

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1136 J. Audio Eng. Soc., Vol. 55, No. 12, 2007 December

sure with the attendance of 125 delegates. He pointed outthat it takes approximately two years of planning and prepa-ration to organize an AES International Conference. He thenintroduced the other members of the committee: Knud BankChristensen, papers chair; Thomas Mørch, secretary; EddyBøgh Brixen, facilities; Subir Pramanik, treasurer; PrebenKvist, webmaster; and Ole Juhl Pedersen, assistant. Afterexplaining the schedule for the the next three days, Pedersenthanked AES Headquarters in New York, represented byRoger Furness, executive director, for their valuable and pro-fessional help in preparing the conference. Then WieslawWoszczyk, AES president, added his words of welcome andpointed out that AES international conferences play animportant role for the Society as venues for high-level scien-tific exchanges in highly focused areas of research.

The keynote address was given by Stanley Lipshitz of theUniversity of Waterloo, who spoke on the topic “The Loud-speaker in the Digital Age.” He started by taking the audi-ence back to the introduction of the electrodynamic loud-speaker in 1877. He moved on to binaural audio, stereo,ambisonics, and wavefield synthesis as examples of differ-ent ways in which signals from soundfields generated byloudspeakers reach the ears of listeners. Lipshitz went ontalking about the directivity of monopole, dipole, cardioid,and and other types of loudspeakers. He mentioned thatroom correction, phase response, and nonlinearities as veryinteresting applications of DSP for loudspeakers.

Following the keynote address, the first papers sessionwas devoted to adaptive systems. Jan Abildgaard Pedersen,Lyngdorf Audio, presented the initial paper, “Fully Auto-

AES 32ND INTERNATIONAL CONFERENCE

Finn Agerkvist

Wolfgang Klippel Bo Rohde PedersenSofus Birkedal Nielsen

Thomas Sporerinvited speaker

Matti Karjalaineninvited speaker

Andrew Bright

Ulrich Horbach

David GunnessMarshall Buck

Fares El-Azm

Jaeyoun Cho

AuthorsAuthors

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matic Loudspeaker-Room Adaptation—The RoomPerfectSystem.” This system for DSP-based room correction isbased on measuring the sound pressure at the listening posi-tion and in at least three random positions scattered acrossthe entire listening room. It enables a fully automatic calcu-lation of a target function, which preserves the basic charac-teristics of the loudspeaker being used.

Fares El-Azm, Lyngdorf Audio, continued with the paper“Natural Timbre in Room Correction Systems (Part II),” inwhich he describes how a certain part of the increased levelat low frequencies due to the influence of a room is naturalto the human ear and should not be removed by room-correction systems.

Next Sofus Birkedal Nielsen, Aalborg University, pre-sented “Time Based Room Correction System for Low Fre-quencies Using Multiple Loudspeakers.” In this system 4low-frequency loudspeakers are placed in specific positions2 at the front end wall and 2 at the rear end wall. The timedomain is used to illustrate how the reflections at the rearend wall are cancelled, leaving a much more even soundpressure distribution in the room.

“Adaptive IIR Filters for Loudspeaker Parameter Track-ing,” by Andrew Bright, Nokia, concluded the session onadaptive systems. He explained how IIR filters are used in asimple method for in-situ tracking of loudspeaker parame-ters as they change over time. In particular he investigatedthe convergence properties and showed a method to opti-mize convergence time and residual error.

Session 2 focused on nonlinear processing. In the firstpaper “Modelling Loudspeaker Non-Linearities,” FinnAgerkvist, Technical University of Denmark, compared dif-ferent techniques for modeling the nonlinear parameters ofthe electrodynamic loudspeaker. He showed that inversepolynomial expansion and localized fitting functions aresuperior to polynomial expansion.

The second and final paper in this session, “Optimal

Design of Loudspeakers with Nonlinear Control,” was pre-sented by Wolfgang Klippel, Klippel GmbH. He showedhow nonlinear control opens up new degrees of freedom inpassive driver design by exploiting the gained efficiency ofloudspeaker drivers with nonlinear motors, such as by usinga short voice coil overhang. Klippel presented the results ofan investigation of the behavior of real loudspeakers undercontrol and the obtained design criteria for active loud-speaker systems.

The first day of the conference was concluded by a com-bined poster session, product demonstrations, and tabletopdemos. Anthony McMahon, Alba Centre, presented thepaper “Design and Implementation of a DSP EnhancedPortable Speaker System” in the poster session. McMahondescribed how DSP enables an enhanced performance oflow-cost portable loudspeaker systems and how it can bedesigned and implemented.

The first of the three demo rooms was used by LyngdorfAudio to present and demonstrate its RoomPerfect room-correction system together with true-digital amplifiers andcompact dipole loudspeakers combined with boundarywoofers. Listeners had a chance to evaluate both a globalfilter and a focused filter. The focused filter is optimal for aspecific position, but still uses the information of the entire3-D soundfield so that everyone in the demo room was ableenjoy the same sound from the focused filter. The global fil-ter enabled people to walk around the demo room and hearvirtually the same sound quality in all positions within theroom.

WaveCapture used another demo room to show its greatvariety of software products: Room-Capture for room instal-lations, Live-Capture for FOH measurements, EQ-Capturefor parametric EQ, Audio-Capture for audio componentmeasurement, and RT-Capture for SPL management.

Sonion used the third demo room to show its newest inven-tions in microphones and receivers. Sonion works with ➥

AES 32ND INTERNATIONAL CONFERENCE

32nd Conference Committee: from left, Jan Abildgaard Pedersen, chair; Knud Band Christensen, papers chair; Eddy Bøgh Brixen, facilities chair;Thomas Mørch, secretary; Preben Kvist, webmaster; and Subir Pramanik, treasurer

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1138 J. Audio Eng. Soc., Vol. 55, No. 12, 2007 December

miniature components to manufacture and market electroa-coustical and electromechanical solutions within hearinginstruments, mobile terminals, headsets, and medical devices.

During this evening part of the program, there were alsotabletop demos in which the authors were able to discuss inmore detail aspects of their papers. These tabletop demossparked many in-depth discussions among the attendees. Therelaxed, informal nature of the event fostered networking andallowed the attendees to make new professional connections.

Oxford Digital discussed its products, research and devel-opment work, and the consultancy services it offers in a table-top demo. Alba Campus, during its demo, provided furtherinformation from its paper presentation on how DSP enablesan enhanced performance of low-cost portable loudspeakersystems.

Wolfgang Klippel, Klippel GmbH, showed his tools fordesigning, assessing, and diagnosing passive transducers.He also demonstrated the DSP used in active loudspeakersystems as described in his paper presentation on the sametopic. Four-Audio presented information on its loudspeakermanagement system on a multirate platform with FIR andIIR filters. The system also includes dual-range AD conver-sion, look-ahead peak-limiters, and RMS limiters.

The tabletop demo of Danville Signal Processing andDSP Concepts demonstrated a new DSP crossover platformbased on SHARC audio modules and a graphical designtool. The platform enables loudspeaker designers to imple-ment a variety of crossover designs combined with opti-

mized audio processing functions, such as time delays andparametric EQs.

DAY 2The second day of the conference started with a session onmeasurement techniques. The first paper, “Musical Trans-ducer-Less Identification of Linear Loudspeaker Parame-ters,” was presented by Bo Rohde Pedersen, Aalborg Uni-versity. He described how the voice-coil current was usedas feedback from the loudspeaker for system identificationin a transducer-less measurement system. In this way hegenerated information on the voice-coil resistance, reso-nance frequency, and damping factor.

Ho Young Sung, Samsung Electronics, presented thepaper “A Method for Objective Sound Quality Evaluation ofHeadphones,” in which he describes how the characteristicsof headphones are different from loudspeakers and howobjective factors that play a key role in sound quality arecarefully selected.

There were four papers in the next session on filter struc-tures and design algorithms. The session began with theinvited paper “Special Digital Filters for Audio Reproduc-tion” presented by Matti Karjalainen, Helsinki University ofTechnology. He gave a great overview of digital-filter categories that have been found important in audio reproduction, covering in particular fractional delays, fre-quency-warping techniques including Laguerre and Kautzfilters, as well as filters used for physics-based modeling,

AES 32ND INTERNATIONAL CONFERENCE

Alba Campus, Danville Signal Processing,Four-Audio, Klippel GmbH, Lyngdorf Audio,Oxford Digital, Sonion, and WaveCaptureoffered product demonstrations during theconference.

DEMOS

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including wave filters and digital waveg-uides. He also talked about the applica-tions of these filter categories in audioreproduction.

David Gunness, Eastern AcousticWorks, presented the paper “Optimizingthe Magnitude Response of Matched Z-Transform Filters (MZTI) for Loud-speaker Equalization.” He explained howMZT filters deviate from analog filtersand how these filters are optimized in away displaying excellent agreement withthe analog target filters, both in magni-tude and phase response.

D. Sookcharoenphol, KMITL, pre-sented “Realization of Linear PhaseCrossover Loudspeaker Network UsingIIR Filters,” in which he described how agroup of IIR filters have their impulseresponses truncated to finite length toachieve linear time-invariance. The sys-tem provides an overall flat magnitude and group delayresponses and also higher slope and stopband attenuationthan prototype elliptic IIR filters.

In the final paper of the session on filters, Rainer Thaden,Four Audio, presented “Loudspeaker Management Systemwith FIR/IIR Filtering.” He described how a combination ofFIR and IIR filters can be freely combined in each outputchannel. The system also includes dual-range AD conver-sion, look-ahead peak-limiters, and RMS limiters.

The final paper session on Day 2 of the conference wasentitled “System Design with DSP.” Marshall Buck, Psychotechnology Inc., presented “Acoustic and TransducerConsiderations for DSP Loudspeakers.” He described how anumber of acoustic and transducer errors must be minimizedto maintain high accuracy in DSP loudspeakers. A two-wayDSP-calibrated studio monitor was used as an example toillustrate and discuss these error sources and their correction.

Ulrich Horbach, Harman Consumer Group, presented thepaper “Application of Linear-Phase Digital Crossover Fil-ters to Pair-Wise Symmetric Multi-Way Loudspeakers—Part 1: Control of Off-Axis Frequency Response.” Heexplained a simple, noniterative linear-phase crossover-filterdesign technique that provides uniform frequency responsesvertically off-axis for a given multiway loudspeaker. Part 1of this paper presents a general method that emphasizes theflatness of arbitrary off-axis frequency responses. Horbachcontinued with the presentation of “Part 2: Control ofBeamwidth and Polar Shape” of this paper. He discussed analternative design procedure based on specifying the totalshape and coverage angle (vertical beamwidth) of the polarpatterns generated by pairs of separated point sources.

After a full day of paper presentations the delegates wereinvited to attend a social event—an excursion to two Danishmicrobreweries, at which the delegates were introduced to adifferent part of Danish cultural heritage. Known as the“Champagne of the North,” beer has long been an importantfood group in Denmark, and local microbreweries are expe-riencing a robust and remarkable renaissance based on the

high quality of the product. The visits started with technicaltours of the facilities and ended with the subjective evalua-tion of their products!

Later in the evening the delegates enjoyed a banquet din-ner of gourmet foods with fine wines. Music was providedby the Danish band Blast Sisters.

DAY 3The final papers session of the conference was devoted tospatial processing. The day started with an invited paper,“SHAPES—A Scalable Parallel HW/SW ArchitectureApplied to Wave Field Synthesis,” presented by ThomasSporer, Fraunhofer IDMT. He discussed a new parallel pro-cessor architecture and the first steps toward an adequateoptimization of wavefield synthesis. Sporer also discusseda software development environment that assists in creatingscalable programs for highly parallel hardware.

Andreas Franck, Fraunhofer IDMT, continued with thepaper “Reproduction of Moving Sound Sources by WaveField Synthesis: An Analysis of Artifacts.” He described howwavefield synthesis can offer spatial audio reproduction overan extended listening area, which, however, is also character-ized by a number of distinct, audible artifacts. Franck classi-fied and described these artifacts, explained their causes, anddiscussed means to reduce audible deviations.

Paolo Martignon, University of Parma, presented “A Digi-tally Controlled Two Dimensional Loudspeaker Array.” Hedescribed a “Sonic chandelier” system, which is a 64-chan-nel, dome-shaped array of 228 speakers. Martignon coveredthe physical structure and algorithm design as well as somelistening tests.

The final paper of the conference, “A Sound Sources andReflection Localization Method for Reverberant RoomsUsing Arrays of Microphones,” was presented by SimonRoper, University of Birmingham. He described the applica-tion of compensation of stereo or multichannel sound repro-duction systems for the variation in room acoustics and non-ideal placement of loudspeakers, which normally requires➥

AES 32ND INTERNATIONAL CONFERENCE

What DSP Can Do for Loudspeakers workshop: from left, Knud BankChristensen, David Gunness, Wolfgang Klippel, Jan Abildgaard Pedersen, andThomas Sporer

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1140 J. Audio Eng. Soc., Vol. 55, No. 12, 2007 December

AES 32ND INTERNATIONAL CONFERENCE

AES presidentWieslaw Woszyczk

offered a toast at theconference banquet,and the Blast Sisters

provide the musicalentertainment.

On Sunday evening attendees toured two Danishmicrobreweries, learning about the production techniquesand tasting the product.

knowledge of the room geometry and the acoustic impedanceof the boundaries. The results were obtained by estimating theimage locations and amplitudes of a source.

The workshop What DSP Can and Cannot Do for Loud-speakers served as the technical conclusion of the confer-ence. The panelists were David Gunness, Eastern AcousticWorks, Wolfgang Klippel, Klippel GmbH, Jan AbildgaardPedersen, Lyngdorf Audio, and Thomas Sporer, FraunhoferIDMT. Each of the panelists gave his view on good and badapplications of DSP for loudspeakers, and the audience par-ticipated by asking many good questions and offering addi-tional comments for the panel.

The conference was brought to a close by Jan AbildgaardPedersen, who thanked his organizing committee, the invitedspeakers, all the authors, the sponsors, the AES Headquarters,the exhibitors, and all the delegates for contributing to theconference. Roger Furness, AES executive director, thankedPedersen and his hardworking team for producing such ahighly successful international conference.

Editor’s note: The conference proceedings book can bepurchased at <www.aes.org/publications/conf.cfm>; a PDFof the book can also be purchased.

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