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PrefaceEvery book tells a story and there is a story behind every book.This story beginsin 1980,in the conference room of the laboratory for the structure of matter atthe Naval Research Laboratory(NRL)in Washington,DC,where Peter D'Antoniowas employed as a diffraction physicist.Knowing Peter's interest in music,a col-league handed him the latest issue of Physics Today with a cover photo of ManfredSchroeder seated in an anechoic chamber.The article suggested using a number oftheoretic diffusers in concert halls.While Peter's interest at the time was not in concerthalls,he became fascinated with the thought of using these diffusers in a renovation ofUnderground Sound,a private studio he originally built in 1972 with Jerry Ressler.The acoustic renovation utilized a new concept called Live End Dead End'proposedby Don and Carolyn Davis of Synergetic Audio Concepts(Syn-Aud-Con)and imple-mented successfully by Chips Davis.At that time,Peter was examining the three-dimensional (3D)structure of matter in various phases using electron and X-raydiffraction techniques.Peter shared the article with John Konnert,a colleague atNRL,and it became apparent that the 'reflection phase gratings'suggested bySchroeder were in effect two-dimensional(2D)sonic crystals,which scatter sound inthe same way that 3D crystal lattices scatter electromagnetic waves.Since the diffractiontheory employed in X-ray crystallographic studies was applicable to reflection phasegratings,it was straightforward to model and design the reflection phase gratings.At this time,Peter's only link to the field of acoustics was a love of composing,recording and performing music.Having scientific backgrounds,John and Peterapproached acoustics as they did the field of diffraction physics,and began research-ing and publishing findings in the scientific literature.The Audio Engineering Societyand Syn-Aud-Con offered a unique forum and community for discussing the research.In October 1983,at the 74th AES Convention in New York,Peter met Bob Todrankfollowing a presentation of Peter and John's first paper on Schroeder diffusers.Bobwas designing a new studio for the Oak Ridge Boys in Hendersonville,TN and wasinterested in utilizing these new acoustical surfaces.The studio was a resoundingsuccess and turned out to be a harbinger of many exciting things to come.In 1983,Peter and John measured quadratic residue and primitive root diffuserswith a TEF 10 analyzer at a Syn-Aud-Con seminar in Dallas,Texas,with theassistance of Don Eger of Techron.Here Peter and John met Russ Berger who wasa pioneer in the use of new products into his firm's recording studios.In 1984,anintensive measurement programme was carried out using Richard Heyser's time delayspectrometry implementation.Farrell Becker was very helpful in the initial evaluationof these exciting new surfaces.Not having access to an anechoic chamber,a boundaryPreface xvmeasurement technique was developed.These measurements were initially carriedout at full scale in large spaces like open fields and parking lots,eventually movingindoors to a sports arena,a motion picture sound stage,and a local high schoolgymnasium.The measurements enabled the theories to be validated.The Oak Ridge Boy's Acorn Sound Recorders project was celebrated with a Syn-Aud-Con control room design workshop in 1984.This project led to many others andcollaborations with a growing community of new studio designers were undertaken.Neil Grant was an early staunch proponent of the research and products.Some of hismilestone designs include Peter Gabriel's Real World Studios,Box,UK;Reba McEntire'sStarstruck Studios,Nashville,TN;Sony Music,New York,NY and CineramaTheater,Seattle,WA.In 1989,John Storyk integrated diffusive technology in manyof his designs,including Whitney Houston Studio,Mendham,NJ;Electronic Arts,Vancouver,BC and Jazz at Lincoln Center,NY highlighting the list.Today much ofthe recorded music you hear is created in music facilities utilizing RPG technology.These fledgling years established relationships that continue to this day and producedmany acoustical landmarks.Interest in recording facilities naturally spread to broadcast facilities,where diffusertechnology is now commonplace.Facilities include BBC,NPR,NBC,CBC and mostof the broadcast networks due to Russ Berger's innovative designs.Being musiciansand audiophiles,led to significant involvement in residential high end audio listeningrooms as well as production studios.In 1989,Peter was introduced to Jack Renner,President of Telarc Records,thecompany that started the classical high end recording industry on a digital journey.Jack was recording the Baltimore Symphony Orchestra at the Meyerhoff SymphonyHall and asked if RPG could assist him.Following initial experimentation,Telarcgraciously credited RPG as Telarc's exclusive acoustical system for control roomand stage use for the Berlioz Symphonie Fantastique in 1990.The somewhat acci-dental stage use and overwhelming acceptance by musicians and conductor promptedan objective and subjective investigation of stage acoustics and acoustical shells bothwith small ensembles and with the Baltimore Symphony Orchestra.These chambergroup studies were conducted with Tom Knab at the Cleveland Institute of Music,where Peter has been adjunct professor of acoustics since 1990,at the invitation ofJack Renner.In 1989,RPG was privileged to provide a custom number theoreticsurface for the rear wall of Carnegie Hall,New York.This installation,along withthe new diffusive acoustical shell development,launched RPG's involvement intoperforming arts applications,which eventually included the Fritz Philips Muziekcen-trum,Eindhoven and the Corning Glass Center,Corning,NY.Many of the acoustical consultants involved in the design of worship spaces beganto include the use of diffusers for rear wall applications and acoustical shells.WhileRPG has collaborated with many acousticians,the relationship with Mike Garrison isnoteworthy for the sheer number and size of the successful worship spaces producedusing diffusers.The crown jewel of this collaboration is the 9,000 seat South EastChristian Church in Louisville,KY.In 1990,RPG funded the DISC Project in an attempt to devise a standardmethodology for evaluating diffuser quality.In 1991,Peter proposed a directionaldiffusion coefficient and the Audio Engineering Society invited him to chair standardscommittee SC-04-02 to formerly develop an information document describing theseprocedures.xvi PrefaceIn 1993,David Quirt Associate Editor of the Journal of the Acoustical Society ofAmerica asked Peter to referee a paper by Trevor Cox entitled 'Optimization ofprofiled diffusers'.(Trevor's research journey had started a few years earlier in 1989when,under the direction of Raf Orlowski and Yiu Wai Lam he completed a PhD onSchroeder diffusers at Salford University,UK.)Trevor's paper outlined a processthat combined boundary element modelling and multi-dimensional optimizationtechniques to make better diffusers.In Peter's view,this paper represented a creativemilestone in diffuser development on par with Schroeder's seminal contribution.Peter and John's review of the paper consumed many months.It required the writingof boundary element codes and developing the first automated goniometer tomeasure these optimized surfaces.During the summer of 1994,Paul Kovitz helpedto complete the measurement software.Trevor's revised paper,accompanied bya refereed paper of Peter and John's review were published in 1995.Since thiswas nearly three years after Trevor submitted the paper to JASA,this must haveseemed to be the peer review from hell,especially as the referees'comments were36 pages long.Peter finally met Trevor in Amsterdam at an AES SC-04-02 standards committeemeeting in 1994 and again in Arup Acoustics'office in London.Our strong mutualinterests led to an informal collaboration.In 1995,Trevor became a research con-sultant to RPG Diffusor Systems,Inc.This relationship started with developing anautomated program to optimize loudspeaker and listening positions in a criticallistening room and blossomed to generate much of the contents of this book.Realizing that good acoustical design results from an appropriate combination ofabsorptive,reflective and diffusive surfaces,as mentioned in the Introduction,Peter(and later with Trevor)began developing absorption technologies as well,includinghybrid abffusive (absorptive/diffusive)systems,diffsorptive (diffusing/absorbing),concrete masonry units,low frequency absorbing arena seating risers,nestableopen-cell foam systems and dedicated absorptive low frequency membrane systems.In 1995,Peter and Trevor became aware of the diffusion research of James Anguson amplitude gratings and modulated phase gratings.James has made significantcontributions to the field of diffuser design and we both have great respect for hisinsight and enjoy our collaborations with him.Also in 1995,we met Eckard Mommertzand Michael Vorlander at the 15th ICA in Trondheim,Norway.It was at thismeeting that we learned of their work developing a procedure to measure the randomincidence scattering coefficient.We have maintained close collaboration to this day,especially as members of the ISO WG 25,chaired by Jens Holger Rindel.To further the development of the diffusion coefficient,RPG co-funded a three yeargrant with the Engineering and Physical Sciences Research Council of the UnitedKingdom,beginning in 1996.Trevor,Yiu Wai Lam and Peter were the investigatorsand Tristan Hargreaves was the doctoral student.This research was very fruitful inthat it produced the first 3D measurement goniometer and yielded a robust diffusioncoefficient which has since been published as AES-4id-2001.This diffusion coefficient has since been used as a metric to develop a range of newdiffusing surfaces,including optimized welled diffusers,profile diffusers,1D and 2Dcurved diffusers,baffled diffusers,genetic binary hybrid surfaces,flat and curvedbinary amplitude gratings,fractal and modulated surfaces,in effect many ofthe topics included in this book.These new optimized custom curved surfaceshave found application in performance spaces like Kresge Auditorium,Boston,MA;Preface xviiHummingbird Center,Toronto,Canada;Edwina Palmer Hall,Hitchin,UK and alsorecording facilities like Sony Music's premier mastering room M1,in New YorkThings began falling into place and all of the relevant diffusion research wascollected into a special edition of Applied Acoustics,entitled 'Surface Diffusion inRoom Acoustics',guest edited by Yiu Wai Lam and published in June of 2000.Lamalso organized a symposium in Liverpool that year.In September of 2001,a specialstructured session on scattering in room acoustics was organized by MichaelVorlander at the 17th ICA in Rome.Having played a pioneering role in makingSchroeder's theoretical suggestions a practical reality,it was personally verygratifying for Peter to be part of a session dedicated to a topic which started as anintellectual curiosity,and has now turned into a diffuser industry and a field ofresearch actively being studied by the leading acousticians of our time.There have been many significant accomplishments over the past 20 years.We nowknow how to design,predict,optimize,measure,characterize and standardize theperformance of scattering surfaces.While there is still much to do,there is a generalconsensus in the architectural acoustics community that a solid theoretical andexperimental foundation has been laid,that diffuser performance can now be quanti-fied and standardized and that diffusers can now be integrated into contemporaryarchitecture,taking their rightful place along with absorbers and reflectors in theacoustical palette.The future holds many exciting possibilities.It is a good time in the history of diffuser development to tell this story.This bookhas allowed us to chronicle developments with sufficient scientific detail,and tocollect in one volume much of what is known about both diffusers and absorbers.In an effort to make this book 'timeless',we are providing a website www.rpginc.com/research,at which we will provide updates,polar responses of 1D and 2D diffusers,and additional diffusion and correlation scattering coefficients for 2D diffusingsurfaces.You can contact us and tell us about technology and techniques we mayhave inadvertently missed in the book.So stay tuned and 'Listen to the Music,Not theRoom'.Peter D'AntonioTrevor Cox