| William J. Broad |
Ancient Instruments Yielding Secrets of Their Music:
The Music Goes Round and Round
Flutes and their kin, including whistles,ocarinas, recorders and pipe organs, are among music's oldestand most versatile instruments. Yet science has long had troubleunderstanding all but the most elementary aspects of how theywork. Now, however, researchers are starting to learn some ofthe secrets. In a way, science is glimpsing the soul of a veryold machine. The tardiness may seem surprising. After all, thescientific revolution has been rolling along for centuries andhas made great progress in understanding time, space, matter andlife, to name a few basics. The physics of wind instruments wouldseem to be passe. But the truth is that the methods of sciencework exceedingly well in some fields and poorly or not at allin others. There are more mysteries than most people realize.
The new insights center on how jets,eddies and waves of air pressure come together at the heart ofsuch wind instruments to form the complex vibrations heard asagreeable tones. Such turbulence is now being photographed andparts of it modeled mathematically with rigor. Even so, the analysesare still sketchy and limited by gaps and approximations. "Theseinstruments seem very simple," said Marc-Pierre Verge, aleader of the research. "But from a physical point of view,they're very complicated, much more so than the piano or violin."
Thenew insights are nonetheless plentiful enough to help music professionalsmake better reproductions of old wind instruments, such as Baroquerecorders, and to invent new ones, scientists say. The advancesalso are aiding electronic music, including that of synthesizers,organs, home computers, games and movies. The goal is to makeartificial tones more realistic and, in other cases, to createsounds and instruments that have no counterpart in the real world.For instance, scientists can now make a virtual flute of almostany size and shape, its tube curled or a hundred feet long. Notescan be very low and sometimes very strange. Electronics giantssuch as Yamaha, the Japanese maker of musical instruments, areincorporating some of the advances into products. The researchis global, with work being done in Japan, Europe and Canada aswell as the United States.
Scientists agree that the physics offlutes and similar wind instruments is a challenging final frontier,despite the field's long history and recent strides. Julius O.Smith III, a Stanford University expert on music, called the theoreticsof wind instruments "perhaps the most slippery in all ofmusical acoustics." Perry R. Cook, a Princeton professorof music and computer science who works on flute simulations,said progress of late had been "really profound," especiallybecause European studies are illuminating what had previouslybeen hidden. "People have been scratching their heads aboutthis stuff since at least Pythagoras and probably before,"Cook said. "Aristotle had a theory. So did lots of acousticsguys."
Flutes, whistles and kindred instrumentsdate to humanity's early days, when they were used in hunting,signaling, magic and ritual. The Maya and Incas made flutes ofclay that modern scholars have found to be surprisingly complexin tone and construction. During the Renaissance and Baroque periods,wood recorders evolved rapidly, with bores becoming tapered andtubes made of two or more interconnecting parts. The more elaboraterecorders had a wide range of pitch, volume and color tone, allowingthem to come alive in expert hands. By the dawn of the IndustrialRevolution, large organs in cathedrals had many hundreds of pipesand a spectrum of tonal characteristics. They were among the age'smost complex machines, rivaled only by clocks.
In general, members of the family workby directing a stream of air against a sharp edge, which causesthe jet to pulsate back and forth, creating waves of sound. Thefamily is thus commonly known as jet or edge winds. With someinstruments, such as the flute, pan pipe and soda bottle, theplayer's lips and skill are central to forming the flow of air,which must be fast but even. With other edge instruments, suchas whistles, recorders and the flue pipes of organs, the formingjob is done by a smooth duct. For the family as a whole, expertshave long known that the basic pitch is determined by the lengthof the tube.
Throughout history, great scientistshave probed the subtleties of such instruments, especially theorigin of the sound and what controls its tonal qualities. Butthe job is very hard. As Lord Rayleigh, an English physicist,showed in 1896, the flow of one gas or fluid past another (orof air moving through still surroundings) is unstable, producingthe kinds of swirls seen in puffs of cigarette smoke. For stringinstruments, the job is much easier. A plucked string produceswaves of vibrations that are easy to see, study and model mathematically,so much so that beginning students of physics often do such analysesto develop their skills. "The math models are extremely good"for string instruments, said Smith of Stanford. But for many ofthe winds, he added, "you're on thin ice."
Despite the edge family's many puzzles,music scientists have made some analytic headway in recent decades.From the 1960s to the 1980s, researchers did so mainly by simplifyingthe problem to its bare bones. For instance, a math simulationof a flute would assume that the instrument had only one dimension,length, rather than the usual three. And the analysis would focuson the instrument's overall characteristics, rather than the riddleof how the sounds start and evolve. Scientists would often makead hoc adjustments to get things going. Then, by trial and error,and careful listening, they would test how well the math and computersimulations matched the real thing, tweaking the model to improvethe sound. Douglas Keefe, head of music for the Acoustical Societyof America, said many of these efforts were toylike and producedrelatively crude sounds. Still, progress was eventually sufficientto attract companies that made electronic instruments.
For flutes, their products had oftenused recorded samples of real sounds. But the companies were eagerto tap the greater flexibility of tone, timbre and quality thatwas possible if they had some insight into the real nature ofthe sound's generation. Yamaha in the early 1990s drew heavilyon Stanford University flute modelers, including Cook, who laterwent to Princeton. The results showed up in such devices as thecompany's VL1 Virtual Acoustic Synthesizer, which used physicalmodeling to create a variety of instrument sounds.
Flute mimicry evolved rapidly as a Europeanresearch group found a way to illuminate many details of the hiddenaction. The work, overseen by Avraham Hirschberg, a physicistat the Eindhoven University of Technology in the Netherlands,was done in concert with IRCAM, the Institute for Research onAcoustics and Music in Paris. To limit variables, the team zeroedin on recorderlike instruments, whichcan be excited by compressed gas rather than human blowing. Atthe heart of the test apparatus was an 11-inch instrument resemblingthe flue pipe of an organ, its central parts made of glass topermit viewing. For visualizations, carbon dioxide was found togive the best contrast with surrounding air. The gas was shotthrough the pipe's duct, and the resulting jets, whorls and eddieswere photographed up close. Eventually, the team was able to takea series of snapshots (and even movies) that laid things bare,in particular how the sounds form and develop. From 1994 to 1997,the team published a series of influential papers based on analysisof the snapshots. And in 1995, Verge, the team's leader, who hadcome from Paris to study with Hirschberg in the Netherlands, receivedhis Ph.D. from Eindhoven University. The work, Verge said in aninterview, is deeply rooted in the team's observations. "Itwas by looking at the pictures that we noticed many, many thingsthat weren't obvious at first," he recalled. For example,the team found that much influence was exerted by tiny vortexes-- little whirlwinds shed from the jet-edge interface in risingnumber and complexity as the sound developed. Such eddies werepreviously presumed to exist, and some scientists had suggestedthat they were key to sound amplification. But the Eindhoven team,which included Benoit Fabre, from the Musical Acoustics Laboratoryof the University of Paris, found otherwise. The vortexes actuallycut the strength of the fundamental, the root vibration that makesup the instrument's lowest note. Moreover, the team discoveredthat the tiny whirls were important in the rise of harmonics,the vibrations more rapid than the fundamental that give an instrumentmuch of its tone and distinctiveness. Finally, the team foundthat vortex shedding helped trigger the instrument's first sounds,especially when the note's onset, or attack, as musicians callit, was fast. An initial eddy that curled off the sharp edge intothe pipe would start a pressure wave that bounced back and forthin the tube, setting up a feedback loop and the instrument's mainvibratory state.
The research so impressed the AcousticalSociety of America, a professional group, based in Woodbury, N.Y.,that it highlighted it last year in its annual "World ofSound" calendar. Splashed atop November were eight of theEindhoven team's photographs showing how a note is rooted in subtlewhorls and vortexes. Despite the advances, the calendar noted,just how such wind instruments work "is still not fully understood."Verge said he and some colleagues are turning their insights intoproducts. In Montreal, they have set up a company, Applied AcousticsSystems Inc. that in a few months is to release music-making softwarebased on physical modeling. Using this software on a computer,a player will be able to be form flute, string, reed, brass andother kinds of electronic notes. "It's fun," Verge said."It's like Lego. You have small blocks and you build whatyou want. The only limit is your imagination." Verge addedthat that he had made virtual flutes more than 30 feet long, somewith side tubes branching off in different directions. "Youget some very low tones," he said, as well as odd harmonics.
Such studies and products are increasinglyof interest to makers of synthesizers, organs, home computers,games and movies, all of whom are eager for sounds that are newand more realistic. Surprisingly, Verge said the flute researchalso had many industrial uses. The team's models of turbulentnoise and note generation, he said, have application to industrialdesign, where ventilation systems as well as gas and water pipesoften develop unwanted sounds.
A last frontier of the field is usingthe new knowledge to illuminate how wind instruments old and neware put together, revealing strengths and weaknesses. Over thecenturies, Verge said, "craftsmen have learned to exploitsubtle acoustical phenomena which make musical instruments veryinteresting as study objects." Hirschberg, the research team'soverseer and an expert in fluid dynamics at Eindhoven University,said that, despite the increasing pace of advance, many of thosemysteries were likely to remain unsolved for years, given thenuances. "I expect," he said, "that the detailsof the physical differences between fair and excellent instrumentswill remain obscure" far into the future.