Weinreich and Directional Tone Colour

by Joseph Curtin

The following article, originally titled “The Violin Finally Speaks,” was featured in the April 2000 issue of The Strad, to whom we are grateful for full reprint rights.

“When I say I study violin acoustics,” says Gabriel Weinreich, Professor Emeritus at the University of Michigan and pre-eminent figure in the world of musical acoustics, “people ask me if I've found the 'Secret of Stradivari'. Well, I suppose it was the instrument's mystique that drew me to it in the first place. Why is it so difficult to make very good violins? I started looking at such questions in the mid 70's, and naively thought that all you needed was a competent person with a year or two to do some serious research.” He leans back in his armchair and laughs. “Now twenty five years have gone by and I still don't know the 'Secret of Stradivari' - which is not to say I haven't learned anything! It's just that the question has changed. The question 'what makes a good violin' has broadened into many separate questions, each requiring its own answer.”

The question of why is it so hard to reproduce the sound of the violin has eluded recording engineers ever since Pablo Sarasate stood in front of a rotating wax cylinder more than a century ago. Though it is now possible to make “high-fidelity” recordings of many musical instruments, even with the best equipment the sound of a solo violin seems to lose something important in the journey between microphone and loudspeaker. Just what is lost has been made brilliantly clear by Weinreich's recent theory of Directional Tone Color - DTC.

“The human voice and most orchestral instruments,” he explains, “send out sound which is either equally strong in all directions or, if it does have a directional pattern - which happens especially at high frequencies - that pattern changes only relatively slowly as the frequency is varied. In the case of string instruments, however, not only are they strongly directional, but the pattern of their directionality changes very rapidly with frequency. If you think of that pattern at a given frequency as beacons of sound, like the quills of a porcupine, then even the slight changes in pitch created by vibrato can cause those quills to be continually undulating. This property - which is the essence of what I call 'directional tone color' - is especially important when one listens to the music in an enclosed space, as one usually does, so that the original sound beacons are perceived as reflections from varying points in the room. The ear is not clever enough to figure out exactly what is happening, but it is certainly clever enough to sense that something is happening! In fact, DTC may be one reason vibrato is used so universally by violinists - as compared to wind players, whose instruments generally lack directional tone color.”

It may also explain, according to Weinreich, both the characteristic “flashing brilliance” of virtuoso playing and the solo virtuoso's uncanny ability of a soloist to be heard above an orchestra, even with whole string sections playing at roughly the same dynamic level. Because the details of DTC vary randomly from violin to violin (or viola to viola), when a dozen are playing in unison the effect is almost completely averaged out. The somewhat mysterious quality of 'projection' may be partly the ear's tendency to notice sounds which have DTC against a background of sounds which do not. As ordinary loudspeakers are not designed for their directional characteristics (in fact, the directional behavior of a loudspeaker is very similar to that of a brass instrument), it is no surprise that they fail to convince the ear it is in the presence of a real violin.

Known to friends and colleagues as Gabi, Weinreich is one of those joyfully articulate people who speak in publishable sentences. He can carry an idea, in the face of digression and interruption, with the agility of a star football player carrying a ball down the field. He was born in Vilna, now capitol of Lithuania, then a part of Poland. After the German invasion of 1939, he and his mother escaped by train across Siberia, eventually rejoining his father and brother in New York City. His father was one of the founders and Director of the YIVO Institute of Jewish Social Science and a linguistic scholar of international repute.

Music was always part of Gabriel Weinreich's life. He took piano lessons briefly as a child, and many years later took up the cello. As a teenager in New York he studied music theory with an elderly gentleman from St. Petersburg - a former student of Rimsky Korsakov. But Weinreich knew from an early age that he would be a scientist. Leaving Vilna under the most difficult conditions, he persuaded his mother to let him take two books by Jules Verne. He eventually enrolled at City College, New York, in chemical engineering. But a meeting with an old school friend changed his direction. “Chemical engineering?” his friend exclaimed. “There is only one place for great minds like ours - physics! Physics is the only place for genius!” Weinreich promptly switched to physics, while his friend made an about face and became a screenwriter.

Weinreich likes to say he came to musical acoustics on a dare, the challenge being to get funded for a subject that was not yet part of university programs in the US. He soon became the first in the field to get what he calls “normal college funding.” As it happened, musical acoustics inadvertently gained the support of the US government thanks to the Cold War. The Pentagon was interested in tracking Soviet submarines using sonar, and so wanted to learn as much as possible about sound. Weinreich's initial work was with the piano. His results made the cover of Scientific American and his reputation was established.

In a sense DTC has brought Weinreich's career full circle. He began his work with violins by looking at their directional properties. The DTC concept developed in his mind for many years, but only after retiring from teaching did he find the time to carry out the necessary research. He also invented a speaker capable of restoring DTC to recorded sound. It works by creating hologram-like interference patterns at the ends of four differing lengths of pipe. The differently vibrating areas of a violin body create the effect in a somewhat similar fashion.

Weinreich takes me down to his brightly lit laboratory, which is dominated by a large inner chamber whose walls, floors, and ceiling are completely covered with geometrical arrays of white and gray foam pyramids. It might be a high fashion hair salon, except for the violin suspended in a metal frame, a phonograph cartridge resting on its bridge, and a tangle of wires leading to a computer outside. In fact the room is an anechoic chamber (literally, a room without echoes), a tool of the trade as useful to an acoustician as a couch is to a psychoanalyst. The lab is otherwise filled with racks of audio gear, test equipment, and electronic devices. Cartoons are taped to the wall near the door. High shelves are filled with dismembered equipment. An Einstein calendar offers a thought for the month.

Weinreich points out the prototype DTC speaker, a plywood box sitting on the floor. Four unequal lengths of gray plastic pipe extend from its top surface. To demonstrate he asks his assistant, Colin Holmes, to remove the top and pipes from the device. Underneath is an ordinary speaker. Weinreich drops a CD into a player and Henryk Szeryng begins to play unaccompanied Bach. The sound seems to emerge quite normally from the speaker, as it would on any good mono recording. But when the section with the pipes is replaced, something remarkable happens. The sound of the violin fills space in a completely different way. It is as though a violin of indefinite size is dancing in the air above the speaker. The sound, like that of a live violin, is enveloping rather than direct. When Weinreich puts on a recording of a pipe organ, the effect is even more impressive.

No less qualified a listener than Pierre Boulez once observed that loudspeakers have the property of “anonymizing” the sound of musical instruments, of making them all sound the same. This is damning criticism in view of the claims to high fidelity made by manufacturers and sellers of audio equipment. The term “high fidelity” rouses a bemused skepticism in Weinreich. “There is a story of a salesman demonstrating Edison's phonograph, switching between a live singer and a recording of her voice. Apparently the audience could not tell when the singer stopped and the phonograph started. Now if you've ever heard one of those phonographs, this seems an extraordinary claim. But we have to remember that in those days people had never heard a recording before. And so the illusion may have been convincing - at least for a while.”

There is a similar story about early motion pictures. The audience was said to have run from the theater in panic when an actor on screen drew a gun and pointed it at them. While it is possible to fool the eye or the ear for a time, our senses seem very good at sorting out such illusions. Of course engineers then scratch their heads and find ways to create more convincing ones. “There are so many people willing to spend money on audio equipment,” say Weinreich, “that progress has been very rapid. In a sense the technology has far outstripped any understanding by its creators of what the technology is supposed to do. Fidelity to what? Do you really want the Berlin Philharmonic in your sitting room? Of course most popular music today is created electronically, and so can be heard only through loudspeakers. The question of fidelity becomes trivial. By contrast, if you want to reproduce perfectly the acoustical experience of being at a string quartet concert, you're out of luck. But the DTC Speaker can add another dimension to the illusion.”

And how does the DTC Speaker fit in with the concept of Stereo? “In a way, stereo stumbled on DTC inadvertently. Though primarily intended to give a spatial sense to music, interference between the two speakers creates a more complex sound field - one which to some extent emulates what a violin does.”

One particularly exciting use for the DTC speaker is, I believe, to enhance the tone of electric violins and electronic synthesizers. Playing a solid body violin through Weinreich's prototype speaker, I was astonished by the degree to which the violin's characteristically undistinguished sound became suddenly responsive to vibrato. The sound took on something of the beauty usually associated with old wood, worn varnish, and Italian labels. As a violinmaker, I have spent years trying to coax this kind of quality from my own instruments. Now here it was, emerging in full flower from the open ends of four plastic sewer pipes.

Although the device has been patented by the University of Michigan, it has yet to be developed commercially. Is Weinreich interested in pursuing such development? “We scientists,” he says in a gently philosophical tone, “tend to lose interest in doing something as soon as we know how to do it. Once we know how to do it, we tend to start looking for something we don't know how to do.” Much like good artists, I think. But with an audio industry intent on selling such musically irrelevant effects as “Surround Sound,” I can only hope manufacturers will begin to take an interest in what Gabriel Weinreich knows how to do.