Top brass

日期：2019-03-07 09:15:09 作者：褚迪鲛 阅读：

By Paul Marks A PAIR of artificial lips is helping to reveal why brass instruments sound the way they do. The researchers who made the latex lips hope they will lead to trumpets, trombones and cornets that are easier to play. The physics of brass instruments are not well understood, says Murray Campbell, trombonist and physicist in the musical acoustics lab at the University of Edinburgh. “You’d be surprised how little is known about the relationship between what a brass player does with their lips, what the instrument tries to do and the note that actually comes out,” he says. Unlike woodwind instruments, which have a vibrating reed in the mouthpiece, in brass instruments a player’s lips are the oscillating device that creates the note. The lips force vibrating air through a funnel-shaped mouthpiece into a resonant tube whose length is chosen to create standing waves of the desired frequency range. But the relationship between the frequencies of lip vibration, the standing wave and the note that is sounded by the instrument is complex, and changes in lip vibration do not produce proportional changes in the note. “It’s strongly nonlinear,” says Campbell. The pressure exerted on the lips by the airflow in the resonant tube can exert note-bending forces back up through the instrument onto the player’s lips. Conversely, he says, variations in how the lips are used can affect the sounding frequency of the tube. But analysing how the lips and instrument interact requires lips to be held in a fixed position for up to ten minutes. And as no player can maintain this, says Campbell, artificial lips are needed. Previous attempts to make artificial lips out of materials such as leather have not been very successful. The playing shape of these lips—known to musicians as the embouchure—could not be adjusted, nor could they reproduce a brass player’s characteristic “buzzing lip”. Now, however, physicists Joël Gilbert at the University of Le Mans and Jean-François Petiot at the University of Nantes have created a pair of artificial lips made from two thin latex tubes (see Diagram). The tubes are filled with water, says Gilbert, so they mimic the density and flexibility of our own lips. And as air is blown through them, it flows through a perforated plastic plate that simulates the effect of teeth. The tension in the “lips” can also be adjusted to mimic different playing styles. Now Gilbert and Campbell, together with their colleague John Cullen, have undertaken a series of tests on trombones and ancient horns—like the Celtic carnyx—with a new version of the French system constructed at Edinburgh. “It seems to work very well,” says Campbell, who will report their early findings at a meeting of the Acoustical Society of America in Columbus, Ohio,