Amateur saxophonists of the world have long been in awe of the piercing high notes jazz legend John Coltrane used to hit, wondering why they can't scale the same auditory heights. Now researchers say they have the answer: Unlike amateurs, pro sax players have learned to flex their vocal tracts in a special way to amplify the high notes.

A sax's sound comes from a flexible reed in the mouthpiece that controls the airflow and pressure through the instrument, setting up strong air vibrations or resonances if a player blows the right way. Musicians and scientists have debated the role of the player's vocal tract—the hollow from the mouth to the glottis (the space between the vocal cords). Some contended that saxophonists have to shape their vocal tracts with their tongues and jaws to create a matching resonance.

"It would make sense for the vocal tract to influence your overall sound, because sometimes just by listening to how somebody plays a line you can tell who it is," says acoustician Jer Ming Chen of the University of New South Wales in Sydney, Australia. But until now, he says, there has been no way to directly measure the acoustics of a vocal tract in mid-note without interfering with the player's sound.

To make a direct measurement, Chen and two of his colleagues in Sydney modified the mouthpiece of a tenor saxophone. They added a device that would emit a combination of 224 tones into the vocal tract of the player, simultaneously recording the intensities of the tones as they bounced back into the mouthpiece, from which they could deduce the vibrations in the tract.

The team recorded five professional and three amateur saxophonists as each played a scale on the modified sax. All players could sound notes below the high, or altissimo, range. For those pitches, there was no strong relationship between resonances in the vocal tract and instrument. Only the pros, however, could break into the altissimo, where a clear relationship emerged: The resonance of the players' vocal tracts was right around that of the note they were playing, the researchers report in Science.

The effect is to amplify the sound of the high notes, which are naturally weak in the sax, Chen says.

But are the pros born with it? Chen thinks not. "They all [said that] to produce the high note at will, they have to think of the note in their head." He takes that to imply a learned relationship born of long, painful hours of practice—which, as we all know, makes perfect.