The same frequencies sound the same


Why do instruments sound different?


It is an everyday experience that the sound of two instruments is different (or the sound of two voices). Even with our eyes closed we can tell whether someone is playing a violin or a flute. This peculiarity of the sound of an instrument, its texture or color, is called a timbre or timbre. But what distinguishes the difference between two timbres? To answer this question, we need to examine in a little more detail what a sound is and what a tone is.

A sound is a mechanical vibration or oscillation. This is generated by a source (such as an instrument or a voice), passed on by a medium (such as air or iron), perceived by our ears and then processed by our brain. This tone has an important property: its frequency. By this we mean the speed of the oscillation. The frequency is measured in Hertz and thus describes the number of oscillations per second. The higher the frequency (i.e. the faster the oscillation), the higher the tone. The other way round applies: the lower the vibration frequency, the deeper the tone. For example, the tone A of a tuning fork has a frequency of 440 Hertz. Likewise, the string of a violin that plays this A vibrates 440 times per second.

If you add energy to an object, it vibrates with its own frequency, the so-called basic frequency. In the case of a violin, the string that is bowed with the bow vibrates. When a musician blows a flute, the column of air inside the instrument vibrates. The fundamental frequency can be increased or decreased by changing the mechanical properties of the instrument. And that's exactly what musicians do when they play their instrument. The violinist, who places his fingers on different parts of the strings, changes the frequency of the strings. The flutist, who closes the holes in his instrument with his fingers, changes the length of the swaying column of air inside.

However, the tones that an instrument produces contain a little more than just the fundamental frequency. Instruments also vibrate with several dozen other frequencies, the so-called harmonics. These are multiples of the basic frequency. Since 440 Hertz is the fundamental frequency of the fundamental tone A, the first harmonic vibrates with 880 Hertz, the second harmonic with 1320 Hertz,….

Scientists have studied the number of harmonics produced by two instruments playing the same note at the same volume. The number varies greatly from instrument to instrument. This acoustic effect is mainly responsible for the difference in timbre of two instruments. But is this explanation enough?

Let's go a step further and try to synthesize the timbre of a violin by letting a computer generate the correct harmonics. Do we recognize the instrument? What we are hearing is definitely not a flute. Still, something is missing. This subtle difference is due to the way the sound is produced. Small instabilities of the bow on the string (or the air flow on the flute) help to increase these differences. These instabilities are very complicated and hardly predictable, so that they are very difficult to generate synthetically. Because of this, a computer will never be able to replace an instrument.

From an aesthetic point of view, the sound made by the computer doesn't sound like that of a real violin. It sounds artificial and boring. You can almost feel that non-physical parameters play a role here. Technique, such as adding a slight vibrato, can make the sound more pleasing. And the intention and the feeling with which a musician creates a tone breathes its very own life into it. The sound thus carries much more than just physical laws.