Sound – resonance

A full-sized acoustic piano is able to play 88 distinct notes. When you press a key on the piano, a number of things happen inside the instrument. First, a damper is raised off the string corresponding to the piano key you have chosen and then a little hammer strikes the string. When you let go of the key, the damper settles back into place.

The sustain/damper pedal lifts all the dampers at the same time to allow the strings to vibrate freely and the notes to keep sounding after you take your finger off the key.

Resonance

It’s important for the piano to have dampers so that the piano strings can be kept quiet. If the dampers were not in place, the piano would softly repeat any sound that comes in contact with it. If you press the sustain/damper pedal on a piano and sing a note, the strings in the piano that make the same note that you sing will vibrate. When you stop singing, you can hear the piano sounding the same note. When the vibration from one object causes another object to vibrate, it is called resonance.

Standing waves

To understand resonance, it is important to understand why a stretched string makes a specific note. Each note on the piano makes a specific sound due to standing waves. When a string on a piano is struck by the hammer, the specific note that is made depends on the length, thickness and tension of the string.

The simplest way that a string can vibrate is known as the fundamental frequency. The string vibrates much like when a skipping rope is stretched and both ends are fixed (held tightly). The pitch of the note depends on the tension, length and thickness of the string.

But why does a strike from a hammer cause the string to vibrate in this way? The hammers in a piano are all nearly the same, yet each string sounds a unique note.

When the hammer strikes the string, the blow sends a burst of energy into the string and causes it to vibrate in many different ways. The vibrations are waves, and they move down the string, echo back off the post at the other end and race back and forth. Most of these waves quickly fade out, but one wave just happens to ‘fit’ on the string. It has just the right energy to make that particular string swing up and down. The strings are attached to a special wooden sound board, which translates the string vibrations into vibrations in the air.

Resonance in string instruments

Stringed instruments rely on standing waves to create music. Plucked instruments, such as the harp, have strings that vibrate much like the piano. Instruments such as the guitar, ukulele and mandolin are designed so that the strings can be fretted and therefore made shorter, thus raising the pitch.

Resonance in wind instruments

Wind instruments also rely on standing waves. However, it is the sound waves in air that create the standing waves in instruments such as the trumpet and flute. Standing waves in tubes act a little differently than waves on strings. Most wind instruments have one open end and one closed end. Sound waves can resonate in tubes with both ends closed, both ends open – and most frequently – one end closed and one end open.

Brass instruments, such as a trumpet, select the standing waves that resonate by using valves to change the length of the instrument. The image below shows three different notes. The top represents the longest wavelength, which corresponds to the lowest note. The middle is a shorter wave, which will be a higher note, and the bottom is the highest note.

Instruments with holes, such as the flute and recorder, operate differently. When all of the holes are covered, the only opening to the outside is the end of the tube. Therefore the lowest note is made when none of the holes are uncovered (top diagram). When a hole is uncovered, the instrument is opened to the atmosphere at that point, and this forces the end of the wave to move to that point. The closer the uncovered hole is to the embouchure hole on the flute (where the musician blows air), the shorter the wave and the higher the note.