All sound comes from vibrations. Sound waves are longitudinal waves, and sound needs a medium, which can be solids, liquids or gases, to carry it. In air at room temperature the speed of sound is about 345 meters per second; it is about 1,500 in water and 5,000 in steel; the speed of sound in air increases with the temperature of the air. Sound is slower than light, so the flash of an explosion is seen well before its sound is heard. Sounds below 20 hertz are called infrasound. The lowest resonant frequency of a tube or string is the fundamental; the higher ones are overtones, and all of them are called harmonics. All resonant frequencies are whole-number multiples of the fundamental; a plucked string makes pulses that travel both ways, and because its ends are fixed, nodes appear at the ends and a standing wave forms; the first mode of a string, the fundamental or first harmonic, shows half of a wavelength; the same note on two instruments has the same fundamental, but the mix of overtones differs, and that mix gives each instrument its distinctive sound, called timbre.
A sound spectrum is a graph of the different frequencies present in a sound, and bowed strings and winds have harmonic spectra. An instrument's timbre is set partly by the relative size of the different spectral components in its sound. Bowing puts energy into the string continuously, which keeps the high harmonics going; a string on its own makes little sound, because it is thin and slips easily through the air. A tube open at both ends has a fundamental frequency twice what it would have if closed at one end; the longest wavelength in an open cylinder is twice its length, but in a closed one, like a clarinet, it is four times the length; a tube closed at one end has only odd multiples of the fundamental, while an open tube also has the even ones, so an open tube sounds richer; the low register of the clarinet has mainly odd harmonics; a trumpet is a closed tube when pressed to the lips, so it naturally makes every other overtone of the harmonic series. In reed instruments, there is an octave hole or register hole which helps players reach higher notes.
On a violin the bridge transfers some of the string's vibration energy to the body, and the body pushes air because it has a large surface area; the bridge is very effective at passing vibrations to the body at frequencies from about one to four kilohertz; the belly, the top plate, is the most important part of the body for pushing air; a small post inside the violin connects the flexible belly to the stiffer back and helps keep the belly from collapsing under the string tension; the bass bar under the bass side of the bridge spreads that foot's motion across a large area of the belly; the air inside the body is important, especially for the low range. In a double-blind test, 21 professional violinists played six violins, and only 8 of the 21 chose an old violin to take home; one of the new violins was chosen most often as the take-home instrument; the test was double-blind: neither the players nor the people handing them violins could know which was which; the scientists, Claudia Fritz and Joseph Curtin, asked 21 players who had played for 15 to 61 years.
A drum has a drumhead, which can be animal skin, fabric, plastic or fiberglass, a body, and a way of attaching the head to the body. Like a string, a drumhead gives a higher pitch when its tension goes up; a drumhead's overtones are inharmonic, which means they are not whole-number multiples of the fundamental; a drum body acts as a resonant cavity: a frame drum with no enclosure sounds loud but lacks depth, while a bigger enclosure gives a fuller, deeper sound. An African talking drum has two drumheads joined by cords, and the drummer squeezes the cords to change the pitch and mimic the tones of speech. In 1914 Erich von Hornbostel and Curt Sachs published a system for sorting instruments by how they make sound; it had four classes: idiophones, chordophones, membranophones and aerophones. An idiophone is an instrument whose own body vibrates, often by being struck; a membranophone is an instrument in which a membrane or thinly stretched material is vibrated. An aerophone is an instrument in which vibrating air produces the sound. In 1940 Sachs added a fifth class, electrophones, whose sound is made by electrical means.
Bartolomeo Cristofori, who worked for the Medici family in Florence, Italy, built the first piano in the early 1700s; a harpsichord's notes came when a plectrum made from a bird's quill or leather struck the string, and the harpsichord could not change how loud a note was. Cristofori made a harpsichord that can play quietly and loudly, and the name was soon shortened to pianoforte. His hammers were covered in deer leather and hit the strings from below; a piano hammer must rebound at once so the string is left to vibrate freely; the soft pedal was invented by Cristofori and so appeared on the very earliest pianos. His early pianos had thin strings and were much quieter than a modern piano; he built only about twenty pianos before he died in 1731. Almost every modern piano has 88 keys, which is seven octaves and a bit.
An octave is a ratio of 2:1 in frequency, and in equal temperament an octave has 12 equal semitones. In equal temperament each semitone has a frequency ratio of about 1.059, which is 2 to the power one twelfth. By convention the note A4 is often set at 440 hertz. In MIDI, the note A4 is number 69, and the number goes up by one for each semitone. Sound levels that differ by less than 1 decibel are hard to distinguish. Robert Moog's 1964 paper described the foundation for a voltage controlled oscillator, which many later synthesizers built on. Robert Moog was fascinated by the theremin as a boy and built his first from plans in the magazine Wireless World. On a theremin the player's hand and an antenna form the two plates of a capacitor; as the hand nears the pitch antenna, the pitch gets higher; the other hand controls volume: the closer it is to the horizontal loop antenna, the lower the volume.
One of the oldest flutes was made from the naturally hollow wing bone of a griffon vulture, with five finger holes and a V-shaped mouthpiece. Its pieces were found in 2008 at Hohle Fels, a Stone Age cave in southern Germany. Mammoth-ivory flutes were hard to make: with only stone tools, the maker split curved ivory along its grain, hollowed out the two halves, carved them and fitted them together with an airtight seal. Researchers say early music may have helped the first modern humans in Europe communicate and form tighter social bonds. In 1995 researchers in Slovenia found a juvenile cave bear's thigh bone with a line of small holes. As of 2026 scientists still disagree about how the Divje Babe holes were made: by people or by ice age hyenas; a replica made by a Boston University biologist suggested hyenas can make one hole, but not a row of nice, round holes all in one bone. Bone flutes between 7,000 and 9,000 years old were found at Jiahu, a site in China. Chinese researchers dated the site with 20 radiocarbon dates from charcoal, plant ash and human bone.
Words to know
- harmonics: overtone family
- fundamental: lowest frequency
- inharmonic: not whole multiples
- temperament: tuning system
- electrophones: electric instruments
- capacitor: stores charge
