Science For Everyone - Classifying Musical Instruments

Copyright © Karl Dahlke, 2022

For thousands of years, we have placed musical instruments into four categories: strings, woodwinds, brass, and percussion. Today there is another system, based on the mechanism of sound production, which is, in some ways, superior. First, let's review the legacy categories.

Strings

A stringed instrument produces sound by bowing or plucking strings under tension. That's pretty clear. Examples going back thousands of years include the lyre and the harp. Orchestral examples are the violin, the viola, the cello, and the base.

Woodwinds

The original woodwind instrument was similar to a flute, perhaps a shepherd's pipe. Drill holes in a wooden or bamboo tube, and blow through it, covering certain holes to produce the desired notes. Most modern woodwinds, such as the oboe, include a reed, to produce a different sound. An orchestra might include a clarinet, an oboe, a bassoon, a flute, and perhaps a piccolo.

Brass

Brass instruments were, not surprisingly, made of brass, and they still are today. Sound is produced by air blowing through a tube, but the large metal bell makes a louder sound, with different harmonics. Examples include the trumpet, the trombone, the french horn, and the tuba.

Percussion

Finally we have the percussion section, from the latin percussiō, to strike. These include drums of all varieties, the xylophone, the triangle, symbols, gongs, and bells.

Introduction to the Orchestra

When I was young, my parents bought me A Child's Introduction to the Instruments of the Orchestra, by Joseph Cooper. I enjoyed it very much, and learned a great deal from his clear explanations and audio examples. I still have it, digitally, on my computer today. If you like, stop and listen to side A, and side B. It's fun to try to identify the pieces being played.

The narrator refers to illustrations, which were included in the art-work of the album. Sadly, my vinyl copy, with its jacket, and informative pictures of the instruments, was lost long ago.

As you will hear, Mr. Cooper ran into some ambiguities, which he tried to gloss over. He says, “The harp doesn't seem to fit into any of these families.” That seemed odd to me, even as a child. The harp is clearly a stringed instrument, however, it is often placed in the percussion section. People in the 1800's felt that bowing was an essential part of the string section. If you can't bow, you're not invited into our clubhouse. Well, you strike bongos with your hands, and strum the harp with your hands, so maybe it is percussion after all; it certainly isn't woodwind or brass.

The piano was also ambiguous, and was introduced as part of a new keyboard section on side B, along with the harpsichord and clavichord. Sound is once again produced by strings under tension, but these strings are struck by hammers, in the case of the piano, or plucked, in the case of the harpsichord. If we are striking strings with hammers, is it a stringed instrument, or is it percussion? Hard to say. thus, Mr. Cooper called them keyboard instruments, and skated past the ambiguities.

Glass Harmonica

The glass harmonica is another instrument that is difficult to classify, relative to these four categories. This is one of Benjamin Franklin's many inventions, though not as well known as the stove, the lightning rod, or the bifocals. Don't be confused by the word harmonica; this is not a mouth organ. (Harmonica comes from the Italian armonia, for harmony.) Instead, the glass harmonica consists of glass bowls that you rub with the hands to produce various notes. This is somewhat like running your finger around a wine glass to make it sing. Mozart was so impressed, he composed several pieces for this new instrument, including K-356 and K-617. Beethoven did so as well. Listen to K-617, and watch the instrument being played.

Franklin's invention is difficult to categorize. I think we can rule out the woodwind and the brass, air is not involved. At first it seems percussion, like a xylophone or bell. However … you strike a bell to make it sing; you don't dare strike these glass bowls. Instead, you rub them with your fingers, and they vibrate on their resonance frequencies. Is this substantially different from rubbing a string with a bow to evoke a particular note? It would be ironic if this hunk of glass was placed in the same category as the strings. That idea was too bizarre, and so, this instrument, like the harm, was quietly assigned to the percussion section.

The glass harmonica faded from popularity early in the 19th century, because it was extremely fragile, and it didn't produce enough sound to be heard throughout a concert hall.

A New System

Appealing to the science of sound production, instruments can now be placed in six different categories: idiophone, lamellophone, chordophone, membranophone, aerophone, and electrophone. These words all end in phone, from the Greek for sound.

Idiophone

These are the simplest instruments. If struck, or rubbed, they vibrate on their own, producing a particular note and a set of harmonics. The simplest example is a bell. Strike it, and it rings. The bell is not under tension, like a string, it simply hangs there, suspended by a rope, waiting to be struck. Other familiar examples include the triangle, the gong, the xylophone, the wooden block, and yes, the glass harmonica, or its cousin the wine glass. These items are manufactured to produce certain notes and certain harmonics when excited by an outside force. Although they are designed with certain sounds in mind, they ring by themselves.

The name idiophone ads the prefix idio, (related to myself), to the word phone. You've seen this in other words, such as idiosyncrasies, and idiomatic.

The violin is not an idiophone, because the string must be held under tension. If a violin string dangles from the ceiling, like a bell, and you pluck it or bow it, it simply flops around, and doesn't make a sound. It can't sing on its own. In the same way, a drum is not an idiophone. The drumhead produces a low note when struck, because it is stretched taut across the drum frame. A free-floating drumhead would simply flop about if struck. Thus the idiophones encompass only half of the percussion section. Drums will be addressed later.

To be fully functional, this new system of classifying instruments should apply to the natural world. Let's look for idiophones in the wild.

Crickets and katydidss, members of the order orthoptera, chirp by rubbing their wings together. The base of the forewing presents a thick, ridged vein that acts as a file. The upper surface of the forewing is hardened, like a scraper. When the male cricket calls for a mate, or advertises his territory, he lifts his wings and pulls the scraper of one wing across the file of the other. The thin, papery portions of the wings vibrate, amplifying the sound. This is called stridulation, which comes from Latin, meaning "to make a harsh sound." (Strident is a related word.) Listen to this cricket chirp at standard speed, and then slowed down 40%. You can hear the individual notes as the scraper goes past the ridges of the file. I've listened to several species; the chirp almost always consists of four notes in rapid succession, perhaps corresponding to four ridges on the wing.

The wings are not under tension, they simply vibrate at a high frequency when scraped. Therefore, the cricket is an idiophone. You can read more about singing insects here.

Lamellophone

A lamellophone is similar to an idiophone, however, one end of the resonating object is attached to a sounding board or frame for amplification. Examples include the jew's harp, the mbira (also called a kalimba), and the music box.

The stem of the jew's harp vibrates when plucked, but it wouldn't make much sound without the frame that surrounds it.

If you have an opportunity, take a music box apart, (hopefully in a nondestructive way), wind it up, and watch it run. A spring turns the music wheel, which has knobs on its circumference, which pluck the tines of the "piano" in sequence. These tines attach to a sounding board, and then to the frame of the music box. A governor, consisting of two thin blades, spins through the air, and regulates the speed of the unwinding spring, so the song does not play too quickly. One windup can play the song three or four times through. In a vacuum, the blades of the governor would not encounter air resistance, whence it would rev up to high speeds. This no longer "governs" the unwinding of the spring, and the song would play through quickly. Of course, in a vacuum, you wouldn't hear it; but you would notice the effect in a partial vacuum, perhaps 5% atmosphere. That's not an easy experiment to perform at home.

Chordophone

A chordophone produces notes when its stretched strings are struck (like a piano), plucked (like a guitar), or bowed (like a violin). This category nicely embraces the piano, the harpsichord, the harp, the violin, the cello, the banjo, and the ancient lyre, without ambiguity.

Anatomically, your voice comprises vocal cords, but this is not a chordophone. These cords are not under tension, and are not plucked or bowed. See aerophones below.

Membranophone

A membranophone is the two-dimensional analog of a chordophone. A membrane is stretched taut across a frame, then struck or brushed to produce sounds. Drums are the obvious example, drums of all types. The humble kazoo is also a membranophone, since the stretched paper vibrates in sync with your voice.

Returning to nature, the cicada, member of the order hemiptera, is a membranophone. The male cicada possesses two ribbed membranes called tymbals, one on each side of its first abdominal segment. These are buckled inward by the tymbal muscle, then released as a loud click with high harmonics. The tymbal then snaps back, producing a secondary click. The two tymbals click alternately. Hollow cavities in the insect's body amplify the sound. Don't put one of these singing creatures next to your ear; volume can reach a dangerous 100 decibels.

Note that the tymbals are under tension, unlike the wings of the cricket; thus the cicada is a membranophone.

Aerophone

As its name suggests, an aerophone produces sound when air passes through it. The air resonates at certain frequencies, based on the size and shape of the instrument. The frequency can be changed by covering holes, or pressing valves, on the instrument. The trombone has a slide that changes the effective length of the instrument. A bugle has no holes or valves, and yet, a skilled player can produce different notes by blowing at different pressures. Only a few harmonics are possible on such a "natural" instrument, thus the songs are limited to 4 or 5 notes along the major triad. Think taps, or reveille.

This category gathers together the woodwinds, the brass, and all the voices of mammals and birds in nature. It seems rather broad, however, the sounds are indeed produced by a common mechanism.

Electrophone

An electrophone produces sounds by activating a speaker. Any sound can be produced in this manner. This is unlike any of the previous categories.

If an electric keyboard is configured to simulate the sounds of a flute, as you press the keys, it does not become an aerophone. It is always an electrophone, because it always produces the sounds electronically.

Mechanophone

This is a category of my own creation. I was trying to classify the high whine of the mosquito, and it doesn't fit any of the previous categories. The sound is produced by mechanically moving her wings, using flight muscles, at a frequency that is audible to human hearing. The vibrations are actively produced, like the electrophone, but of course the energy is biological, not electrical. Other insects also produce audible sounds as they fly, but these sounds creep me out, so let's not talk about this any more.

The hummingbird also produces a low hum, (thus its name), as it hovers over a flower.

This seems to be a necessary category; perhaps the name mechanophone will catch on.