To understand how an instrument works you need only to take a look at how they are constructed and the player physically interacts with them. Below you’ll find how instruments are grouped into 5 major musical instrument families.
Brass
As the name suggests, this family of instruments is (mostly) made of brass. Some scholars, however, have given a slightly different classification to brass instruments. Known also as "labrosones", the sound of a brass instrument is generated by the player’s lips as they blow into the instruments as they contort and expand their facial structure to achieve different harmonics.
The sound of this manipulated wind, (not unlike blowing raspberries) travels through channels in the instrument, past valves which can be covered or uncovered to achieve different pitches.
Once the wind passes the valves, it is released through the "bell" of the instrument, meaning that, unlike woodwind instruments, the sound travels straight. Due to the need for a consistent tone, the player should have excellent breath control, including in some cases, "circular breathing".
Some notable examples of brass instruments are the cornet, french horn, tuba, trombone and, of course, the trumpet. Some instruments within the brass family are actually made out of wood. These include the Didgeridoo, the Alphorn, and the Vuvuzela.
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Woodwind
Like the Brass family, the woodwind instruments utilize the player’s natural breath patterns to manipulate sound. Unlike Brass, it relies less on the player’s ability to use their lips, but a variety of ways to "split" the wind. For many, this is the reed or double-reed. This is perhaps one of the oldest forms of musical production and can be emulated on a piece of grass, for instance.
The wind is pushed passed the reeds, causing them to resonate, achieving a texture particular to that kind of reed. Much like the brass instruments, this sound can be manipulated via keyed valves. But instead of a bell (again in most cases) they often have a flat sound hole, which creates a more omnidirectional sound wave than the brass.
Examples of woodwinds are the Harmonica, Recorder, Flute, Oboe, Clarinet, and Bassoon. It may surprise that despite mostly being constructed of brass, the Saxophone is classed as woodwind, for the simple fact of the reed in the mouthpiece.
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Percussion
It’s very easy to expect the Percussion family to include only types of drums or cymbals. But getting into the mechanics of percussion, it is broadly speaking, "that which can be struck (externally or internally) to create a pleasing sound."
Believed to be the oldest musical family, beyond the human voice, the percussion starts really with the human body itself. When you clap or click your fingers along to some music, you are adding a ‘percussive’ element!
The percussion family is very diverse. For many percussionists, they can spend their lifetime chasing interesting new sounds to add a percussive element to the music they play and can take in a whole host of disciplines.
There are surprising inclusions to the family, which don’t strictly adhere to any particular rules. Whistles, which use an internal beater - and glockenspiels, which utilize the standard musical stave, but are used with beaters.
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Strings
Stringed instruments may date back as far as there have been hunting bows. Some ancient cultures would use the hunting bow much like you might play an elastic band. Noticing the vibration of the string of a hunting bow, the early man may have discovered the pleasing sound of stretching and releasing the string whilst plucking. Additionally, some musical anthropologists suggested that the hunting bow was used on sheer rock faces to create sound, much like one would with violin strings.
Things have come a long way since then, and of course, we now have violins, dulcimers, and guitars. Guitars in the twentieth century have become a staple for aspiring musicians, and many such as Nicolo Paganini, found other stringed instruments easy to learn having played guitar first.
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Keyboards
Arguably one of the most popular musical instrument families in the 21st century, the instruments which require a keyboard are, with the advent of digital technology, one of the most versatile. These days one can recreate any recorded sound and manipulate it using software and a MIDI keyboard.
Some have suggested that the keyboard layout may go back to the Greek hydraulic pipe organ in the third century BC. The heyday of the keyboard in Europe was during the periods between the late 17th century and the early 20th, and the harpsichord and piano have strong associations with the Enlightenment.
Other than the digital keyboard, the keyboard family includes the harpsichord, organ, clavichord and the piano.
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Unconventional Instruments
As time has progressed, mankind has found ever more inventive ways to create musical instruments. Some don’t fall into any particular family or are simply unique in how they generate sound.
Included here is the Theremin, often to be heard in old suspenseful movies, in part for its ghostlike, eerie sound. But aside from the spookiness, it can be a singularly extraordinary piece of equipment.
It’s powered by electricity but unlike any other instrument, it is controlled without having any physical contact at all. The pitch and the volume are controlled by the player placing his hands near antennae which sends an electronic signal to create an oscillating frequency resulting in a beep. It can also be hooked up to other instruments, whose signal can also be manipulated by the thereminist's hands.
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Conclusion
While I’ve attempted here to give you an overview of what constitutes a musical instrument family, you’ve probably seen that there are always exceptions to the rules. But if you want to get an understanding of how are musical instruments grouped, there is no better way than to get your hands on one and play!