The origins of the Harmonica
Types of Harmonica

In contrast to a single-reed harmonica, where sound is produced from one reed when a note is played, a harmonica in which two reeds simultaneously produce a sound when the same note is played is called a double-reed harmonica. Double-reed harmonicas have reeds on the top and bottom.

The notes of a single-reed harmonica

The notes of a single-reed harmonica

The notes of a double-reed harmonica

The notes of a double-reed harmonica

A double-reed harmonica projects sound better. This is because the frequencies of the two reeds differ from one another slightly, making the notes produced tremoloF.
Frequency refers to the number of times a wave repeats during one second and is measured using a unit called hertz (Hz). For instance, when the A that is the fifth note from the left on a double-reed harmonica is played, the frequency of the reed on the side of the fundamental pitch (the fundamental pitch is the instrument's key note; it arises from the lowest number of vibrations) is around 440 Hz, meaning that it vibrates about 440 times in a second.
The wavering reed (the one that produces vibrations that add a wavering sound to the note) is tuned about 2 Hz higher, at 442 Hz. Thus, the reed buzzes out a sound about twice per second. In other words, there are about two tremolos per second.

The higher the pitch, the more tremolos

It is not the case that there are always two tremolos per second, regardless of the note played. It sounds better if the number of tremolos increases progressively as notes go from lower ranges to higher ranges, and so harmonicas are built so that the higher the note, the more tremolos.

Chromatic harmonicas

As can be seen by looking at the layout of the scales, it is impossible to produce semitones with an ordinary single- or double-reed harmonica. Therefore, you need many instruments to match the different keys in which you might want to play: one for the key of C, one for the key of A, and so forth.
The chromatic harmonica, which can produce semitones, was designed to fix this issue. The term "chromatic" refers to the production of semitones. Because a chromatic harmonica can produce chromatic-scale notes like piano keys, all keys can be played in using one instrument.
As a result of videos and other media posted online, in recent years, more people are being entranced by the chromatic harmonica's distinct tones and are picking the instrument up.
(Reference: Zettai! Umaku Naru Haamonika 100 no Kotsu)

Chromatic harmonicas

Chromatic harmonicas

Four types of sound can be produced from one hole

Because a chromatic harmonica has two reeds for each hole, two sounds can be produced from the same hole, one by blowing and the other by drawing.
For example, for the lowest notes, on the far left, a blow produces a C, and a draw produces a D. Furthermore, when the player presses the slide lever on the right of the instrument, the instrument switches from the top layer to the bottom layer, and then a blow sounds a C♯, and a draw sounds a D♯.
In other words, two sounds, each, can be produced using the top and bottom layers, meaning that, considering the hole overall, four sounds can be played using it. Because there are multiple reeds, the chromatic harmonica is characterized by its wide range of more than three octaves.

The structure of a chromatic harmonica

Removing the top cover from a chromatic harmonica, the reeds and black objects (valves) are lined up alternately on a plate. There are apertures under the valves, with reeds hidden under them, meaning that there is a pair of reeds for every hole in the mouthpiece.
Removing the bottom cover, there is a similar plate, and so there are actually four reeds for every mouthpiece hole.

When the slide lever has not been pressed

When the slide lever has not been pressed

When the slide lever has been pressed, raising all notes by a semitone

When the slide lever has been pressed, raising all notes by a semitone

A harmonica with the top and bottom covers removed

A harmonica with the top and bottom covers removed

What is the function of the valves?

The valves used to be made of goat skin. Today, imitation leather or rubber sponges are used to make them. The role of the valves, both when blowing and when drawing, is to block the gap between the reeds and the apertures so that the reeds that the player does not intend to sound do not move, that is, so that no air is released.
When the player draws in, the top reed and valve are sucked toward the plate, sealing the gap. When the player blows, the bottom reed and the valve fly up from the plate, allowing the bottom reed to sound. Thus, the valves act to ensure that air has an effect on only the reed that is being sounded.

The ten-hole harmonica is also called the Blues Harp or the blues harmonica. It is a small harmonica with ten holes to blow into. Blues Harp is the name of a product from the German company Hohner. The ten-hole harmonica is used in genres such as rock, folk, and the blues.