The Structure of the Harmonica
[Experiment]How notes are produced

How does the air move when the reeds vibrate?
There are rectangular apertures in the plate beneath the reeds. These apertures are one size larger than the reeds, with gaps of about 0.03 mm to the left and right and of around 0.1 mm on the ends.
Because of this construction, air flows in one direction from the top when the player breathes in through the mouthpiece. When this happens, the speed of the air flowing through the aperture is higher than that of the air flowing on top of the reed, giving rise to a difference in pressure. The air pressure decreases when air moves through a narrow spot. Because of this, the reed is sucked in toward the aperture, bending downward.
Thus, the flow of air is temporarily cut off, but, because the reed is flexible, it returns to its original position. The repetition of these two movements makes the reed oscillate up and down. As the reed moves up and down, the aperture opens and closes, and the flow of air entering and exiting the aperture continues cyclically. The vibrations of the reed also cause the air surrounding the reed to vibrate cyclically. These vibrations combine to give rise to compressional waves that spread across the air and reach the listener's ears, being heard as sounds.

Diagram of the principles of how sound is produced

Diagram of the principles of how sound is produced

The larger the vibrations of the reed, the louder the volume.
In addition, the thicker the plate, the greater the amount of air that escapes through the aperture, increasing the oscillations, thus also increasing the volume. However, if the reed is small, and the plate is too thick, then it will be difficult for vibrations to occur. Thus, the thickness of the plate is determined by the size of the reed.

The first specification that determines a note's pitch is the length of the reed. As low-pitched notes become higher-pitched notes, the length of the reed gradually grows shorter.
The next-most important quality is the breadth of the reed. Typically, higher notes are very slightly narrower.
The thickness of the reeds is also related. The reeds would not produce sound well if they were just simple boards, and thus the central portions are shaved to create a finish with gentle curves. The base and end portions are left thicker without being shaved too much. During tuning, these areas are shaved down, making slight adjustments to the pitch of the notes.

The reeds for the lower notes, on the left side, are longer

The reeds for the lower notes, on the left side, are longer

The ends of the reeds for the lower notes also play the role of weights. Instead of making the reeds longer, leaving them unshaven makes the ends heavier, lowering the pitch of the sound. Several of the reeds for the lower notes are of the same length, with the weight changing the pitch.
The reason that the reeds are not made longer is because the manufacturer takes playability and portability into consideration and attempts to make the instrument as compact as possible. This consideration makes sure that the appeal of the harmonica-that it can be played easily whenever and wherever-remains untarnished.

The quality of the notes is most determined by the way in which the reed lifts up. It is crucial that it not shoot up in a straight line, and that it also does not start dipping down into the plate at some point, but that it rises at just the right angle.
The distance between the end of the reed and the plate should be made roughly the same as the thickness of the end of the reed. If it is less high than that, then the reed will be sucked into the aperture, and no sound will be produced. If, instead, it is higher than that, the reed will be less reactive and will not produce notes as readily.

Diagram of reed from the top and cross section

Diagram of reed from the top and cross section

The reeds are very important parts. During normal use there is a cover that protects the reeds from any damage, but careful attention should be paid if the instrument is disassembled.

Cross-section of reed and plate, Reed, Plate

Cross-section of reed and plate, Reed, Plate