The Structure of the Celesta
The resonator boxes control the volume and pitch

Merely striking a sound bar does very little-the sound will be very quiet and won't reverberate at all. Celestas require resonator boxes for this very reason. Give the action model a try-you can see for yourself the effect a resonator box has.
The volume of a resonator box increases as its pitch decreases. This is because, if the size or shape are not changed to match the pitch of the sound bar you want to resonate, it will resonate less-or may not resonate at all.

In other words, each resonator box resonates at a different pitch.
Although resonator boxes may look like simple square boxes, they actually have necks and mouths on the bottom, and there is an ellipsoidal hole running partway through on the inside. They look quite like a bottle of liquor, just turned upside down. When air is blown into the box from the mouth, it produces its unique sound.
If the ellipsoidal hole (i.e. the "neck") is thin and long it will produce a low pitch, with the pitch falling as the cross-sectional area of the mouth gets smaller. Therefore, the volume of the box and the cross-sectional area of its mouth are adjusted so that it produces a certain sound.