why a small reed gap leads to no sound

When a first tried to adjust the gap of the reeds I tried to reduce the gaps. What happenend then was an absence of sound when I was blowing a little hard.

Going to the Reed gaping videos, I learned that a larger gap was needed. It took me a few days to understand why. This is maybe common knowledge but since I don't recall having seen it somewhere I'll explain why here.

As explained elsewhere on this site, the reed is located opposite side to the flow of air. For practicality let us take a blow reed. This blow reed is located in the chamber. As the air flow goes into the chamber, due to the little air gap the pressure does increase on the chamber side of the reed pushing the reed towards the reed plate and through the reed plate until a point is reached where the air gap is big enough to let the air exit the chamber. At this time the air pressure inside the chamber decreases and the reed comes back through the reed plate due to its own spring force. If conditions are well, then an oscillation starts : 

• with air blow pressure does increase inside the chamber
• reed travels through the reed plate until there is enough space to let some aire escape the chamber reducing the chamber air pressure
• natural reed spring and reduced air pressure bring the reed back in the chamber
• as air continues to flow and the reed being in the chamber, the pressure does increase inside the chamber
• reed travels through the reed plate until there is enough space to let some aire escape the chamber reducing the chamber air pressure
• natural reed spring and reduced air pressure bring the reed back in the chamber
• and so on.

Of course this explanation is a simplification but it is enough for my explanation.

When the reed is on the other side of the plate (not the rest side), it is its own spring force that makes it come back. If this force is too little, then the reed will remain in actual position. There will be an equilibrium : reed remains bent, openning remains big, air flows continously, reed remains bent : no sound.

The laws of physics tell us that a spring force is proportional to its displacement (important point!). So if the gap of the rest position is small, there will be lesser displacement between the rest position and the air release position therefore less spring force. Then, according to the conditions, the reed might find an open position where the air pressure compensate the spring force : no sound. 

To increase the spring force that gets the reed back in the chamber, increase the gap of the rest position. This is why there is a minimal gap on the rest position of the reeds.

Obviously and this has been explained on this site, if the rest gap is too big, then a little air will be able to flow through this opening and there will be no sound until the air flow does increase : the harp is less responsive.

I hope that you were able to read to the end.

Bod