Ok, got it! Melodica tubes seem cheap to me, so it’s surprising that people would be looking for something cheaper. For example, I bought a tube and a short, hard mouthpiece for $USD 8.88 on Amazon recently.
In any case, you identified a hole in my knowledge about an interesting topic, so thanks.
Some background: in fluid dynamics, gasses and liquids are considered fluids. Air is a fluid and so is water. If you look up “is air compressible”, you’ll find a lot of references to air being incompressible at low velocities (below Mach 0.3). The bike pump example is a counter-example, so that’s where the hole in my knowledge was. I found a detailed explanation at https://physics.stackexchange.com/questions/257851/can-air-be-considered-incompressible-as-long-flow-velocities-are-less-than-100-m. The short answer seems to be that below Mach 0.3, the error in fluid dynamics formulas that assume air is incompressible is small and air can be treated as incompressible when studying its flow.
Fluid dynamics is what you would use to analyze air flow through a melodica–at least, when one or more reeds is open.
Pressure and compression are two different things. Pressure is a force applied over an area while compression is an increase in fluid density, where fluid density is mass per volume. In a sealed container, for example, you could increase the pressure just by heating the container but it won’t change the fluid density.
When you blow into a mouthpiece, you are definitely applying a force, so you are applying pressure.
But can I compress air with just my lungs? I’m still working to find an answer. I think it’s “yes”, but you can’t compress it much. As a test, I tried to see if I could compress the air in my melodica. I put on a hard mouthpiece and blew as hard as I could without pressing any keys. No luck: this just leaked air through the mouthpiece opening. How about blowing directly into the melodica? No luck again: the air forces its way out past the reeds. How about using the hard mouthpiece and plugging it (the mouthpiece) with my finger? Ok, some luck. I couldn’t blow any air into this. For practical purposes, I suspect air compression is not a factor in melodica playing.
If you use a soft mouthpiece, then you might increase its volume–you aren’t so much increasing the fluid density of the air as just increasing the volume of air in the system. I suspect that when at least one reed is open, you would be hard pressed to inflate the mouthpiece. An interesting test for this would be to create a mouthpiece from a balloon.
Is the melodica more responsive with a hard mouthpiece? When you blow into a mouthpiece, you are creating a pressure wave. This is exactly the same as a sound wave and it travels at the speed of sound. This speed is only dependent on distance, not on the cross-sectional area of the tube. The time it takes for a reed to sound is based on the pressure wave, not the speed of the air you blow in (in fact, the air speed will increase or decrease as it travels through the system). Will the pressure wave slow down because some pressure is used to expand the tube? I don’t know; it doesn’t seem like it should. The pressure wave might be robbed of some energy which might attenuate the sound.
The balloon experiment sounds more interesting by the moment.
If you have a soft mouthpiece and continue to blow when all reeds are closed, you will increase the volume of stored air. When you press a key, what’s the effect? I don’t know.
I still don’t think there is any significant “responsiveness” difference in soft vs. hard mouthpieces, unless we get to highly stretchable mouthpieces. If I could find a balloon and a way to connect it to my melodica, it might help find the answer.