A reed instrument makes sound through a feedback loop: the player's breath pushes a thin, flexible reed against a mouthpiece opening, and the reed flaps open and shut. Each puff of air it lets through travels down the tube, reflects off the far end, and returns to nudge the reed's own vibration. The system locks onto whichever frequency makes those reflections arrive back in step with the reed's motion — a resonant standing wave of air pressure inside the tube. That resonant frequency, not the reed alone, sets the pitch you hear.
A clarinet behaves acoustically like a tube closed at one end, so its lowest register spans a twelfth (an octave plus a fifth) before the first overblown note — unlike the flute or saxophone, which overblow to a plain octave because their air columns behave as open at both ends.
A vibrating reed and a resonating tube lock together into a standing wave — set the reed stiffness, tube length, blowing pressure and tube type to see how the resonant harmonic, and therefore the pitch, is chosen.
Closed-open tubes (reed instruments) resonate only at odd harmonics and overblow to a twelfth; open-open tubes resonate at every harmonic and overblow to an octave. Tube length sets the fundamental; the reed just supplies the energy that keeps the resonance alive.
Drag tube length to change pitch, switch tube type to compare harmonic families, adjust blowing pressure to see the wave amplitude respond, and press Overblow to jump the sounding harmonic the way a player forces a register change.
A clarinet's cylindrical, closed-reed bore is why its lowest register spans a twelfth before the first overblown note, while a conical-bore saxophone with the same reed mechanism behaves acoustically like an open pipe and overblows to a plain octave.