Open Tube - False Tones

False Tones

Some large conical instruments like tubas have a strong and useful resonance that is not in the well-known harmonic series. For example, most large B♭ tubas have a strong resonance at low E♭ (E♭1, 39 Hz), which is between the fundamental and the second harmonic (an octave higher than the fundamental). These alternative resonances are often known as false tones or privileged tones.

The most convincing explanation for false-tones is that the horn is acting as a 'third of a pipe' rather than as a half-pipe. The bell remains an anti-node, but there would then be a node 1/3 of the way back to the mouthpiece. If so, it seems that the fundamental would be missing entirely, and would only be inferred from the overtones. However, the node and the anti-node collide in the same spot and cancel out the fundamental.