Speaker midrange errors can stem from resonant diaphragm flexure and be fixed by damping.

Evidence

As good as it was, there was some midrange misbehavior which was identified to be the result of resonant flexure along the long axis of the racetrack shaped B139 woofer. An absorbing phase plug over the center 1/3 of the diaphragm eliminated most of the cancellation and a new crossover resulted in an improved loudspeaker. — Toole, Floyd E, Sound reproduction- the acoustics and…, 71