As with most things in acoustics, labels are useful but there's sometimes more than a simple tag line. For example, horns usually act like quarter-wave resonators near their lowest frequency extremes. The line between pipes and horns becomes blurred in the bass. Similarly, quarter-wave pipes aren't immune to acting as Helmholtz resonators if the conditions are right. Bass-reflex speakers may use Helmholtz resonance, but that doesn't prevent standing waves from forming. The laws of physics don't prevent standing waves from existing in a cabinet simply because it is a horn or a bass-reflex speaker, nor do they prevent Helmholtz resonation in a ported quarter-wave pipe. So my point is that sometimes there are more than one of these acoustic properties at work in any kind of loudspeaker.Two π towers are bass-reflex cabinets with Helmholtz resonance tuned to 40Hz. But they are not immune to standing waves along their long axis. Quarter-wave resonance is controlled by stuffing and by the placement of the port and speaker. I would imagine that other similar looking speakers designed to be used as quarter-wave pipes control Helmholtz resonance by using port dimensions that shift the Helmholtz frequency out of the passband or using it to advantage as a secondary acoustic device. But the primary acoustic feature of a two π tower speaker is the Helmholtz resonator of its bass-reflex cabinet.
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