Ported speaker enclosures exploit the 180-degree phase shift at resonance.

Evidence

Ported loudspeakers are a clever application of Helmholtz resonators and resonance theory. The speaker enclosure is converted to a Helmholtz resonator by cutting a port in it. The design strategy (figure 13.8) exploits what we have learned about resonances and phase shifts of the drive relative to the resonator. At the top, the speaker is oscillating in and out at very low frequency; it is obvious that as the speaker moves in slowly, it must force air out of the port. This dictates the relative phase of the port and cone at low frequency. A short-circuit dipole results. So far, the port seems like a bad idea. … Above the resonance frequency (figure 13.8, bottom), the situation reverses, since the drive (the speaker cone) and the resonator must shift phase by 180 degrees. This means that the speaker cone and the port will be in phase above the Helmholtz resonance frequency, enhancing the sound intensity outside the loudspeaker. By using a fairly long neck, which can be placed internally, the Helmholtz resonance frequency can be lowered to the desired frequency. — Eric J. Heller, Why You Hear What You Hear- An…, 1244

Categorization

• XREF: The 180-degree phase flip across resonance echoes the earlier pitch-shifting and phase material from this book’s resonance chapters.
• SEED: The phase-flip explanation for why ports work is a clean, counterintuitive hook for an explainer on speaker design.