A speaking-trumpet flare can boost lower frequencies when wall dissipation dominates resonance enhancement.

Evidence

The speaking trumpet shares some of the same issues as a bell on a trumpet: shall we add a tube, starting at the lips, with a more gentle flare ending in a wide mouth to let more sound out? The competing issue is resonance enhancement, which requires confinement. We argued that because of strong dissipation (friction and energy loss at the walls), adding a bell can boost the power output at some frequencies, if friction and dissipation are indeed trumping resonance enhancement, putting them on a rising part of the resonance advantage curve (section 10.8). The horn will thus boost the power at lower, strongly damped frequencies. — Eric J. Heller, Why You Hear What You Hear- An…, 1708