Horn confinement amplifies siren output via proximity resonance.

Evidence

The NFC mechanism was well understood by Lord Rayleigh. However, it seems that ever since Rayleigh, the usual casual explanations, something like “there is a resonator near a source and it resonates! Resonators get loud on resonance!” have gradually dulled the mind and the level of understanding. The difficulty with the casual statement is that it is not an explanation, and, worse, it is false in some circumstances. For example, there can be a suppression of the sound if a monopole source, instead of a dipole or quadrupole source, is brought near the resonator. Capture, phase shifting, and reradiation still occurs, but now the phase-shifted, reradiated sound finds itself more or less destructively interfering with the direct sound from the monopole, depending on its distance to the resonator. It is quite shocking to hear this failure if one believes that any sound source should be amplified by proximity to a resonator. Dramatic amplification follows only from breaking a short-circuiting sound source. — Eric J. Heller, Why You Hear What You Hear- An…, 1889 (#contradicts, via linked claim)

This humming sound is so delicate that it cannot stand up to the slightest noise intrusion. Even the listener must remain absolutely quiet—no talking, no foot or arm movements, and quiet, slow breaths with open mouth. I believe an open mouth improves hearing for two reasons: it straightens the auditory canal and it enables the mouth to serve as a resonant chamber, amplifying faint sounds to more audible levels. This is exactly what children do instinctively when the lights go out: the jaw drops and faint sounds are more easily heard. — Gordon Hempton, John Grossmann, One Square Inch of Silence- One Man’s…, 191 (#exemplifies, via linked claim)

Categorization

• XREF: connects to: A horn amplifies sound without increasing diaphragm motion — Eric J. Heller, Why You Hear What You Hear- An…, 539; Proximity to a resonator suppresses, rather than amplifies, monopole sound sources — Eric J. Heller, Why You Hear What You Hear- An…, 1889; Horn bells boost lower frequencies where wall friction dominates resonance enhancement (+8 more)

Related

• A horn amplifies sound without increasing diaphragm motion (similarity 0.88)
• Horn confinement forces air bursts to do more acoustic work (similarity 0.85)
• Horn loudspeakers amplify sound through proximity resonance in a narrow throat (similarity 0.85)
• A horn’s main benefit is source loading, not directional aiming. (similarity 0.83)
• Sound near walls or large vessels is enhanced by scattering regardless of resonator tuning (similarity 0.82)
• Proximity to a resonator suppresses, rather than amplifies, monopole sound sources (similarity 0.82)
• Helmholtz resonators serve as transducers, amplifiers, and sound controllers. (similarity 0.82)
• Horn throats boost output by constructive interference with the source (similarity 0.81)
• A vocal tract constriction at the mouth forms a Helmholtz resonator (similarity 0.80)
• Confinement reflections amplify sources only when wave sign is preserved (similarity 0.79)
• Throat chamber echoes raise impedance by in-phase amplitude buildup (similarity 0.78)
• An open mouth amplifies faint sounds by serving as a resonant chamber (similarity 0.78)
• Blowing across a bottle mouth produces a Helmholtz resonance (similarity 0.78)

Broader Connections

• Proximity to a resonator suppresses, rather than amplifies, monopole sound sources #contradicts (0.90) — Eric J. Heller, Why You Hear What You Hear- An…, 1889
• On resonance, releasing sound reduces its loudness #contradicts (0.85)
• Concentrating sound into a smaller solid angle raises horn sensitivity #expands (0.80)
• An open mouth amplifies faint sounds by serving as a resonant chamber #exemplifies (0.80) — Gordon Hempton, John Grossmann, One Square Inch of Silence- One Man’s…, 191
• Horn bells boost lower frequencies where wall friction dominates resonance enhancement #qualifies (0.75)
• Adjacent compression and rarefaction regions can short-circuit and kill radiated sound #complicates (0.75)
• Resonance requires confinement of previously emitted amplitude #expands (0.75)
• Instrument bodies act as coupled resonators that shape and radiate sound #expands (0.70)
• Operating a loudspeaker near three room boundaries imposes a 9 dB low-frequency rise #expands (0.70) — David Howard, Jamie Angus, coustics and Psychoacoustics, 2193