The Physics of Open-Air Acoustics: Dispersion and Resonance

Designing loudspeakers for the outdoors represents a fundamental departure from indoor acoustic engineering. In a living room, sound waves reflect off walls, ceilings, and floors, creating a reverberant field that reinforces bass and fills the space. Outdoors, these boundaries vanish. Sound energy radiates into an infinite “half-space” (hemispherical radiation) or “full-space,” dissipating rapidly according to the Inverse Square Law: for every doubling of distance from the source, sound pressure level (SPL) drops by 6 decibels. The Bose 251 Environmental Speakers address this energy loss not through brute force, but through geometric precision and resonance manipulation.

The Mechanics of Articulated Arrays

To combat the lack of reflective surfaces, Bose employs an Articulated Array® design. Unlike a conventional speaker that fires sound in a single direction (monopole), the 251 utilizes two 2.5-inch full-range drivers mounted at precise angles within each cabinet.

This geometry creates a specialized dispersion pattern.
1. Wide Coverage: By angling the drivers, the speaker projects distinct lobes of sound energy across a broader horizontal plane. This effectively widens the “sweet spot”—the area where stereo imaging is perceptible. In an outdoor setting like a patio or pool deck, listeners are rarely stationary. This array design ensures consistent tonal balance as the listener moves through the coverage area, mimicking the wide dispersion characteristics of live instruments.
2. Phase Coherence: The spacing and angle of the drivers are calculated to minimize destructive interference (comb filtering) between the two sources, ensuring that the overlapping sound waves sum constructively rather than cancelling each other out.

Multi-Chambered Bass Architecture

Reproducing low frequencies outdoors is notoriously difficult. Bass wavelengths are long—a 50Hz wave is roughly 22 feet long. Without room boundaries to contain and pressurize these waves (room gain), bass energy dissipates omnidirectionally.

The Bose 251 mitigates this through a multi-chambered enclosure design. Internally, the cabinet acts as a complex acoustic waveguide. * Helmholtz Resonance: The internal chambers function as Helmholtz resonators. The air mass inside the ports vibrates against the “spring” of the air volume within the cabinet. By tuning the dimensions of these chambers, engineers can boost specific low-frequency bands. * Distortion Control: The multi-chamber structure also serves to dampen internal standing waves and brace the cabinet walls. This rigidity prevents the plastic enclosure from vibrating sympathetically (cabinet resonance), ensuring that the 5.25-inch woofer’s energy is converted into sound projection rather than mechanical noise. This efficient coupling allows the relatively compact driver to produce perceptible bass impact in an open field.

The Challenge of Free-Field Response

In an anechoic (echo-free) environment like a backyard, the frequency response of a speaker changes. High frequencies are absorbed by air and foliage more readily than low frequencies. The 251 is voiced with a specific free-field EQ curve, likely boosting high-frequency output to compensate for this atmospheric absorption, ensuring that cymbals and vocals retain their crispness even at a distance.