Half-Space (2π)
Definition
Half-Space (2π)
Half-space (2π steradians) describes sound radiation above a single large reflective surface, such as a speaker on the ground. The surface acts as an acoustic mirror, doubling the effective output (+3 dB) compared to full-space (4π) radiation. Subwoofers commonly exploit half-space loading for increased low-frequency output.
When a sound source is placed on or very close to a large reflective surface, the surface prevents radiation into the lower hemisphere. All acoustic energy is redirected into the upper hemisphere, effectively doubling the intensity in that half-space. This increases the sound pressure level by 3 dB compared to the same source suspended in full space.
The concept extends to quarter-space (π steradians) when the source is at the junction of two surfaces (floor and wall), giving +6 dB. Eighth-space (π/2 steradians) at a tri-corner (floor, two walls) gives +9 dB. These loading effects are most significant at low frequencies where the surfaces are large relative to the wavelength.
Subwoofer placement routinely exploits boundary loading. A subwoofer on the floor is in half-space. Pushed against a wall, it approaches quarter-space. In a corner, eighth-space. Each boundary adds approximately 3 dB of output at no additional amplifier cost. However, the loading also changes the directivity pattern and can excite room modes more strongly.
For measurement, the testing standard matters. Speaker sensitivity measured in half-space (2π) reads about 3 dB higher than the same speaker in full-space (4π). Specifications must state the measurement condition, or comparisons between products become meaningless.
SonaVyx's SPL meter captures the actual level at the measurement position, which includes all boundary loading effects. Understanding these effects helps explain level differences between positions and guides optimal speaker placement.
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