Driver Efficiency & Sensitivity
Calculate the reference efficiency and 1 W / 2.83 V sensitivity of a subwoofer from Fs, Vas and Qes.
Inputs
Where your driver sits (1 W / 1 m)
Derived values: Qts ≈ 0.44 · Qes ≈ 0.50
How it works
The Thiele/Small parameters hide a surprising number: the driver’s reference efficiency η₀, the fraction of electrical power converted to acoustic power in its passband. Subwoofers are very inefficient, typically 0.1% to 1%. The rest becomes heat in the voice coil.
η₀ = (4π² / c³) · Fs³ · Vas / Qes SPL(1 W, 1 m) = 112.2 + 10·log₁₀(η₀) Qes = Qts·Qms / (Qms − Qts) SPL(2.83 V) = SPL(1 W) + 10·log₁₀(8 / Z)
Notice how efficiency depends on the cube of Fs: a driver tuned lower is dramatically less efficient. That is the fundamental trade-off of subwoofer design, a large, low-tuned, low-sensitivity driver, or a smaller one with higher efficiency but less extension. The three “levers” are Fs (low), Vas (big = more efficient) and Qes (low = stronger motor = more efficient).
Sensitivity changes by 3 dB per doubling of power: a 3 dB more sensitive driver needs only half the amplifier power for the same loudness. The “watts for 100 dB at 1 m” readout makes that concrete. The formula assumes half-space (2π) radiation and a flat passband; rooms and cars will change what you hear.