SPL: how loud, and under what conditions

Sound pressure level is a ratio expressed in decibels, against a reference pressure of 20 micropascals. That is about the quietest sound a healthy young ear picks up at 1 kHz. It is why the figure is a level rather than a pressure, and why it means nothing until you say at what drive, at what distance, and into what space.

What the column on the driver list is

The driver's sensitivity: the level it produces at 2.83 V, at 1 meter, into half space. All three conditions matter.

• 2.83 V, not 1 W. See drive level for why the convention is a voltage, and why quoting watts flatters a 4-ohm driver by 3 dB.
• 1 meter, in a free field. Twice the distance is 6 dB less.
• Half space, meaning the driver is in a large flat surface radiating forwards only. Full space, a driver hanging in the open, is 6 dB lower for the same input. A figure measured one way is not comparable with a figure measured the other.

The number in the list is what the source publishes. The driver's own page also shows what Baffle computes from the parameters, and flags the two disagreeing by more than 2 dB. Published sensitivity figures are frequently optimistic.

Sensitivity is not quite efficiency

This is the distinction worth having. Reference efficiency η₀ is the fraction of electrical power that leaves as sound, and for an ordinary woofer it is well under one percent:

\eta_0 = \frac{4\pi^2}{c^3}\cdot\frac{f_s^3 V_{as}}{Q_{es}}

Sensitivity is that, plus a term for how much current the driver draws at the fixed voltage:

\mathrm{SPL}_{2.83V,1m} = 112.1 + 10\log_{10}(\eta_0) + 10\log_{10}\!\left(\frac{8}{R_e}\right)

The second logarithm is the whole difference. Two drivers of identical efficiency but different Re do not play equally loud from the same amplifier: the lower-impedance one draws more current, takes more watts, and is about 3 dB louder for a halving of Re. It is not converting power any better. It is being given more of it.

The same trap catches driver arrays. Wiring a pair in parallel halves the impedance and so gains 6 dB at constant voltage, where the same pair in series gains nothing at all; their efficiency is identical either way. If you compare two designs, compare them at the same voltage or at the same power, and say which.

What it does not tell you

Sensitivity is a small-signal figure, measured where the driver is behaving linearly. It says nothing about how loud the design gets before the cone runs out of travel, which is usually what actually limits a woofer. And it is tied to bass extension and box volume by Hofmann's Iron Law: sensitivity is one of the three things you are trading against each other, never a free win.