Shure Aonic 3 sensitivity: the guide says «108 dB SPL/MW» — MW is the megawatt
Shure's Aonic 3 guide prints «Sensitivity at 1 kHz — 108 dB SPL/MW» in upper case, while the SE215 and SE846 guides on the same server print «dB SPL/mW». On a FiiO KA13 the milliwatt reading gives +15.0 dB, the volt reading −0.8, and the literal megawatt asks for 6419 V.
One capital letter, three answers: +15.0 dB, −0.8 dB, and a source that would have to produce 6.4 kilovolts.
The Shure Aonic 3 user guide, hosted on pubs.shure.com, prints «Impedance at 1 kHz — 26 Ω» and «Sensitivity at 1 kHz — 108 dB SPL/MW». In SI notation MW is the megawatt: one million watts. The intended unit is mW, the milliwatt — a factor of 10⁹ away.
Read as milliwatts, which is what we store. S_V = 108 + 10·lg(1000/26) = 123.85 dB/V. A 110 dB peak needs 0.203 V, 7.81 mA and 1.58 mW. The FiiO KA13 delivers 1.22 V behind an output impedance under 1.7 Ω, which across 26 Ω leaves 1.145 V: 125.0 dB SPL, +15.0 dB, verdict comfortable, damping ratio 15.3.
Read as volts. The peak would need 1.259 V, 48.42 mA and 60.96 mW. The KA13 reaches 109.2 dB SPL, −0.8 dB — «enough, without headroom», and one decibel from failing outright. The gap between the two plausible readings is 10·lg(1000/26) = 15.85 dB, and on this dongle it straddles the verdict boundary exactly.
Read literally. 108 dB per megawatt is 108 − 10·lg(10⁹) = 18 dB/mW, i.e. S_V = 33.85 dB/V. A 110 dB peak would then require 6419.285 V, 246 895.59 mA and 1.58 gigawatts. The KA13 would produce 35.0 dB SPL — quieter than a library. This reading is not refuted by taste; it is refuted by arithmetic.
How to judge. Shure's own documents settle it. On the same server, the SE215 guide prints «107 dB SPL/mW» and the SE846 «114 dB SPL/mW», both in correct lower case; all three Aonic guides — 3, 4 and 5 — carry the upper-case form. It is a capitalisation slip in one document family, not a different quantity. On this same KA13 the SE215 (17 Ω, 107 dB/mW) computes to +15.6 dB, within 0.6 dB of the Aonic 3 — which is what you would expect from two in-ears of the same house.
The pearl: the reason to record the slip is not the megawatt, which nobody would believe, but the comparison it poisons. A reader holding an Aonic sheet and an SE sheet side by side sees «108 dB SPL/MW» and «107 dB SPL/mW» and has no way to know, from the documents alone, whether the same quantity is being quoted. The one-decibel difference looks like a measurement. It is a typography difference until you go and check a third document.