Read enough printer and computer spec sheets and you meet two kinds of acoustic figure that look like relatives and are not. One says "Acoustic power emissions: 6.6 B(A)". The other says "51 dB(A)". Both describe the same machine. Neither converts into the other by moving a decimal point.
Our printer and mini PC guides lean on this distinction constantly, so this page explains it once, using only figures the makers publish.
Sound power: the machine
Sound power is the total acoustic energy a machine emits, in every direction, regardless of the room it sits in or where you stand. It is a property of the machine, the way wattage is. Declaration standards write it A-weighted in bels: B(A), where one bel is ten decibels.
HP declares the LaserJet MFP M234dw at "Acoustic power emissions: 6.6 B(A)". Dell declares the OptiPlex Micro 7020 at 2.7 bels idle. Apple declares the Mac mini at 1.3 B. These numbers can be compared with each other, because they do not depend on a microphone position.
What they cannot be compared with is anything in dB(A) at a distance, which is the second quantity.
Sound pressure: your ears
Sound pressure is what arrives at a specific point: a microphone one metre away, or at the operator position, the standards' phrase for where a person sits. It is written in dB(A), and it is the number closest to what you actually hear.
Xerox publishes the B230 as "1-meter Average Sound Pressure, dBA": 53 printing one-sided, 16 in ready. Dell publishes 15.4 dB at the operator position at idle, with a separate bystander column. Apple publishes 4 dB at the operator position for the Mac mini. Same kind of number, stated position, honest.
The two quantities travel together in a proper declaration. HP's international datasheet for the M234dw carries both: 6.6 B(A) of power, 51 dB(A) of pressure. Neither is derivable from the other without knowing the room and the distance, which is exactly why the standards require the pair.
The bel trap
One bel is ten decibels, and this is where spec sheets bite people.
A machine declared at 6.6 B(A) of sound power is, numerically, 66 dB of sound power. Put that beside a 51 dB(A) pressure reading and the machine appears to disagree with itself by 15 dB. It does not: the two numbers measure different things, and the 66 never was a pressure you would hear.
Canon springs the trap in the other direction, writing sound power in decibels. Its MF275dw specification gives "Sound Power" as "Active: 69.3 dB or less". Beside HP's "6.2 B(A)" that looks like a machine three times as loud. It is 6.93 B, sitting exactly where a small laser printer belongs. The row label, not the number, tells you what you are reading.
The small print that means something
Declared tables carry a statistical adder written KV. Dell's OptiPlex declaration reads 2.7 bels with "KV, (bels)" of 0.4; Apple's Mac mini reads "1.3 (KV = 0.3)". The declared value plus KV is what ISO 9296 calls the value for verification: the level a sampled unit should not exceed. A maker printing its KV is showing its working, and in our guides that maker outranks one printing a bare number at the same level.
The standards you will see cited are ISO 7779 (how to measure), ISO 9296 and ECMA-109 (how to declare), and ECMA-74 (the operating modes, idle against disk against processor load). A figure naming any of them is worth more than the same digits naming none.
How we handle the two quantities
In our tables, we always say which quantity a figure is, quote the unit as the maker printed it, and never rank a power figure against a pressure figure or either against a bare marketing number. Where a maker publishes both, the pick's noise line carries both.
Loudness units are a third thing again: bathroom fans and range hoods are rated in sones, which is a perceptual scale, and that has its own page. For the general problem of comparing any of these across brands, see why you cannot compare dB ratings across brands.
We did not measure anything on this page. Every figure is the maker's own, quoted as published.
