Materials & compliance · 7 min read
Reading a noise specification comparatively
Same number, different world. A decibel figure without distance and speed attached is not a specification — it is material for a marketing claim.
Three variables make or break the number
Published noise figures in this category are almost impossible to compare, and not because suppliers are dishonest — because they measure different things. Three variables explain nearly all of the difference, and a figure without all three stated is unusable.
| Variable | Effect | Practical consequence |
|---|---|---|
| Distance from the source | Roughly 6 dB per doubling of distance | One metre reads far quieter than 30 cm for the same unit |
| Operating state | Higher speed means more mechanical noise | A figure at low intensity flatters the product |
| Frequency weighting | A-weighting follows human hearing | Unweighted readings are not comparable with dB(A) |
The scale itself
The decibel is logarithmic, so intuition misleads: a 10 dB increase is perceived roughly as a doubling of loudness, and 3 dB is close to the smallest difference most people notice. That is why chasing small differences between two suppliers' specifications is wasted effort, while a 10 dB gap is immediately audible.
A-weighting matters for the same reason. It de-emphasises frequencies human hearing is less sensitive to, so it reports what is perceived rather than what is present. A measurement quoted without its weighting is not comparable with one that used it — [Beranek & Mellow, 2012] set out the underlying acoustics if you want the derivation.
The conditions we publish
Distance 30 cm from the motor axis, A-weighted, maximum intensity, unit held free of contact surfaces, instrument specified to IEC 61672. The full conditions — including why the distance is 30 cm rather than the more flattering one metre — are laid out in the noise test method.
Published figures: dual-motor models measure at or below 60 dB(A), single-motor models 45–55 dB. Every unit is checked against the limit rather than a sample being drawn, because motor noise is the defect a customer notices first and cannot be argued with afterwards. The same figures sit on the technology page alongside the other measured values.
Where noise comes from
In a motorised device, noise is partly mechanical — bearing quality, rotor balance, gear meshing — and partly structural, transmitted through the housing and amplified by resonance. That second part is why two products with the same motor can measure differently, and why improving noise often means changing the chassis rather than the motor.
It is also why we measure noise rather than promising it: the failure mode is a unit-level variation that only shows up when you check every unit. Ask any supplier what percentage of finished units are checked for noise, then decide whether their figure is a specification or an average.
References
- Beranek, L. L. & Mellow, T. J. (2012). Acoustics: Sound Fields and Transducers. Amsterdam: Elsevier.
- Rossing, T. D. (ed.) (2014). Springer Handbook of Acoustics, 2nd ed. New York: Springer.
- International Electrotechnical Commission (2013). Electroacoustics — Sound level meters — Part 1: Specifications, IEC 61672-1. Geneva: IEC.
Questions on this guide
What counts as quiet in this category?
Consumer personal devices are conventionally kept below 65 dB(A). Below about 60 dB(A) at 30 cm sits under typical conversational speech, which is the comparison most buyers find meaningful.
Should I measure samples myself?
If noise is central to your positioning, yes — under conditions you specify, so you are comparing like with like rather than trusting two sets of marketing numbers.
Why do two factories quote different decibel figures for the same motor?
Because the conditions differ. Distance, speed setting, mounting and background level all move the number, and a figure quoted without them is not comparable with anything. Ask for all four before comparing two quotations.
Is a lower figure always better?
Up to a point. Below roughly the low fifties at a stated distance, how the motor is mounted inside the housing usually matters more to what a user hears than the motor's own rating does.
What should I write into a brief about noise?
A distance, a speed setting, and whether the figure is a limit or a typical measurement — then ask for the method. A number in a brief with no method attached cannot be checked when the goods arrive.
How is the figure in our own range taken?
In free field at 30 cm at maximum speed, with a stated limit applied to every unit rather than a typical value quoted once. The test method is published so it can be reproduced.
Should a noise figure be quoted at 30 cm or at 1 m?
Whichever the test used — and the distance has to be stated, because loudness falls with distance. The same device measured at 1 m reads lower than at 30 cm, so a smaller number without a distance is not a quieter device. Ask for distance, weighting and setting together.
Does a lower figure mean a better motor?
Not on its own. A lower figure can mean better balance and mounting, or a smaller motor at lower speed doing less work. Read it beside the runtime and the speed range: quiet at the cost of output is a different product, not a better one.
Still open: how a noise figure is quoted to make it look better — or read the method we measure to.