A decibel figure was measured in a room you do not have

There is a short answer about appliance noise figures and a useful one, and they are not the same. What follows is the useful one.

Power and pressure are not the same figure

Sound power describes how much acoustic energy a machine produces and is a property of the machine alone, independent of the room. Sound pressure describes what a microphone measures at a stated distance, and it changes with room size, wall finishes and where the meter stands. European appliance labelling generally uses sound power, which is why the numbers look higher than the pressure readings people expect.

Comparing a sound power figure from one brand with a sound pressure figure from another produces a difference of roughly ten units that means nothing. Any noise number without a stated quantity and, for pressure, a stated distance is not comparable with anything.

The scale is not linear and the arithmetic surprises people

The decibel scale is logarithmic, so an increase of three units represents roughly a doubling of acoustic energy. Perceived loudness follows a different curve again, and a change of about ten units is generally taken as sounding roughly twice as loud.

On the bench, this is why a four-unit difference between two dishwashers is a genuine and audible improvement rather than a rounding error. It also means two identical machines running together are not twice as loud, but only about three units higher than one. Adding a quiet source to a loud one changes almost nothing, which is why the loudest component sets the character of a whole appliance.

The test conditions are deliberately unlike a kitchen

Standardised measurements take place in acoustically controlled rooms designed to remove reflections or to distribute them evenly. A real kitchen has hard floors, tiled walls and a cabinet the appliance is bolted into, all of which raise the level and change the character. Installation matters enormously, since a machine on a resonant floor transmits vibration into the structure in a way no acoustic chamber reproduces.

Testing is usually done on a specific programme, and appliances have other programmes that are considerably louder. None of this makes the figure dishonest, but it does mean the number predicts relative rather than absolute experience.

What actually makes a noise irritating

Steady broadband noise is far easier to ignore than noise with a strong tonal component, such as a whine at a fixed frequency. Modulation is worse still, because a sound that rises and falls repeatedly keeps recapturing attention in a way a constant hiss does not.

Impulsive sounds, such as a pump priming or a valve clicking, are noticed at levels well below the average figure. Low-frequency energy travels through building structure and is felt as much as heard, which is why a distant machine can still be intrusive.

No single-number rating captures any of this, and manufacturers rarely publish the spectrum that would.

Where quiet operation comes from

Mass damps vibration, which is one honest reason a quieter appliance is usually heavier and more expensive to ship. Brushless and inverter-driven motors run at variable speed, allowing long gentle cycles instead of short violent ones.

Isolation mounts, sound-absorbing linings and stiffer chassis all cost money and consume internal space that could have been capacity. Aerodynamic design of fans and impellers reduces tonal noise substantially, and it is one of the areas where engineering investment is genuinely audible. Because quiet costs money in several places at once, a low noise figure at a very low price deserves a sceptical read of the test conditions.

Manufacturer figures are measured under conditions the manufacturer chose.

Using the number sensibly

Compare figures only within one measurement convention, and prefer sources that state both the quantity and the programme tested. Treat differences of one or two units as noise in the data, and differences approaching ten as a genuine change of category. For anything installed near a living space or a bedroom, weight tonal character and night-mode behaviour above the headline figure.

Ask whether the quoted figure applies to the programme you will actually run, since eco cycles are often both longer and quieter. This site does not measure sound, so recordings and spectra published by reviewers are the practical complement to the rating on the box.

The takeaway

A noise rating ranks machines against each other; it does not predict how a machine will sound in your room.

Anything you cannot buy a spare part for is a rental with a long term.