Two suppliers quote what looks like the same fan. One states 84 dB(A), the other 96 dB(A). The second fan is not louder. The two numbers were measured in different places and describe different things, and comparing them directly is how acoustic problems get designed into a building.
Sound power is not sound pressure
Sound power is a property of the source: the total acoustic energy the fan emits, in watts, expressed in decibels. It does not change with distance or with the room. Sound pressure is what a microphone or an ear experiences at a particular position, and it depends entirely on distance, on the room’s absorption and on what else is nearby.
A fan quoted at 96 dB sound power and a fan quoted at 68 dB sound pressure at three metres may well be the same fan. If a datasheet gives a single figure without saying which quantity it is and, for sound pressure, at what distance, the figure is not usable.
Inlet, outlet and casing radiated
A ducted fan radiates sound in at least three directions and they are not equal. Inlet sound travels back up the supply duct. Outlet sound travels downstream. Casing-radiated sound comes off the fan body into the plant room or the ceiling void it sits in.
Which one you need depends on your problem. If the complaint is in the room served by the duct, you need the in-duct figure for that side and you will be solving it with an attenuator. If the complaint is from the office next to the plant room, you need the casing-radiated figure and you will be solving it with lagging or a heavier plant room wall. Quoting the wrong one leads to an expensive attenuator that changes nothing.
Octave bands, not a single number
An A-weighted single figure compresses the whole spectrum into one number and hides the shape. Two fans at the same dB(A) can sound completely different: one with its energy at 500 Hz will pass through a light partition, while one with its energy at 4 kHz will be absorbed by soft furnishing but will be more annoying to people nearby.
Blade passing frequency, the running speed multiplied by the blade count, is usually the tonal peak, and a pure tone is judged as more intrusive than broadband noise at the same level. You cannot see any of this in a single number, and you cannot design an attenuator from one either. Ask for octave bands from 63 Hz to 8 kHz.
The test method matters
ISO 13347 describes the methods for determining fan sound power under standardised conditions. A figure produced by that method is comparable across suppliers. A figure measured in a workshop with the fan on the floor is not comparable with anything, because a hard floor adds several decibels through reflection alone.
What happens after installation
Published data assumes the fan is installed with the inlet and outlet conditions the test used. A bend immediately at the inlet distorts the flow into the impeller and both the aerodynamic performance and the noise get worse, often by several decibels. The system effect from bad inlet conditions is a real, predictable penalty, and it is not the fan manufacturer’s fault when it appears on site.
What to ask for
Ask for sound power, in octave bands, stated separately for inlet, outlet and casing radiated, at your actual duty point rather than at the fan’s best point, with the test method named. If a supplier can only give you one A-weighted number, you have no basis to compare their fan with anyone else’s.