Sound power and sound pressure are both quoted in decibels, which is why they get mixed up constantly.
They are not interchangeable. One describes the fan, and the other describes what you happen to hear from where you happen to stand.
Sound power is the total acoustic energy a fan emits, and it belongs to the fan regardless of surroundings. Sound pressure is what a microphone measures at a point, and it depends on distance, room absorption and reflections. Specify and compare sound power in octave bands. Use sound pressure only to describe a specific location.
The difference in one sentence
A light bulb analogy works well here. Sound power is the wattage of the bulb, and sound pressure is how bright it looks from where you are standing.
Move closer and the brightness changes. However, the wattage does not.
So a fan quoted at 78 dB sound power is a fixed property. A fan quoted at 62 dB sound pressure tells you nothing until somebody states the distance and the room.
Why datasheets should quote sound power
- It is measured under a defined standard, so results are comparable.
- It does not change with room type or measuring distance.
- Acoustic consultants need it to calculate room levels properly.
- Attenuator sizing works from power, not pressure.
- Two suppliers can be compared directly on the same basis.
- Finally, it splits into octave bands, which is what design calculations need.
Consequently a datasheet quoting only a single A-weighted sound pressure figure is giving you marketing rather than data.
Octave bands matter more than the total
A single number hides where the energy actually sits, although that is exactly what decides how hard it is to treat.
| Band | Character | Ease of treatment |
|---|---|---|
| 63 Hz | Deep rumble | Very hard, needs mass |
| 125 Hz | Rumble | Hard |
| 250 Hz | Low hum | Moderate |
| 500 Hz | Mid | Straightforward |
| 1 kHz | Mid-high | Easy |
| 2 kHz and above | Hiss and tone | Easy with lining |
Low-frequency energy travels further and passes through construction more easily, which is why plenum fans sometimes feel louder than housed fans measuring the same total.
Converting between the two
You can estimate pressure from power, provided you know the acoustic environment.
In a free field, sound pressure falls by 6 dB every time distance doubles. Indoors, room absorption and reflections change that considerably, so a proper calculation uses the room constant rather than a simple distance rule.
At one metre in a typical plant room, sound pressure often lands 8 to 14 dB below sound power. Treat that as a sanity check rather than a design figure, because room absorption varies enormously.
What to write in a specification
- Ask for sound power in octave bands from 63 Hz to 8 kHz.
- State the duty point, since noise at free air is meaningless.
- Ask for inlet and outlet sound power separately where relevant.
- Name the test standard, normally AMCA 300 or an ISO equivalent.
- State any A-weighted limit as a room requirement, not a fan requirement.
- Finally, ask what speed the quoted data corresponds to.
That last question matters on variable speed fans, where quoted data sometimes corresponds to a speed nobody will ever use.
Common mistakes
Three errors cause most acoustic disappointments, and all three are easy to avoid.
Comparing one supplier sound power against another supplier sound pressure. Sizing an attenuator from a single A-weighted number. Specifying a room noise limit and expecting the fan datasheet to guarantee it. All three are avoidable with the right data.
Our fan noise guide covers the mechanisms, our noise diagnosis guide covers the sounds themselves, and AMCA publishes the measurement standards.
One habit prevents most of them. Put the words sound power, octave band and duty point into the enquiry itself. Suppliers who have the data will send it, and suppliers who do not will start explaining, which tells you something useful either way.
Fan acoustics FAQ
Which number should I compare between suppliers?
Sound power in octave bands at your duty point. Anything else depends on the measurement conditions, so it cannot be compared fairly.
Why is my fan louder than the datasheet says?
Usually because the datasheet quotes sound power and you are measuring sound pressure in a reflective room. Check also whether inlet conditions are adding noise.
How much quieter is 3 dB?
Three decibels is half the acoustic energy, although it is only just noticeable. A 10 dB reduction sounds roughly half as loud to most people.
Does A-weighting matter?
It approximates human hearing sensitivity, which is useful for a single summary figure. However, it under-weights low frequencies that often cause the real complaints.
Sound power describes the fan and sound pressure describes the room. Specify the first, measure the second, and never compare one against the other.
