Equipment sound data: sound power, sound pressure and how to compare them

A decibel figure means little until you know whether it is power or pressure, and how it was measured.

Noise is one of the most common reasons an installed fan, chiller or condensing unit is rejected, and one of the least comparable lines in Chinese catalogues. One sheet prints 62 dB(A) at 1 m, another 78 dB(A) with no distance, and both may describe similar machines. The difference is usually between sound power and sound pressure.

Two quantities with the same unit

Sound power is a property of the source: the acoustic energy the machine emits, independent of the room or the distance. It is referenced to 1 picowatt.

Sound pressure is what a microphone or an ear receives at one position. It depends on distance, direction, reflecting surfaces and the room. It is referenced to 20 micropascals.

Both are expressed in decibels, which is why they are confused. A sound pressure figure without a distance and a measurement condition cannot be compared with anything. A sound power figure can be compared between products and used by an acoustic consultant to predict the pressure at a neighbour's window or in an occupied room.

Test methods to ask for

ISO 3744 determines sound power from sound pressure measured on a surface around the source in a largely free field over a reflecting plane. It is an engineering-grade method; ISO 3745 is the precision method in anechoic rooms and ISO 3746 a survey method. The current edition was approved in Europe as EN ISO 3744:2026. A supplier quoting sound power should name the method.

For large air-cooled outdoor equipment such as air-cooled chillers and condensing units, ANSI/AHRI Standard 370 sets the sound rating method; its 2025 edition superseded the 2015 standard. Smaller unitary outdoor equipment falls under AHRI Standard 270. Fans are often rated by fan-specific test codes, so ask which code applies.

What a usable sound line contains

A useful datasheet gives the A-weighted sound power level, the octave-band spectrum from 63 Hz to 8 kHz, the operating point at which it was measured and the test method. For fans, the operating point matters as much as the speed, because sound changes along the fan curve; our duty-point guide explains how to fix it. For chillers and condensing units, ask for sound at full load and at the part-load condition that runs at night.

Sound pressure figures are still useful when they come with a distance, a measurement surface or room description and the corresponding sound power.

Where noise questions arise on this site

Fan pages often show the problem directly. The Sihe SHG duct fans sheet prints sound pressure at 3 m, while other Sihe sheets use different bases, so the figures cannot be ranked as printed. The Qinye SWF mixed-flow selection and the Gerui T35 axial fans both depend on the operating point you choose.

Large-space and outdoor equipment adds distance and direction. Bigfans' ceiling fans for commercial rooms and its column-supported large-space fans are judged by people standing under them. The Envicool dry coolers page publishes low-noise fan wording without a full sound basis, and air-cooled machines such as the Xinwuxin air-cooled screw chillers are often judged against night-time limits at the site boundary.

Inside buildings, terminals are close to occupants: the Yilaide concealed horizontal fan coils and the Langri cassette fan coils should be compared at the same fan speed and external static pressure, and ducted terminal assemblies add duct-borne noise to that comparison. Cooling towers bring fan and falling-water noise outdoors; our cooling tower selection guide lists the sound questions to ask.

Questions to ask

Is the figure sound power or sound pressure? If pressure, at what distance, over what surface and in what environment? Which standard was used, and was the test done by the factory or a third party? At which airflow, pressure, speed or load was it measured? Can you provide the octave-band spectrum? Do the figures include accessories such as silencers, louvres or acoustic enclosures, and are those in the quoted scope?

Common questions

Can I convert a pressure figure at 1 m to sound power myself? Only roughly, and only if the measurement conditions are known. Ask the supplier for sound power measured to a named standard instead.

Why does a fan get louder when the duct system changes? The fan moves to a different point on its curve. Sound data are only valid at the stated operating point.

Is an A-weighted single number enough? For a first comparison, yes. For design near occupied spaces, ask for octave bands, because low-frequency noise is treated differently.

By HVACDeck Editorial · Updated 2026-11-04