Sound Power and Sound Pressure: Compare Fan Noise

Sound power and sound pressure: the distinction

Sound power level, often written L_W, characterises a source under stated operating and test conditions. Sound pressure level, L_p, describes sound at a measurement position. A fan can have one declared sound-power result for a duty while producing different sound-pressure levels at different places.

Distance, direction, reflections, barriers and other sources all affect the receiver result. This is why a fan's catalogue number cannot be compared directly with a handheld meter reading unless the quantities and conditions match.

ASHRAE's sound and vibration guidance distinguishes source power from pressure at a receiver and sets out the reference quantities used for each level.

The two decibel equations

Sound power level is L_W = 10 log₁₀(W/W₀), where the usual airborne-sound reference power W₀ is 10⁻¹² watts. Sound pressure level is L_p = 20 log₁₀(p/p₀), with reference pressure p₀ = 20 micropascals for airborne sound.

The different multipliers are part of the definitions. They do not mean that a sound-pressure number should be doubled to become sound power. Both are logarithmic levels relative to different reference quantities.

A doubling of sound power increases its level by about 3 dB. A doubling of pressure amplitude increases sound-pressure level by about 6 dB. Neither statement alone describes how loud a person will judge the sound.

Why distance changes the receiver result

In an ideal free field, for a compact source observed sufficiently far away with the same directivity, doubling distance reduces sound-pressure level by approximately 6 dB. A real laboratory or roof installation may have reflections, shielding, multiple sources and directional discharge that make this simple relationship unsuitable.

For an illustrative free-field comparison, a level of 60 dB at two metres would be about 54 dB at four metres under those assumptions. This is a propagation example, not a prediction for a particular fan or site.

Do not apply a fixed subtraction to sound power to obtain sound pressure. The acoustic model must include the distance, directivity and environment relevant to the receiver.

Read the complete fan noise specification

Look for the quantity, frequency bands or overall weighting, operating point and test arrangement. A-weighted sound power L_WA and A-weighted sound pressure L_pA are still different quantities. Our dB versus dBA guide explains weighting.

• Is the result inlet, outlet, casing-radiated or a combined source rating?
• What airflow, pressure and speed were used?
• Does a pressure result state distance and measurement position?
• Are the compared values weighted in the same way?
• Is background noise or measurement uncertainty reported?

Ask for frequency-band data when selecting a silencer. An overall level can hide a tone or frequency band that needs attention.

Turn a noise concern into a design brief

Identify who hears the sound, where they hear it and when it occurs. Record the fan's operating state and whether other equipment is running. A competent acoustic assessment can then separate duct-borne sound, casing radiation and vibration transmission.

We can discuss PVC duct silencers as part of the extraction design. Match the acoustic requirement with airflow, pressure loss and chemical compatibility. Keep the agreed receiver condition in the brief so the final check measures the outcome that matters.