What is sound power level?

Sound power level (SWL or L_W) is a decibel quantity that expresses the total acoustic power radiated by a source relative to the reference power of 1 picowatt (10⁻¹² W). Unlike SPL, SWL does not depend on distance or the acoustic environment.

What is the difference between SWL and SPL?

SWL describes how much power a source emits in total (like a light bulb wattage), while SPL describes the pressure at a specific point in space (like the brightness at your eye). SPL depends on distance and room reflections; SWL does not.

Why does SPL drop by 6 dB when distance doubles?

A point source radiates energy equally in all directions over a sphere. When you double the distance, the sphere area quadruples (proportional to r²), so the intensity — power per unit area — drops to one quarter. A factor of 4 in intensity is 10 log10(4) ≈ 6 dB.

What reference power is used for SWL?

The international standard reference is 10⁻¹² watts (1 picowatt), denoted W₀. All SWL values in this calculator use that reference unless otherwise noted.

Does this calculation account for room reflections?

No. The SPL formula used here assumes free-field conditions — an ideal point source radiating in all directions with no reflective surfaces. Real rooms will produce higher SPL due to reflections. Use room-acoustic simulation tools for enclosed-space predictions.

How accurate is 0 dB SWL?

0 dB SWL equals 10⁻¹² W (1 picowatt) — an extraordinarily small amount of power, below the threshold of audibility for most frequencies. Conversational speech is around 65–75 dB SWL; a jet engine is around 160 dB SWL.

Sound Power Level Calculator

Name: Sound Power Level Calculator
Availability: InStock
Author: Nham Vu

Convert between sound power level (dB re 1 pW) and acoustic power (watts), and calculate sound pressure level at any distance from a point source.

Sound Power Inputs

Start from

Sound Power Level (dB re 1 pW)

Enter L_W in decibels. Reference: 10−12 W. Range: 0–200 dB.

dB SWL

Distance from Source (m)

Distance at which to calculate SPL. Free-field, point source assumed.

Common SWL Presets

Sound Power Level

90.0 dB

re 1 picowatt (10−12 W)

Acoustic Power

1.000 mW

radiated by the source

Sound Pressure Level at Distance

59.0 dB

SPL at 10 m from source (free field)

Inverse-square drop

−31.0 dB

from SWL to SPL

0 dB SPL 140 dB SPL

Formulas Used

L_W = 10 × log₁₀(W / 10⁻¹²)

L_p = L_W − 10 × log₁₀(4πr²)

W = acoustic power in watts, r = distance in meters, reference intensity = 10⁻¹² W/m². The SPL formula assumes a point source in a free field (no reflections).

SPL at Various Distances

Based on current SWL of 90.0 dB. Free-field point source.

Distance	SPL	Drop from SWL

Summary

Convert between sound power level (dB re 1 pW) and acoustic power (watts), and calculate sound pressure level at any distance from a point source.

How it works

Choose your starting point: enter a sound power level in dB or a power in watts.
The calculator instantly converts between the two using the formula L_W = 10 log10(W / 10^-12).
Enter a distance from the source in meters to compute the resulting SPL at that point.
SPL is calculated with the inverse-square law: L_p = L_W - 10 log10(4π r²).
The results panel shows both values with the inverse-square drop-off clearly stated.
Adjust any field in real time; all outputs update immediately.

Use cases

Compare loudspeaker efficiency specs given in sound power level.
Estimate noise levels at a given distance from industrial machinery.
Verify whether a sound source meets environmental noise regulations at a site boundary.
Convert manufacturer wattage ratings to decibel sound power for acoustic modeling.
Understand how SPL drops by 6 dB each time distance from a point source doubles.
Plan PA system placement by back-calculating required SWL from target SPL at distance.
Study the relationship between acoustic power and perceived loudness.
Crosscheck acoustic reports using the reference intensity of 10^-12 watts.

Frequently Asked Questions

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Last updated: 2026-05-29 · Reviewed by Nham Vu