Distance Attenuation Calculator
Enter a reference sound pressure level and two distances to find how many decibels are lost as sound travels further from its source. Choose between a point source (speakers, engines), a line source (traffic, pipes), or a plane source (large walls), and optionally include atmospheric absorption. The result updates as you type.
Formula
Worked example
A compressor emits 90 dB at 1 m. At 10 m (a point source): geometric loss = 20 x log10(10/1) = 20 dB, so SPL = 90 - 20 = 70 dB. With 1 kHz atmospheric absorption (0.0048 dB/m) over 9 m: extra 0.04 dB, giving 69.96 dB. At 20 m the geometric loss is 26 dB, giving 64 dB.
What is distance attenuation?
Distance attenuation is the reduction in sound pressure level (SPL) as a sound wave travels further from its source. For a point source radiating equally in all directions (omnidirectional), the sound energy spreads over an ever-larger spherical surface. Because surface area grows with the square of the radius, the intensity at any point falls as 1/r^2, and the SPL drops by 20 x log10(r2/r1) decibels. This is the inverse square law: every time you double the distance, you lose approximately 6 dB. A rock concert speaker at 1 m measured at 110 dB will reach about 104 dB at 2 m, 98 dB at 4 m, and 86 dB at 16 m - all in open, free-field conditions.
Point, line, and plane sources
Not all sources radiate like a sphere. A line source (a long row of traffic, a stretched pipe, a live-band PA rigged as a line array) spreads its energy cylindrically rather than spherically. The intensity falls as 1/r instead of 1/r^2, so the SPL drops by only 10 x log10(r2/r1) - roughly 3 dB per doubling of distance. A plane source (a large vibrating wall or a distributed array designed to maintain a planar wavefront) causes no geometric spreading at all: the wave travels without diverging, so the level stays flat regardless of distance (until air absorption and other losses take over). Choosing the correct source model matters greatly for long-distance predictions: using the point-source formula for a road will overestimate how quickly the noise fades.
Atmospheric absorption
Even in perfectly open air with no reflections, the air itself absorbs acoustic energy through viscous and thermal losses and by exciting rotational and vibrational states of oxygen and nitrogen molecules. This absorption increases with frequency: bass frequencies travel almost unimpeded across kilometres, while high-frequency content above 4 kHz can be significantly attenuated over hundreds of metres. The absorption coefficient alpha (dB/m) depends on temperature, humidity, and frequency, and is standardised in ISO 9613-1. At 1 kHz, 20 C, and 50% relative humidity, alpha is approximately 0.0048 dB/m - negligible for short distances but meaningful at 500 m or more. This calculator uses the ISO 9613-1 typical-condition coefficients for seven octave-band centre frequencies.
How to use this calculator
Set the distance unit (metres or feet), then select the source type. Enter the reference SPL in dB at the reference distance - this is the level you measured or found in a data sheet. Enter the target distance where you want to know the level. The calculator returns the SPL at the target, the geometric loss, and the total attenuation. Toggle "Include atmospheric absorption" to add air absorption loss; then pick the frequency band that best represents the dominant content of the sound. Use the chart to see how SPL decays across a range of distances. The reference table on the page shows where the computed level sits among common real-world sounds.
Typical sound pressure levels (dB SPL)
| dB SPL | Source or environment | Exposure risk |
|---|---|---|
| 0 | Threshold of hearing | None |
| 20 | Quiet whisper at 1 m | None |
| 30 | Quiet bedroom at night | None |
| 50 | Quiet office, library | None |
| 60 | Normal conversation at 1 m | None |
| 70 | Vacuum cleaner at 3 m | Low (long exposure) |
| 80 | Alarm clock at 1 m | Moderate |
| 90 | Pneumatic drill at 15 m | High (>8 h/day) |
| 100 | Angle grinder at 1 m | Very high (>2 h/day) |
| 110 | Live rock concert, front rows | Hazardous (>30 min) |
| 120 | Jet engine at 100 m | Pain threshold |
| 140 | Gunshot at close range | Immediate harm |
Reference levels for common sound sources and environments. Values are approximate.
Frequently asked questions
Why does sound lose 6 dB every time the distance doubles?
A point source radiates equally in all directions, so its energy spreads over a sphere. The surface area of a sphere is 4*pi*r^2, so when the radius doubles the area quadruples. Intensity (power per unit area) therefore falls to one quarter, which is a factor of 4 reduction. In decibels, 10*log10(4) is about 6 dB. This is the inverse square law applied to sound.
What is the difference between sound pressure level and sound intensity level?
Sound intensity is the power carried per unit area (W/m^2). Sound pressure level (SPL) is measured in decibels relative to a reference pressure of 20 micro-pascals (the threshold of hearing). For a free progressive wave in air, SPL and intensity level are numerically equal, so the inverse square law gives the same -6 dB/doubling result for both. This calculator uses SPL, the quantity measured by a standard sound level meter.
When should I use the line source formula instead of the point source formula?
Use the line source formula (10*log10(R2/R1), -3 dB per doubling) when the source is much longer than the distance at which you are predicting. A motorway seen from 50 m is a classic line source. At very large distances (much greater than the source length) the source begins to look like a point and the -6 dB rule gradually takes over. In practice, many noise models switch from the line to the point formula at a transition distance equal to roughly 1/pi times the source length.
Does this formula work indoors?
No, not directly. Indoors, sound reflects off walls, floors, and ceilings, building up a reverberant field that adds to the direct-path level. The reverberant SPL is roughly constant throughout the room regardless of distance from the source. Room acoustics models such as Sabine's equation or the direct-plus-reverberant formula are needed for indoor predictions. This calculator assumes free-field (anechoic) conditions typical of an outdoor open space.
How accurate is the atmospheric absorption coefficient used here?
The coefficients are representative values from ISO 9613-1 at 20 degrees Celsius and 50% relative humidity. Actual absorption varies with temperature and humidity. Hot, very dry air absorbs high frequencies more strongly; cold, humid air absorbs less at most frequencies. For precision outdoor noise assessments (roads, industrial sites, wind turbines) use site-specific meteorological data and a full ISO 9613-2 propagation model.
What does a negative atmospheric loss mean?
In this calculator, atmospheric loss is always zero or positive: it only applies when the target distance is greater than the reference distance. If you set a target closer than the reference (to predict what a source sounds like up close), the geometric loss becomes negative (a gain), meaning the level is higher than the reference. Atmospheric absorption is suppressed in that case because the wave has not travelled farther through air.