🔈 Predicted Level
📏 Distance Change
Calculation Breakdown
Full Calculation Table
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How to Use the Sound Level Calculator
Use Single Source Distance to project a known level to a new distance. Use Combine Multiple Sources when several pieces of equipment contribute to the same receiver point.
Enter a known sound pressure level and the distance at which it applies, then enter the target distance where you want the prediction.
Click Calculate Sound Level to see the predicted level at the target distance, or the combined total for multiple sources, along with a full breakdown.
Check the result against the noise criterion for the receiving space using the noise criteria selector to confirm whether further noise control is needed.
Predicting Sound Pressure Level at a Receiver Point
Sound pressure level is not a fixed property of a source; it changes with distance, direction, and the surrounding environment. When you have a known sound pressure level at one distance, whether from a manufacturer's data sheet or a field measurement, you can project that level to a different distance using the inverse square law, as long as the sound is radiating into an open or free-field-like environment without significant reflections.
The Distance Attenuation Formula
The relationship between sound pressure level at two distances from the same source follows a simple logarithmic formula: the new level equals the reference level minus 20 times the base-10 logarithm of the ratio of the new distance to the reference distance. This formula reflects the fact that sound energy spreads over an increasingly large area as it travels away from a point source, so the energy per unit area, and therefore the sound pressure, falls off with the square of the distance. In practical terms, every doubling of distance reduces the level by about 6 dB, and every halving of distance increases it by about 6 dB. The reliability of this kind of calculation depends on starting from a correctly calibrated reference measurement, which is why traceable calibration of sound measurement instruments, the kind of work the National Institute of Standards and Technology carries out as part of its measurement science mission, underlies every accurate sound level reading in the field.
When This Simple Method Works Well
Distance-based prediction from a known reference level works best for outdoor conditions, large open indoor spaces, and close-range predictions where the direct sound path from source to receiver clearly dominates over any reflected sound. This makes it well suited for predicting property-line noise from outdoor condensing units and rooftop equipment, a scenario many municipalities regulate directly, such as the City of Vancouver's published maximum allowable noise limits for mechanical equipment, or for quick checks of equipment noise a few metres away in an open mechanical room.
When to Use a More Complete Model
Indoors, at greater distances from the source, reflected sound from walls, ceiling, and floor begins to add to the direct sound, and the simple inverse square relationship becomes less accurate. In these cases, the level eventually flattens out rather than continuing to fall at 6 dB per distance doubling. For this reverberant condition, use the sound power calculator, which includes a room constant term to account for reflected sound buildup, or the room acoustics calculator for a full reverberation time and absorption analysis.
Combining Multiple Sources at One Receiver
When several pieces of equipment are audible at the same point, such as a mechanical room with a boiler sized using a boiler sizing calculator, circulating pumps sized with a pump sizing calculator, and an air handler all contributing to the noise level in an adjacent office, the individual contributions must be combined using logarithmic addition, not simple arithmetic addition. Two sources each at 60 dB do not combine to 120 dB; they combine to approximately 63 dB. The multi-source mode in this calculator, and the standalone decibel calculator, both perform this combination correctly. This is essential for accurately predicting the total noise environment when multiple mechanical systems operate simultaneously.
A Note on Accuracy
This calculator applies standard logarithmic distance attenuation and sound level combination formulas. It does not account for atmospheric absorption over long distances, wind effects, barriers, or complex reflecting geometries. For property-line noise studies subject to municipal noise bylaws, or for critical noise-sensitive projects, confirm the prediction with field measurement or have an acoustical consultant complete a detailed analysis.
Frequently Asked Questions
Many HVAC equipment manufacturers rate sound pressure level at 1 metre or 3 feet from the unit under standardized test conditions, though the exact reference distance and test standard vary by equipment type and manufacturer. Always check the equipment submittal data to confirm the reference distance and whether the published value is sound power level or sound pressure level, since the calculation depends on using the correct starting point. If the data sheet only lists sound power level, use the sound power calculator instead, which converts Lw directly into a distance-based prediction.
Yes, this calculator is designed for exactly that situation. If you have a sound pressure level measured or rated at one distance and need to predict the level at a different distance, this tool applies distance attenuation directly to the known pressure reading. This differs from the sound power calculator, which starts from a sound power level rating and calculates pressure at any distance from scratch, including directivity and room effects.
Simple distance attenuation calculations are reasonably accurate for outdoor, open-space conditions and close-range predictions where the direct sound path dominates. Accuracy decreases indoors at greater distances where reflected sound from room surfaces begins to dominate, since basic distance attenuation formulas do not account for reverberant buildup. For indoor predictions at distances beyond a few metres from the source, use the sound power calculator's room-aware calculation or the room acoustics calculator for a more complete result.
Calculate the predicted sound pressure level from each individual source at the receiver point separately, then use the Combine Multiple Sources mode above, or the standalone decibel calculator, to combine them using logarithmic addition. Decibels cannot be added arithmetically because they represent a logarithmic ratio, so two sources each producing 60 dB at a receiver combine to approximately 63 dB, not 120 dB. Use the equipment sound level calculator for a mechanical-room-specific version of this same combination.
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