🔈 Predicted Sound Pressure
📏 Distance Attenuation
Calculation Breakdown
Full Calculation Table
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How to Use the Sound Power Calculator
Enter the equipment sound power level (Lw) from the manufacturer's data sheet or AHRI certified rating, in decibels referenced to 1 picowatt.
Enter the distance from the sound source to the point of interest, such as an occupied desk, an adjacent room, or the property line.
Choose free field, a mounting position near reflecting surfaces, or a room type. This sets the directivity factor and room constant used in the calculation.
Click Calculate Sound Pressure to see the predicted level at the receiver distance and a breakdown of the distance and directivity adjustments driving the result.
Converting Sound Power Level to Sound Pressure Level
Sound power level (Lw) describes the total acoustic energy a source radiates, independent of distance or surroundings. Sound pressure level (Lp) is what a person or a meter actually experiences at a given point, and it always depends on distance, the acoustic environment, and any nearby reflecting surfaces. Equipment manufacturers publish Lw because it is a fixed property of the machine, and for certified equipment those published values are verified through AHRI's independent third-party certification program. Engineers then calculate Lp for the specific installation using the source-to-receiver distance and the surrounding acoustic conditions.
The Basic Free-Field Relationship
For a point source radiating into free space, the relationship between sound power and sound pressure at a distance r follows the inverse square law: Lp equals Lw minus 20 times the base-10 logarithm of the distance in metres, minus 11, plus a correction for directivity. This means every doubling of distance from a source in open space reduces the sound pressure level by approximately 6 dB. A source at 88 dB Lw produces roughly 77 dB Lp at 1 m in free space, roughly 71 dB Lp at 2 m, and roughly 65 dB Lp at 4 m.
Directivity and Reflecting Surfaces
Real HVAC equipment rarely sits in true free space. A rooftop unit, selected with an RTU sizing calculator and resting on a roof deck, radiates into a hemisphere rather than a full sphere, since the roof surface reflects sound back upward instead of letting it pass through. This hemispherical radiation is captured with a directivity factor Q of 2, which adds about 3 dB compared to free-field radiation at the same distance. Equipment placed in a corner where two hard surfaces meet uses Q of 4, adding about 6 dB. A three-surface corner mounting uses Q of 8, adding about 9 dB. Selecting the correct directivity factor for the actual equipment mounting location is essential for an accurate prediction.
Indoor Rooms and the Room Constant
Inside an enclosed room, sound reaches the receiver by two paths: the direct path from the source, and reflected sound that bounces off walls, ceiling, and floor before arriving. Close to the source, direct sound dominates and the level falls with distance similar to free field. Farther from the source, reflected sound dominates and the level becomes nearly constant regardless of further distance increases, an effect controlled by the room's total absorption, summarized in the room constant. Rooms with more sound-absorbing surfaces (acoustic ceiling tile, carpet, soft furnishings) have a larger room constant and produce lower reverberant sound pressure levels than hard-surfaced rooms of the same size. This calculator's room presets apply typical room constants for small mechanical rooms, medium offices, and large warehouse-type spaces, the last of which is the situation you will meet when equipment is selected with a warehouse HVAC calculator for a big, hard-surfaced building. For a more precise room-specific calculation using actual room dimensions and surface materials, see the room acoustics calculator.
Using This Result for Noise Criteria Compliance
Once you have a predicted sound pressure level at the receiver, compare it against the target background noise level for that space type using the noise criteria selector. If the predicted level exceeds the target, options include relocating the equipment farther from the occupied space, adding a duct silencer sized with the attenuator sizing calculator, or selecting quieter equipment. When multiple pieces of equipment contribute to the same receiver point, use the equipment sound level calculator to combine their individual contributions into a total predicted level.
A Note on Accuracy
This calculator uses standard acoustic propagation formulas and typical room constant values for common HVAC installation scenarios. Actual sound pressure levels can vary due to atmospheric conditions, unusual reflecting surfaces, background noise masking, and equipment installation quality. Field verification should follow a recognized procedure such as ANSI/ASA S1.13, the standard for measuring sound pressure levels in air, so that a measured result can be compared fairly against the prediction. For critical noise-sensitive projects such as hospitals, recording studios, or noise-sensitive residential adjacency, an acoustical consultant should verify the prediction with field measurements or detailed room modelling.
Frequently Asked Questions
Sound power level (Lw) is a fixed property of a noise source describing the total acoustic energy it radiates, independent of the surrounding environment or distance, analogous to the wattage rating of a light bulb. Sound pressure level (Lp) is what a microphone or the human ear actually measures at a specific point, and it depends on distance, the acoustic environment, and nearby reflecting surfaces. The same piece of HVAC equipment has one sound power level but produces a different sound pressure level at every distance and in every room. Use the decibel calculator if you need to combine several sound pressure readings from multiple sources at one receiver point.
For a point source radiating into free space with no reflecting surfaces, doubling the distance reduces sound pressure level by approximately 6 dB, following the inverse square law. Near a hard reflecting surface the reduction per doubling is similar, though the starting level is higher due to the reflection. Inside a room with multiple reflecting surfaces, the reduction per distance doubling becomes smaller once the receiver is far enough that reflected sound dominates over direct sound, an effect the room constant in this calculator accounts for. Use the room acoustics calculator to model this reverberant behaviour more precisely for a specific room.
Sound power level is a single, fixed number describing the source itself, so it can be published once regardless of installation. Sound pressure level changes with distance, room size, surfaces, and nearby reflecting objects, so a single published pressure value would only be valid for one specific installation. By publishing sound power level, manufacturers give engineers the raw acoustic output, and the engineer calculates sound pressure level for their specific layout using this calculator. See the fan sound calculator for estimating Lw directly from fan airflow and static pressure when no data sheet is available.
A source radiating freely with nothing nearby uses a directivity factor Q of 1 (full sphere). Equipment sitting on a hard floor or ground surface, like a rooftop unit on a roof deck, radiates into a hemisphere and uses Q of 2, adding about 3 dB compared to free space. Equipment in a corner where two surfaces meet uses Q of 4, adding about 6 dB. A three-surface corner mounting uses Q of 8, adding about 9 dB. Select the option in this calculator that matches your actual equipment mounting location for the most accurate prediction.