Quiet room50 dB
Office70 dB
Conversation90 dB
Loud110+ dB
Very loud
| Parameter | Value | Notes |
|---|
Equal-Level Source Combination Reference
| Number of Identical Sources | Level Increase Over One Source | Example: Five 65 dB Sources Combine To |
|---|
Increase = 10 × log₁₀(n) dB, where n is the number of identical sources. This applies to sources of equal level only; use the Combine Sources mode above for sources with different levels.
Why decibels don't add like ordinary numbers
The decibel is a logarithmic unit. A sound pressure level in dB is defined as 20 times the base-10 logarithm of the ratio between the measured sound pressure and a fixed reference pressure. Because the underlying physical quantity (sound energy or sound pressure squared) follows a linear scale while the decibel compresses that scale logarithmically, two sources at the same decibel level don't sum to double the number. Two identical 70 dB fans running together produce approximately 73 dB, not 140 dB, because doubling the sound energy corresponds to a 3 dB increase, not a 100% increase in the decibel reading itself.
To combine decibel levels correctly, each level must first be converted back to a linear energy ratio, the linear ratios summed, and the sum converted back to decibels. The formula for combining any number of levels is: Ltotal = 10 × log₁₀(Σ 10(Li/10)), where Li is each individual source level in dB. This calculator performs that conversion automatically for any number of sources, so you always get a physically correct combined level rather than an incorrect simple sum.
The 3 dB rule and why it matters for HVAC equipment selection
A useful shortcut: adding a second source of equal level always raises the total by approximately 3 dB, regardless of the starting level. This holds whether you're combining two 40 dB sources into 43 dB or two 90 dB sources into 93 dB. This matters directly for HVAC equipment selection — if a single rooftop unit produces 75 dB at the property line and the project requires two identical units, the combined level rises to about 78 dB, not 150 dB and not simply 75 dB. Conversely, halving the number of operating units (for example, running one fan instead of two identical fans) reduces the sound level by 3 dB. This 3 dB relationship is symmetric in both directions and is one of the most useful rules of thumb in HVAC acoustics.
Combining sources of different levels: the dominant source effect
When two sources have significantly different levels, the louder source dominates the combined total almost entirely. A rule of thumb: if two sources differ by 10 dB or more, the combined level is within about 0.4 dB of the louder source alone — the quieter source is effectively masked. If they differ by exactly 3 dB, the combined level is about 1.8 dB above the louder source. If they're equal, the combined level is 3 dB above either one. This means that in a mechanical room with one dominant piece of equipment and several much quieter items, reducing the noise from the dominant source has far more impact on the total room level than addressing the quieter items. Use the Combine Sources mode above with your actual equipment sound power levels to identify which source is driving the total and where noise control effort should be focused. Pair this with the equipment sound level calculator for a full mechanical room analysis.
Frequently Asked Questions
The decibel is a logarithmic unit representing a ratio to a fixed reference value, not a linear measurement. Because sound energy adds on a linear scale but decibels compress that scale logarithmically, two equal sources don't combine to double the reading. Two identical 70 dB sources together produce about 73 dB, not 140 dB, since doubling sound energy corresponds to roughly a 3 dB rise on the logarithmic scale. Correct combination requires converting each dB value to a linear power ratio, summing those ratios, then converting the sum back to decibels using Ltotal = 10 log₁₀(Σ10L/10). This calculator performs that conversion automatically. Use the octave band calculator when you need to combine levels across multiple frequency bands rather than single overall values.
Adding a second source with the same decibel level as an existing one increases the total by approximately 3 dB, regardless of the starting level — true for two 50 dB sources (becomes 53 dB) or two 80 dB sources (becomes 83 dB). Adding a third identical source raises the total by roughly 1.8 dB more, since the existing combined level already dominates. Each additional identical source contributes progressively less increase. Ten identical sources combined increase the level by 10 dB over a single source, following the pattern 10 × log₁₀(n). Use the reference table above to see this pattern for 2 through 10 sources, and the Combine Sources mode to calculate any specific combination of equipment sound levels for your project.
Decibel subtraction reverses the combination formula to isolate one unknown source when you know the combined total and one known source. Convert both to linear power ratios (10 raised to level/10), subtract the known source's ratio from the total's ratio, then convert the remaining ratio back to decibels. This is the standard field method for isolating HVAC equipment noise from background: measure total sound level with equipment running, measure background alone with equipment off, then subtract to find the equipment's individual contribution. Use the Subtract mode above for this calculation. The result becomes unreliable when the two levels are within about 3 dB of each other. Pair this with the room sound level calculator to verify the isolated equipment level against your target noise criterion.
Differences below about 1 dB are generally not perceptible under normal listening conditions. A 2 to 3 dB difference is typically the smallest change most listeners can just notice. A 5 dB difference is clearly noticeable to nearly everyone. A 10 dB difference is widely used as a practical benchmark for a perceived doubling or halving of loudness, though the exact relationship depends on frequency content and listener sensitivity. For HVAC noise control, a design change that only reduces equipment sound level by 1 dB is unlikely to produce a noticeable improvement for occupants, while a 5 to 10 dB reduction is meaningful. Use the Level Difference mode above to check any two levels, and the noise criteria selector to find the target background level for your space type before deciding how much reduction is actually needed.