Add and subtract decibel values, combine noise levels from multiple HVAC sources, and convert between dB scales. The foundational acoustics math tool for any HVAC noise analysis.
Convert between sound power level (Lw) and sound pressure level (Lp) at any distance and room type. Essential for translating HVAC equipment data sheets into room noise predictions.
Calculate sound pressure level at any distance from HVAC equipment using sound power level, directivity, and room absorption. Verify noise levels against occupancy limits.
Sum octave band sound power or pressure levels across all frequency bands. Analyse and combine spectral data from fans, ducts, and terminal units for NC and RC rating inputs.
Determine the Noise Criteria (NC) rating from octave band sound pressure levels. Compare the measured spectrum against standard NC curves to identify the applicable rating number.
Calculate Room Criteria (RC) rating with quality assessment suffix (N, R, or RV) from octave band measurements. RC captures low-frequency rumble that NC misses, making it more useful for HVAC.
Determine the Noise Rating (NR) number from octave band sound pressure levels. NR curves are widely used in European and international HVAC specifications for background noise assessment.
Look up recommended background noise levels for any occupancy type, such as offices, classrooms, hospitals, hotels, theatres, and mechanical rooms. Returns target NC, RC, and NR values with guidance notes.
Calculate room reverberation time (RT60), total absorption, and critical distance from room dimensions and surface materials. Essential for auditoriums, classrooms, and speech-critical spaces.
Calculate steady-state sound pressure level in a room from multiple HVAC sources. Accounts for room constant, directivity, and source-to-receiver distance. Compare result against NC and RC targets.
Estimate fan sound power level from airflow rate, total static pressure, and fan type. Output the octave band spectrum needed for duct path attenuation and room noise calculations.
Convert duct or plenum air velocity to an estimated NC (noise criterion) rating. Check whether supply or return plenum velocity keeps occupied-space noise within target.
Calculate regenerated noise from air velocity at duct fittings, transitions, dampers, and diffuser necks. Find the maximum velocity to keep self-noise below the room noise criterion.
Calculate sound power radiated through rectangular duct walls into spaces the duct passes through. Determine whether duct lagging or re-routing is needed to protect sensitive adjacent rooms.
Calculate cumulative sound attenuation along a duct run, from straight lined sections and elbows to unlined bends, area changes, and duct end reflection, to find the noise level at each diffuser.
Determine required insertion loss for a duct silencer from noise budget analysis. Calculate the trade-off between insertion loss, face velocity, and pressure drop for pod-type and splitter attenuators.
Combine sound levels from multiple HVAC equipment items, such as air handlers, chillers, cooling towers, and condensing units, and calculate the total sound pressure level at any receiver point.
Calculate sound transmission loss through walls, floors, and ceilings using STC ratings and octave band TL data. Determine whether a partition meets the noise isolation requirement between mechanical and occupied spaces.
Calculate the noise reduction provided by acoustic lagging on pipes and ducts. Determine the insulation thickness and material needed to meet a target insertion loss at each octave band.
Calculate vibration isolator natural frequency, transmissibility, and required static deflection for HVAC equipment. Size spring or rubber mounts for fans, pumps, and air handling units on sensitive floors.
Convert between sone, phon, and dB(A) loudness scales. Sones appear in fan and terminal unit specs; this tool translates them into NC-comparable dB(A) values for room noise assessments.
HVAC Noise: What Actually Matters
HVAC noise is the most common occupant complaint in commercial buildings, and one of the most misunderstood design parameters. The calculations involved are well established, but tedious without the right tools.
NC vs. RC: Which One to Use
NC (Noise Criteria) is the older standard, still widely used in North America. It rates background noise on a single number. RC (Room Criteria) is more rigorous: it includes a quality assessment (N for neutral, R for rumbly, RV for rumbly with vibration) that tells you what kind of noise problem you have, not just how loud it is. The RC method was developed and is maintained by ASHRAE Technical Committee 2.6, Sound and Vibration Control. For HVAC work, RC is more useful. Offices typically target RC-35, executive spaces RC-30, open-plan offices RC-40, a range checked with the office HVAC calculator alongside cooling and heating load. Use the NC curve calculator and RC rating calculator to compare both against the same octave band spectrum.
Where Duct Noise Actually Comes From
Three sources dominate. Fan noise travels down the duct and radiates at the diffuser. Breakout noise passes through duct walls into rooms the duct passes through, often a problem in corridors above ceilings and a factor the National Building Code of Canada's sound insulation provisions, published by the National Research Council, address for adjoining spaces. Regenerated noise is created at elbows, transitions, and dampers when air velocity is too high. Each requires a different fix. The breakout noise calculator and duct self-noise calculator address the second and third sources specifically.
Air Velocity and Noise
Keeping main trunk duct velocity below 900 FPM in quiet spaces, and branch ducts below 600 FPM, prevents most velocity-generated noise before it starts. If you're sizing ducts with the duct sizing calculator, check the resulting velocity against these limits.
Frequently Asked Questions
NC (Noise Criteria) and NR (Noise Rating) both use a worst-case single octave band compared against a family of curves to produce a rating. RC (Room Criteria) instead averages the mid-frequency bands and separately flags low-frequency rumble or high-frequency hiss with a quality letter. NC is standard in North America, NR is common internationally, and RC is often preferred by acoustical consultants for its more complete low-frequency assessment. Compare all three directly with the NC curve calculator, RC rating calculator, and NR curve calculator.
The decibel is a logarithmic unit representing a ratio to a reference value, not a linear measurement. Combining sound levels correctly requires converting each level to a linear power ratio, summing those ratios, then converting back to decibels. Two equal sources combine to roughly 3 dB above either one alone, not double the reading. The decibel calculator and equipment sound level calculator both handle this combination automatically for any number of sources.
Common options include lined duct sections, duct silencers (attenuators), reducing air velocity to lower self-generated noise, increasing distance between the noise source and occupied space, and adding acoustic lagging to duct walls to reduce breakout noise into adjacent rooms. Use the sound attenuation calculator to trace the full duct path and the attenuator sizing calculator to size a silencer for the specific reduction needed.
Recommended noise targets vary considerably by space type, since a classroom, a private office, and a mechanical room all tolerate very different background levels. Use the noise criteria selector to look up the recommended NC, RC, and NR range for your specific building category and space type, then verify your design against that target with the appropriate rating calculator.
Absolutely. All AskHVAC.ca tools are fully responsive and work on any smartphone or tablet without an app install. They run entirely in your browser with no internet connection required after the page loads. Results can be exported to PDF or CSV directly from your phone, and the interface switches between Imperial and Metric with a single tap.