HVAC Noise Analyzer

HVAC noise has a handful of fingerprints. Fan noise is usually the loudest: a fan pushes air with a set of blades, and every time a blade sweeps past a fixed point (the housing cut-off, a strut, a grille) it makes a pressure pulse. Repeated at the rate the blades pass, this produces a tone at the blade-passage frequency, BPF = (RPM ÷ 60) × blades, plus harmonics at 2×, 3× and so on. The built-in BPF calculator lets you enter a fan's RPM and blade count to predict where those tones land in the spectrum. Find a sharp spectrum peak, compute the BPF for your fan, and if they line up you have identified the source — this part of the tool is exact arithmetic and is calibration-independent. Common culprits include supply fans, return-air fans, and variable-speed drives (VSDs) whose switching frequency can also show up in the octave band spectrum as an audible tonal component.

A duct behaves like an organ pipe: standing waves form at frequencies set by its length and end conditions. A length open at one end and closed at the other resonates at f = c ÷ (4L) and its odd harmonics; a length open (or closed) at both ends resonates at f = c ÷ (2L) and all harmonics, with c ≈ 343 m/s at room temperature. Temperature affects the speed of sound (roughly +0.6 m/s per °C above 0°C), so a hot plenum or cold outdoor duct will shift the resonance slightly from the nominal prediction. If a peak in the spectrum matches a predicted duct resonance, that run of duct is amplifying the noise and may need acoustic lining, a plenum, or a length change. You can use the room frequency analyzer to cross-check whether a low-frequency peak is originating inside the duct or is a room mode excited by the HVAC system.

The NC (Noise Criteria) curves are a family of standardised octave-band limits (here NC-15 through NC-65 at the published octave centres 63, 125, 250, 500, 1000, 2000, 4000 and 8000 Hz) used by mechanical engineers and acoustical consultants to rate steady background noise in occupied spaces. The NC rating of a sound is the lowest curve that is not exceeded in any band. Typical design targets are NC-20 to NC-30 for open-plan offices, NC-25 to NC-35 for conference rooms, and NC-35 to NC-45 for classrooms. This tool overlays your measured octave bands on the NC family. The honest catch: a true NC rating needs calibrated dB SPL. Without calibration the comparison is a shape check — useful for seeing whether your noise tilts toward low-frequency rumble or high-frequency hiss relative to the curves, but not a real NC number. Calibrate against a sound-level meter and the SPL mode becomes quantitative (still an estimate from a consumer mic). A related standard, RC (Room Criteria), adds a subjective quality descriptor (Neutral, Rumble, Hiss) and is preferred by some ASHRAE guidelines; this tool uses NC because it is more widely referenced.

Finally, the character hint reads the spectral balance: energy bunched in the low bands tends to be rumble (low-frequency fan imbalance or structure-borne vibration transmitted through the building fabric); a broad mid-band hiss is whoosh (high air velocity at grilles/diffusers, often reduced by upsizing the grille or lowering the fan speed); and a strong narrow tone up high is whine (blade or bearing). A steady low-pitched HVAC hum — the kind tenants most commonly complain about — usually shows up as a tonal peak near the fan's fundamental BPF or at 100/120 Hz from motor pole-pass frequency; it is also a common symptom of air handler noise transmitted through ducts or the building structure. These are guidance, not a diagnosis — a certified acoustical engineer should be engaged for compliance, legal disputes, or tenant complaints. To capture the broader background noise floor in a space, the noise floor analyzer records octave-band levels across a longer dwell time.