Human hearing isn’t equally sensitive at every frequency — we hear the midrange (around 1–4 kHz) most easily, and need much more energy in the deep bass or extreme treble for it to sound equally loud. That sensitivity is captured by equal-loudness contours (the Fletcher–Munson curves and their modern ISO 226 successors). The standard weighting filters approximate this: A-weighting rolls off lows and very highs to match how we judge loudness at moderate levels; C-weighting is much flatter, suited to loud sounds and peaks; Z-weighting is flat (no weighting). A-weighting is the basis for dBA measurements used in occupational noise regulations and environmental noise ordinances worldwide.
This analyzer splits the mic signal into standard octave bands — typically centred at 31.5, 63, 125, 250, 500, 1k, 2k, 4k, 8k, and 16 kHz — then applies the chosen weighting to each band. The bars show the perceived loudness of each band rather than its raw energy. Switching to Z reveals the unweighted octave-band spectrum; switching to A shows why a bass-heavy signal can measure high yet not sound that loud. For a finer-resolution view of the same signal, try the audio spectrum analyzer which plots the full FFT. Because it’s a relative, uncalibrated mic reading, treat the shape — which bands dominate — as the takeaway, not the absolute dB numbers. If you need to track the overall level alongside band weighting, pair this with the live decibel meter.
Common use cases include checking which frequency region dominates a room’s background noise (HVAC hum typically peaks in low-mid octave bands), verifying that HVAC or mechanical noise meets an NC curve target, and confirming whether a broadband noise source leans toward bass or treble under A-weighting before EQ correction.
What’s the difference between A, C and Z weighting?
A-weighting matches the ear’s reduced sensitivity to bass and extreme treble at moderate levels (the most common for noise). C-weighting is much flatter and used for loud sounds and peak measurement. Z-weighting is flat — the raw, unweighted level.
Why does the bass band shrink under A-weighting?
Because the ear is far less sensitive to low frequencies at normal levels, A-weighting attenuates them heavily (about −16 dB at 125 Hz, −26 dB at 63 Hz). So a band with lots of bass energy can still sound modest — which is exactly what A-weighting reflects.
What are the Fletcher–Munson curves?
Equal-loudness contours — maps of how much level each frequency needs to sound equally loud to a listener. The weighting filters here are simplified, single-curve approximations of that idea.
Are these calibrated dB SPL?
No. They’re relative dBFS from an uncalibrated mic, so use them to compare bands and weightings, not as absolute dBA/dBC sound levels. A calibrated meter is needed for real measurements.
Is my audio recorded?
No. The signal is analyzed in real time and is never recorded, saved, or transmitted. The microphone is released when you press Stop or close the tab.
Which octave bands does the analyzer show?
The analyzer displays the standard one-octave bands centred at approximately 31.5 Hz, 63 Hz, 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, 8 kHz, and 16 kHz — the same centre frequencies used in acoustic engineering and most sound-level meter standards. The highest and lowest bands depend on your browser’s FFT resolution and sample rate.
When should I use C-weighting instead of A-weighting?
C-weighting is nearly flat across the audible range and is best for measuring high-level or impulsive noise — concert venues, industrial equipment, gunshots — where the ear’s frequency sensitivity shifts (per the ISO 226 equal-loudness model). A-weighting suits ordinary environmental and occupational noise below about 85 dB. Comparing dBA vs dBC for the same sound reveals how much low-frequency content is present: a big dBA–dBC gap means substantial bass energy.
Can I use this for HVAC or room noise assessment?
Yes, with the caveat that readings are relative and uncalibrated. The octave-band shape is useful for identifying which frequency regions — such as the low-mid rumble of HVAC or the broadband hiss of air handling — dominate the background noise. For a formal noise criterion (NC) or room criteria (RC) assessment, you need a calibrated sound-level meter; use this analyzer to get a quick directional view before investing in that.
How is ISO 226 different from the original Fletcher–Munson curves?
The original Fletcher–Munson measurements (1933) were the first systematic equal-loudness contours. ISO 226:2003 updated them with better methodology and more listeners, producing curves that differ noticeably below 500 Hz and above 10 kHz. The A-weighting filter predates ISO 226 and is a simplified approximation, so the shape is similar but not identical. For this tool’s relative comparison purposes the distinction rarely matters.