# Ohmforge Sound Level (WAV → Leq, LAeq, Lmax)

> Upload a WAV recording: A/C/Z-weighted Leq, Fast/Slow Lmax, peak and L10/L50/L90. Measures a WAV recording like a sound level meter: equivalent continuous levels with A, C and no (Z) frequency weighting, maximum levels with Fast (125 ms) and Slow (1 s) time weighting, minimum, C-weighted peak and the statistical levels L10, L50 and L90, plus a level-over-time chart. Without calibration the levels are relative to a full-scale sine; with an offset from a reference meter they read dB SPL. Same engine as the sound level meter in Ohmforge's Pocket instruments.

Part of usefulweb.app (https://usefulweb.app/). Part of [Ohmforge](https://ohmforge.usefulweb.app/) (ownerApp: ohmforge). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://ohmforgesoundlevel.usefulweb.app/ (interactive tool at https://ohmforgesoundlevel.usefulweb.app/tool)
- JSON API: POST https://ohmforgesoundlevel.usefulweb.app/api/v1/calculate with a JSON object of inputs (schema: https://ohmforgesoundlevel.usefulweb.app/api/v1/tool)
- OpenAPI: https://ohmforgesoundlevel.usefulweb.app/openapi.json
- MCP (Streamable HTTP): https://ohmforgesoundlevel.usefulweb.app/mcp - tool `ohmforge_sound_level`

## Inputs
- file (file, required): WAV recording - PCM 8/16/24/32-bit or 32-bit float, 8 to 192 kHz, up to 10 minutes. Stereo is mixed to mono. Processed in memory and not kept.
- offset (number): Calibration offset - Added to every level. 0 = uncalibrated (dB relative to a full-scale sine). Get it by recording a known level, e.g. a 94 dB calibrator or a reference meter.
- channel (select): Channel

## Method
- Frequency weighting: the analogue A and C curves of IEC 61672-1 (poles at 20.6 Hz, 107.7 Hz, 737.9 Hz and 12.2 kHz) are turned into digital biquad filters with the bilinear transform; the 12.2 kHz pole pair is partly pre-warped so the response stays within 0.3 dB of the curve from 10 Hz to 10 kHz at 44.1 and 48 kHz (above 10 kHz it falls off faster than the curve). Z means no weighting.
- Time weighting: the squared weighted signal is smoothed exponentially with τ = 125 ms (Fast) or 1 s (Slow); Lmax is the highest smoothed value. Leq is the energy average over the whole recording. Peak is the largest absolute sample of the weighted signal. L10, L50 and L90 are the LAF levels exceeded 10, 50 and 90 % of the time, sampled 100 times a second after the first second (the meter needs that long to settle).
- Levels are 10·log10(mean square / 0.5) + offset: a full-scale sine reads 0 dB, so an uncalibrated recording gives relative levels. The calibration offset is the difference between a reference meter and this reading.
- Known answers: a 4 kHz tone burst of 200 ms reads 1.0 dB below the steady tone with Fast and 7.4 dB below with Slow; 2 ms reads 18.0 dB below with Fast — the 10·log10(1 − e^(−T/τ)) values of IEC 61672-1.

## Limits
- This is not a calibrated or certified sound level meter. Phone and computer microphones, automatic gain control and noise suppression change levels by many decibels; use the results for comparisons and orientation, not for legal, workplace or hearing-protection decisions.