How to Calibrate a Phone Microphone for SPL Measurement

TL;DR

Phone microphones can achieve ±2 dB accuracy when properly calibrated, making them viable for screening measurements and trend monitoring. This guide covers two calibration methods: using a 94 dB acoustic calibrator (preferred) and the comparison method against a reference meter. SonaVyx stores per-frequency correction curves for repeatable results.

Why Calibration Matters

Uncalibrated phone microphones typically have ±5-10 dB error compared to a Class 2 sound level meter. The error is not uniform — most phones roll off below 100 Hz and above 8 kHz due to MEMS microphone design and automatic gain control (AGC). Calibration compensates for these known frequency-dependent errors and establishes an absolute reference level.

Method 1: Acoustic Calibrator (Recommended)

Obtain a 94 dB or 114 dB acoustic calibrator. Pistonphones and electronic calibrators produce a known SPL at a known frequency (typically 1 kHz). This is the gold standard for establishing an absolute reference.
Open SonaVyx SPL Meter at /tools/spl-meter and navigate to Settings → Calibration.
Position the calibrator. Place the calibrator directly over the phone microphone opening. For phones with multiple microphones, identify the primary microphone (usually on the bottom edge). Create an acoustic seal if possible — some adapters are available for common phone models.
Tap "Calibrate" and enter the reference level (94 dB or 114 dB). SonaVyx captures the raw signal level and calculates the correction offset. This single-frequency calibration corrects the absolute level.
Verify the calibration. Remove the calibrator and check the ambient reading against a known reference or expected value. Readings should now be within ±2 dB of a Class 2 meter.

Method 2: Comparison Against Reference Meter

Obtain a calibrated Class 2 sound level meter (even a borrowed one for 10 minutes is sufficient).
Set up a stable noise source. A speaker playing pink noise at a moderate level (70-80 dBA) works well. The noise must be stable — avoid music or speech.
Place both devices at the same position. The reference meter and phone should be within 10 cm of each other, facing the same direction, at the same height.
Read the reference meter LAeq over 30 seconds and note the value.
In SonaVyx, enter the reference reading in the calibration dialog. SonaVyx calculates the offset between its raw reading and the reference.
For frequency-dependent correction, repeat the comparison using octave band pink noise (one band at a time) or use SonaVyx's auto-calibration which captures the full spectrum correction curve in one pass.

Phone Microphone Limitations

Limitation	Impact	Mitigation
AGC (Automatic Gain Control)	Compresses loud signals, inflates quiet signals	Disable AGC if possible; SonaVyx attempts raw access
Low-frequency rolloff	Underestimates bass below 100 Hz	Per-frequency calibration curve
Dynamic range	Clips above ~100-110 dBA, noise floor ~30 dBA	Use for 40-95 dBA range only
Wind sensitivity	Outdoor readings unreliable without windscreen	Use a foam windscreen or measure indoors

Common Mistakes

Calibrating with the phone case on. Cases can attenuate high frequencies by 2-5 dB. Remove the case for calibration, or calibrate with the case you always use.
Assuming calibration is permanent. Phone firmware updates can change AGC behavior. Recalibrate after OS updates and at least monthly for ongoing projects.
Using calibration for compliance. Calibrated phone readings are suitable for screening and trend monitoring, not for regulatory compliance. Always note "approximate measurement — phone microphone" in reports.
Covering the microphone. Fingers, tables, and pockets all affect readings. Hold the phone with the microphone exposed and pointed toward the source.

Tool Bridge

Open the SonaVyx SPL Meter and go to Settings → Calibration. The calibration is stored locally and persists across sessions.