How an iPhone Microphone Is Turned Into a Decibel Meter

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A developer at W Lab describes building Noise Meter, an iPhone app that displays sound levels in decibels and logs them overnight. The core challenge is disabling iOS's default microphone processing, such as gain control and noise suppression, which would distort readings. The app applies standard frequency weightings (A, C, and Z) using filters derived from IEC 61672-1, adapted to the device's sample rate. Measurements are averaged into per-second records that track both energy-based equivalent levels and short-term peaks. The post also explains that dBFS readings must be converted to dB SPL to match standard sound level meter output.

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