Built-in Microphone

The iPhone's built-in microphone (not available on iPod touch devices) supports units of FS (full-scale, or maximum input before clipping) and Pa (pascals). It does not support any other engineering units, like the headset or dock inputs do.

• Using the built-in microphone on the original iPhone is not recommended for measurements which require full audio bandwidth or a flat input frequency response. The built-in microphone of the original iPhone appears to be routed through a speech-processing codec, which limits the sample rate to 8 kHz and significantly degrades the frequency response.
• The iPhone 3G rolls off the low-frequency response of the built-in microphone below 100 Hz.

The default pressure sensitivity of the built-in mic is 3.85 Pa/FS, which should get most iPhone 3G devices within 1 or 2 dB of the actual sound level. The accuracy of the measurement does depend on the nature of the acoustic signal, however. For example, an acoustic signal that is dominated by low-frequency energy (below 100 Hz) will appear to have a lower sound level when measured by the iPhone 3G's built-in microphone (see above).

The pressure sensitivity of the built-in microphone is synchronized with the device sensitivity (the two are always the same, so calibrating one calibrates the other).