What is a VU meter?
The VU (volume unit) meter shows the audio signal level from any connected audio input. Audio inputs can be from any of the iPhone's internal microphones, external Lightning connected sources or external analog sources connected via the headphone connector.
The VU meter is designed to enable you to aim the audio signal level at a target level so your captured sound is audible and not distorted. The VU meter measures an input signal using decibels relative to full scale or dBFS. Using dBFS you should aim to target your signal around -12 dBFS with maximum peaks around -6 to -3 dBFS.
The VU meter has 3 main readouts.
- 1. The main signal level readout
- 2. The peak level indicator which is displayed as a small line above the main signal level
- 3. An overload indicator which is displayed as a red line above 0 dBFS. This only becomes active when the audio peaks above 0 dBFS
A good signal
The animation above shows a good input signal level from a connected audio input. The main signal level is hovering around -12 dBFS with peaks around -6 dBFS. Looking at the animation we can see the scale on the left is non-linear (-60 to -40 dBFS is displayed as a small range whereas -12 to -6 dBFS is displayed as a large range). This non-linear scale has been designed to make it easier to see the range closer to 0 dBFS and to give better control over any adjustments to the input level.
The animation also shows the peak level indicator is often quite far above the main signal level. This is because the number of audio input samples being analysed is more than the time we have to gracefully animate the main signal level and, because of this, any fast peaks are only shown using the peak level indicator. The peak level indicator is also held at any high points for a short amount of time so it is easier to see what level the peaks are reaching. During the time the peak level indicator is held the main signal level can fall back down.
A poor signal
The animation above shows a poor input signal level from a connected audio input. The main signal level is regularly above -6 dBFS with peaks hitting 0 dBFS. Using decibels relative to full scale, 0 dBFS is assigned to the maximum possible digital level and anything over 0 dBFS will result in clipping or distortion to your audio. With signal levels over 0 dBFS the overload indicator is activated. This indicator is a warning that your audio is being clipped and probably distorted.
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