A mix can sound wide on headphones and collapse in mono. The meter reads an audio file in the browser and measures the correlation between the left and right channel, the share of the side signal, the level a mono sum loses and the balance of the channels, in total and in three frequency bands, plus the correlation over time.
Drop an audio file here
The file stays in the browser; nothing is uploaded. WAV, FLAC, MP3, M4A, OGG and Opus, as far as the browser plays them.
The correlation compares the left and right channel: +1 means the same signal (unchanged in mono), 0 means independent, −1 means opposite phase, where the mono sum cancels out. In the vectorscope of FL Studio a centred mono signal appears as a vertical line, while a horizontal line means the channels are 180 degrees out of phase.
Mid is (L + R) ÷ 2, side is (L − R) ÷ 2. The value given is the distance between both RMS levels: −6 dB is narrow, 0 dB means side and mid are equally loud. According to the FL Studio manual, stereo tools work on exactly this difference between the channels.
The mono sum is the level of (L + R) ÷ 2 against the RMS level of the stereo signal. 0 dB means nothing is lost; independent channels lose around 3 dB, out-of-phase content more.
The bands come from second-order high-pass and low-pass filters, identical for both channels. This shows whether only the highs are wide or the bass drifts apart as well.
Sources: Wave Candy · Fruity Stereo Enhancer
Everything is calculated in the browser; the file never leaves the computer. The whole file is measured, so short out-of-phase passages hardly show in the total and appear in the curve instead. Channels beyond the first two are ignored.