ios - FFT and decibel scales -

ios - FFT and decibel scales -

if take sound info on iphone (i.e. real) data, perform fft , take magnitudes (re^2 + im^2).

these vary >0 big numbers, 10log(n) in db.

this gives me outputs negative (for inputs < 1) positive.

but examples i've seen (and drawing spectrum in sonic visualiser) have positive spectrums when measured in db.

so have missed?!

on wider note, understand decibels ratio, in context when turning fft magnitudes db, ratio to?

the simple reply that, part, you can add together arbitrary number db value create values positive, or negative, or whatever prefer. uncalibrated microphone, on iphone, makes sense anyway, since know relative values.

for more advanced technical approach, using calibrated microphone, reference using db (spl), reasonable standard, hassle, , not meaningful in utilize case anyway.

rationale: main reason shifting arbitrary amount log doesn't study units of measurement. example, if know input amplitude 0.1 pascal, it's valid 100 millipascal, you'd taking log of 100 rather 0.1 (so log values either 2 or -1). both valid , selection exclusively arbitrary. when comparing standard reference, in db spl, note it's done ratio, log(p/pref), removing impact of changing units.

ios signal-processing fft