Spectroid is a real-time audio spectrum analyzer with reasonable frequency resolution across the the entire frequency spectrum.
Q: Why are the dB values negative?
A: Spectroid uses dBFS (Full Scale) where 0 dB is the maximum power that the microphone can measure, so the decibel values are negative because the measured power is less than the maximum power.
Q: Can I zoom in on the spectrum plot?
A: Yes, do a two-finger pinch-to-zoom gesture.
Q: Why are there discontinuities/gaps in the spectrum plot and waterfall?
A: Spectroid uses multiple FFTs overlapped in frequency in order to provide better frequency resolution at lower frequencies than a single FFT. The caveat of this method is varied impulse response and minor discontinuities in frequency. The upside is that it can efficiently produce a spectrum that better matches the frequency resolution of human audio perception. It's still probably not as good as your ears though!