Logarithmic Scale Question

I just have a small theoretical question that buggers me to be honest.

I have a sound file that I found its spectrum on linear and on logarithmic scale. Alright, I have done this, it's a plain standard procedure. The problem is that I cant really understand the plot on logarithmic scale. (it just looks like a typical logarithmic plot with dB, and it is correct by the way) .

Well, the visual differences are obvious, the linear plot frequencies are quite scattered and they peak in a non homogenous way, while I notice that on log scale the frequencies peaks are very close to each other and they don't seem to have so many extreme values. Could someone explain me this a bit ?

Cheers

• Can you upload your plots somewhere? – Phonon Oct 22 '13 at 18:09
• @Phonon yep sure. (logarithmic here) imgur.com/BNue8ko and linear here:imgur.com/dY7GjXL – Thomas Brd Oct 22 '13 at 19:04
• I added your images to the post. Please annotate them accordingly. – Phonon Oct 22 '13 at 19:14

2 Answers

The meaning of the logarithmic plot is that the signal power above 250 Hz is about ${10^{-10}}$ times less that the starting values (most of the power is concentrated at the lowest frequencies), so it's almost zero. The capability of a logarithmic plot is to show both, the high and low values at the same time, specially useful in acoustic field. The negative aspect is the visual distortion of the signal, you must always keep in mind that what are you looking at is a logarithmic plot.

I have a little addition to Sturm's response you can use as a rule of thumb to better imagine what is happening in the logarithmic domain. When looking at powers, you can say that -3dB means half the power, -10dB means 1/10th of power etc. Or, vice versa, 3dB means double, 10dB means ten times the power and so on.

This will give you a way better impression of how low the powers really are.