Understanding the Plot Spectrum graph better on Analysis

Hi, I build mandolins and use Audacity Vers 2.0.5 on Windows XP to record frequencies of components of the instruments when I am shaping them by tapping the component and recording ten or so taps, I them view the Plot Spectrum graph in Analysis, to record the frequency of the major tone. I also record plucking of the individual strings when the instrument is completed to check that there are no or minimal overtones (or undertones). Up until now, I have usually just looked at the graph and can see the major frequency easily and any over and under tones, but notice that the vertical axis of the graph in dB’s shows negative dB values, with what I considered to be the loudest or most prominent frequency to be the smallest negative value (if that makes sense). My question is why are the readings negative dB’s and also sometimes when I look at a “peak”, the graph shows a lower value than listed in the panel below the graph e.g., Peak looks like say -40dB on the graph, but is recorded as a different value in the panel below the graph.

I hope this is clear, I really would like to understand the Plot Spectrum better.

Greg Dunn

Yes, you do have to pay attention. Plot Spectrum changes range to make the display give you the maximum information possible. That’s usually more confusing than it needs to be. That might be an Audacity change request.

The numbers go from zero which is the maximum loudness before the digital system “runs out of numbers” and stops following the sound. Maximum quietness depends on the bit depth. 16 bit sound stops getting quiet at -96dB which is super quiet.

I saw recently where somebody posted the sound volume upside down. The problem with that is most of the show ends up in hard-to remember big numbers. Maximum volume is 96 and general recording volume should be between 85 and 90.

Come back in ten minutes and tell me what those three numbers were without looking. You get used to the negative numbers.

I’m wondering why you think overtones are a bad thing. Overtones are the difference between violin qualities. With no overtones, everything would sound like a flute.

I need to come back.