Hi cuibono,

I wouldn't worry about the different noise levels you mentioned (-75dB, -87dB, ...). Trust your ears and you'll be more than satisfied.

I have done a lot, doing right now some and will possibly doing in the future some more FFTs (programming and applications using it) and it's most of the time a question of the used window type (before you calculate the FFT itself) the used "processing unit" (i.e. an 8 bit micro controller, a fixed point DSP or, meanwhile, a floating-point DSP) and further methods (like time averaging of the FFT bins and so on).

Since you mentioned the Zoom H2 (where I'm not familiar with) as the recording source, capable of recording with 24 bit at 96kHz (that's all I could figure out in a hurry about this little thingy), I assume the issue will not be the ADC. It's more likely the algorithm/method of deriving the FFT from the input data (this points directly to the window application). There are certain types of windows used prior to run the FFT. For instance Blackman-, Hamming-, von Hann- (aka Hanning), Kaiser-Bessel-, etc., just to name a few of them. These windows determine, beside other things, the hight of the sidelobes - or the noise floor. Whatever your special interest is about the signal you want to put your magnifying glass on, this determines the proper selection of your window type.

As long as you don't have the choice to select the window type, or to get an info about what window type is used in a particular window application, I would be very carefully when interpreting such results.

If you're still interested / keen on it in reading more about FFTs and why windowing of the input data is necessary just 'google' and look for "FFT" and "windowing". You should come up with a couple a pages explaining it for you.

After all, my final rule would be: Never trust what you haven't cheated or manipulated yourself