Import the normalised “low level” file into a fresh Audacity Project.
Select the entire track, then select the “Amplify” effect.
The Amplify effect should offer to amplify by +2 dB.
Apply the effect, then look carefully at the waveform. I suspect that somewhere in the track there is a small spike that has an unusually high amplitude (note that “spikes” may be up or down).
Could you see if you are able to reproduce the problem? - perhaps generate some short tones from the Generate menu so that you know exactly what there peak amplitude is and export them as files. If you are able to reproduce the problem, please give step by step details of what you are doing.
I think I’ve found the problem.
As a workaround, set the following Preference and I think Normalize will work correctly:
Edit menu > Preferences > Import/Export
“When importing audio files” > “Make a copy”
It’s a pretty obscure bug that requires a specific series of conditions (hence it took me a while to pin down how to reproduce the issue).
I’ve added a bug report and the details are here: http://bugzilla.audacityteam.org/show_bug.cgi?id=489 so the bug will be picked up and fixed in due course.
Unless you are frequently working on very long audio files it is probably best to leave the Preference setting to “When importing audio files” > “Make a copy”. Not only will that work around this bug but it is a generally safer option.
Thanks again, Steve; sorry to create more work for you but I appreciate the further info.
I guess what is troubling me is that the the Normalise function appears to work properly as a standalone function, suggesting to my inexperienced eye there may be something broader wrong with the way Chains are initialised.
Anyway, I think I’ll proceed with caution for now and see what the developers come up with.
I don’t think the problem is with “the way Chains are initialised”.
The fact that you saw the problem with the first file in the batch was a red herring.
The conditions that make the issue occur are:
“Read Directly” is selected in Preferences.
The warning message for importing uncompressed audio is disabled.
The imported file is in an uncompressed format (for example .WAV)
The file is long enough that it is not automatically copied.
The original peak level is not 0 dB (otherwise the bug is invisible).
The Normalize effect is used in a Chain.
The Chain is applied to files (not the current project)
What I think is happening (or rather “not happening”) is that the Chain command is failing to pass the peak level to the Normalize effect, so Normalize assumes a default peak level of 0 dB. The Chains code was updated a while back and I suspect that the developer simply forgot about passing this data to Normalize. I think that Normalize is the only effect that actually needs to know the peak level of the audio. The only other effect that I think may possibly have a similar problem is the Noise Removal effect but I would imagine that it will be obvious to the developer if this is the case or not when they re-examine the code.