Null testing

The idea behind null-testing is simple.
Record a tune with a AD converter and you end up with a file.
Change something in the system, add an audiophile power cord, an audiophile switch.
Record again and you have a second file  and the only difference is the change you made.
Compare these two files by subtracting them.
If the result is null, there if of course no difference.
If there is, you can listen to this difference, tells you what this change has done.

Practice is a bit more complex.
Both recordings must be perfectly time aligned.
Clock drift is another phenomenon.
In the past, AudioDiffmaker was the only one known to me, being able to do this kind of null testing.
However, it latest version is from 2008.
There is a new kid on the block: DeltaWave


Matching analysis

To perform a full matching operation, simply select the two files, as before, but now press Match button. This will take a while, depending on the speed and memory of your computer, as well as the size of files you are trying to match.



Notice the results displayed in the status below the plots, in the status. These are the parameters that DeltaWave was able to compute as the differences between the two files:

Value Explanation
Volume diff (in dB) is the amplitude difference between Reference and Comparison file. 0dB means the two files have the same level
Phase Offset (in milliseconds), is the fractional offset between the samples of the two files. 0 means there is no offset
Difference (rms) (in dB) is the difference between the two files after phase and amplitudes have been matched, computed as root-mean-squared value. The larger the negative number, the better the match (-300dB means the files are the same).
Correlated Null Depth (in dB) indicates how closely the files null each other. The greater the number, the better null match.
Phase Drift (in ppm) is the detected clock drift between the two files. 0 means no drift. Any value above zero means the clocks in the two files diverge.


Beta-test: DeltaWave Null Comparison software - Audio Science Review

Audio DiffMaker

Audio DiffMaker is a freeware tool set intended to help determine the absolute difference between two audio recordings, while neglecting differences due to level difference, time synchronization, or simple linear frequency responses.

The difference recording that results is only what has changed between the two recordings. If anything - a change of component, a treatment, mechanical damping, etc. - is having any audible effect on the audio signal in a system, the difference recording will have audible content. The end result is primarily intended to be evaluated by ear.