How to Null Test Two Audio Files in Audacity

Did your editor change the audio when you exported it? A null test lets you listen to the difference between two files. Flip the polarity of one file and play it together with the other. Wherever the samples match, a positive value and its negative add up to zero.

Run a control test first

Import one WAV into a new Audacity project twice. Leave both tracks at their original gain and pan, and start both clips at the same point. Select only the second track and choose Effect → Special → Invert. Play the tracks together at a comfortable listening level. You should hear silence and see no output on the meter. If you hear the original audio, check that both tracks are playing and that you inverted only one. The Audacity manual explains how Invert works.

Need a file to try? Open the WAV format examples and download the 24-bit mono sample. Import the same download twice for the control. For the next test, export a copy of the original and compare the two files.

Compare the original and the export

1. Start with matching settings. Open both WAVs in the WAV file inspector to check their sample rates and channel counts. A mismatch can cause conversion during playback or mixing. Keep left and right channels paired in the same order.
2. Line up the same sound. Import the original and the export. Zoom in on a sharp sound that appears in both, such as a clap or a drum hit. Align that point as closely as you can. Matching the clip edges may leave an offset if one export has leading silence.
3. Invert one track and listen. Leave track gain and pan unchanged when testing whether the exports match at their original levels. Play them together or use Audacity’s Mix and Render to New Track command to inspect the difference waveform.

Try one change you can hear

After your duplicate WAV cancels, undo the inversion and lower the gain of one track by 1 dB. Invert that track again. You should now hear a quieter copy of the original because the two signals differ in level. Restore the gain and check that the tracks cancel again. Then try nudging one track slightly. The difference you hear depends on the timing shift and on the sound itself. These two controls give you a reference for diagnosing a real export.

Make sense of the difference

• You can hear most of the original. Check alignment, gain, channel order, and whether you inverted the correct track.
• You hear a thin or echo-like sound. Check for a small timing offset before blaming an export setting.
• You hear a faint difference after an MP3 export. Lossy encoding changes samples. Compare the decoded result by ear at matching playback levels if you’re deciding which export to use.
• The result is silent. The audio cancels under your current settings. The two files can still differ in metadata, file size, or other information outside the audio samples.

Know what the test can prove

Keep the untouched export and note the project sample rate and any gain or timing changes you made to run the comparison. If you aligned the clips by moving one of them, the test tells you how closely the aligned samples match. It cannot tell you that the exports started at the same time. Check the original clip positions separately when sync is part of the delivery requirement.

A null test tells you where audio differs. It takes a separate listening test to decide whether that difference matters. If you’re comparing two recordings from different microphones, even a small change in timing or placement can leave a clear residual.