Audacity is an open source audio editing software which supports multiple platforms, such as Linux, Mac and Windows. That’s my favorite audio editing tool.

Today I found a problem when using Audacity: when I concatenate 2 mp3 audio files into one mp3 file, one of the two parts in the concratenated file sounds very odd, like being played in a very slow velocity.

Actually what I did is to open the two files in Audacity, select all of file-2, copy and paste it at the end of file-1. After that, the file-2 part in the concatenated file got the problem I described. I tried another way: copy all of the file-1 part and paste it to the beginning of file-2, the same problem happened except this time the file-1 part in the concatenated file sounds like being played in the a very fast velocity.

I tried changing the format of the two files from mp3 to wav and concatenating the wav files, and I got the same problem. And I also tried using the same zoom rate of the two editing windows(It will be ridiculous it it works, but I did try it). Neither of them works.

And finally I found this is caused by the “sampling rate“. It seems that: when concatenating two files if they’re using different sampling rate, Audacity will use the sampling rate of the first file, and adjust the second file’s to fit it by zooming in/out its playing velocity. In my case, file-1’s sampling rate is 8000Hz, file-2’s sampling rate is 44100Hz, when copying file-2 to file-1, the playing velocity of the file-2 part in the concatenated file is about 18% (8000/44100) as the original velocity.

And the fix is simple when I know the root cause: change the sampling rates of the files to make them using the same value, such as 44100Hz, by choosing it in the dropdown list of the Audacity file window in the left-bottom corner, like this: