Because we insert ads into the mp3 files, it is incredibly important for every episode to have the same “apparent loudness”. This is done through two basic audio editing techniques. Dynamic Range Compression and Normalization.

In general, compressing and normalizing to 0dB is an audio engineering “rule of thumb”, so that all your files will be at the same apparent loudness. In this case it is more critical because the ad engine is going to be inserting ads that have all been compressed and normalized to 0dB into your files so this becomes more than just a good idea, it must become a standard practice.

Most audio editing programs have this option as a built in feature since this practice is an audio engineering “standard”.

In Audacity (free download for both Mac and PC) you can compress and normalize the entire file in one step.  Simply highlight the track, click on “Effect” the select “Compressor”. Leave the default settings the way they are (including Normalize to 0dB after compressing), click “OK” and you are done.

If you don’t “compress and normalize to 0dB” here’s what can happen after the ad insertion engine puts the ads in.  In the waveform below the first 30 seconds is where the ad insertion engine put a pre-roll ad. The rest of the waveform is the “show” or “audio  program”. When the user listens to this the audio ad will sound approximate twice as loud as the rest of the audio program. The only way to avoid this is to compress and normalize to 0dB so your show is the at the same level as the ads that will be inserted. (click on the image to see full size)

As you can see the first 30 seconds looks very solid. All of our ads are compressed and normalized to 0dB. This is why the programs must be compressed and normalized to 0dB too.