iTunes Radio Normalizes Playback Volume with Sound Check

I’ve seen some reports online suggesting that iTunes Radio uses Sound Check, Apple’s volume-normalizing feature, to keep music streamed on iTunes Radio at a consistent relative volume. Since these claims present no data or examples to back them up, I decided to have a listen and see if I could determine whether iTunes Radio is, indeed, using Sound Check.

For those unfamiliar with Sound Check, it’s a feature in iTunes that normalizes the volume when you play music through the app. You can activate it be checking a setting in iTunes’ Playback preferences, and it’s also available on iOS, in Settings > Music > Sound Check, and on the Apple TV, in Settings > Audio & Video > Sound Check.


If you turn Sound Check on, iTunes examines all your music to determine how much it deviates from a norm:


When it’s finished, you can see, for each track, whether the music’s volume needs to be increased or decreased when using Sound Check; this song needs to be reduced by 5.0 dB if Sound Check is active:


When you play music or podcasts from your iTunes library, iTunes takes into account those volume changes, playing music softer or louder as required. Volume differences also show up for audiobooks,

You can notice Sound Check if you play, say, an opera, where tracks segue into each other. You’ll notice obvious changes in volume from track to track, if their average volume is very different.

So what is iTunes Radio doing? Quite simply, it’s lowering the volume of loud songs, sometimes drastically. In order to show this, I recorded some iTunes Radio streams, and compared them with previews from the iTunes Store, where Sound Check is certainly not turned on. I then compared some CD rips and purchased music I have with iTunes Radio streams. In all of these cases, iTunes’ volume was the same.

I’ll just show you one example; Miley Cyrus’ Wrecking Ball, an ode to overly loud production techniques. The first part of the waveform below is the chorus of the song, from an iTunes Radio stream; the second part is the middle of the chorus, then the quieter part, from an iTunes Store preview:


As you can see, the volume is drastically higher in the iTunes Store preview. It’s also a bit more flexible; look at the arrows, which show the same points in the song. That little notch is where Miley’s voice pauses: “Yea you … Wre-e-eck me.” You can see in the second sample that the waveform is less flat than in the iTunes Radio sample.

None of this is definitive. You could argue that it’s really not clear if I’m comparing the same things. That there’s no way to know exactly what the iTunes Radio stream contains; whether it’s the same format as iTunes Store previews, or the same format as the music in my iTunes library that I compared (AAC 256 kbps). But listening to iTunes Radio, it is very obvious when certain songs come on – again, the current hits by Lady Gaga and Miley Cyrus – that they are muted.

But it’s also obvious in tracks such as The Allmann Brothers’ Ramblin’ Man, Bob Weir’s Black Throated Wind, and several others that I compared. The volume change isn’t only for the latest over-produced hits; it’s for anything that has a volume that is a bit higher than normal.

iTunes Radio also raises volume of softer music. Here’s a piece by Steve Reich, the second movement, Slow, of his Double Sextet. The first part is from an iTunes Radio stream, the second is from my iTunes library, a file that I bought from the iTunes Store. With Sound Check on, iTunes says that this track needs a 6.0 dB boost, and you can see it well.


The arrow highlights the louder first chord of the movement. The first one is the iTunes Radio stream, the second the beginning of me playing my own file. There is a marked difference both in the volume of that chord, and in the entire playback.

Given both anecdotal evidence (loud songs by certain artists sound softer on iTunes Radio), and the above tests, it’s pretty clear that iTunes Radio is using Sound Check. Is this a good or a bad thing? If you like the sound of Miley Cyrus and Lady Gaga, you’ll probably be turning up the volume when their songs come on. You’ll also find that quieter music is played a bit louder.

I don’t use Sound Check because I’ve found that some music gets distorted if Sound Check alters its volume too much. But this is radio; I’m less likely to care exactly how something should sound.

On the other hand, Apple is clearly saying that overly loud music doesn’t have a place on iTunes Radio. Will music suddenly become less loud? I doubt it; iTunes Radio doesn’t have the power to change the way music is produced. However, it’s possible that some music producers will take note of this, and some may just reconsider how much volume they’re putting into their music.