Throughout the nine month history of the 25+ newsletter, we’ve been flagging the landmarks of a changing Adult Top 40 format, from the week that Eminem’s “Love The Way You Lie” debuted on the Hot AC chart to the week that the only two debuts on that chart were rap records. And now we can add one more.

The souped-up pop and rhythmic sound that has made its way over from Mainstream to Adult Top 40 in recent years was the biggest piece of Radio-Info/Mediaguide’s Hot AC chart last week, finally edging out the pop/acoustic Modern AC sound that had not only defined the format for most of the past 15 years but, in some cases, had become the entire format.

Call the new sound of Adult Top 40 (and Mainstream CHR) “Turbo-Pop”—the aggressive pop/techno sound defined by producers Max Martin and Dr. Luke that lives somewhere on the cusp of pop, dance, and R&B. It’s a coinage that I’m surprised hasn’t been used more heavily already; (the only reason it’s not perfect is that it invokes a turboprop aircraft when the music is more like a G6, like a G6).

Even if straight-ahead pop without a rock element had become the dominant sound of the format, it would be big news. Instead, most of the artists who had established a beachhead for pure pop followed their producers to a more aggressive sound. When Katy Perry’s “Hot N Cold” became a major record for not just Hot AC but Mainstream AC, it became a foregone conclusion.

Here’s how last week’s Adult Top 40 chart breaks down:

Turbo-Pop – The aggressive Max Martin/Dr. Luke-driven sound on the cusp of pop, dance, and R&B (current hits by Avril Lavigne, Pink’s “Raise Your Glass,” Katy Perry, Enrique Iglesias, Usher, Lady Gaga, Chris Brown, etc.) – 13 songs;

Modern AC – The center lane of Hot AC/Adult Top 40 for more than a decade, usually pop/rock with acoustic elements (Script, Christina Perri, Train, Onerepublic, Colbie Caillat, Sara Bareilles) – 12 songs;

Alternative Crossovers – Bands with an edgier sound or more rock cred than the Modern AC group (Linkin Park, Stone Sour, My Chemical Romance, etc.) – five songs;

Mainstream Pop – Straight ahead pop that’s not quite aggressive enough to go in the “Turbo-Pop” stack, although the line is pretty blurry (the two Bruno Mars hits, Pink’s “F**kin’ Perfect,” at least sonically, Cee-Lo) – four titles, and, again, they could go with the first category;

Indie Pop – Often songs that started at Alternative and Triple-A more because of the artist’s pedigree than because they were really rock (Adele, Mumford & Sons, Florence + the Machine, etc.) – four songs;

Country – Either Taylor Swift or Sugarland’s current hits could easily fit in different categories, but together songs that started at Country account for two titles.

Categorizing songs is always highly subjective, of course. Some might argue that some of the subcategories here could be merged. And there are still individual stations that stick more to the traditional pop/rock feel. But there’s an overall sea change any way you look at it. And then the question becomes, does it sustain?

There are already early signs of another possible change in the offing. Adele’s “Rolling In The Deep” is a potentially significant record for this format. “Rolling In The Deep” has the advantage of being a great pop song at the core with an aura of something different surrounding it. It sounds like a hit record, but, unlike a lot of songs on both CHR and Hot AC, it doesn’t sound like any other hit record.

It will be interesting to see what kind of inroads “Rolling” is able to make at Mainstream Top 40. It will also be interesting to watch the indie pop/rock sound in general at Adult and Mainstream Top 40. Through the years, there have always been a handful of pure pop records that petered out halfway up the Alternative chart without ever making it to pop radio because of artist image. Now the Florence + the Machine-type records are going further at Alternative than at any time since the pre-Nirvana era.

If “Turbo-Pop” represents about a third of the Hot AC chart, it controls most of the real-estate on the Mainstream Top 40 chart. And that’s probably good for Adult Top 40. After 20 years as a positioner, “Today’s Best Music” has finally become the most potent thing that stations can offer even an adult audience, as well as the thing that unifies a family reunion’s worth of age groups, none of whom love the music of each other’s high-school years. The “yesterday and today” mix that used to set Hot AC apart from Adult Top 40 is harder to pull off. But “variety” is probably still viable and will now manifest itself at Hot AC not through a mix of eras, but through a mix of styles, in which both Usher and Mumford & Sons can be accommodated.