If you suspect that songs today tend to sound the same, it turns out you're right. A group of Spanish scientists looked at a huge database of songs and analyzed their trends, publishing their results in the scientific journal Nature. What they found was proof positive that, over the last few decades, songs have progressively gotten louder, decreased their pitch transitions, and generally become more homogeneous.
The scientists looked at the Million Song Dataset, a database maintained by Columbia University that collects data from over a million songs recorded since 1955, including tempo, volume and the pitches of the notes used in each song. By comparing the evolution of this data over time, they found three remarkable trends.
First off, it seems like popular music is getting louder: in other words, if you play two recordings, with your stereo set at the same volume, chances are the latest recording will make the most noise. The researchers say this isn't due to higher-quality equipment but because of a conscious decision by producers and sound engineers to catch the attention of more listeners.
The second trend that was detected was the restriction of pitch transitions, with metrics showing less variety in pitch progressions. Pitch roughly corresponds to the harmonic content of the piece, including its chords, melody, and tonal arrangements.
Lastly, the researchers detected a trend of homogenization of the timbral palette. Timbre is what makes a particular musical sound different from another, even when they have the same pitch and loudness. It is essentially the difference between different instruments playing the same note at the same loudness. They found that, after peaking in the mid 60s, timbral variety has continued to narrow.
The researchers also go as far as to say that you could probably take a typical song from the 60s and modify it – i.e., increasing the average loudness, using common harmonic progressions and changing the instruments – to make it sound remarkably like the latest summer hit. Or, if you were so inclined, you could do the opposite just as easily.
Harder to explain is the motivation behind those trends. It could be argued that, after the experimentation of the 60s, musicians have been gradually, to some extent, "figured out" what sells the most records. Perhaps increasing the loudness really could be likened to a sort of marketing strategy that exposes more listeners (and potential buyers) to a music product. And if you believe the advertiser's maxim that "if you hear something enough times, you will eventually buy it," the homogenization and repetition of the sound transitions could be also be playing a role in this sense.
Whatever the cause, we can probably expect a few "I told you so" comments from baby boomers who have long claimed there is a lack of creativity and originality in modern music and that "things were better in my day."