CDs have been getting louder since the mid-80s, as record labels fight to get their artists heard. Trevor Cox, Professor of Acoustic Engineering at the University of Salford, investigates
Why is modern music so loud?
You might think the answer is simple: People have turned the volume up to eleven. But it isn't just that, since the late 1980s, the music industry has been using a production trick to make songs appear louder. It created a "loudness war", as the industry pushed to make each track more impactful than the next.
"It's kind of a sonic arms race, where everyone is trying to be louder than everyone else," says Ian Shepherd, a mastering engineer, who has worked with the likes of Deep Purple, Tricky, New Order and King Crimson.
Here is an example using Michael Jackson's Thriller (first released in 1982). The music will fade through three versions; the middle and end are re-releases from the 1990s and 2000s. When the music was re-mastered for the later compilation albums, the music was made louder.
The trick being used is called dynamic range compression. It boosts quieter passages of music so that, overall, the music sounds louder.
The waveform shown in the video contains all the tell-tell signs of compression. At the beginning, you can see has peaks and troughs. By the end, everything is at an almost constant level.
Although louder music is generally more exciting, the problem with over-compression is that the sound has no light and shade. For music to work, it needs to have dynamics; loud music is more impressive when contrasted with something quieter.
The compression also adds artefacts to the music that can sound nasty. The following audio is from a piece of research by a colleague of mine at Salford University, Dr Bruno Fazenda. It gradually adds more compression onto a guitar. Initially the instrument sounds quite natural but, as more and more compression is added, a nasty buzzing sound is heard.
What studies like Dr Fazenda's have shown is that a bit of compression is good, as it evens out the sound, but too much is unpleasant.
It isn't just music producers and other audiophiles who complain about the overuse of compression - some consumers have also been protesting. But it may come as a surprise to hear that the best-known example is from heavy metal fans.
In 2008, Metallica's album Death Magnetic was made available both on CD and also via the computer game Guitar Hero. This gave fans an unusual opportunity to compare versions, with many preferring the less compressed game soundtrack over Metallica's own CD.
More than 10,000 fans signed a petition to get the album re-mastered. Here is a short snippet from one of the tracks, starting with the version from Guitar Hero and then the same section from the notoriously compressed CD.
But lately, it has seemed like the loudness war could be over.
Compression leaves distinctive signs on recordings that audio engineers can use to analyse forensically how the loudness war has progressed through the decades. Scientific studies show that the loudness of hit records kept increasing from about 1989 to around 2004. After that, the increase in loudness seems to have slowed. It is hard to be sure that the war is over, but there does at least seem to have been a ceasefire.
The move from downloading tracks and buying CDs, to listening to streaming services such as Spotify, seems likely to change things for the better.
Streaming services are usually set so that every music track appears to be equally loud. This means that if a music track is overly compressed while it is being recorded, to make it seem louder, once it is passed through the streaming service, the loudness advantage will be gone, and all that would be left is the nasty distortion.
The incentive for ever louder tracks may soon disappear.
A BBC Radio 4 documentary on the Loudness Wars, called Compression versus Art, is available on the iPlayer.