The fictional band Spinal Tap could make their instruments louder with the help of amplifiers that went up to 11. Lesser musicians must find other ways to pump up the volume. One well-established trick is compression, which makes music sound fuller by hushing the loudest parts of a track and making quiet parts noisier.
Used since the 1930s, compression is now common in the music industry, streaming, radio and television. Long suspected to have links with hearing damage, there has been little experimental evidence to support concerns. Now research in guinea pigs has shown that compressed music can damage the ears in ways that regular music does not. The research, though preliminary, suggests that there may be cause to worry about the harmful effects of compression.
The composer Claude Debussy called music the space between the notes. As well as offering structure and distinctive phrasing, these pauses give the listener’s brain vital rests that help auditory neurons recover. Compressed music interferes with this recovery because making the quiet parts louder can fill many millisecond-long gaps in the signal with noise. As a result many music aficionados find listening to compressed tunes exhausting.
To test whether compression can negatively affect hearing, scientists turned to Adele’s 2015 single “I Miss You”, a song whose distinctive frequency spectrum results in all restorative pauses being eliminated after compression. They also turned to guinea pigs, animals that can hear similar sound frequencies to people while also being content to sit placidly for four hours while played the same song on a loop.
The guinea pigs were split into two groups. One group listened to the regular track, while the others were played a compressed version. Importantly, the music was played to both groups at an average volume of 102 decibels—uncomfortably loud but just below Britain’s Health and Safety Executive’s recommended maximum average for live music.
Tests of the cochlea, damage to which is the leading cause of hearing loss, showed some mild temporary impairment in both groups immediately after the tests, as would be expected. But compression caused more lasting damage to the middle ear’s stapedius muscle, which contracts to protect the inner ear from loud noises. At just 1mm long it is the smallest skeletal muscle in the body.
Both normal and compressed music reduced the strength with which this muscle reflexively contracts to 40% of its pre-Adele state. Though the animals who heard the standard track recovered fully within a day or so, those that endured the compressed version did not. Their stapedius muscle reflexes were still at less than half their strength by the time the experiment ended a week later.
Writing in the journal Hearing Research the scientists, led by Paul Avan, an audiologist at the Pasteur Institute in Paris, suggest the constant stimulus of the compressed music overwhelmed nerve cells in the animals’ auditory processing pathways, affecting their ability to use the muscle.
Although the study does not address the level at which compression starts to be harmful, nor how long the effects could last (nor, for that matter, whether humans react as guinea pigs do), the results do suggest that average decibel level might not be the only harmful property of music.
