Compressed audio is not new and affects both digital and analog domains. The rise of portable systems, however, has accentuated the use of this audio compression so that the listener continues to perceive all the musical information, even in a noisy environment. A common practice adopted by producers and DSP systems, but which is not without risks for hearing as a study on guinea pigs has just shown.
What is compressed audio?
In the collective unconscious, compressed music refers to a digital audio file from which part of the musical information is removed to reduce the file size. This is the case of the MP3 format, for example. The result is generally unflattering, with an imprecise, shrill sound and an absence of vitality that quickly unmasks the applied compression.
However, there is a second type of compression that is much more difficult to detect, because it preserves all of the musical information. This compression acts on the dynamic range of the song by increasing the level of low intensity sounds and/or decreasing those of higher intensity. In other words, the variation between loud and quiet sounds are reduced. It is this second scenario that presents the highest risk for hearing.
Why compress dynamics in music?
Dynamic range compression in music allows the listener to hear whispers and more subtle details more effectively, without having to crank up the volume when in a noisy environment. This can be compared to the dynamic range compression modes used in home theater. Without this compression, music can suffer a significant difference in volume between voices, instruments and different nuances. In a noisy environment, it may then be necessary to increase the volume to hear the quietest sounds. Therefore, one might think that dynamic range compression preserves hearing by reducing the volume necessary to be able to properly hear all the information. However, the reality is much more complex and less positive.
How does compression affect hearing?
Sound compression has become commonplace, be it on the radio, on television, in concert halls, on video game consoles or even on certain online music services. An omnipresence that does not give your ears a break, with weaker nuances and increased acoustic energy. A potential danger that researchers from Inserm and the Faculty of Medicine of Clermont-Ferrand wanted to demonstrate.
To find out whether compressed sounds are a danger to hearing, researchers made 90 guinea pigs listen to music for several hours. The latter have a very similar hearing system to humans. One part of the panel was given music with a compressed dynamic range, while the other half of the guinea pigs listened to music without compression. The playlist was very diverse, with pop, classical and electronic music to cover all tastes and to not bias the experience with a single genre.
Listening to compressed music caused auditory fatigue that lasted for more than 48 hours in guinea pigs
Subsequently, an ENT examination revealed auditory fatigue that lasted for more than 48 hours for the guinea pigs having listened to compressed music. None of the guinea pigs suffered hearing loss or irreversible damage, but the protective muscles inside the ear were weakened. Professor Paul Avan who led the research explains this result by the absence of silence in the compressed sounds. There are no more variations, no more pauses, not even a millisecond of silence. The auditory system is continuously stimulated and has no time to recover. It is like looking at a fixed spot without blinking.
Sound compression in high fidelity
Dynamic compression affects all fields, but high fidelity remains the most spared, provided you opt for high quality media and files such as SACD, DSD or Hi-Res music. Even the latter are not entirely exempt, as producers and sound professionals are increasingly resorting to this compression. An older production is therefore more likely to maintain a full dynamic range compared to current pop songs.
The same track can also undergo a different dynamic compression depending on the system in charge of its reproduction. For example, the filtering of some speakers will enhance the treble a little, so that the level of all the high frequency sounds reproduced will be a little higher than initially. This characteristic is also found with certain horn speakers for which the highs and/or midrange is always a little higher and louder. The result is very flattering, but for prolonged listening sessions, the risk of hearing fatigue is increased. You should also be wary of certain DSP modes that modify the response curve and compress the dynamics. This is notably the case of many connected or Bluetooth speakers that deliberately accentuate the bass level.
To avoid this while on the go, you can opt for a DAP with reference files whose dynamics are preserved. Choosing a Hi-Res music service that does not apply additional filters to music is also a safer bet depending on the productions. Finally, as the music regains its full dynamic range, the more subtle nuances may become less audible in a noisy environment. Active noise cancelling headphones can be an excellent way to enjoy the full range of information without having to turn up the volume.