Millions of people are risking their hearing with prolonged exposure to loud music from MP3 players and smart phones. A new earphone technology developed by dB Logic of Indianapolis, Indiana, aims to prevent such damage by limiting the maximum volume of music, and is able to do so without distorting louder passages or making soft sounds inaudible.
People have been listening to portable music players via headphones since 1979, when Sony introduced the Walkman. But the widespread adoption of digital music players over the last decade represents a new public-health hazard, because digital devices store so much music that users can listen without interruption for hours. In the era of portable CD players, users “listened to seven hours, at most,” per week, says Brian Fligor, director of diagnostic audiology at Children’s Hospital Boston and an expert on acquired hearing loss from portable media players. Fligor recently studied digital-player users in New York City and found that “the average person was listening for 18 hours per week. We had some people who were listening upwards of 70 hours per week.”
Users risk hearing loss when they combine long listening times with high volume. An investigation sponsored by the European Commission found that digital music players produce maximum sound levels ranging from 88 to 113 decibels, with levels of up to 120 dB possible depending on the type and positioning of the earphone. (The quietest audible sounds are zero dB, normal conversation occurs around 60 dB, and 120 dB is equivalent to the noise of a nearby airplane taking off.) It is estimated that between 2.5 million and 10 million music-player users in the European Union are at high risk of hearing loss.
The system from dB Logic limits headphones’ volume with a circuit that is powered by the audio signal, eliminating the need for batteries. At the heart of the circuit is a tiny transformer that steps up the small voltages of the music player’s audio output to levels that can operate the transistors that regulate volume. The system only attenuates the volume if the average loudness is consistently over 85 dB. This means both that soft sounds can pass through at full volume and that brief passages greater than 85 dB are not abruptly attenuated. Two other popular methods for limiting volume—using a resistor to attenuate the audio signal regardless of volume, and using a pair of diodes to clip all sounds above a given threshold—aren’t capable of this.
Although dB Logic’s system could have been set to begin limiting at any particular loudness level, 85 dB was chosen because that’s the threshold at which the U.S. Occupational Safety and Health Administration requires employers to start protecting workers’ hearing, says Med Dwyer, dB Logic’s CTO and cofounder. “Very few users will listen at that level and say ‘That’s not loud enough for me,’ especially in our target demographic of younger people,” says Dwyer.