However, even a seemingly painless, tolerable level can be damaging after say 8 hours of exposure. Your local OHS authority no doubt has the tables.

What I propose here is a system of two devices.

1. The Decibel Dosimeter.

A tiny dB meter and data logger on a chip, which can be worn either behind the ear or as a natty ear-ring. It logs dB level against time exposed.

2. Exposure limiting headphones.

These would be integrated ambient noise cancellation 'phones and stereo 'phones you can plug into your walkperson/iDevice. An embedded CPU in the earmuff reads the dB exposure log from the 'meter.

As the exposure approaches damaging amounts, the 'phones crank down a limiter which keeps your personal stereo at a safe level, accounting for what you've already been exposed to.

So if you've already been exposed to 8 hours in a foundry, you won't be able to destroy your hearing (further) by cranking Def Leppard (Def Jam/Def Penalty Kru) on the bus home.

Having the logger separate from the 'phones allows it to log exposure from any source, including that played through the 'phones.