Standard headphones that come with iPods and other digital music players are, by and large, ordinary. Every time you hear a really loud sound, your hearing acuity decreases. Thankfully, the effect usually isn't permanent -- audiologists refer to it as temporary threshold shift -- but repeated exposure to high volumes will likely reduce your hearing ability. If you occasionally experience ringing in your ears after listening to loud music or other sounds, take heed. Nature's telling you to turn it down. If you don't, you will suffer some hearing loss -- and you'll never get it back.

That's why it's a bad idea to block out external noise by cranking your portable audio device's volume up to 11. Noise-cancelling headphones alleviate this widely ignored problem. By cancelling out the sound of airplanes, buses and other sources of aural aggravation, they enable you to hear every note in your personal soundtrack without blowing out your eardrums.

Noise cancellation: Passive vs Active
The low-tech, or passive, way to cancel noise is simply to wear closed-back, circumaural (around the ear) headphones or very snug earbuds. These shield your ears by sealing them off from the outside world. Active noise cancellation, on the other hand, is far more complicated and involves some serious science.

First, tiny microphones, one on each earpiece, detect ambient noise before it gets to your ears. Then the noise-cancellation circuitry, usually housed in an external module, essentially inverts the captured signal, turning the noise's sound wave upside down. Before you know it, the noise-cancellation system adds the sonic opposite of the external noise to whatever you're listening to, thereby eliminating most of the pollution and leaving you with just your music.

Physical limitations make absolutely perfect noise cancellation impossible, but some of the noise-cancelling headphones we tested did a great job without adversely affecting the music. Read the reviews below to find out which model offers the best combination of design, performance and affordability.

Bose QuietComfort 3 Read CNET.com.au's review With a smaller form factor and rechargeable battery, Bose has extended the portability and appeal beyond air travel with the QuietComfort 3 headphones. Jabra C820s Read CNET.com.au's review For about a quarter of the price, Jabra's C820s noise-cancelling headphones deliver much of what the Bose QuietComfort headphones offer. Bose QuietComfort 2 Read CNET.com.au's review Bearable 14-hour international flights? With these Bose noise-cancelling headphones to cut down the engine noise, we may have found a jetlag antidote. Sennheiser PXC-250 Read CNET.com.au's review Highly effective noise-cancelling circuitry mated with detailed sound and a comfortable fit -- what's not to like? Sony MDRNC11 Read CNET.com.au's review Should frequent travellers invest in a set of Sony's Noise Cancelling Headphones? Sennheiser PXC-300 Read CNET.com.au's review It packs almost the same performance as the Bose QuietComfort 2 in a much smaller package and at about half the price.