2. Isolation: Good Or Bad?

By nature, earphones send music directly into your ears, but that doesn't mean that the outside (sonic) world disappears. Earphones may attenuate external sounds to a greater or lesser degree, but they don't cancel them out. The degree of attenuation depends on design and technology. In many cases, earphones are intentionally designed to let sounds from the outside world in. Under normal conditions, simply adding music to the existing sound environment is more natural and enjoyable, and that's more or less what certain top-of-the-line home headphones do.

Conversely, if the environment is noisy, isolating the hearer as much as possible from the noise is a legitimate goal. Of course, one most keep in mind that the resulting isolation can be dangerous - as in getting run over by a bus you never hear - and can also be psychologically unpleasant after a certain amount of time. (Some people can't stand to wear headphones that isolate them too much.)

Be that as it may, circumaural and supraaural earphones exist in two types - closed models that feature no acoustic communication between the exterior and the interior, and open models, with little or no acoustic isolation. The appearance of the earphones may be almost identical for these two categories, incidentally; the difference is in the construction. By the way, don't expect too much of the acoustic isolation provided by closed earphones. It mostly affects the upper part of the audio spectrum, and is always fairly limited in the lower frequencies.

By their nature, earbuds provide only a low level of acoustic isolation. Intraaural earphones, however, act as real "earplugs," resulting in a high level of attenuation - provided they fit the user's ear properly, of course!

Noise Is The Enemy

In many situations, such as listening to music while using in public transportation, noise is the enemy. This is especially the case since it has the effect of prompting the user to increase the volume, which is not good for the ears. One solution that's existed for many years, but is still not widespread or well known, is Active Noise Reduction.

The principle is simple; exterior noises are captured by a microphone and re-injected (via the earphones) into your ear after being phase-inverted. It's a known fact that two sounds that are identical but whose phase is inverted cancel each other out - like what happens when you tow a car in one direction while somebody else is towing it in the opposite direction; as long as the forces are equal, the car doesn't move.

Obviously, electronic circuitry and a power supply are needed to operate the system, but the noise reduction is significant: approximately 15 dB (though it's only effective in the lower end of the spectrum, bass and low midrange). Fortunately, for the upper part of the spectrum, passive attenuation (as provided by closed-type headphones) is effective. The combination of the two is highly effective, and a good set of ANR phones is really nice to use if you're in a noisy environment. This is the type of headphones airline pilots use, and also passengers on some long flights - those seated in first class, at least!