The term "earphones" covers a range of products that vary widely in appearance and in technology, so we'll narrow the field down to models that can be used on the go. Even so, there's still a wide range of diversity in that category. There are many different ways of "injecting" sound into a pair of ears; each has its advantages and disadvantages, and is better suited to a particular type of user's tastes and listening habits.
Acoustic Coupling - An Essential Factor
Remove the earphones slightly from your ears and you'll notice that the sound is totally altered. The bass frequencies disappear - and even the midrange - and all you hear is the high notes. This phenomenon is well known, and very unpleasant when the person next to you is listening to music with earphones. All you hear is "chicka-chicka-chicka" and hissing - rhythmic enough, but not really enjoyable for the unwilling listener. For a set of earphones to reproduce the entire audio spectrum, it has to be coupled perfectly to your ear in exactly the way its designer intended.
The most traditional design approach, especially for home listening, is called "closed-ear" or "circumaural" - where the earphone surrounds your entire outer ear and rests on your skull. Obviously this type of earphone is bulkier, but more comfortable, since generally the ear itself isn't subject to any pressure. And with well-designed models, the acoustic coupling is nearly perfect.
"Supraaural" earphones, which have become extremely common, rest directly on the ear itself. This approach makes it possible to make lighter, less bulky phones. On the other hand, the acoustic coupling can be less perfect than with circumaural models, and comfort also suffers if the phones are a bit heavy or press too hard on your ears. This type of earphone can be fitted with various attachment systems, such as a standard headband, neckband, or earclip.
The most common type of earphones for use with portable audio players is the earbud; its small, slightly flattened round shape fits right into your ear opening. This type of earphone has the advantage of being practical, very light (obviously), and not at all bulky. On the other hand, while good ones do exist, there are a lot of very poor-quality earbuds out there. Coupling with the ear is often very poor, and in general, this type of earphone does a poor job of reproducing the lower frequencies.
Last comes a more recent category - "intraaural" earphones, which go directly into the ear canal. This approach is derived from hearing aids and the "ear monitor" systems used by musicians on stage, but one major difference is that hearing aids and ear monitors are very carefully fitted to their user's ear anatomy (using silicone molding). This is obviously not the case with earbuds sold to the general public - the price of ear monitors starts at several hundred dollars and can reach thousands. This means that good acoustic coupling is a problem here as well, especially since it's very important with this type of listening device; performance can go from very good to inaudible with a shift in positioning. Various solutions are being offered, with some better than others and some more suited to a given individual's specific case, as we'll see when we take a look at the different models.