If headphones don't necessarily have advantages over loudspeakers in terms of overall response flatness, they can easily score in terms of distortion. Because of the tiny amount of movement required to shift a few cc of air, distortion levels (especially in the bass) can be an order of magnitude or more better than even quite fancy loudspeakers. Bass quality is also much easier to achieve: there may be some rolloff of bass response in some models, but there is almost never the sort of lumpy and resonant bass that plagues far too many loudspeakers. Basically (sorry!) there just isn't anything in a typical headphone that resonates at bass frequencies.
Stereo imaging is much more consistent than with speakers as the relationship between transducers and ears is fixed. 'Ideal' loudspeaker listening (speakers equidistant from the listener with symmetrical room placement etc.) may give better imaging with most recordings as it is normal to master for loudspeakers rather than headphones, but headphones will certainly beat speakers randomly dumped into a domestic environment.
But what about the differences between types of headphone? There are several areas where they can differ, including the type of drive unit, whether the drive unit is enclosed at the back and how they sit on the head. By far the most common drive unit is effectively a miniature loudspeaker drive unit, with a voice coil in a magnet gap and a conical, or possibly domed, membrane attached to it. Other systems have been used, though, including the electrostatic principle – see the boxout 'headphones or earspeakers?'.
Assuming a regular electromagnetic driver is used, the surrounding body can be open at the back or closed off, giving better isolation from the world, but potentially creating a resonant chamber which may colour the sound. Open and closed headphones alike can incorporate earpads which surround each ear ('circumaural') or sit on it ('supra-aural').
The difference between the last two is largely one of comfort, though some listeners find that the positioning of headphones on the head is critical to the sound, and relatively tight-fitting circumaural headphones are the most consistent here.
In-ear monitors
There's a further approach to headphones which is very much a recent development, the 'in-ear monitor' or earphone. These tiny devices first entered the collective consciousness as a practical accompaniment to portable audio devices and most early examples (and indeed many cheap current ones) are sonically vile.
Thanks to the efforts of such manufacturers as Etymotic Research, in-ear transducers have achieved standards that can legitimately be called hi-fi and they certainly have advantages on the noise-exclusion front, since they function as earplugs at the same time as music-playing devices.
It must be said, though, that some listeners never get on with them, and getting decent sound depends critically on fitting them correctly in your ears.
Mention of reducing ambient noise brings us to another modern headphone variant, the noise-cancelling variety. The principle here is to use a microphone somewhere near the earpieces to detect ambient noise and then amplify this and add it in antiphase to each ear's signal so that it is effectively cancelled out at the ear.
This can work quite well (especially with lowfrequency sound) and Sennheiser and Bose, in particular, have sold a lot of such models to frequent flyers. The disadvantage is the need for electronic amplification and with it a battery to power everything – and, of course, the extra circuitry has something of an impact on quality, in principle at least.
What about wireless?
Wireless headphones also need a battery or two. The idea of freeing the listener from the headphone lead – which apart from being a literal tie to the hi-fi system is also a liability when one gets up in a hurry and snags it on something – is clearly attractive and wireless headphones linked to the sound source via a miniature radio transmitter have been around for a while.
Quality has never been quite what one might hope, though, and now that so many people have some kind of portable music player (MP3 etc.) it may end up more attractive to load the music on to that and have done with the limited range, tendency to drift off channel and other tiresome side effects of wireless models.
Perceived headphone quality, not surprisingly, is much like any other bit of hi-fi: the more familiar you become with it, the more demanding you are likely to be. It's therefore hard to say just how much money one should spend to get 'good' headphone quality, but certainly compared with loudspeakers the cost is modest.
You can spend over £1,000, especially on electrostatic models, but some very, very fine headphones cost less than £500. In a recent group test, we were particularly impressed by Sennheiser and Beyerdynamic models at little over £300 and £200 respectively.
In a world where a 'mid-price' interconnect cable costs £50-£100, we don't feel the least bit embarrassed at recommending the headphone user to part with at least £70. Below that price, you are likely to miss out on a good deal of musical pleasure: above it, the virtues of good hi-fi typically start to become apparent.
Headphones are not for every listener, nor perhaps for every day, but a good pair is a great investment for any keen audiophile.
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