Logically, a good set of hi-fi headphones should equal or better a professional monitoring system in all respects other than, perhaps, deep bass reproduction. And, unlike loudspeakers, headphones are independent of the acoustics of the room in which they are used -- so why isn't all monitoring conducted on headphones? After all, it would reduce the amount of noise escaping from the studio and would completely do away with the need for acoustically-treated control rooms. Sadly, life just isn't that kind; headphones behave differently to loudspeakers in several key areas, and as most music is mixed for loudspeaker playback, relying solely on headphones can mislead you into producing a mix that sounds quite wrong when replayed over loudspeakers. But why is this, and if it's true, are headphones any use at all for monitoring?
When a stereo mix is heard over loudspeakers, because the speakers are physically in front of us our natural hearing mechanism perceives the soundstage as being in front of us. With headphones, because the 'speakers' are on either side of us, there's no real front-to-back information, which can make the stereo image seem as though it's passing through the head rather than being on a virtual stage in front of the listener.
There's a further difference between loudspeakers and headphones: when you're listening to natural sounds or stereo from loudspeakers, some of the sound from the left loudspeaker is picked up by the right ear, and vice versa. By contrast, headphones provide a very high degree of separation between the left and right channels, which produces an artificially detailed stereo image. This characteristic is useful when checking the positions of various sounds in the stereo mix, but the same mix heard over speakers will not seem to have such a wide or clearly focused stereo spread.
Discounting for a moment the problems caused by discrepancies in stereo imaging, a more serious problem when using phones is that different people perceive different amounts of bass -- factors such as the distance between the headphone diaphragm and the listener's ear will change the level of bass. The way in which the headphone cushion seals around the ear also plays a part, which is why pushing the phones closer to your ears produces a noticeable increase in bass. The proximity effect also causes an increase in bass energy as the headphone moves closer to the ear drum. This factor alone is very worrying in the context of monitoring, where one of the main aims is to achieve a proper tonal balance across the full audio spectrum.
The seal problem between the headphone and the head can be circumvented by making the headphones acoustically open -- the transducer is mounted in an acoustically transparent support spaced away from the head. Open phones usually make use of a foam-filled enclosure which is ventilated with slots or holes, though the cushion part of the headset is similar to that of an enclosed headphone. Another advantage of an open design is that the sound is less coloured than in the case of an enclosed phone, but one trade-off is that isolation between the inside of the phones and the outside is lost. In other words, outside sounds can enter the headphones with very little attenuation, and perhaps more importantly in the studio, signals such as click tracks or monitor mixes can leak out of the phones and be picked up by nearby microphones. Of course this is only a real problem when using the headphones to provide a cue mix.
The sound quality of open headphones can be surprisingly good, and ineffective sealing of the cushions to the head ceases to become a major issue. However, there are still inconsistencies in bass perception, due mainly to the proximity effect. This means that even the best headphones are not suitable for monitoring while mixing, though their ability to resolve fine detail or show up small amounts of distortion make them idea for double-checking a mix that already sounds balanced on loudspeakers. In the case of the home studio operator, it is possible to use headphones at times when noise might be a problem, but ultimately, a mix should be checked on both loudspeakers and headphones.
Both open and enclosed headphones come in a variety of impedances, ranging from 8 ohms or so up to several hundred. Most headphone amplifiers will happily drive any impedance you're likely to encounter with no trouble, but if for any reason you want to split the output from a headphone socket to feed two sets of phones, it's a good idea to choose phones of the same impedance, otherwise one set will be louder than the other.
Fully enclosed phones generally produce higher sound levels due to their more efficient coupling with the ear, and by the same token, they can provide a deeper sense of bass. In theory, enclosed headphones must colour the sound to some extent, but in practice, some of the better models are extremely good in this respect. Professional studios invariably use enclosed headphones for providing performers with their cue mixes, due to the low amount of sound leakage in and out of these models, but they are seldom useful for judging the finer points of a mix.