The Quick Headphone Guide
Headphones are in some ways the speaker world's equivalent to motorcycles; smaller, cheaper, and more efficient. But not suited to all uses. And like motorcycles, there are a variety of different types of headphones. Unlike motorcycles, 10stripe can explain the different types of headphones.
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There are several basic types of headphones. These differ primarily in how much isolation they provide (how much they block out external sound), their efficiency, and their size and shape.
Although they come in a huge variety of shapes, "open" headphones are all fairly similar from an acoustic standpoint. They are called "open" because they do not create a sealed ("closed") chamber around the ear. In essence, all open headphones just suspend a small speaker next to your ear. They may take different approaches to that task, and they may use speakers of differing quality, but the basic task is always the same.
Open headphones can also be called "supra-aural" headphones if you are feeling pretentious, because they sit "on top" of the user's ear. There are several major categories of these headphones:
"Ear clip" headphones (no common term for this type)
"Street style" headphones with a wraparound band (these happen to be wireless)
A relative rarity these days, "closed" headphones create a sealed chamber around each ear. Air cannot (easily) escape from this chamber, and so vibration is transfered better from the speaker to your ear drum, while external sounds are damped out. This allows closed headphones to provide better isolation, higher efficiency, and significantly better bass reproduction.
Nothing in life is free, though; closed headphones have to be larger (so they can wrap all the way around your ear, not just sit on top of it), and they tend to squeeze the wearer's head slightly to help maintain their seal. But since they press against the wearer's head, not ears, and are usually well-padded, this may not be such a problem. It's also significantly harder to design them competently, which means decent ones tend to be pricey.
Better closed headphones also tend to have uncommonly high impedance, which we will discuss later.
These days, any headphones described as "full-sized" or "circumaural" (in other words, big) are likely to be closed because it's otherwise hard to justify having headphones that are so large. But neither of those terms necessarily means that the 'phones are actually closed; larger headphones can still be open, or "semi-open"/"semi-closed" (close to sealed, but not quite).
If you're comfortable with the idea of sticking a foam-padded piece of plastic into your ear canal, in-ear headphones may be for you. They are the smallest and most portable sort of headphones, and offer the highest isolation (not unlike jamming your fingers in your ears). They essentially take the basic concept of sealed heaphones even further than "circumaural" designs, and create a tiny sealed chamber inside your ear. This leads to very high efficiency (and again, exceptionally high isolation), and makes life easier for the people designing the things. Which has the happy result that good-quality in-ear headphones are typically cheaper than other types of 'phones of similar quality.
But all is not sunshine and roses. Tiny transducers ("speakers" hardly seems appropriate for something that almost directly vibrates your skull) are not able to reproduce bass very well. Many people find these headphones uncomfortable, especially when worn for extended periods. Some manufacturers provide several different foam pads to deal with this problem, but that is still only a partial solution. And if you tend to share headphones with friends, well, something you insert into your ear may not be the most hygenic option.
These headphones are also sometimes called earphones (in particular, manufacturer Shure uses this term), canal phones, and in-canal earphones.
Shure e2c earphones, photo by Flickr user MyLifeStory
Like most electronics, headphone spec sheets tend to include quite a variety of things. And like most electronics, many of those things are completely unimportant.
In theory, this defines the frequency range of sounds that the headphones can reproduce. In practice, it is usually listed as "20 Hz to 20 kHz" or something similar. As it happens, 20 Hz to 20 kHz is the nominal range of human hearing. This is usually a completely made up range, and not worth paying attention to. It's also a drastic oversimplification, because it implies that frequency response is flat across that range, which is never completely true.
If you want to see actual useful frequency response information, see if the headphones you're interested in are listed at HeadRoom. They have the equipment and knowledge to take accurate measurements and produce pretty little graphs. They also happen to sell headphones, but their evaluations are uncommonly honest.
Impedance specs are, again, an over-simplification. Impedance is similar to resistance, but takes into account the capacitance and/or inductance of the load. A headphone speaker is basically a non-ideal inductor, and it will have an impedance that varies with frequency. Headphones (and all speakers) are rated with some nominal impedance, which is essentially just what the manufacturer measured at one specific frequency.
For headphones, 32 Ohms is considered a "normal" impedance rating. Output amplifiers that expect to drive headphones are normally built on the assumption that they will be driving something like a 32 Ohm load. 64 Ohms is not uncommon, and values all the way up to 600 Ohms are not unheard of. In general, closed headphones will have higher than average impedance ratings.
For most users, the major significance of higher impedance is that higher-impedance headphones are more difficult for a typical audio amplifier to drive, especially the output amplifier in a portable device like an MP3 player. The end result is that high-impedance headphones may be too quiet when used with portable devices. This problem has led to a number of headphone amplifier projects intended to provide some extra "juice".
If you're buying something with a more "ordinary" impedance value, though, there is usually no reason to worry about impedance.
Efficiency is a measure of how much power the headphones are able to convert to sound for a given amount of electrical power input. This becomes important mainly when dealing with high-impedance 'phones, where higher effiency can help reduce the loss of volume that can come with that high impedance.
Not always quantified by a number (some manufacturers provide an often-poorly-documented single number in decibels), isolation is how well a set of headphones manages to block out ambient noise. Open headphones provide none, closed headphones provide quite a bit, and in-ear headphones will provide even more. As a bonus, high isolation also means that less sound will leak out as well, which decreases the odds that you will annoy the person next to you if you happen to like your music loud.
Headphone makers love to try to differentiate their products from the rest of the market, and they have tried all sorts of ideas to that end. Some are better than others.
Wireless headphones are increasingly popular, because they are convenient and a little bit cool. The wireless link can take several forms. Infrared (IR) requires line-of-sight and generally has terrible audio quality. Most products marketed as "radio frequency" (RF) wireless, often in the 900 MHz band, also have significant quality and signal dropout issues. If you're going to go wireless, your best bet is a Bluetooth device that uses the A2DP profile. Audio quality over A2DP is pretty good, although range can vary quite a bit depending on your particular device. Finding such devices (especially those that include an adapter that contains a Bluetooth transmitter and plugs into a 3.5 mm audio jack) can be difficult, unfortunately.
Noise canceling is also a much-desired, though misunderstood, feature. Noise canceling headphones contain a built-in microphone and some support electronics; they listen to the ambient noise around you and insert an out-of-phase version into the audio. This is used to get rid of low-to-mid-frequency noise; higher-frequency noise is meant to be attenuated by old-fashioned isolation. This means that noise canceling is very helpful for certain types of noise (frequenct air travelers seem to love it), and roughly useless for certain others. Given that it is a fairly expensive feature, it pays to know how well-suited noise cancellation is to the sort of noise you personally deal with when you wear headphones.