When you're looking to buy a set of headphones you will encounter a list of specifications that provide basic information on their requirements and abilities. While nothing is as important as using the headphones yourself to determine whether they sound good enough for your needs and feel comfortable, understanding the specifications will help you narrow the field.
Closed headphones vs open headphones
This is the most important distinction to make when buying headphones or earphones. A closed set of headphones — Audio-Technica's ATH-A500 headphones for example — block out external noise, whether through active noise-cancelling hardware or simply by sealing well around the ears.
Canalphones (also known as in-ear monitors — IEMs — or in-ear headphones) like the Apple In Ear Headphones also fall into the closed category, as they seal against the ear canal to prevent ambient noise from disrupting your music. While closed headphones have the advantage of blocking out external noise, they generally have a more 'shut-in' sound than their open counterparts.
Another important advantage of closed headphones is that they allow you to have your MP3 player's volume lower than with open headphones — since you're not trying to drown out outside noise — which will preserve your hearing as well as saving a little of your MP3 player's battery life. Closed headphones are often uncomfortable when worn for long periods of time
Most headphones fall into the open category, including earbuds that don't seal against your ear canal, clip-on headphones, and full-size headphones that sit on your ears rather that over them. It is easier to produce open headphones so they are generally cheaper than equivalent closed models, and sound quality is also usually slightly better. If you are in an area free from ambient noise, open headphones will offer a wider soundstage so your music sounds more enveloping and '3D'. Closed headphones are generally more comfortable as they don't clamp tightly against your head to create a seal.
Whether you buy closed or open headphones should come down to where you'll be using them most frequently and how highly you rate comfort, noise cancelling and sound quality.
Frequency response
Frequency response represents headphones' ability to reproduce audio frequencies. Measured in hertz, the smaller number is the headphones' bass response cut-off, while the higher number represents the roll-off of treble reproduction. Most digital music has a theoretical frequency range of 20-20,000Hz. While not all of this frequency range will be used, it is good to have a set of headphones covering as much of this as possible.
However, we advise you don't put too much stock in this particular figure. Unless you're buying headphones from a specialised, respected manufacturer, it's likely that this figure is, to be frank, bogus. In a perfect world, all headphone manufacturers would quote figures of ±3dB — meaning they'd only tell you what frequencies the headphones could play while maintaining a volume within a three decibel range — but some manufacturers are a little creative with these numbers.
Impedance
Put simply, impedance is the headphones’ electrical resistance to the current being pushed through them. Measured in Ohms, impedance is a factor influenced by the components used in the design of the headphones — high quality headphones that have low electronic interference (hiss) levels usually have high impedance levels as a by-product.
The higher the impedance of a set of headphones, the harder your MP3 player or computer sound card has to drive them to attain any particular volume. High quality headphones sometimes have such high impedances that a dedicated headphone amplifier is required to supply enough power to create a listenable volume level. Most MP3 players are able to easily power headphones with impedance levels of up to around 100 ohms, while the 300 ohm nominal impedance of models such as the AKG K701, Sennheiser HD 650 and Sennheiser HD 800 means they require a standalone headphone amplifier to power them.
Sound pressure level
A measurement is taken by playing a 1 kilohertz note at a power level of 1Vrms (one volt root mean square) and recording the decibel level of a pair of headphones’ reproduction of the sound. Headphones with a higher sound pressure level will generally be louder when supplied with any given audio source. This figure varies depending on the material used when building the headphones — more expensive headphones do not necessarily have a higher sound pressure level.
Total harmonic distortion
When driving headphones at high volumes, it is possible for the diaphragm (the ‘speaker’ inside the headphones) to be unable to move fast enough. This leads to distortion, manifesting itself as crackling, popping or the alteration of musical notes. When you see a set of headphones with a high percentage of total harmonic distortion, this means that they will more easily distort. Aim for a set of headphones with as low a total harmonic distortion (or THD) level as possible. Most reputable manufacturers’ headphones have total harmonic distortion levels below one per cent, while high-end headphones have distortion levels below 0.1 per cent.
Weight, size and cable length
Depending on where you’ll be using the headphones, you will want a different mix of weight, bulkiness and cable length. If you only intend to use the headphones at your PC, a cable length of around three metres is standard and allows a good deal of movement without requiring unplugging. If you’re running with the headphones on you will want a pair as light and small as possible. The heavier a pair of headphones are the more likely they are to be fatiguing over time and hurt your ears.
Most headphones will terminate in a standard 3.5mm headphone jack, which is suitable for use with all MP3 players and computer sound cards. High-end headphones will sometimes have ¼in plugs designed for plugging in to headphone amplifiers — this makes them unsuitable for portable use.
