Headphone basics



There are two basic types of headphone based on the size of the ear cups: over-ear and on-ear. Over-ear headphones are more comfortable than on-ear headphones, but they can be less practical for travellers.

Open headphones
In open headphones, the cavity between the driver and the ear is open and connected to the outside world. This is achieved by porous ear pads or by the plate that holds the transducer consist of porous material. In the latter case the back of the headphone is open as well. These are generally the most comfortable and best sounding type of headphone. On the downside, they have minimal sound isolation and high level of sound leakage, therefore, these are best for listening alone in a quiet environment.

Open headphones have a broad peak in their impedance curve at the resonance frequency and the bass response of open headphones varies according to the value of the source impedance (output impedance of the amplifier). As a rule of thumb the output impedance of the headphone amplifier may not exceed 25% of the headphone's nominal impedance. (Closed and semi-open headphones have almost flat impedance curve, hence they are less sensitive to the output impedance.)

Closed headphones
Closed headphones (sealed headphones) have ear-pads that seals the acoustic chamber between the ear and the driver and the back of the driver is closed. Closed headphones provide isolation from the outside world, blocking out some of the surrounding noise while avoiding sound leakage from the headphones. The attenuation of the outside noise is very good at high frequencies (above 1kHz), but rather poor at low frequencies. Only closed headphones with active noise cancelling have complete attenuation in the full audio band. They are recommended in moderately loud environments (e.g. in an office).

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Semi-open headphones
This is a very small group of headphones. Semi-open headphones have airtight ear-pads (closed front) and only the back of the driver is open. One likely advantage over open headphones is the well-behaved impedance response curve which is important for portable audio players and integrated sound cards. Just like open headphones semi-open headphones are best for listening alone in a quiet environment. Open back planar magnetic headphones (HiFiMAN HE-400S, Oppo PM-2, Audeze LCD-2) are semi-open types.

How should a headphone sound?
Probably the most important question about headphones is how a headphone should sound. A headphone should sound like good speakers in a good room and the best approximation of this is the Harman headphone target curve. The frequency response of a natural sounding headphone has a broad peak at 3 kHz to mimic the response of the human ear. (Maybe one day the manuals will publish the correction curves for all headphones.)

Comfort
One of the most important aspect of any headphone is the comfort. The size and the shape of the ear cups, the material of the ear pads and the headband have a big impact on the comfort. Some over ear headphones have smaller ear cups than others and they can be uncomfortable for people with larger ears.

External DACs and headphone amplifiers
There are a lot of hype surrounding high-end DACs and headphone amplifiers. Some audiophiles attribute magical qualities to high-end electronics, we can read reviews about how an external DAC changes the listening experience, sometimes dramatically... However, there are very few problems with on-board DACs & amplifiers in notebooks, tablets and even cell phones today. Investing in an external DAC or amplifier is questionable and the number of cases when switching to an external DAC can improve the listening experience is rather small. Here is a short list of the possible errors in modern audio players, cell phones and notebooks.

  1. Noise
    Noise is a real problem in analog electronics and not in digital electronics, however, some low-end products may have higher noise level which can be audible. The headphone out in cell phones have SNR (signal-to-noise ratio) of 85-95 dBA and these are pretty good values. The noise floor of 95 dBA SNR is very unlikely to be heard even with a sensitive over-ear headphone, and the noise floor of 85 dBA is just barely audible in a quiet room.

  2. Low volume
    Portable audio players and sound cards have low maximum voltage that is not enough to drive high impedance headphones to loud levels.

  3. The output impedance of the amplifier is critical with low impedance open headphones
    Open headphones have a broad peak in their impedance curve at the resonance frequency and the bass response of open headphones varies according to the value of the source impedance (output impedance of the amplifier). As a rule of thumb the output impedance of the headphone amplifier may not exceed 25% of the headphone's nominal impedance. (Closed and semi-open headphones have almost flat impedance curve, hence they are less sensitive to the output impedance.)



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