Headphones

The vast majority of all headphones are moving-coil transducers in which a small, stiff diaphragm is coupled to an electrical coil wound in the gap of a powerful magnet. The arrangement is similar to a loudspeaker; although the dimensions and design mean that the transducer looks more like a moving-coil microphone capsule than a traditional loudspeaker. Headphones come in two main types, circumaural and supra-aural. Circumaural headphones fit snugly around the ears and form a good air seal with the head by the combination of pressure imparted by the headband and "ear-cups" which totally enclose the outer ear. This arrangement provides for a good isolation from external sound and guarantees a tight, physical coupling so that the changes of pressure imparted by moving of the diaphragm couple directly with the pressure changes in the auditory canal and eardrum. Circumaural headphones may be of the open-back type, in which the diaphragm is free to radiate into the adjacent environment, but these are uncommon. More usually, for reasons given below, they are of the closed-back type in which the diaphragm is enclosed at the rear.

The second type of headphones are known as supra-aural, meaning that they fit against the ear. Nowadays, this type of headphone is the more popular type, having been pioneered for the personal stereo cassette player. They are characterised by a lightweight design in which very powerful, rare-earth magnets are employed and in which the headband imparts a small pressure to hold an open-backed transducer against the outer ear with some intervening reticulated, acoustically-transparent foam for comfort. Because the headphone diaphragm is light and open-backed, the air is unsealed in the auditory canal and this type therefore offer virtually no acoustic isolation from the environment. In addition, because there is no seal, the ears cannot respond to absolute pressure but respond instead to the velocity of the diaphragm displacement. This would mean that the response of the system would rise at 6dB per octave were it not for the fact that the design of the diaphragm and its mounting is arranged to compensate for this.

Each type of headphone has its advantages and disadvantages. In the circumaural type, the combination of a pressure-operated transducer tightly coupled to the pressure-sensitive transducer in the ear, ensures that there is a reliable and extended bass response down to very low frequencies. The same cannot be said for the supra-aural type, where bass response can be light and variable and depends on the positioning of the transducers on the external ear. On the other hand, treble response is usually better from the supra-aural type due to the light mechanical arrangement. One unusual drawback of the circumaural type is the appearance of significant noise of physiological origin caused by constraining the ears inside the pressure of the ear-cups. This vascular noise is significant enough to affect masking levels at low frequencies.

In the end, these rather subtle performance effects have probably have less effect on choice than simple matters of comfort. Sealed ear-cups can be hot and unpleasant and supra-aural types are generally more pleasant to wear for extended periods. In any case, in recent years the supra-aural type has relegated the closed-back, circumaural type to second position and to specific roles where acoustic isolation is required; either where external noise is to be isolated from the signal present in the headphones or where the signal inside the 'phones must be isolated from the environment. An example of this latter situation is in the recording studio where foldback signals or tempo "clicks" must be prevented from being picked up by open microphones.

Impedance and efficiency

The two important electrical specifications for a pair of headphones are the impedance of the transducers and their sensitivity. Commercial headphones may vary from around 30 ohms up to about 300 ohms. Higher impedances, such as 600 ohms, are more useful in studio installations where headphones may be used directly as a monitor across a professional balanced-line signal (without an accompanying amplifier).

Headphone sensitivity is usually stated as the in-the-ear sound pressure level produced by one milliwatt (mW) of audio input. Typical sensitivity ratings run from 88 dB per mW to 105 dB per mW. But these figures must be viewed in conjunction with the impedance figure. Whereas a 0dBu voltage drive will produce 1mW in a 600 ohm headphone, it would produce 8mW in a 75 ohm headphone! So headphones must be selected for the purpose they are intended to serve. The normal problems being low-power, battery-run equipment, with inevitably low voltage rails and output voltage swing, failing to provide adequate output from higher impedance models.

Electrostatic headphones

Although the majority of headphones operate on the moving-coil principle, there is nothing to prevent the use of other motor systems used elsewhere in loudspeaker design. The advantages (and disadvantages) of electrostatic transducers have been discussed above and there have been a few commercially successful headphones based on electrostatic techniques.

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