Headphone Guide

Last updated: 31 March 2010
Published in: Digitising analogue media | Creating new digital media |
Tags: audio | binaural | digitisation | equipment | headphones | mobile | music | sound recordings |

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Summary

A guide to the types of headphones available for various listening tasks, their features, strengths, weaknesses and relative merits. This guide is intended to help users choose the best type of headphones for their needs.

Introduction

The increasing use of personal media players and smartphones for listening to digital audio, as well as audio playback from personal computers both at home and in the workplace, has meant that listening to music and other audio on headphones has in recent years become as common as listening through speakers - if not more so. Since the advent of the Sony Walkman in 1979 the trend towards private listening has snowballed, and while mobile media devices have in some senses come full circle, now often being used as the primary audio source for a complete stereo system, headphones are firmly established as a key element of the audio playback chain.

Headphones are also vital to the recording process, allowing performers and artists to monitor their performances in a way which will not be picked up by microphones or other performers, and headphones can even be used for monitoring in some live performance situations.

In this guide we look at the various different types of headphones available, and their suitability for these different environments, workloads and applications.

Headphones - Key Facts

• Main goals: good sound quality, and isolation from external noise 
• Headphones give good sonic detail but poor soundstage 
• Over-ear, On-ear or In-ear types
• Closed back over-ear gives better isolation
• New ear canal phones are efficient, isolated, and can also sound very good
• Even very good headphones cannot replace speakers for some critical tasks

Common tasks

The raison d'etre of headphones is of course listening to audio - be it music, spoken word or any other type of amplified or recorded sound. For the final user of an audio object, listening can be for pleasure and/or for information - in either case an optimal listening experience is the goal.

In the production environment however, where the focus is on making audio resources for use by others, monitoring performances and recordings and making critical audio judgements, headphones can be required to offer more specific properties to fulfil different practical roles in the audio lifecycle. There are three main profiles which have particular requirements:

Listening for pleasure

The simple use of headphones for listening to music or audio for personal enjoyment. To create an engaging listening experience, a pleasing sound quality is the primary requirement;

• Goal: Optimal subjective listening experience
• Primary requirement: Pleasant audio character (often including extended frequency response)
• Secondary requirement: Reduction of external noise
• Recommended: Open-back over-ear (possibly featuring noise cancelling technology) or ear-canal in-ear

Critical and analytical listening

Using headphones to listen to digitised material, and to make judgements about processing, mixing, editing and other critical tasks.

• Goal: Objective sonic accuracy (reference monitoring)
• Primary requirement: Even frequency response and minimal colouration (often with good isolation from external noise)
• Secondary requirement: Comfort for extended listening sessions
• Recommended: Closed or open-back over-ear professional headphones or high quality ear-canal in-ear monitors

Monitoring

Monitoring the performance of a recording artist or live performer. Can be either monitoring by a sound engineer or the performer themselves for confidence and/or technical purposes.

• Goal: Isolated reproduction of a monitor signal, to aid performance (for performers) or system configuration (for sound engineers) or enable monitoring of audio signal during recording (for recording and video engineers)
• Primary requirements: Isolation from external sound and minimal 'spill'
• Secondary requirement: Accurate and clear sound reproduction
• Recommended: Closed-back over-ear or ear-canal in-ear monitors

Headphone advantages and disadvantages

Physical effects

The headphone cable ties you to your sound source (excepting wireless headphones - see below). This will limit your range of movement, and can be awkward - make sure when choosing a set of headphones that you allow enough cable length to suit the proposed usage.

Headphones isolate you from the surrounding environment by partly (or even completely) cutting out ambient noise. This isolation can be advantageous for reducing unwanted external noise, but is problematic if interaction with surroundings is desired. Open-backed, earbud and on-ear designs give less isolation from your surroundings.

Sonic effects

Midrange and high frequency detail on headphone audio is usually good, and can be exceptional. Additionally, the lack of room resonances and external noise can give a very uncoloured sound which enables detailed identification of unwanted sonic artefacts such as noise, hiss, hum, reverberation etc, and assessment of correctional and creative audio tools. Headphones make a useful analytical and forensic audio tool.

However, invariably* the left signal goes into left ear, right signal to right. The sound of each channel is literally piped into its respective ear, and this lack of crossover can create a 'disembodied' sound, with little impression of physical presence, stereo 'image' or 'soundstage'. Speakers on the other hand place the sound within a listening space, and can create a convincing soundstage. With headphones - as one analogy has it - "musicians and singers seem to be sitting on a clothesline stretched from ear to ear inside the head"¹

The bass frequency response of headphones is often limited, and even when extended, it lacks the physical power of bass frequencies generated by a speaker.

[* there is in fact a little used technique - crossfeed - whereby a small amount of each channel is fed into the opposite side, which for some listeners is claimed to improve the impression of stereo soundstage. Crossfeed can be combined with micro delays and filters to simulate the tiny delays between sound arriving at each ear]

Headphone anatomy

Headphones, 'phones or 'cans' come in many varieties and designs. Many of these - especially traditional headworn types - share common elements.

An adjustable headband holds the earpieces over each ear, which can sometimes swivel on articulated joints to enable a snug and comfortable fit. Each earpiece is fed a signal by a two-core cable, which either exit each enclosure separately, or one is passed through the headband to be combined with the other, thus requiring only a single cable connection point. The signal is fed to one or more speaker drivers in each earpiece, which are situated in a small enclosure which sits on or just above the ear on a cushion.

In a heavy workload environment parts can wear out or fail, and some professional headphones (such as the industry standard Beyerdynamic DT100) are fully modular, in that each part can be ordered and replaced individually, and as such are largely user-serviceable.

Types of headphones

On-ear

Many lightweight headphones use foam cushions to surround the two driver enclosures, which then sit on the outside of ear. This design is called on-ear or 'supra-aural', and while making them smaller and often cheaper to produce, it has some weaknesses. Acoustic isolation is poor, as is spill from the headphones. Also, prolonged use can cause discomfort from the pressure on the outside of the ears, and heat trapped by the pads resting on the the ear. For these reasons, while they can be a cost-effective solution, on-ear headphones are not recommended for critical audio tasks or heavy use.

Sennheiser HD437 on-ear headphones. Image used with permission

Pro:

• Lightweight

Con:

• Can be uncomfortable and hot for long listening sessions
• Poor isolation

Over-ear

Over-ear or 'circum-aural' headphones have doughnut-shaped cushions which sit on the skull surrounding the ear, and raise the headphone drivers off the ear. This provides superior comfort, and forms a seal around the ear, and also allows the use of large (and therefore often high quality) drivers and enclosures. Over-ear and on-ear headphones come in two types:

1. Open Back

The backs of the headphone enclosures incorporate grilles, vents or holes. This can reduce resonance within the headphone enclosure, and thus have acoustic benefits, and most of the best 'hi-fi' audiophile headphones are of the open-back over-ear type. The vents do however allow the penetration of external noise into the listening enclosure, and the spill of the headphones' output to the immediate vicinity, making open-back headphones unsuitable for use in situations where spill into a microphone, or penetration of external noise is undesirable (e.g. record monitoring).

Sennheiser HD650 open back audiophile headphones. Image used with permission

Pro:

• Open sound
• Comfort

Con:

• Lack of isolation

2. Closed Back

Closed-back over-ear headphones provide the best acoustic seal of any type of headworn (i.e. not in-ear) headphones. The sealed enclosure, combined with a cushion which surrounds the ear in constant contact with the skull, gives the best acoustic isolation from external noise, and also minimises any spill of the headphones' own output. Almost all professional recording or television studio headphones, and those used by live sound engineers, are of this type. [Note: the new type of in-ear ear-canal headphones are beginning to find favour for their similarly excellent isolation properties, as well as their often excellent sound quality - see below]

Beyerdynamic DT150 closed back monitoring headphones. Image used with permission

Pro:

• Isolation - sealed enclosure gives best isolation of any traditional headphone design
• Comfort - no contact with ear, so comfortable for long sessions. You may need the occasional break to let your ears cool down though!
• Sound quality - many professional closed back headphones have excellent sound quality
• Spill - closed back and constant contact with the skull around the ear minimises overspill into microphones etc.

Con:

• Closed back enclosures can resonate and can have a more 'boxy' sound and limited soundstage than open-back equivalents

In-ear

1. Ear-bud

The small in-ear headphones which are commonly included with mobile audio devices, MP3 players etc.

Ear bud in-ear headphones, incorporating in-line microphone for use with mobile phone

Pro:

• Discrete - no headband, and small size make them a reasonably discrete solution
• In-line controls - some headphones included with specific devices incorporate in-line controls for that device, and/or an in-line microphone
• Price - low price, often included free with portable players and mobile phones

Con:

• Sound quality - often quite poor
• Isolation - also poor
• Spill - 'tinny' overspill into immediate vicinity

2. Ear canal

Very small in-ear headphones which are wedged actually into the ear canal (like earplugs).

Shure E3C ear canal headphones

Ear canal headphones were developed from professional in-ear monitors, first used for on-stage monitoring by live performers. Though they can be considerably more expensive that 'ear bud' type in-ear headphones, they are sonically massively superior. Frequency response is wide and flat, isolation is excellent and there is virtually no spill from them whatsoever.

They have several empirical advantages over traditional headphones, and their disadvantages are by comparison quite subjective:

Pro:

• Discretion - small size and positioning inside the ear mean they are very discrete - one reason that they are favoured for theatrical and live performance monitoring.
• Isolation - 'ear-plug' design minimises penetration of external noise
• Spill - Most types cannot be heard by anyone other than the listener, even very close up
• Low power - can be used at much lower level than most other types of headphones for same perceived volume
• Sound quality - ear canal positioning allows excellent reproduction of full frequency range with little or no resonance or colouration

Con:

• Some users may find the 'earplug' sensation uncomfortable
• Ear wax can build up in the ear pieces - regular cleaning is required
• Intimate positioning means that these headphones should not be shared
• Isolation from ambient and environmental sound may be a disadvantage in certain situations
• Ear canal positioning gives a very 'dry' sound, which can lack a feeling of soundstage, and feel like the music is being piped directly into the brain!

Headset

Headsets incorporate both headphones and a microphone, mounted on a small arm attached to one of the earpieces. Some applications - conferencing, aviation etc - require two way communication with a headworn microphone and headphones, and a headset is an easy way of achieving this.

Logitech Clearchat USB headset

Noise cancelling

Some headphones incorporate noise cancelling technology. Electronic noise cancelling involves the use of a small battery powered circuit to listen to the external background noise with a microphone, then reproduce an exact but phase-reversed copy, which is then added to the headphone output signal, thus largely cancelling out background noise. While this can be a very useful and effective feature for some noisy listening environments, including aeroplanes, trains etc, it is not recommended for critical tasks, where an unadulterated signal is required.

Noise cancelling headphones will require a power source - usually a battery - to power their noise-cancelling circuits.

Wireless headphones

Wireless headphones comprise a base station transmitter and a pair of battery powered headphones, incorporating the receiver. They can work either by radio transmission or by infra-red; infra-red communication requires line-of-sight contact between transmitter and receiver, whereas radio frequency (RF) communication does not.

While wireless headphones can be appealing for home listening, they can be susceptible to interference, and rely on an amplifier built in to the headset, whose battery power supply needs charging, and contributes to the weight of the headset. These factors make them unsuitable for critical work.

However, if wireless headphones are suitable to your needs. make sure that they are powered by Lithium ion rechargeable batteries, which have smaller size, longer life and higher power than NiCad Nickel Cadmium units. RF wireless is also generally considered a more robust system than infra-red.

Impedance

Like any electronic circuit, headphones have an electrical resistivity, or 'impedance', which is measured in ohms - symbol Ω. The higher the impedance, the greater the current required to drive the headphones to the same level. Most consumer headphones, and many professional headphones are therefore low impedance (<80Ω), enabling them to be driven easily by the majority of devices' headphone outputs.

For some specific monitoring tasks however, especially where multiple pairs of headphones are to be used in parallel, higher impedance is required to avoid overloading the output circuits, and to preserve sensible listening levels and bass response (which can be compromised by low impedance in some instances). For this reason some professional headphones are high impedance (>250Ω), or are available in high and low impedance versions - e.g. the Beyerdynamic DT250, available in 80Ω or 250Ω. High impedance headphones are more difficult to drive, and may not give sufficient level if used with some consumer or mobile equipment with low output levels.

Care should be taken not to mix headphones of significantly different impedance if using a passive headphone splitter.

Safe Listening Levels

Many headphones, especially ear-canal types, can produce levels capable of causing permanent hearing damage. Prolonged listening to levels of over 85dB is strongly discouraged.

Of course it can be difficult to know what exact level you are subjecting your ears to, so an element of common sense is required! Over a long session, or when out and about in a noisier environment, try to resist the urge to keep nudging up the volume as your ears get tired and accustomed to a louder level, or to compensate for background noise. Take breaks to give your ears a rest (this also has the added benefit of improving your ability to analyse sounds with 'fresh' ears). Watch your output meters (if you have any) and volume settings.

As an example, when using my ear canal headphones with my laptop, I try never to turn the master volume above 50%, as I have established that, all other things being equal, these phones produce plenty of volume at this drive level. Experiment with your regular set-up when your ears are fresh, and try similarly to establish reasonable ballpark limits for output levels.

Binaural recording

Binaural recording is a specialised recording technique, designed for headphone playback. Miniature microphones are placed either in the recordist's ears, or in the ears of a dummy head, thus closely approximating the positions of a listener's eardrums, and thus a 'realistic' reproduction of their listening experience. When played back on headphones, binaural recordings can be disconcertingly convincing!

Headphone amplifiers and splitters

Headphones are essentially small headworn speakers, and as such they require an amplifier - although one with a far smaller output level than one for use with speakers. The volume the amplifier can produce will depend directly on the impedance of the headphones you are using (see Impedance above). Headphone output level is greater than line level output, but is also designed to drive a different impedance load. Some devices have a single output which, while not ideally suited to either task, is capable of either role. A dedicated headphone amplifier or output will give better sound quality, where available.

If using a passive headphone splitter (with a single plug and multiple sockets), all headphones used must be of similar impedance, or wildly varying levels between different sets will result. Additionally, as more headphones are connected the overall level will drop, as the amplifier is called upon to drive more speakers.

Studio headphone amplifiers have single or multiple line level inputs, and offer separate level controls for each set of headphones. Some offer additional features like monitor inputs, which allow individual monitor signals to be mixed with the master monitor feed, allowing an element of customisation of individual monitor mixes.

Connectors

Headphones deliver a stereo signal (i.e. two independent channels) and therefore need a 3-core connection cable - one to carry the left signal, another the right, and the third to carry a common shared ground. Some headphones additionally feature a microphone or in-line controls, which require further cable cores and multiple or custom connectors. These are the most common connector types:

1/4" stereo jack plug

A single plug with 3 terminals - Tip, Ring and Sleeve (TRS). Common on professional audio equipment.

3.5mm stereo mini jack

As above, but a smaller version. Found commonly on consumer audio equipment and compact and mobile professional devices, due to its size.

Adaptors are readily available which convert 1/4" to 3.5mm jack, and vice versa.

3.5mm gold plated stereo headphone jack and matching threaded 3.5mm-1/4" adaptor

USB

Some newer devices for use specifically with computers (e.g. audio conferencing headsets) terminate in an oversize USB plug which incorporates AD/DA conversion, enabling use independent of a host soundcard. While very convenient for simple computer-based applications - chat, conferencing, simple recording and listening to music - they are not compatible with standard headphone outputs, nor can they be used with your choice of audio interface, and as such are too inflexible for critical audio work.

Unusual connectors

• 2.5mm mini jack - as above, but smaller still
• TRRS (Tip-Ring-Ring-Sleeve) mini jack - incorporates a fourth terminal, usually used for in-line microphone, for use with mobile phone or similar. TRRS output is compatible with TRS headphones, where the fourth terminal is simply unused.

3.5mm TRRS jack - note the additional ring connector on the plug shaft for in-line mic

References

1 HeadRoom Supreme headphone amplifier - John Atkinson 1994

Last updated: 31 March 2010
Published in: Digitising analogue media | Creating new digital media |
Tags: audio | binaural | digitisation | equipment | headphones | mobile | music | sound recordings |

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