Is it really an art to monitor and mix with headphones? There is certainly no dearth of discussion on the matter. Based on hours of reviewing newsgroup postings, articles, white papers and other pro audio literature, it must be so, since opinions on the subject range from gung-ho support of headphones for all monitoring and mixing to banning headphones outright from the studio and the stage. Performers worry about one set of concerns, while recording engineers fret over another. Musicians tend to be more accepting of headphones than recording engineers. Since consumers currently prefer loudspeaker fidelity over headphone fidelity, engineers who successfully monitor and mix with headphones must be able to correlate the two very different soundfields.
This article does not take a stance on whether headphones are better than loudspeakers in professional settings (even the most ardent champion of cans will say “it depends”), and instead examines techniques for using headphones in all monitoring situations. It may one day be possible to dispense with monitor speakers entirely, but the consensus agrees that day is not here yet. Nevertheless, when used with forethought and grounded in practical experience, headphones can challenge loudspeakers on every front. Further, in particularly demanding applications, a little technological wizardry can go so far as to fool the ears into believing that headphones are loudspeakers.
SETTING UP FOR A LIVE PERFORMANCE
Equipping The Performers
Hearing preservation as well as higher quality sound have been the driving forces behind the popularity of headphones and in-ear monitors for performers and audio engineers alike. Monitor speakers (wedges and sidefills) are the traditional means for musicians to hear themselves during a performance. The wedges are the primary sound sources, and the sidefills help to maintain uniform coverage across the stage. Most of the time, the sound image from such setups is washed out stereo, the image being best at a “sweet spot” on stage, which degrades if the performer moves around. Monitor speakers also have the bad habit of “spilling” off stage (due to wall reflections and the radiation pattern of the loudspeakers themselves) and compromising the soundfield from the house system. They are also prone to feedback if the microphones are not carefully placed. Perhaps the biggest concern is that monitor speakers (and concert sound systems generally) play so loudly that the musicians suffer hearing damage. Ear plugs are not an option since musicians onstage need to hear each other clearly.
Headphones can avoid most of these problems. The acoustic isolation of headphones allows each performer to listen to a monitor mix at a comfortable volume, with improved fidelity and dynamic range, and, when the equipment provides, to customize the monitor mix without affecting what others hear. During live performances, monitoring with standard headphones is pretty much restricted to rhythm sections and instrumentalists. Closed-ear headphones improve the intelligibility of a monitor mix and provide some attenuation of ambient noise as well. There is no feedback since the output from the canalphones is not audible to onstage microphones. For greater realism and interactivity, one or two stage monitors may be installed to expand headphone sound, or a performer may wear the headphone with one earcup off. However, these techniques can place hearing at risk.
Performers, such as vocalists, who like to move around the stage are not good candidates for headphone monitoring – primarily because headphones are too bulky. Instead, in-ear monitors permit mobility and provide acoustic isolation that is better than closed-ear phones. The popularity of in-ear monitors has displaced most, if not all, on-stage wedges and sidefill monitors. The earpieces of in-ear monitors are typically canalphones with excellent attenuation of ambient noise. (See A Quick Guide To Headphones and Preventing Hearing Damage When Listening To Headphones for more information about canalphones.) Performers are free to roam about the stage, toting wireless receivers to drive the canalphones and listening to a personalized mix in aural privacy. Without the higher SPLs of stage monitors, vocalists additionally benefit from reduced vocal fatigue, as they no longer have to strain their voices singing over the mix. See STUDIO MONITORING AND MIXING for a special discussion about vocalists and headphone monitoring.
Tips on Setting Up The Monitor Mix
A monitor mix for headphones and in-ear monitors has different requirements than one for loudspeakers. Headphones sound more detailed, offer true stereo (as opposed to the washed out image of stage monitors), and isolate the listener from ambient sounds. Consequently, a good headphone mix is more difficult to achieve. At the same time, since they can hear better and are listening to higher quality sound, performers are more likely to demand custom mixes. Here are some tips from Steven McCale and other engineers familiar with the art of mixing with ear monitors:
- The mix must have everything that a musician would hear without headphones: drum overheads, left and right keyboard feeds, video sound.
- Use audience microphones to lessen the isolation of headphones.
- Always provide a stereo mix, for greater intelligibility and so that each musician has a spatial location.
- For a more natural sound, add special effects such as reverb, harmonizers and delays to the mix.
- Be prepared to provide for a large number of custom mixes.
- Every ear monitor should have a limiter to prevent hearing damage – especially important in live performances where spurious noise in a sound system could amplify to deafening levels.
- Install gates on drums and compress bass guitars slightly for lively, controlled sound.
A setup designed for headphone monitoring is vital for creating a good mix. The average console may not have enough inputs or outputs. Headphone mixer amplifiers have multiple inputs for a main stereo mix, various subgroup mixes (such as drums, background vocals or keyboards) and possibly a separate effects loop. Individual levels controls let musicians dial their own custom mixes for each headphone output. Remote mixers, such as the The Psychologist from Intelix, are the most convenient of all, and free up console outputs besides. As performers may be using headphones with different efficiencies and listening requirements, booster amplifiers can augment the individual outputs of the mixer. The better headphone distribution amplifiers have two inputs per output – one for the main mix and another for a custom mix, and musicians can select between them at the flick of a switch.
Enhancing Headphone Sound
Two major sources of dissatisfaction with headphone monitoring are the quality of the low-end response and the distorted spatialization of headphone soundfields (“in-each-ear” and “inside-the-head”), which many performers find uncomfortable. Regardless of the frequency response, headphones do not convey the strong sensation of bass of loudspeakers, which some musicians (like bass players) demand, because low frequency perception is more physical than aural. Bass notes are conducted through bones in the body, and merely hearing them lacks impact. In that case, headphone monitoring is still an option if supplemented with vibration transducers such as “shakers” and subwoofers, which add a physical sensation to bass. See A Quick Guide To Headphone Accessories for more information about vibration transducers.
The distorted perspective of headphones can be mitigated by first processing the mix through an acoustic simulator such as a crossfeed filter. Where crossfeed processing is not sufficient, an auralization processor (virtualizer) applies more complex processing to achieve true 3-D spatialization. Virtualizers were once implemented with expensive computers and software, but are now available in consumer audio gear. They can be added as an outboard to an existing monitoring system. Acoustic simulators are sold separate devices or as components of headphone amplifiers, of surround sound decoders and even as accessories with headphones. Many PC sound cards feature 3D sound outputs for headphones. Be careful to distinguish between acoustic simulation for for headphones and for loudspeakers (acoustic simulation for loudspeakers generates surround sound from stereo loudspeakers). Some in-ear monitors, such as AKG’s IVM1, have a built-in virtualizer. For more information about acoustic simulators, see A Quick Guide To Headphone Accessories, An Acoustic Simulator For Headphone Amplifiers and Technologies for Surround Sound Presentation in Headphones .
STUDIO MONITORING AND MIXING
More Monitoring Options For Performers
In the studio, setting up the monitor mix for performers may be slightly less complex, since the performance is not live. Most of the principles of setting up a mix for a live performance still apply. In this less formal atmosphere, there is more flexibility in configuring the headphone system. If hearing conservation is not an issue, then open-air phones are more comfortable than closed-ear types. However, if played too loudly, open-air types are prone to leak or bleed sound into microphones, so should be offered to performers as a second choice. Remote mini-mixers, which could be a distraction onstage, are a blessing-in-disguise in studios and let musicians instantly customize their mix, thus freeing engineers to focus on other things.
Vocalists are most comfortable hearing their own voices. Where hearing conservation is not an issue, vocalists can monitor with open-air headphones (again, being careful to avoid sound bleeding into the mike). Closed-ear headphones are also workable, with one earcup off the ear to let in ambient sound (mute the channel to the floating earcup to minimize sound bleed). If a vocalist wears in-ear monitors or closed-ear headphones without compromising the acoustic isolation, avoid making the voice so prominent in the mix that it sounds close-miked and unnatural – which can then cause singers to restrict their sound. In particular, vocalists who use in-ear monitors (canal-type headphones) can hear their own voices very clearly due to the occlusion effect. A compressor in the mix can help in these situations. (Apparently, some engineers raise the level of the vocalist’s mix as a natural form of compression for vocalists.) If vocalists report that they cannot hear themselves in headphones, try reversing the phase on the microphone to see if the vocalist’s voice is in phase with the voice in the headphones.
Acceptance From Recording Engineers
Headphone monitoring is also gaining converts among recording engineers, many of whom have discovered the advantages of monitoring with headphones over loudspeakers. From the console operator’s point of view, the soundfield of headphones is more detailed, so that any problems in a mix are easier to spot. However, engineers will often draw a line between using headphones for tracking and for mixdown. Headphone mixes can sound terrible when played back over loudspeakers, due to the different characteristics of the soundfields such as frequency response, interchannel crosstalk and spatialization.
Whatever the reason (hearing conservation, budget, equipment, preference), there are success stories about mixdowns done through headphones. While it isn’t easy to correlate headphone sound with loudspeaker sound, it can be done. An understanding of psychoacoustics is a good beginning. Good mixdowns with headphones are a matter of practice (and a few tips don’t hurt either). Of course, the final result should always be checked on loudspeakers.
The Challenge of Mixing with Headphones
The close proximity of headphone transducers to the ears affects how the audio spectrum is perceived. The lack of physical sensation of deep bass in headphones was discussed earlier. Headphones also tend to be brighter than loudspeakers, because the air attenuates high frequencies from speakers before they reach the ears. Headphones direct all sounds straight to the eardrums, bypassing the acoustic shaping that occurs when sound interacts with the listener’s head. Many headphones are now “diffuse-field” equalized so that they sound flat from within the ear canal – although that equalization is based on an average head shape and may not be a good match with every listener. For more information on HRTFs and diffused-field equalization, see A 3-D Audio Primer and A Quick Guide To Headphones.
Loudspeakers play in a real acoustic space. Headphones sound artificial because each audio channel is isolated to one ear. Sound waves from loudspeakers interact with each other (interchannel crosstalk), with wall reflections and with the listener’s head before they reach the ears. The resulting soundfield is a complex amalgam of phase-shifted amplitudes, which may amplify, cancel and/or delay select frequencies. It is impossible to determine through standard headphones how the phase and amplitude variations in one audio channel will affect another when played back over loudspeakers. Consequently, an otherwise smooth headphone mix can have a decidedly rough quality when heard through loudspeakers.
Acoustic simulators can improve the distorted perspective in headphones. Even the simplest of acoustic simulators, a crossfeed processor, can recreate interaural crosstalk in headphones for a more natural presentation. Beyond mere crosstalk is the whole issue of true spatial perception. Binaural recordists must monitor with headphones to hear spatial information, but the narrower soundfield of headphones can result in regular stereo mixes sounding almost monaural over loudspeakers. Until surround sound came into vogue, few audio engineers spoke of mixing for true spatial placement. Yet, with a good microphone configuration to capture localization cues, a stereo soundfield from quality loudspeakers can reproduce a sense of spatial depth and height as well as left-right width. The average headphone is not capable of re-creating these spatial artifacts in a stereo recording.
In terms of spatial editing, engineers have had a limited set of spatial options for stereo recordings: pans, delays, reverberation and other special effects. Headphones are perfectly good for auditioning spatial effects, unless the effects phase shift signals so that they sound different when heard over loudspeakers. And of course, standard headphones trap the soundfield in a straight line between the ears, so are of little value for directing placement of voices and instruments in a 3-D surround field.
Engineering A Realistic Acoustic Environment In Headphones
While many of the same techniques for making headphones sound more natural to musicians are easily adapted to recording engineers, the importance of having a close correlation between headphone and loudspeaker sound demands careful selection of headphone equipment and application of techniques. First and foremost, if they don’t already own a pair, engineers should audition diffuse-field equalized headphones, which are designed to sound flat inside the ear canal. Diffuse-field equalization is a fairly common product feature nowadays (for example, many of the AKG and Sennheiser phones are diffuse-field equalized).
If more than one engineer is participating in a session, giving everyone the same brand and model of phones set at the same gain will help with consistency of perception (if not peace). Closed-ear phones and in-ear monitors have the clearest and most extended reproduction, while attenuating ambient noise. Also, phones with good acoustic isolation are better for monitoring low-level (85-90db) mixes, which sound better on consumer audio systems. See A Quick Guide To Headphones for more information about diffuse-field equalization.
When monitoring with headphones, Fred Ginsberg of the Equipment Emporium recommends learning to set levels by ear instead of eye. The headphone level should be adjusted so that the 0 VU reference tone sounds as loud in one’s head as a loud telephone conversation – uncomfortable, but NOT painfully loud. Shouts and emphasized vocals should only briefly jump into the zone on a VU meter. LXH2 in his article Thoughts And Processes On Mixing With Headphones suggests the use of a tuned bass circuit when mixing low frequency content.
Whether or not the phones are diffuse-field equalized, binaural recordist Ron Cole suggests equalizing headphones with the biophonic curve (shown above) as a guide to compensate for ear canal resonances and other spectral differences between loudspeakers and headphones. Biophonic EQ (as well as any other signal processing mentioned below) is for listening purposes only, so the equalizer should be inserted just before the headphone amplifier. The biophonic curve is only a guide, and experimentation is encouraged. For more information about the biophonic curve, see Taking Audio In Another Direction.
Most headphones tend to image between the ears or in the back of the head. A binaural microphone system can help to create a realistic headphone sound field (see Thoughts And Processes On Mixing With Headphones). A simple technique for pulling the soundstage forward with supra-aural (on-the-ear) phones is to wear the earcups slightly lower and forward on the ears. Try out various positions to get the best localization and depth. The goal is to get the sound to enter the ears at an angle and engage more of the HRTFs of normal hearing. Unfortunately, this trick does not work as well with circumaural phones, which are designed to remain in a fixed position on the ears.
Acoustic simulators (crossfeed filters and virtualizers) electronically recreate the properties of a true acoustic space in headphones. The inability to hear interchannel phase effects on a recording is a major obstacle to using headphones for mixing. A simple crossfeed processor can mimic this aspect of acoustic space by introducing crosstalk between channels, so that phase effects can be heard. In fact, a good crossfeed processor tries to avoid overemphasizing phasey artifacts on recordings. At the same time, the processing smooths out the sonic image inside the head – no more stereo echos bouncing off the ears or holes in the soundstage. Moreover, by reducing the exaggerated stereo effect of headphones, crossfeed can help produce a better stereo mix.
For the most part, crossfeed simulators are electronic devices, but there are other options: headphone designs that provide an acoustical form crossfeed (such as the AKG K1000s) and PC-based crossfeed applications. See A Quick Guide To Headphone Accessories and HeadWize Projects Library for more information on crossfeed processors.
Note: Spatial enhancers are circuits that phase-invert the crossfeed to achieve a more spacious sound in headphones by adjusting the amount of ambient sound (such as reverberation) in a recording. Because these types of acoustic simulators dramatically alter the phase of the crossfeed, they are NOT suitable for checking interchannel phase interaction in recordings. In general, audition and experiment with acoustic simulators to become familiar with the characteristics of the sound fields that they create.
If an engineer insists on the utmost realism when mixing with headphones, then a headphone virtualizer (auralization processor) may be just the ticket. Virtualization takes acoustic simulation a huge leap beyond crossfeed to completely externalize the headphone soundfield outside the listener’s head. Virtualizers simulate a virtual, loudspeaker array inside regular headphones. Virtual reality headsets would not be convincing without them. For example, a stereo virtualizer will simulate a soundfield of two in-front loudspeakers. A surround virtualizer recreates five (or more) virtual speakers around the listener’s head.
Virtualizers also let the listener adjust many of the acoustic characteristics of an audio signal to mimic a variety of acoustic spaces – ranging from a large, reverberant concert hall to a small nightclub (add a vibration transducer to enhance the physical sensation of low frequencies for greater realism). Some virtualizers may be calibrated for each listener’s head-related transfer functions to generate highly accurate spatial cues. There are also special headphones with motion sensors (or standard phones with an add-on motion sensor) to vary the perspective of the sound field as the listener’s head moves.
Acoustic simulators are sold as separate devices, as a feature of headphone amplifiers and surround sound decoders, as headphone accessories, and more recently, as plugins for PC-based music players. Many PC sound cards offer 3D sound outputs for headphones, based on technologies from companies such as Aureal Corporation, SRS Labs and Creative Labs. (Note: Be careful to distinguish between acoustic simulation for for headphones and for loudspeakers. Acoustic simulation for loudspeakers generates surround sound from stereo loudspeakers). There are also advanced PC cards and software (from companies like Lake Technology and WaveArts, Inc.) that turn PCs into full-blown acoustic modelling workstations.
When purchasing an acoustic simulator and, in particular, a virtualizer, careful and extensive auditioning is a must, as the quality of the image depends heavily on how well the processor approximates an individual’s HRTFs. For example, head-movement tracking may be critical for 3-D hearing for one person, but not another. Check the environmental adjustments to make sure that the various simulations are realistic. All Dolby Headphone virtualizers are pre-configured to simulate a Dolby Reference Room 1, which acoustically models a small, well-damped room appropriate for both movies and music-only recordings. Additionally, DH virtualizers may simulate a Room 2 (a more acoustically live room particularly suited to music listening) and/or a Room 3 (a larger room, more like a concert hall or movie theater).
Fortunately, the popularity of PC-based music players (especially MP3-type players) has spurred the creation of low-cost and no-cost acoustic simulation plugins from most of the major 3D technology companies. QSound, SRS Labs and Lake Technology are just a few of the companies that have written plugins for PC-based music players such as Winamp and RealPlayer. These plugins are an inexpensive (often free for evaluation and less than $30 to purchase) means of evaluating the various competing 3D sound processing algorithms. However, poor performance from a plugin by a particular vendor does not necessarily reflect on the performance of any hardware implementations of that same technology.
Prices have fallen on hardware simulators as well. New (and improved) consumer devices, such as the Sennheiser DSP360, are appearing with MSRPs of less than $100. AKG’s Hearo wireless headphones have virtualizer circuitry inside the wireless transmitter. The Hearo 999 Audiosphere is full-featured enough for studio use. For more information about acoustic simulation and virtualizers, see A Quick Guide To Headphones, A Quick Guide To Headphone Accessories, A 3-D Audio Primer and Technologies for Surround-Sound Presentation in Headphones.
Engineers involved in surround-sound mixing have an alternative to virtualizers: surround-sound headphones. The standard 4-channel headphone, run directly from a 4-channel amplifier, does not produce a realistic surround field, because it does not integrate the listener’s HRTFs. Research indicates that these phones could sound more like loudspeakers with interchannel crosstalk and a small delay to the rear channels (around 30 to 50 ms) and the crosstalk feeds (5 to 10 ms). Another non-electronic solution for virtualization is Sennheiser’s Surrounder sound collar, which projects a sound field around the listener’s head. However, the Surrounder provides no acoustic isolation and is a different experience in feel and fit from headphones. For more information about surround headphones (including recent developments in 4-channel phone design), see A Quick Guide To Headphones and Technologies for Surround-Sound Presentation in Headphones.
8/15/98: added section re: technique for improved in-front localization with supra-aural headphones.
7/10/99: updated section Engineering A Realistic Acoustic Environment In Headphones.
12/18/99: revised sections discussing acoustic simulation technologies.
5/1/00: added discussion of using Dolby Headphone from a mixing console.
8/4/00: added section on acoustic simulation plugins for PC-based music players.
References: Much of the research for this article came from a long (and laborious) review of audio newsgroups, where gabby engineers enthusiastically share their tips and techniques. There is no way to list them all, so I must thank them en masse.
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