In-Ear Monitors: Why Do They Sound Like That?
Jay Kadis and Stephen Ambrose
Providing good sound monitoring to performers is a critical part of producing a great experience for the audience as well as for the musicians. When people on stage hear what they need to play their best, everyone benefits. Providing such cues is the job of monitor mixers and their monitor playback system. Traditionally this has meant floor wedge speakers aimed at the performers. Unfortunately, these systems produce the potential for acoustic feedback as well as an unpleasantly loud stage environment. These problems have driven a move to personal monitoring using drivers placed in the ear canal to replace the stage monitor loudspeakers.
This solution has definite advantages, but the performance of in-ear monitors (IEMs) has been limited by changes caused by the insertion of a physical device into the ear canal. Closing the normally open end of the canal changes the acoustics of the outer ear and drastically alters the behavior of the middle ear as a result. Various attempts have been made to ameliorate these effects but so far, they have been only partially successful. In order to understand why, we need to look at the auditory system and how it behaves in the normal open-ear condition. We can then see how this behavior changes when we place a driver in the canal and how that change might be minimized without compromising the sonic performance of the system.
When the ear canal is un-occluded, as it is when we listen to loudspeakers in a room, sound waves reach the tympanic membrane, or eardrum, by propagating through the ear canal. The sound waves are tiny alterations of the static, or barometric, pressure of the air around us. This static pressure is about 101 kPa or 14.7 lb/in2 at sea level. Pressure changes that produce a just audible sound are 20 mPa while painfully loud sounds are about 20 Pa. Just audible sounds vary the pressure by less than one-billionth of the atmospheric pressure! That is the sound pressure variation that normally reaches the eardrum.
When we seal the ear canal with an IEM device, we fundamentally change the conditions that affect the pressure in the canal. By closing the canal, we create a sealed space that contains air. The air pressure is the same as the atmospheric pressure until we turn on the driver. When operating, the driver membrane moves in and out, slightly changing the volume of the closed space. This generates a pneumatic pressure that we do not encounter in the open-ear; the driver acts as a pump. The pressure generated by this pumping action corresponds to a sound pressure level of 120 dB SPL, close to the threshold of pain. This high level then triggers a physiological response that attempts to reduce the level, the acoustic reflex.
The human auditory system has a built-in compressor-like reflex designed to protect the cochlea from excessive stimulation. When a loud sound reaches the inner ear, it causes the stapedius muscles of both ears to contract, reducing the transfer of sound energy to the cochlea. This changes the quality of the sound we perceive as well as its loudness. Even listening at normal levels with IEM drivers triggers the acoustic reflex due to the pneumatic pressure it generates. In order to deliver high-quality sound, this effect must be reduced. Attempts to reduce the pneumatic pressure include venting the IEM to the outside with tubing or passages to the outside. These approaches work to some extent, but they do not perfectly counter the pneumatic effect and they do affect the sound quality. The IEM produces relatively little output at low frequencies, though that is countered by the increase in low frequency pressure cause by the pneumatic pressure contribution to the overall pressure in the ear canal. Thus, too much venting destroys the low frequency efficiency of the device. We need a better way of countering the pneumatic pressure without losing the low frequency efficiency.
A novel idea has recently been developed by Asius Technologies and is in the final testing phase. The idea is to use an inflatable airbag-like partition between the driver and the eardrum. With the proper choice of materials, the tiny balloon can be inflated to just fit the ear canal, sealing it with the user-adjustable pressure used to inflate the device. The inflated device provides a semi-elastic seal that allows the pneumatic pressure to reach the outside while maintaining a sealed ear canal that transfers low frequency energy efficiently. By eliminating the pneumatic overpressure, sound is transferred to the inner ear without eliciting the acoustic reflex. Lower sound levels then produce a louder perceived sound without the alterations caused by the middle ear muscular contractions. By adjusting the inflation pressure, the listener can determine the amount of outside sound blockage as well as the fit of the device.
The new technology is not only ideal for stage monitoring; it is applicable to recreational music listening and to hearing aids as well. The device reduces the occlusion effect for all listeners, making the listening experience more like listening to sound from loudspeakers rather than hearing sound as if it originated within the head, a common effect of conventional IEM devices. Finally, in-ear monitors can sound like loudspeakers without the threat of feedback and without creating an unnatural sensation of sound in the ear!
Jay Kadis is the author of the just published book The Science of Sound Recording. Jay has played guitar since his school days, written and recorded original music, built studios and done research in psychoacoustics and music technology. He teaches sound recording at Stanford University’s Center for Computer Research in Music and Acoustics.
Stephen Ambrose is an industry pioneer and inventor of the wireless in-ear monitor (IEM) systems currently used in nearly all rock concerts today. He is the founder and president of Asius Technologies.