**2. Literature review**

#### **2.1 Delivering synchronization auditory cues to music performers**

The use of a click track in music performance was first documented for the soundtrack recording of *Fantasia* (Disney, 1940). Maestro Leopold Stokowski, who was an audio engineer at Bell Labs, experimented with new recording workflows to synchronize different sections of orchestra and principals on a multitrack device [12]. While the need for a click track was justified by such a creative innovation, its extensive use in music performance comes with downsides. Like sirens or fire alarms, click sounds are designed to grab attention with a lot of high-frequency energy. Therefore, long exposures to high levels of click tracks contribute to the risks of musicians' hearing loss [13]. Although click samples can be changed in digital audio workstations to accommodate musicians' preferences, the mechanical nature of the click implies that "overall, playing with a click track means playing with the metronome" [14]. According to Cardassi [15], "a click track is likely the most dreaded synchronization tool in music," and it generates performers' "angst and unpleasantness." Drawing upon Blauert [16]'s theory, spatial audio applications offer greater source discrimination possibilities than a stereo image. Therefore, binaural technologies provide sound engineers with more mixing space than stereo, which implies more source-positioning options and the need for less equalization and dynamic range compression [17] to avoid masking effects among sound

sources. Consequently, we suggest that binaural headphone monitoring solutions allow for lower click track levels and less processed instrumental and vocal sources for performers to synchronize with each other, on a soundtrack or a movie, meanwhile protecting their aural health and improving their creative experience.

Previous research suggests that a generalized use of click tracks has homogenized creativity and globalized music cultures. For instance, an analysis of tempo across the past 60 years of U.S. Billboard Hot 100 #1 Songs revealed that a 5-beat average standard deviation from 1955 to 1959 decreased to 1-beat between 2010 and 2014 [18]. Moreover, Éliézer Oubda, a music producer and sound engineer who owns *Hope Muziks Studio* in Ouagadougou, Burkina Faso, trains his assistants in explaining to Western African musicians how to perceive the downbeat in the click track in the same way Europeans and North Americans do.1,2 To minimize "the straightjacket feeling" [14] induced by click tracks, composers, performers, and studio professionals can collaborate on developing alternative cue tracks and monitoring systems. For instance, customized tracks may combine pre-recorded fragments from the parts to be performed with vocal instructions or relevant pitches. These may also feature excerpts of embedded click tracks within the pre-existing layers of audio to provide additional guidance at specific times only. These cue improvements can be accompanied by a context-dependent choice of the monitoring system. While highfidelity technologies may not always be the best solution3 , the selection of a wearable device requires some attention. Typical studio headphones consist of closed-back headphones that are "designed to block out environmental noise using a passive acoustic seal" [19]. Mostly found in live scenarios, in-ear monitors provide a more drastic acoustic isolation, with visual discretion and stability benefits in situations where the performer frequently moves their head. With non-isolated ear cups, openback headphones offer a more natural or "speaker-like sound" [19] with a more pleasant spatial image, and less risk of performers feeling isolated and disconnected from the environment. Whereas we did not consider using open-back headphones for our monitoring applications because their audio content would leak into the microphones, their benefits have inspired our binaural solutions to overcome the auditory feedback challenges of stereo closed-back headphones and in-ear monitors.

Two types of technologies exist to deliver synchronization auditory cues to musicians while providing them with direct access to their own sound production and acoustic environment, namely acoustic-hear-through and microphone-hearthrough monitoring systems [20]. Primarily developed to improve the safety of outdoor runners when they are listening to music, acoustic-hear-through monitoring systems, also known as bone conduction headphones leave the users' ear canal free by conveying the auditory cues "from the vibration of the bones of the skull [or jaw] that is transmitted to the inner ear" [21]. Whereas this technology eliminates

<sup>1</sup> See 15:00-20:10 of the roundtable discussion about "De-colonizing the Digital Audio Workstation" with Éliézer Oudba and Eliot Bates facilitated by Menon and organized by Pras and Kirk McNally: https://www.canal-u.tv/chaines/afrinum/ roundtable-discussion-about-de-colonizing-the-digital-audio-workstation

<sup>2</sup> During a jembe workshop taught by Issa Traoré alias Ken Lagaré, an arranger and sound engineer who owned *Authentik Studio* in Bamako, Mali, graduate student Leo Brooks and percussion instructor Adam Mason explained the fact that European and North American musicians struggle to hear the downbeat in Western African music (see 45:50–47:10): https://www.canal-u.tv/chaines/afrinum/ percussion-workshop-with-ken-lagare

<sup>3</sup> For example, Oubda gave the example of rural musicians from Burkina Faso who got intimidated when they heard themselves through high-fidelity headphones for the first time (see 17:00–17:40): https://www.canal-u.tv/chaines/afrinum/roundtable-discussion-about-de-colonizing-the-digitalaudio-workstation

### *Binaural Headphone Monitoring to Enhance Musicians' Immersion in Performance DOI: http://dx.doi.org/10.5772/intechopen.104845*

disconnection feelings from the acoustic surroundings, like open-back headphones, the monitoring mix may leak into the microphones. Indeed, May and Walker [22] reported "approximately 12 dB A (total) of 'leakage'" in the context of listening tests. Also, Cardassi, who tested a bone conduction headphone to record an electroacoustic album on piano and vocals, could only use it for pieces that did not require the use of a close vocal microphone, and whose cue tracks did not include any click.4 Primarily used as hearing aids devices, microphone-hear-through monitoring systems consist of mounted microphones on the users' headset that capture what they would hear without headphones [23]. Cooper and Martin [2] designed a microphone-hear-through monitoring system named Acoustically Transparent System (ATH) that combines the binaural rendering of the signal captured from two headset-mounted microphones with the synchronization cues. In performance situations, they observed that the ATH has "a notable impact on both quality of tone production and the confidence of the [trumpetist]" [2]. Their findings confirm the relevance of designing binaural technologies to improve musicians' experience while performing with headphone monitoring.
