Wearable Neck Surface Accelerometers for Occupational Vocal Health Monitoring: Instrument and Analysis Validation Study

Abstract

Background: Neck surface accelerometer (NSA) wearable devices have been developed for voice and upper airway health monitoring. As opposed to acoustic sounds, NSA senses mechanical vibrations propagated from the vocal tract to neck skin, which are indicative of a person's voice and airway conditions. NSA signals do not carry identifiable speech information and a speaker's privacy is thus protected, which is important and necessary for continuous wearable monitoring. Our device was already tested for its durable endurance and signal processing algorithms in controlled laboratory conditions.

Objective: This study aims to further evaluate both instrument and analysis validity in a group of occupational vocal users, namely, voice actors, who use their voices extensively at work in an ecologically valid setting.

Methods: A total of 16 professional voice actors (age range 21-50 years; 11 females and 5 males) participated in this study. All participants were mounted with an NSA on their sternal notches during the voice acting and voice assessment sessions. The voice acting session was 4-hour long, directed by a voice director in a professional sound studio. Voice assessment sessions were conducted before, during, and 48 hours after the acting session. The assessment included phonation tasks of passage reading, sustained vowels, maximum vowel phonation, and pitch glides. Clinical acoustic metrics (eg, fundamental frequency, cepstral measures) and a vocal dose measure (ie, accumulated distance dose from acting) were computed from NSA signals. A commonly used online questionnaire (Self-Administered Voice Rating questionnaire) was also implemented to track participants' perception of vocal fatigue.

Results: The NSA wearables stayed in place for all participants despite active body movements during the acting. The ensued body noise did not interfere with the NSA signal quality. All planned acoustic metrics were successfully derived from NSA signals and their numerical values were comparable with literature data. For a 4-hour long voice acting, the averaged distance dose was about 8354 m with no gender differences. Participants perceived vocal fatigue as early as 2 hours after the start of voice acting, with recovery 24-48 hours after the acting session. Among all acoustic metrics across phonation tasks, cepstral peak prominence and spectral tilt from the passage reading most closely mirrored trends in perceived fatigue.

Conclusions: The ecological validity of an in-house NSA wearable was vetted in a workplace setting. One key application of this wearable is to prompt occupational voice users when their vocal safety limits are reached for duly protection. Signal processing algorithms can thus be further developed for near real-time estimation of clinically relevant metrics, such as accumulated distance dose, cepstral peak prominence, and spectral tilt. This functionality will enable continuous self-awareness of vocal behavior and protection of vocal safety in occupational voice users.

Keywords: mechano-acoustic sensing; neck surface accelerometer; voice monitoring; wearable device.

Figure 1

The NSA Wearable Device. (A) Hardware instrument, and (B) Schematic design. Adapted from “Figure 1. The physical prototype and schematic of the NSA”, by Lei et al, 2019 [4] and licensed under CC BY 4.0. PCB: printed circuit board.

Figure 2

Human Protocol of Voice Assessments and Voice Acting. Voice assessments included Self-Administrated Voice Rating questionnaire (SAVRa) and neck surface accelerometer (NSA)-derived acoustic voice evaluation.

Figure 3

Means and standard errors (error bars) of Self-Administrated Voice Rating questionnaire (SAVRa) as functions of Time and Gender Group. The voice acting session is highlighted in the pink region. Asterisks denote statistically significant differences between a specific time point and Day 1 (**P≤.01, *** P≤.001). DISC: laryngeal discomfort level; EFFT: current speaking effort level; IPSV: inability to produce soft voice; n.s.=no significant differences.

Figure 4

Means and standard errors (error bars) of accumulated distance dose (Dd) as functions of Study Group and Gender Group. (A) Total 4-hour sessions. (B) First and second parts of session. n.s.=no significant differences, ie, P>.01.

Figure 5

Means and standard errors (error bars) of neck surface accelerometer-derived acoustic metrics in the Rainbow Passage Task as functions of Time and Gender Group. Asterisks denote statistically significant differences (1) between the female (F) and the male (M) participant groups, as well as, (2) between a specific time point and Day 1 (*** P≤.001). CPP: cepstral peak prominence; f₀: fundamental frequencyo; H1 – H2: difference between the first and second harmonic magnitudes; HRF: harmonic richness factor; SAL: skin acceleration level; SE: spectral entropy.

Figure 6

Means and standard errors (error bars) of neck surface accelerometer-derived acoustic metrics in the Sustained Vowel Task as functions of Time and Gender Group. Asterisks denote statistically significant differences (1) between the female (F) and the male (M) participant groups, as well as, (2) between a specific time point and Day 1 (*** P≤.001). CPP: cepstral peak prominence; f₀: fundamental frequencyo; H1 – H2: difference between the first and second harmonic magnitudes; HRF: harmonic richness factor; SAL: skin acceleration level; SE: spectral entropy.