Objective: This study investigates the relationship between vocal pitch (frequency) and amplitude (loudness) in professional singers, focusing on differences between chest and head registers. The primary aim is to quantify amplitude variability as a function of fundamental frequency. A secondary aim is to analyze pitch and volume control, measured through jitter and shimmer, respectively, as a function of vocal register and loudness.

Methods:
A cross-sectional study was conducted with 22 healthy, professional singers (8 male, 14 female). Participants underwent a screening nasolaryngoscopy to rule out vocal pathology. Acoustic recordings were collected as participants produced pitches at major third intervals across their vocal range in both chest and head registers. For each pitch, participants produced their quietest and loudest volumes. Data analysis included regression analysis to model the frequency-amplitude relationship and statistical comparison of jitter and shimmer values across registers and loudness conditions.

Results:
Results indicate that amplitude is positively correlated with frequency, but the relationship is register-dependent. In chest register, amplitude increased with frequency at a higher rate for females at all loudness levels (R^2=0.84 for loud volume, R^2=0.93 for quiet volume). In head register, the correlation between frequency and amplitude was stronger than in chest register for both genders (R^2=0.98 for loud volume in males, R^2=1 for quiet volume in males, R^2=0.91 for loud volume in females, R^2=0.98 for quiet volume in males), but the increase in amplitude was at a lower rate across volume levels. Furthermore, jitter was significantly higher for females when singing quietly in head voice versus chest voice (p=0.05 for A3, p=0.006 for C#4, p=0.01 for F4, p=0.03 for A4, p=0.01 for C#5).

Conclusions:
Chest and head registers employ distinct mechanisms for loudness control. The chest register produces a steep, powerful increase in amplitude with pitch, while the head register offers a more moderated response, allowing for superior dynamic independence in the upper range. Significantly elevated jitter during quiet head voice in females reveals a register-specific challenge for pitch control in soft, high-frequency singing. These findings inform vocal pedagogy for register transitions and can guide the development of targeted exercises to improve pitch accuracy.