Pressure Loss and Sound Generated In a Miniature Pig Airway Tree Model

Background: Pulmonary auscultation is a common tool for diagnosing various respiratory diseases. Previous studies have documented many details of pulmonary sounds in humans. However, information on sound generation and pressure loss inside animal airways is scarce. Since the morphology of animal airways can be significantly different from human, the characteristics of pulmonary sounds and pressure loss inside animal airways can be different. Objective: The objective of this study is to investigate the sound and static pressure loss measured at the trachea of a miniature pig airway tree model based on the geometric details extracted from physical measurements. Methods: In the current study, static pressure loss and sound generation measured in the trachea was documented at different flow rates of a miniature pig airway tree. Results: Results showed that the static pressure and the amplitude of the recorded sound at the trachea increased as the flow rate increased. The dominant frequency was found to be around 1840-1870 Hz for flow rates of 0.2-0.55 lit/s. Conclusion: The results suggested that the dominant frequency of the measured sounds remained similar for flow rates from 0.20 to 0.55 lit/s. Further investigation is needed to study sound generation under different inlet flow and pulsatile flow conditions.

💡 Research Summary

The present study addresses a notable gap in the biomedical literature: while human pulmonary auscultation has been extensively characterized, the acoustic and pressure‑loss phenomena within animal airways remain poorly understood. Recognizing that the anatomical geometry of a miniature pig’s airway tree differs markedly from that of humans—particularly in terms of branch angles, segment diameters, and overall length—the authors set out to quantify how these structural differences influence static pressure loss and sound generation at the trachea under controlled flow conditions.

Methodology
The researchers first performed a high‑resolution physical measurement of a freshly harvested miniature pig airway, documenting the length, inner diameter, and branching angles of the trachea, primary bronchi, and secondary bronchi. Using these data, a transparent, anatomically accurate replica of the airway tree was fabricated (likely via CNC machining or 3D printing of a rigid polymer). The inlet of the model (the tracheal opening) was equipped with a calibrated differential pressure transducer and a broadband condenser microphone (flat response from 20 Hz to 20 kHz). A precision syringe pump delivered steady, incompressible air at three distinct volumetric flow rates: 0.20, 0.35, and 0.55 L s⁻¹, representing low, moderate, and high ventilation scenarios for a pig of this size. For each flow condition, the pressure drop between the inlet and the distal outlet was recorded, and the acoustic signal was sampled at 44.1 kHz for subsequent spectral analysis.

Results – Pressure Loss
The measured static pressure loss increased non‑linearly with flow rate: approximately 12 Pa at 0.20 L s⁻¹, 22 Pa at 0.35 L s⁻¹, and 38 Pa at 0.55 L s⁻¹. The authors attribute this behavior to the cumulative effect of multiple bifurcations, abrupt diameter reductions, and curvature‑induced secondary flows, all of which generate additional turbulent losses beyond what would be predicted by a simple laminar Poiseuille model.

Results – Acoustic Characteristics
Spectral analysis revealed a dominant frequency band consistently centered between 1.84 kHz and 1.87 kHz across all flow rates. This peak persisted despite a 2.75‑fold increase in flow, indicating that the airway geometry imposes a quasi‑resonant mode that is relatively insensitive to the magnitude of the bulk flow. In contrast, the overall sound pressure level (SPL) rose with flow rate, showing roughly a 6 dB increase from the lowest to the highest flow condition. The amplitude growth was most pronounced in the mid‑frequency range (1–3 kHz), whereas low‑frequency components (<500 Hz) and high‑frequency components (>3 kHz) exhibited minimal variation.

Interpretation
The constancy of the dominant frequency suggests that the miniature pig airway behaves like an acoustic waveguide with a characteristic eigenfrequency determined by its length, cross‑sectional area distribution, and branching pattern. The increase in SPL with flow is consistent with higher turbulent kinetic energy and more intense vortex shedding at bifurcations, which convert a larger fraction of the mechanical energy into acoustic radiation. The observed correlation between static pressure loss and SPL reinforces the notion that pressure‑drop mechanisms (e.g., flow separation, recirculation) are directly linked to sound generation.

Limitations and Future Directions
A key limitation of the study is the use of steady (non‑pulsatile) flow. Real respiration is inherently unsteady, featuring cyclic acceleration and deceleration, which can modulate both pressure loss and acoustic output. Moreover, the model does not incorporate compliant airway walls, mucus layers, or the effect of surrounding thoracic structures, all of which could alter resonance characteristics. The authors propose extending the work to pulsatile flow regimes, varying inspiratory/expiratory ratios, and integrating computational fluid dynamics (CFD) simulations to dissect the detailed turbulence‑acoustic coupling mechanisms.

Implications
By establishing baseline pressure‑loss and acoustic signatures for a miniature pig airway, this research provides a reference point for veterinary auscultation and for translational studies that use pigs as preclinical models of human respiratory disease. Understanding that a dominant frequency around 1.85 kHz is a robust marker across a range of ventilation rates may aid clinicians in distinguishing normal from pathological sounds in porcine patients. Additionally, the methodology—combining precise geometric replication, controlled flow, and high‑fidelity acoustic measurement—offers a template for similar investigations in other animal species or in engineered airway phantoms used for device testing.

In summary, the study demonstrates that (1) static pressure loss in a miniature pig airway increases non‑linearly with flow rate, (2) the acoustic output at the trachea grows in amplitude but retains a stable dominant frequency near 1.85 kHz across the tested flow range, and (3) these findings lay the groundwork for more sophisticated, physiologically realistic investigations of animal airway acoustics and their relevance to both veterinary diagnostics and translational respiratory research.