Mechanism of Vocal Fremitus Based on Frequency of Waves
The frequency of sound waves significantly affects vocal fremitus transmission through the respiratory system, with lower frequencies (below 300 Hz) being transmitted more effectively through the lung parenchyma compared to higher frequencies, which experience exponential attenuation.
Frequency-Dependent Transmission Characteristics
- Vocal fremitus transmission follows a low-pass filter pattern, with relatively flat transmission of energy up to 300 Hz at the anterior right upper lobe, after which there is an exponential decline to 600 Hz 1, 2
- At the right posterior base, transmission remains flat only up to 100 Hz with exponential decline to 150 Hz, demonstrating regional differences in transmission properties 1
- The frequency dependence of respiratory system impedance (Zrs) is a critical factor in understanding how sound waves propagate through lung tissue 1
Wave Propagation Mechanism
- Sound transmission in the respiratory system is dominated by wave propagation through the parenchymal porous structure rather than through free gas 2
- Transit times for sound waves from trachea to chest wall are approximately 1.5 ms to the right upper lobe and 5.2 ms to the right posterior base, indicating different pathways and tissue properties 2
- The propagation of sound waves is not significantly affected by gas density changes (as demonstrated in studies comparing air vs. Heliox breathing), confirming that transmission occurs primarily through the tissue matrix rather than airspaces 2
Physiological and Pathological Implications
- In patients with intrapulmonary airway obstruction, respiratory resistance (Rrs) increases at lower frequencies and falls with increasing frequency, creating a negative frequency-dependence pattern 1
- This negative frequency-dependence is explained by mechanical inhomogeneities of the lungs, which affect how different frequency components propagate 1
- In healthy subjects, Rrs exhibits an increase with frequency above 10-15 Hz due to airway wall compliance, gas compressibility in central airways, and inertial distortion of the velocity profile 1
Clinical Applications and Measurement Techniques
Forced oscillation technique (FOT) uses different frequency ranges to assess various aspects of respiratory mechanics:
The first antiresonant frequency (far,1) at high frequencies correlates better with forced spirometry indices than medium-frequency parameters and carries information about airway wall compliance 1
Frequency-Related Diagnostic Patterns
- Vocal fremitus has been applied in other clinical contexts, such as breast imaging, where vibrations from vocalization create distinct patterns in benign versus malignant lesions 3, 4
- In pulmonary assessment, the pattern of change in respiratory impedance in various pulmonary function abnormalities consists of increased resistance (Rrs) at lower frequencies and decreased reactance (Xrs), associated with an increase in resonant frequency 1
- The frequency dependence of Zrs allows for structural exploration of respiratory mechanics, with multiple-frequency measurements providing more comprehensive information than single-frequency assessments 1
Practical Considerations
- When using vocal fremitus for clinical assessment, lower frequency sounds (such as saying "99" with a low voice or humming a deep sound) provide better transmission through lung tissue 3
- The frequency range of 75-300 Hz appears optimal for clinical assessment of vocal fremitus, as higher frequencies are significantly attenuated 2
- Changes in fundamental frequency during speech are achieved through two mechanisms: increasing vocal-fold tension (primarily raising frequency) and varying subglottal air pressure (primarily lowering frequency) 5