Overview
Sound is used in medicine both to examine the body and to treat it. This topic covers the physical characteristics of sound and how we hear it, then the diagnostic methods built on it (auscultation, blood pressure measurement, audiometry, phonocardiography) and the therapeutic use of shock waves in lithotripsy.
Physical and psychophysical characteristics of sound
Psychophysical acoustics deals with the relationship between the physical parameters of sound (frequency, wavelength, intensity, pressure) and their subjective perception by humans (loudness, pitch, timbre).
- Objective (Physical) Characteristics:
- Frequency (f): Number of oscillations per second (Hz). Determines pitch.
- Wavelength (λ): Distance a wave travels during one oscillation (λ = v/f).
- Intensity (I): Energy per unit area per unit time (W/m²).
- Sound Pressure (P): Root mean square variation in pressure due to the sound wave.
- Subjective (Perceptual) Characteristics:
- Sound Level (Loudness): Perceived magnitude of sound, measured in decibels (dB).
- Pitch: Human perception of frequency.
- Timbre: Quality or color of sound that distinguishes sources with the same pitch and loudness (e.g., violin vs. flute).
- Tones:
- Simple: One frequency.
- Complex: Combination of multiple harmonics.
- Acoustic Resistance (Za) = ρ × c (density × velocity of sound).
- Noises: Irregular sound waves with broad frequency spectrum.
- Intensity I = (P_{max}^2) / Z_a.
- Sonic Booms: Produced when an object moves faster than sound; wavelength λ = c/f.
- Hearing Range: 20 Hz – 20,000 Hz (infrasound < 20 Hz, ultrasound > 20 kHz).
- Intensity Level (E):
- Sound Level (L):
Measured in decibels (dB).
- Frequency Resolution: Smallest detectable difference in pitch (~2–3 Hz for musicians).
- Spatial Resolution: Ability to distinguish between two sources based on angular separation.
- Vocal Apparatus: Vocal cords produce vibrations; frequency determines pitch.
- Auditory System: Outer ear → tympanic membrane → ossicles → cochlea → auditory nerve.