Sound in Medicine Lecture

Characteristics of Sound Waves

• A sound wave is a pattern of disturbance caused by energy traveling away from the source of the sound.
• Sound is a mechanical disturbance from a state of equilibrium that propagates through an elastic material medium with a definite velocity.
• Sound travels fastest in solids, relatively slower in liquids, and slowest in gases.

Frequency and Wavelength of Sound Waves

• The number of rarefactions and compressions that occur per unit time is known as the frequency of a sound wave (f).
• The distance between successive compressions and rarefactions is known as the wavelength of a sound wave (λ).
• The speed of sound is given by: vs = fλ.

Sonic Spectrum

• The sonic spectrum can be classified into three frequency ranges: infrasound, audible sound, and ultrasound.
• The human ear can hear sounds in the range of roughly 20 Hz to 20 KHz.
• Infrasound refers to sound frequencies below the normal hearing range (<20 Hz).
• Ultrasound refers to sound frequencies above 20 KHz.

Infrasound

• Infrasound can travel long distances without losing much power due to its low absorption and large wavelength.
• Infrasound can produce clear symptoms including respiratory impairment and aural pain.
• Infrasound is used in the study of heart mechanical function.

Ultrasound

• Ultrasound is used clinically in various specialties.
• Ultrasound gives more information than an X-ray and is less hazardous for the fetus.

Intensity of a Sound Wave

• The intensity (I) of a sound wave is the energy carried by the wave per unit area and per unit time.
• Intensity is expressed by the maximum change in pressure (p0) as: I = p02 / 2Z.

Sound Intensity Level [Ratio]

• The absolute value of sound intensity (I) cannot be measured, but it can be compared to a reference intensity (I0).
• The intensity ratio is given by: 10 log (I/I0) = 10 log (10^12 W/m²).

Effect of Sound on Human Hearing

• Loudness (or volume) is the degree of sensation of sound produced in the ear, which depends on intensity.
• Pitch refers to whether a sound is high (sharp) or low.

Sound Reflection and Transmission

• When a sound wave is applied perpendicularly to the interface between two media with different acoustic impedance, a portion of the wave will pass through and another will reflect.
• The ratio of reflected to incident waves can be measured as: Iref/Iin = (Z2 - Z1) / (Z2 + Z1).

Applications of Audible Sound in Medicine

• Stethoscopes are diagnostic instruments that amplify sounds made by the body from the heart, lungs, or other body sites.
• Modern stethoscopes consist of a bell with a thin diaphragm, tubing, and earpieces.