Sound in Medicine

General Properties of Sound

• A sound wave is a pattern of disturbance caused by energy traveling away from the source of the sound.
• Sound is a mechanical disturbance that propagates through an elastic material medium with some definite velocity.
• In air, sound can be defined as a local increase (compression) or decrease (rarefaction) of pressure relative to atmospheric pressure.
• Sound travels fastest in solids, relatively slower in liquids, and slowest in gases.

Characteristics of Sound Waves

• The frequency of a sound wave is the number of rarefactions and compressions that occur per unit time.
• The frequency of a wave is mathematically represented as f = 1/T.
• The wavelength of a sound wave is the distance between successive compressions and rarefactions.
• The wavelength is mathematically represented as λ = c/f.

Sound Intensity Level [Ratio]

• The absolute value of sound intensity (I) cannot be measured, instead, it can be compared with a reference intensity (I0).
• The intensity ratio is defined as 10 log (I/I0).

Effect of Sound on Human Hearing

• The human ear can distinguish two characteristics of sound: loudness and pitch.
• Loudness is the degree of sensation of sound produced in the ear, and it depends on its 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 (Z1 and Z2), a portion of the wave will pass through and another one will reflect.
• The ratio of reflected (Iref) or transmitted (Itran) and incident waves (Iin) can be measured.
• If Z1 = Z2, there is no reflected wave, and transmission to the second medium is complete.
• If Z2 < Z1, the sign change indicates a phase change of the reflected wave.

Applications of Sound in Medicine

• Audiogram: a graph that shows the faintest sounds that can be heard (hearing threshold) and the loudest sounds that can be heard without pain (pain threshold).
• Stethoscope: a diagnostic instrument that amplifies sounds made by the body from the heart, lungs, or other body sites.

Sonic Spectrum

• 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 or less than 20Hz.
• Ultrasound: is the frequency range above 20KHz.

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, aural pain, fear, visual hallucinations, and chills.
• Infrasound can be used in the study of heart mechanical function.

Ultrasound

• Ultrasound is used clinically in a number of specialties.
• It often gives more information than an X-ray and is less hazardous for the fetus.