 # What is Sound?

Sound is a pressure wave that is generated by a vibrating object. This vibration makes particles in the medium vibrate, this vibration allows for the transmission of energy through the medium.

When particles move in a parallel direction to the propagation direction it is referred to as a longitudinal wave. The result of longitudinal waves is the creation of compressions and rarefactions within the medium. Variations in the molecular density and pressure. Compression is the formation of the high-pressure region. Rarefaction is the formation of a low-pressure region. Amplitude

When a wave passes through a medium, the particles of the medium get displaced temporarily from their original undisturbed positions. When a wave passes through the medium, the maximum displacement of the particles of the medium from their original undisturbed positions is called amplitude of the wave. The displacement is directly proportional to the amount of work done to generate the energy that sets the particles in motion.

Frequency

The frequency of a wave is measured as the number of complete back and forth vibrations of a particle of the medium per unit of time.
10 hertz = 10 vibrations per second Wavelength

Wavelength is the distance of one complete wave cycle. Sound travels at different speeds in different mediums but if we know the speed “c “, then the wavelength is defined as:

### λ=𝑐𝑓

The speed of ultrasound in the human body is 1540 m/s

Time period

The time for one complete wave is called the time period of the wave. Therefore

### 𝑇=1𝑓 𝑓=1𝑇

Acoustic Velocity

The speed at which a wave propagates through a medium is called the acoustic velocity. Velocity is a vector quantity, magnitude and direction. Acoustic velocity we will consider as magnitude only (a scalar quantity), The rate at which the wave is transmitted through the medium. Acoustic velocity of ultrasound in the human body is 1540 m/s.

To summarise, sound waves in body tissue are longitudinal waves. They originate from a source and transmit out at a known velocity. This velocity remains constant and is independent of frequency. The greater the amplitude the more “power” we are transmitting.

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