Sound Field Characteristics

Investigation of the sound field of an ultrasonic probe in water using a hydrophone

In the experiment, the sound field of an ultrasonic probe in water is investigated by determining the sound pressure distribution in axial and lateral direction using a hydrophone and characterizing sound field quantities are discussed.

Keywords: sound field, near field, far field, focus zone, sound pressure, sound pressure distribution, sound speed, sound intensity

The area in a medium in which sound waves propagate is called the sound field. Depending on the material and the sound generation or coupling, it has a specific geometry, significantly limits the lateral resolution of an ultrasound probe and can influence sound attenuation. The sound field can be described by sound field variables such as sound pressure and sound velocity or sound energy variables such as sound energy and sound intensity. Using a hydrophone, the sound field in a liquid can be examined by determining the sound pressure amplitude along and across the sound field axis. Characteristic features such as near field length and sound field width can be derived from the amplitude distribution.

Amplitude scan along the switching axis

Amplitude scan across the switching axis

For a 2 MHz probe (16 mm diameter) in water (c = 1497 m/s, T = 25 °C) the theoretical near field length is 85 mm. The hydrophone measurement along the sound field axis (upper diagram) shows a slightly backward shifted maximum at approx. 100 mm. The measurements of the lateral sound field distribution at different probe distances (lower diagram) show a local modulation of the signal amplitude in the near field region.

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