Final answer:
The frequency of sound in an ultrasound transducer is determined by the material composition of the transducer's piezoelectric crystal. This is because the physical and piezoelectric properties of the crystal dictate the frequency of vibrations it can produce when voltage is applied.
Step-by-step explanation:
The characteristics of the active element that determine the frequency of sound in an ultrasound transducer are primarily related to the material composition of the transducer itself. The piezoelectric properties of the material define how it responds to electrical voltages, generating specific frequencies of ultrasound. Thus, the correct answer is A) The material composition. Other factors, such as the shape of the probe, are indeed important, but they mainly affect the focusing and direction of the ultrasound beam rather than its frequency. The color of the casing and the length of the cable do not affect the frequency of the ultrasound produced by the transducer.
When used for imaging, ultrasonic waves are emitted by a crystal in the transducer that vibrates at high frequencies when voltage is applied. The frequency and intensity of these vibrations, or waves, are crucial ultrasound characteristics affecting image resolution and penetration depth. Hence, it's the physical and piezoelectric properties of the crystal that are key in determining the operating frequency of the ultrasound.