Originally Posted by
CatFan
1. The shape of the transducer crystal determines the shape of the beam.
2. The size of the transducer crystal determines the beam angle. A very small transducer transmits in almost all directions. As it gets larger, the beam becomes more focused, so a narrow cone angle transducer is larger than a wide angle transducer.
3. The crystal vibrates best when the thickness of the crystal is equal to half of the wavelength of the signal driving it. So like Kosmo said, since a lower frequency has a longer wavelength, a thicker element has a lower resonant frequency.