Section 2: Physics of Ultrasound

where D is the diameter of the transducer
and λ the wavelength.
K= is the spread factor
K=1.22 for null edges
K=1.08 for 20dB down point (10% of peak)
K=0.88 for 10dB down point (32% of peak)
K=0.7(0.56?) for 6dB down point (50% of peak)
Source for K, ASNT Study Guide UT by Matthew J Golis