Some forty years ago, in 1975, Mostofa Fahim and his group of researchers from the University of Missouri published a paper on male contraception--they were looking at contraceptive methods without resorting to condom.
Those had their scrota immersed in 140-degree Fahrenheit water didn't conceive for 35 days. Infrared-radiated scrota didn't conceive for 75 days. Microwave-radiated scrota weren't consistent, sometimes they worked and sometimes they didn't.
The ultrasound bunch, after a 5-minute exposure, went sterile for seven months.
No one knows why ultrasound kills sperm. But we do know that heat does, which explains why testicles are located outside the body, but sound?
In Fahim's study, the tissue temperature after ultrasound was only 100-degree Fahrenheit, much lower than the 140-degree Fahrenheit water bath, but with much deadlier effect. He hypothesized that the ultrasound treatment affected the ion exchange of fluid of the testicles, which distorted sperm development.
info: Therapeutic ultrasound as a potential male contraceptive: power, frequency and temperature required to deplete rat testes of meiotic cells and epididymides of sperm determined using a commercially available system. James K Tsuruta et. al, Reproductive Biology and Endocrinology 2012, 10:7 doi:10.1186/1477-7827-10-7