What does it mean that the fetal heart bought online shows 2mhz?

This is the working frequency of 2mhz, and the general power of home fetal heart is 2-4mhz. Exceeding this will not meet the national standard. Because I am worried that my mother will not use it, I am afraid that it will affect the health of the fetus, but it does not mean that the high frequency is good, nor does it mean that the low frequency is good. Mainly depends on the sound conversion rate of the probe. Generally, the conversion rate of all-in-one machines will be higher because of short delay. At present, there are low-end models such as comper and Edan, which can hear the fetal heart rate, but the frequency is generally unstable, and the frequency error is about 15%, which is about the fetus. It is recommended to buy a professional one, which will not affect the baby's health.