In engineering notation, it's usually preferred to use the appropriate SI prefixes (pico, nano, micro, 1, kilo, mega, giga...) in the unit of measurements, each SI prefix represents three orders of magnitude (1000). When the number is greater than 1000, you always upgrade your unit, so that the numerical value is never greater than 1000 [0]. Instead of "1500 Hz", You say "1.5 kHz". OP is an engineer and I guess she has been trained to always do it.

[0] These rules are not always enforced, and there are alternative notations, but the 1-1000 rule is the most common convention, writing "3500 MHz" instead of "3.5 GHz" is just awkward.