# Rated power vs power output

Could anyone please explain to me the difference between rated power and power output. It says in the instruction book that the power output is 20 mW/ square centimeter but at the back of the machine (fetal doppler) it says: rated power less than 0,7 W). So it operates at 20 mW Or 0,7 W?

Thank you so Much for explaining this to me.’yes Inwas referring to the fetal doppler. I am trying to understand the technical Details of the devices .

Ina different doppler there is a value: Max. Audio output 500m W rms.

What is this exactly? Can it be a power output? Or is it the same as rated power?

Do you know how to determine a thermal Index and mechanical index for a doppler operating at:

Ispta 10.8, Power output less than 20 mW square cm ( or are other values needed as well)?I can provide them

I am Wondering if the transducer has to be aligned with a heart to Detect it? I used a transducer with a beam Range of 300 square mm. So if The baby measuring 7cm is in the range of that beam even if the beam is not in a straight line to a heart will i be able to hear the heartbeat or Will i only hear it when transducer is in the straight line to heart ( aligned with heart). Im trying to find the answers to those questions for two weeks already...

It seems that you are asking about the power used by a fetal doppler device such as a baby heart monitor.

It says in the instruction book that the power output is 20 mW/ square centimeter ...

That tells you that the mechanical or sonic energy output by the device will be 20 mW/cm2. Therefore the total output power will be area × 20 mW where the area is measured in cm2. This figure will be useful in determining the amount of energy going through each unit area of the subject.

... but at the back of the machine it says: rated power less than 0,7 W.

That is the maximum power consumed by the device from the mains or battery power supply.

If the device does nothing else other than generate the ultrasonics then from these two figures you can calculate the efficiency of conversion of electrical power into sonic power: Efficiency = output power / input power. (Remember to multiply the specified output power/cm2 by the area first.) The efficiency is likely to be quite low.

If the device has other circuitry such as a display or an audio amplifier then that will consume some of the input power.