...The inductor and piezo is in the 5v output of the arduino.

When i measure across the piezo i get around 3.3v

...However when measuring the AC voltage i only get half of what's inputtet through the waveform generator, any guess what's wrong?

How do you know what's coming-out of the waveform generator?

DMMs are calibrated to measure the RMS voltage of a sine wave. If you don't have a sine wave it won't be correct.*A 0 - 5V square wave has a peak of 5V (and 5V peak-to-peak). The average is 2.5V and the RMS is also 2.5V.

The RMS of a sine wave is 0.707 times the peak and the average** is 0.637 x peak. Since

the meter assumes a sine wave, some meters will simply filter the rectified & filtered voltage to get the (rectified) average, and then multiply by (0.707/0.637) to get RMS.

Or, it might read the peak and multiply by 0.707. It looks like that's what your meter is doing...It's just the wrong formula for a square wave.

* There are "True RMS" meters. I have one at work.

** That's the average of the rectified voltage or the average of the absolute values. The

*true* average is zero, since a sine wave is positive half time and negative half the time.