Why is the phase shift not within 360 degrees when measuring with WinMLS?

A typical question from people with experience from traditional measurement equipment is:

Question: “I have started to try to measure phase responses now, but it seems like WINMLS always presents a huge phase shift with increasing frequency that is far from reality. Several thousand degrees while reality is maybe 360 degrees. It looks like this phase shift is somehow "built in" and always present in any measurement.”

Answer: It is true that the phase shift is built-in. If you measure the impulse response of a loudspeaker you will notice that there is a certain flight time (given by the distance from the loudspeaker to the microphone). This flight time gives a delay and this delay is the same as a linear phase shift. A delay of as little as 1 sample will give a phase shift that linearly increases with frequency and is as much as 180 degrees at half the sampling frequency.

This phase shift can be removed in several ways.

The easiest way is to go to Plot->Plot Type Settings->Phase Response/Function…. In the dialog make sure the check box shown below is checked.

Alternatively, you may type in the time of flight delay in milliseconds in the same dialog box.

You may also make sure the time window is set so the initial delay is not included.