I do not understand why a simulation where you calculate the 50 lowest modes between 3kHz and 3.5kHz should be without a result?
Let’s take the 3D-printer project as an example. If you calculate for example the first 10 eigenfrequencies/eigenmodes it will look like this (copied from your project):
You can see that there are the 7th and 8th right below 700 Hz and 2 modes above 700 Hz. If you are interested in frequencies above 600 Hz, you can for example run the same analysis changing the interval of interest to 600 up to some very high number if you definitely want to get 20 results. Then the solver calculates the 20 lowest eigenfrequencies that are above the limit of 600 Hz. If there are more than 20 in the interval, the rest will be disregarded. See below:
(the 4 first eigenfrequencies are matching to the last result + the 16 next frequencies).
If you would define a range which contains less than the specified max number (20 in this case) of frequencies, the result will contain only those eigenfrequencies.
(only two frequencies between 600 and 700Hz)
Hope it is clearer now!