What relationship between the time recording length and the frequency bandwidth?

Question

In model testing for ocean waves, the data recording duration is usually longer the better especially for irregular waves. The recording length shall be long enough to achieve sufficiently small bandwidth in the spectral analysis.

In ideal wave spectrum, the bound waves can also present, so the bandwidth can be large, but does it mean the recording duration can be shorter even?

On the other end, for a regular wave (zero bandwidth), then only the one period length of recording is needed, right?

So is confused completely. The time recording length is neither inversely proportional to the frequency bandwidth nor otherwise.

There must be a way to follow the bandwidth of the wave spectra. It might be a stupid question, please help with some basics.

Answer 1

There are two relevant times, not just one. One time is the sampling duration, and the other is the time between successive samples, the dwell time. From those two times you get two frequencies.

The dwell time is small, so the inverse of the dwell time is a large frequency. Half of that frequency is the Nyquist frequency. It is the highest frequency that can be accurately measured.

The total sampling time is larger, so the inverse of the total sampling time is smaller. This is your frequency resolution. It is the smallest difference between frequencies that you can discriminate.