Is there a device that can measure a sound or musical note in hertz?

Question

I'm looking to record a note that is 100 hertz specifically, probably with a synth or an old organ.

Answer 1

There are many ways to measure frequency

Ex: Frequency spectometer, modern scopes. However, if 100 Hz is a standard freq in instruments - a simple electronic tuner may do the job.

Make sure the recording medium can actually record such a low frequency (that is: both the microphone and the input and the recording medium support that freq. response)