Can the frequency of an electrical current be heard and distinguished from other frequencies just by connecting circuit to a speaker?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. (a) Simple test speaker. (b) Attenuator for PC speaker input.

Figure 1a might work adequately enough for you. C1 removes the DC component. R1 limits the current to a safe but possibly inaudible signal.

Figure 1b would allow you to plug into a set of computer loudspeakers with built-in amplifier. Start with the volume turned down.



Figure 2. gStrings for Android.

Many of the guitar or piano tuner apps have frequency readouts on them. Hold the phone up to the speaker and you can read the frequency.