I adopted a similar "beat method" used by Watanabe and Takeda to observe the effects of stimuli on the frequencies of the fish. This method takes advantage of wave interference. When two waves of slightly different frequencies interact, they will produce an alternating constructive and destructive interference pattern -- with sound waves, this is interpreted as alternating loud and soft sounds. Each of these fluctuations is known as a beat and the number of beats that occur in a second is known as the beat frequency.
Beat frequencies can be easily calculated -- it is simply the absolute value of the difference between the two waves. For example, if I stimulate the water with 850 Hz and the fish's resting electric organ discharge frequency is 860 Hz, the beat frequency would be 10 Hz.
Now, to visualize the beat frequency as well, I used the Backyard Brains Spike Recorder application found on their website. To start, I used the recording electrodes as input into the SpikerBox. After turning it on, I could estimate the fish's resting frequency.
Next, I turned on the function generator, setting it 5 Hz below the fish's resting frequency -- this resulted in a beat frequency of 5 Hz. The recording electrodes now pick up the wave interference caused by the stimulation frequency. On the Spike Recorder display, the beat frequency shows up as rhythmic bumps. This relates back to the idea of alternating constructive and destructive interference with the peaks being "loud sounds" and the troughs as "soft sounds".
Using the Spike Recorder, I can now record and watch the effects of certain stimuli on the fish's electric discharge frequency over time.