Speaking of frequency response, I have one of these. Hours of fun.
https://www.amazon.com/Otoscope-ScopeAround-4-5-Inch-Endoscope-Included/dp/B085L6HT76/ref=mp_s_a_1_1...
https://www.amazon.com/Otoscope-ScopeAround-4-5-Inch-Endoscope-Included/dp/B085L6HT76/ref=mp_s_a_1_1...