I do the same thing with toads except instead a patch on the skin, I use a small electrode just beneath the skin since toad skin is sensitive to chemicals and sometimes slippery. Also, instead of a trailer, I perform these experiments in field stations, hostals, apartments and at Centro Jambatu
in Ecuador and Peru. I also give them a little bit of paralytic that keeps them still throughout the test (~50 min per test). Then put them in a large trunk, lined with acoustic foam and play tones at varying frequencies. I increase the sound level at each frequency until I see the hearing nerve fire and viola! We know what the frog ear is sensing (or not sensing).