Quite correct. The disc looks like a speaker and is in fact a tympanic membrane.
A surface that picks up sound waves much like the diaphragm of a microphone. It does not
need an auricle (ear cartilage) to collect and reflect the sound like we have, which would
offer some sense of direction of sound.
Frogs may have tried that route during evolution but those frogs looked ridiculous.
Mother nature does not have a sense of humour. Besides, frog ears are tuned into
low frequency sound and quite possibly infrasound and such long wavelengths have
no effective directionality. Long wavelengths tend not to reflect from an auricle
anyway thus that kind of anatomical structure would be useless because such low
frequency sound waves would just bend around a small auricular structure.
I know what you're thinking. No, not those thoughts ... the other ones. Elephants have
very large ears and yet also hear infrasound that can travel kilometers, so why the big
ears? Whales communicate with infrasound as well as higher frequencies and yet they
have no auricles either. I let you ponder that a bit.
