I remember standing waves from MW lesson in university about 20 years ago. I think it is something like it.Acoustics is a hard to learn science.
That's good. Remember your radiation patterns, this is just lobing. Avoid short wavelengths and as kazap is showing, there are more than 2 sources at higher frequencies as well, so short has multiple criteria. If you do this you'll be working to lower that frequency and extend the clean radiation.