About 25 years ago, I had a friend who tried this experiment, it actually worked. He opened a portal to an unknown area, it appeared to be a room on the other side of the portal. He was very frightened and to test it, that what he was seeing was real. He decided to throw a broom that was nearby. He picked the broom up and through it through the portal, it went right through the portal and landed in the room on the floor on the other side of the portal. This also made him very scared as to what he had done. He then quickly shut his equipment down, the portal slowly closed and disappeared and he also noted there was a delay of the portal closing which was extremely slowly.
He was using an audio transducer driven and a few watts of audio power, in the audio range of human hearing. He never told me the audio frequencies he was using or the type of amplifier or transducer used. He said, “you never know what will come through the portal and escape into his world”