I found this android app for my galaxy note phone very interesting:https://play.google.com/store/apps/deta ... HZpZXciXQ.
The $5 pro version shows an audio spectrum up to 40khz or so in an attractive moving spectrum graph. It is so sensitive it will show an audio spectrum of your breath even if you are breathing quietly near it.
We talk here often about finding modern things that radiate one way or another. With this i set it quietly on a table for example, and saw two weak lines at 18khz and 12khz in my living room. Hmmmm. by moving the phone around I located the 18khz line to my electronic dog fence controller, very strong. (i guess i could have heard that 55 yrs ago myself). The 12khz was coming from my shut off plasma tv.
You get the idea. it is fascinating. in a review of it someone said it would show bats echolocating outdoors. The rain this morning out the window showed up as a big prettty white noise spectrum.