Jerry Lande's speaker bracket thingie....
When I was a younger man- teens until about 35, I could hear above 20K, 23-ish IIRC measured with a frequency generator.
I even used specific Radio Shack "death Star" tweeters to get that extra range in most of my speaker builds. Very durable as well- never had any fail.
Now I am lucky to hear 18K
Dammit I also used to see better than 20/20, and now I need readers.......