I’ve been meaning to do this for a long time - now the SUNY New Paltz students got me motivated.
I installed audacity on my Windows laptop, with a so-so USB mic, and clicked “record”.
I installed “sox” on Linux and ran the command “play -n synth 10 sine 20-20000” to play the audible spectrum with the volume of the boombox at about a third.
I clicked “stop”, selected the sound wave, and clicked “Analyze” => “Plot spectrum”. It seems to drop off markedly after 6000 Hz (sheesh, we use a $50 pair of tweeters):
Wait - it was probably a cheap mic. I try a better mic and get better results:
One of the students used an even better mic and got a fairly flat curve all the way up to 20k (so they are good tweeters :))