I have access to a digital piano which has a feature which stretches the tuning at the top and bottom ends to make it sound more like a real acoustic piano (which apparently has the bottom notes flattened a bit and the top notes sharpened to suit the vagaries of human ears).
I want to check whether it's working by turning it on and off, and measuring the change in frequency.
I have an electronic tuning device which is designed as an aid to tuning acoustic instruments such as guitars and violins. It clips on to the instrument and displays the name of the note being played together with how much (cents) it varies from the correct pitch - based on A=440 etc.
If I could get this device to pick up the output from the digital piano, it would do exactly what I want. But it only works by being clipped to something which is physically vibrating - it can't pick up vibrations from the air. So I need something which I can connect to the piano's line output, which generates enough vibration to be picked up by my tuning device. I've tired various little loadspeakers, etc. to no avail.
Any ideas?