My first, and last, audio reference involved calibration whilst beating against WWV.
That got real old, real fast, and started making recordings of decades and octaves from the tuner in band hall. Took 45 minutes to warm up, but never more than a fraction of a cent off.
That got old too, so ended up buying audio test CD. Pretty sure was from W5YI.
Rock stable signal out of headphone jack of any *1x* cdplayer in a computer.
Now ? Last time I needed 1K I took an oven heated / gps derived & corrected 10Mhz reference and divided until I got what I wanted. /10, /10, /10, /10.
And if I am in a rush, I will just grab my 1khz tuning fork. Pulling that out after getting stopped and questioning calibration has gotten me out of ALOT of speeding tickets.
Most, if not all, were rightfully deserved.
~SD
