After showing my audiophile husband this post:
http://www.zity.biz/index.php?mx=forum;ox=display;msg=624092
He started playing with his iPhone, on which he has an app, "Audio Tools", by Performance Audio which has various wave form generators, including sine wave generator. The Audio Tools from the iTunes App Store wasn't free, but wasn't too expensive either. He doesn't remember what it cost.
He connected the headphone output on his iPhone to the audio input on our ET312B, but was not able to successfully test out the 20-25 Hz signal referenced on the article,
http://www.rehab.research.va.gov/jour/86/23/3/pdf/warner.pdf
with his rectal probe because, as I mentioned recently in a post, the battery in our ET312B has gotten a lot of use over the last almost ten years and is about dead.
Are there any other members who could try this out and let me know how it worked out?