How long do I have to wait to hear the heartbeat at an ultrasound?

Anybody know approx. how far along I’d need to be for me to be able to hear my baby’s heartbeat through the actual ultrasound itself? In my last 2 trips to the doctor, the first time I had no u/s, but we were able to hear the heartbeat with that little handheld doppler thing (sorry I’m not too great with this “lingo”, this is my first baby!), and the second time (about 2 weeks ago at 12wks6days), we saw the ultrasound, but again had to hear the heartbeat on the same little doppler machine… I am now 14wks and 6 days and will be going back to the doctor in 2 weeks (although not for a sono). How long do I have to wait? Thanks in advance!!!