Sufficient ambient noise will confuse any speech recognition system. I have not found DNS 12 particularly susceptible to external noise ... however you need a good directional microphone. The one that came in the box with the product works well. The mike is a standard connection (not USB). I would recommend staying away from USB or Bluetooth for DNS. For the record I run DNS on Win7 x64 with 8GB RAM, quad-core Intel chip and dual Samsung 128GB SSDs configured for RAID-0.
I record on my laptop using its built in microphone while the laptop plays music from its built in speaker (speakers?) and the open windows have occasional noisy traffic. The problems I have with it do not seem to be related to ambient noise.
Whether it was Tv in background or just its habit of getting bored with same words, it just got crankier and not liking basic words it hadn't had oroblem with before. Even to not even bringing them up in correct, making have to spell the words out each time. No different in this problem then ladt time I used Dragon 9 years ago.
Had problems with tv on full volume but not sure that problem was different once tv off - got worse and worse on recognizing same word til wouldn't bring it up in correction box even. Has to spell it out each time. Training words is DEFINITELY WORSE than Dragon version I used 9 years ago. Back than it gave you ability to train it to tell difference between word it preferred and word you wanted, now it doesn't. So useless if it likes a word better than the one you want it to recognize.