In some countries it's enough for one party to consent, which can be you, so it's legal.

In the US it even varies by state, which has gotten people in trouble with recorded phone calls.

I am using it with friends and family, and tell them beforehand about the experiment

What about going out to a restaurant, a party, a date, or even somebody you bump into on the street? You're recording a lot of people, which to me is a huge invasion of privacy. What do you do if someone is uncomfortable with being recorded?

I mean, at a practical level, you don't tell them and ignore it. The fact that it is illegal to record people doesn't mean it's against your personal ethics.

Same question applies for any photograph you take in public. Isn't the fact that the people are in public material to whether or not they can be recorded? That's different than calling them on their phone and recording that conversation.

I mentioned it in another reply but this is discussed in The Every by Dave Eggers, there is essentially an entire area of San Francisco that is deliberately kept free of any and all devices that can record A/V and in order to enter you have to deposit your phone and be wanded to detect surreptitious recording devices.

In New Hampshire and a few other states it is illegal to record audio from other people without permission. I know because I was part of a team building a social network (CenceMe) that shared sensor data from phones so people could see what others were doing. We wanted to use machine learning on audio and aside from the fact that it would have greatly reduced phone battery life we found it was illegal in the state we were doing experiments in.

Is it illegal to record it, or is it simply inadmissible in court?