Detecting hidden voice recording device is absolutely impossible by legal way because such type of spy devices doesn't transmit any signal. So if person has voice recording pen or fitness band with built in mic you will never detect it using any detector. But there is one way which 100% can protect your conversation from spy recording - white noise generator and audio jammer. Such type of counter surveillance devices is effective against any microphone based eavesdropping device including: tape recorders, RF transmitters, hard-wired microphones and shotgun microphones. It even protects against microwave or laser reflection pickups. The Audio Jammer varies it's frequency and amplitude randomly, making it so that it cannot be easily filtered. The only sound an eavesdropper will hear is the Audio Jammer noise.