As with all listening devices (including your ear) when you begin placing material between you and the audio source it will start to degrade the audio quality. You might still be able to hear the conversation, but the sound will become weak and the movement of the cloth over the microphone will make a loud rubbing sound on the audio file. For best results it is better to keep it out of sight, but still in the open air. Overall, this is something that the customer will have to play around with themselves to see what works best for them.