A. Why would one have to 'explain' it to friends and relatives who visited?
B. It doesn't have to be approached as a 'tickle room', while discussing it with builders.
When I lived on the east coast for a few years, my landlords built a soundproof music room in the basement. I saw that it might lead to other applications.
It wasn't truely soundproff, since they had the bad habit of canking the volume up, and beating themselves up with their own sound waves. They would leave after a half hour, with headaches, and feeling drained. Self mutilation it looked like to me.
But back to the original topic, just building a room seperate from the house is all one would need to do. One could add their own eyehooks for suspention use, and add whatever 'furnature' one desires.
Any friends or relatives who stumbled upon it, (like if it was in the basement) could just be told it was a work space. If someone entered it unawares by gum, you got yourself a new victim. (Locks would be a good idea.) But otherwise, if you were discreet about it, one wouldn't need to 'explain' it to anyone.
Like that very early Frank Zappa song goes,
"No one will know if you don't want to let them know. No one will know lest it's you that might tell them so."
