Sometimes a hero needs a little peace and quiet.
Much of the research for The Duke and The Enchantress focused on music, namely such topics as soundproofing, patronage, style and tonality, and reputation.
Soundproofing research proved especially difficult because there was no such concept at the time, at least not that I could find. That said, there was still a good deal of information on acoustics and the density of building materials. Should someone have been motivated at the time, they could have crafted a soundproofed room or home with a bit of ingenuity.
Want to reduce sound? Create a thick barrier. Want to stop sound? Use a material that absorbs it. Want to scatter sound? Create an air gap between walls.
I wanted the soundproofing mentioned in the story to be as realistic as possible for the time when it came to materials, logistics, cost, aesthetics, and inspiration. I wanted it to be something that anyone with motivation and means could have accomplished.
The answer? A mixture of stone and cob.
The idea behind noise suppression is to reduce, absorb, and scatter sound. Use the right material, and you'll dampen noise to a small extent or a great extent, possibly to the point of complete soundproofing. This article has an interesting and simple explanation of how soundproofing works.
Stone walls (think of a castle or a Cornish hedge) are great ways to create that thick barrier for noise reduction. Couple that with cob construction, and you have a match made in heaven for Georgian era soundproofing. Read a little about this cob-lover's discovery of cob as a soundproofing device here.
I'm a little in love with cob after all of this research! To simplify it to the most basic of materials, cob is a mixture of straw, subsoil, and water. It is the true-to-form Earth home. This makes it fireproof, inexpensive to construct, and fairly resistant to earthquakes. Most importantly, it's been used for house construction since the 11th century, though how we're most familiar with it for English property construction comes from the 17th century. Those adorable English cottages you've always wanted to stay in? Made of cob.
While I doubt the duke would have gotten his hands dirty, he could have completed the soundproofing himself if he desired. Another reason I went with stone and cob for his soundproofing technique is so that the home would not undergo any structural changes or renovations. I didn't want the duke to add a room or demolish a room, rather use the available space in the home, namely an inconspicuous place to prying eyes. It would have been all too easy to add stone and cob layers against the existing inner walls of the manor.
The bookshelf he uses to enter and exit the soundproofed room wasn't only for the aesthetics (and mystery) of a hidden door, rather was another form of soundproofing. If he had used a traditional door, all of his soundproofing efforts would have gone to waste because the door would have leaked sound. Wood, no matter what type, is too porous to stop sound, though a dense wood could help absorb some sound.
In the duke's case, he has two custom fitted bookshelves of the densest wood connect to each other with a gap between, creating an air pocket to scatter sound. It would have been possible for him to insulate the inner sides of the bookshelves with cob, as well, to absorb and scatter sound. I would imagine, being that he's opening a double-sided bookshelf made of heavy wood, cob, and that oh-so-important air gap, it is no light feat. He must have nice pecs and biceps, then, eh?