Sound deadening material?

[2020 mustang]

I was ASSUMING that the wood was a replacement for the rear seat back which creates a separator between the trunk and the cabin area. Typically the noise is coming through the floor and quarters. When I did the wagon, I did the inside of all the external sheetmetal - doors, quarters, floors, firewall ....
I know your are not redoing the entire car but the way I understand the sound deadening stuff - it keeps the sheetmetal from vibrating/transmitting noise. I doubt (and am often wrong) that the wood is actually causing the noise itself.

I don't know about his Mustang, but mine, the factory seat backs fold dowm to allow you to put long items in the trunk and pass through to the cabin.
There isn't any factory metal panel behind the seats , like in say a car with a seat that doesn't fold down, Regal, caprice for example.
He could line the back(trunk side) of the wood with a thin sheet of metal and then put the matt on it.

[CTX-SLPR]

I used a cheap soldering iron to fuse the carpet where I poked the holes for bolts and such through to keep it from fraying around the penetration.

[nonhog]

Just watched this video. Long but interesting on this very topic. https://www.youtube.com/watch?v=rnHbHJ1BQXA

I added sound (Kilmatt) and heat(Noico) to my DD Tundra after adding off road tires. Worked well! Still hear the tires but it's better!

[pdub]

Just watched this video. Long but interesting on this very topic.

That is a very good video on the subject! Loaded with facts and procedure. That's pretty much "it" right there. I've been filming for video on this job but now I realize that's been more for my own entertainment compared to that video. Thanks for posting it, I enjoyed watching the whole thing.

[pdub]

So I just got through putting in Red's carpet, however may days that's been. It fought me every step of the way, like it was made for some other model of Mustang or something. On the back panels going up to the backseat, forget it. Lumps and molded humps and I don't mean wrinkles, like what the hell. I'd never done carpet before When I threw it in the car the molded part fit the tunnel fit so well, I just started there and went in all directions with it. Well, I was just now vacuuming up the mess and getting ready to put the front seats back in and only then did I notice - the whole carpet is crooked, counterclockwise by about 20 degrees. No wonder it was such a dick to put in. All of those lumps on the back panel were supposed to be in the back CORNER where there are in fact lumps in the car. This is the view looking straight down the dash at the padded area underneath the pedals.

[pdub]

Here's 8 days of work boiled down to 18 minutes. I'm humbled by the really excellent video nonhog posted on the subject above. That makes me realize how useless my videos are from an instructional standpoint. But I guess I can't instruct when I don't know about the subject, eh? At least with the camera and the video editing software I get to entertain myself while I'm working on something I don't know how to do, so...

[DanStokes]

Here's 8 days of work boiled down to 18 minutes. I'm humbled by the really excellent video nonhog posted on the subject above. That makes me realize how useless my videos are from an instructional standpoint. But I guess I can't instruct when I don't know about the subject, eh? At least with the camera and the video editing software I get to entertain myself while I'm working on something I don't know how to do, so...

To the contrary. Somebody struggling thru the process is relatable and very useful to anybody trying a task for the first time. Besides, you're just SO darn lovable!