Home DIY Building a Threshold for a Soundproof Theater Door

Building a Threshold for a Soundproof Theater Door

Building a Threshold for a Soundproof Theater Door

When I originally built my theater door, I only made the threshold for one half of the opening and left the other half (roughly six inches) with just a bare floor. I needed to finish the threshold in order to install tile right next to it so now was the time.


My second reason for building it now was to finally allow the door to seal air-tight. The door already had weatherstripping on three sides which do an admirable job, but the bottom has been problematic all along. It does have an “automatic door bottom” but that’s never worked as perfectly as I hoped. One possibility for that is that the existing threshold was not parallel to the door and thus the door bottom’s rubber gasket wasn’t evenly distributed along it. A goal of mine, then, was to make the entire thing coplaner to the door and see if that worked.

If it still didn’t work, then my plan was to build a door stop for the bottom as well.

The Video

Miller Time

The existing door threshold was made from poplar a full inch thick and thus the new threshold needed to be that thick as well. This made getting a pre-milled board from Home Depot a non-starter since the AZ Home Depots don’t carry any boards like this over 3/4″. I found myself reaching into my rough lumber stack and found this one (hard to miss):


I started by cutting it to very rough length on my miter saw. This required two cuts since the entire board is wider than my cut capacity. It’s not terribly accurate, but all I really wanted at this point was to get the piece to even a tiny bit reasonable size.


Next up was ripping it to rough width on the table saw. This board was straight line ripped on one side by the lumber yard, although it has since moved a little. It was still straight enough on this little bit of length, though, so no problems there.


The board was technically flat enough for my use since I was just going to glue it down to the slab and then flatten it to be in line with the other side of the threshold. But it did have a mild cup and an equally mild front to back skew.


I was thinking that it would be nice to joint it but in the past, that required making a custom sled for my planer and I didn’t really feel like spending that effort this time. It would be very handy to have a 6″ jointer since that would make this so much easier. Alas, I don’t have…

Wait! I do have a 6″ jointer!


Yeah, it turns out that the previous homeowner left his jointer in the workshop when we bought the house and I just never used it. How random is that? I never used it because it was in pretty rough shape; dirty, covered in spider webs, and with all non-painted surfaces very notably rusty. Maybe it didn’t even work at all?

I spent a little bit of time cleaning it off and working at the rust with WD-40 and a scouring pad. That worked okay. The blades still had a tad bit of rust on them and they were nicked in places but didn’t look too terrible. I fired it up and sure enough, it at least turned on and the blades spun around. Worth a try!

It had been a long time since I watched a “how to use a jointer” video and so my technique was completely wrong — I was putting far too much pressure on the board. It did still concentrate on the cupped portion. I couldn’t afford to go too nuts on the jointer, though, since I was only at an eighth over an inch and didn’t have a lot of material to get rid of. So I got close enough.


Then I ran it through my thickness planer to get me with a 32nd or so of where I need to be.


After that, I was finally ready to rip the board to its final width and cross-cut it to its final length.

I also did a first rough sanding using a trick I learned a few years ago to first scribble pencil lines on the piece and when those are sanded away, you know it’s time to go to the next grit.


Looks like a proper milled board now! It’s almost too bad that it’ll never be seen.


Leveling the Base

I attached the board using PL375 construction adhesive. I had used Red Guard waterproofing compound under the other side’s threshold but didn’t this time since I’m not really convinced it’s necessary in our super dry AZ climate. If I’m wrong, then I’ll fix it when the future problem arises. I weighted it down with a random block of concrete I had lying around.


The two sides of the threshold were a little close in height but not completely flush. Far more importantly, though, the hinge side was notably higher than the other side. I decided to kill two birds with one stone and while sanding the two pieces flush, I also would lower the hinge side by nearly an eighth. The vast bulk of this was done with my belt sander, since the cross-grain nature of this mostly precluded planing it.


I could only get so far with the belt sander, though, so even after all that, the bits closest to the door stop were still notably higher and so the automatic door bottom still wouldn’t completely seal.



That meant getting some flap pads for my angle grinder and going to town with that. I could get right up to the door stop that way and so it was level all the way across after that.

I still wanted the entire threshold to be roughly an eighth higher in order to fit better with my automatic door bottom in the door. So I cut out a piece of hardboard and then cut out the lower part of the stops to fit the hardboard piece under it.


This also got a dose of construction adhesive. And was also weighed down with the random chunk o’ concrete.

Alas, even after all that, the automatic door bottom still didn’t work as well as I hoped. It sealed a lot more consistently than before but I could still hear air movement (whistling) when the AC was on. That meant that I needed to install a door stop on the bottom after all.

Installing the Door Stop

The door stop is only 3/4” thick so I didn’t need to mill any rough lumber this time.

I did need to cut an angle into the stop but not at any particular degree. So I just measured off 1/2″ at the max height of the blade and figured that whatever angle that was was going to be good enough.


The actual cut is thoroughly unsafe! The problem starts because I can’t put my fence on the opposite side of the blade, meaning that the piece is going to be trapped between the blade and the fence. I also haven’t yet made a tall fence so there was nothing inherently to keep the piece from tipping over.

I tried to minimize the risk by clamping another piece to the back in an attempt to create a wider and more stable footprint. This still has a very high risk of kickback. I was gambling that the friction of the pieces would keep them mostly in check and then I just made sure I didn’t stand directly behind it.


It worked! No, I don’t recommend that at all, but I wasn’t forced to learn a hard lesson or anything and the end result was just what I was hoping for.


Next up is cutting an 1/8” rabbet on the stop to accept the weatherstripping kerf. My blade is a thin kerf one and so I just made the proper depth cut and then just broke off the thin bit that was remaining. It cleaned up nicely after some thorough sanding.


I used plain wood glue for this step since it’s wood to wood contact. This deserved an additional concrete block weight to accompany the original random block.


Installing the Weatherstripping

The weatherstripping needs to go all around the door stops and so the existing side stops needed to be cut to extend the bottom kerf.


The weatherstripping needed to be cut at a fairly precise 45 degrees and so I used a square cut piece of lumber to hold it and then cut the angle using a speed square and utility knife.


It fit like a glove after all that!


And it worked, too! There’s no visible light coming through any side of the door now and I don’t detect any moving air when the AC is cycling. I consider that a complete success.

Finishing Up

The edges of the threshold were in pretty rough shape so I sanded them flush and slightly rounded first with my oscillating tool and then by hand. It got a coat of flat black paint.


Then while I was at it, I painted the rest of the door assembly in black. This would have been far easier to paint with the door down but that’s incredibly heavy and considering how much of a pain it was to install the first time, I certainly didn’t want to do that again! So I just removed all the hardware; taped up the hinges; and went to town.


Future Work

I am probably not going to create a second layer of stops and weatherstripping in the style of a “bank vault” door. Instead, I’m going to create a second door that will provide some level of soundproofing. Not sure when that’ll be.


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