"There's a slappy echo, reverb, 3 big windows on the front wall... I'm really confused."

That's from an email a reader sent me last year, and windows come up all the time. One on your left, one on your right, a big one right behind the speakers. The question is always some version of "is this going to cause problems?"

You can relax. Of everything on that list, the windows are the part you need to worry about least. They will change where your setup ends up, though, and that's what I want to get into today.

For my latest video I asked Mike Senior to grill me on exactly this. (You met him here two weeks ago with his Helmholtz resonator.)

Back in February I wrote that glass behaves a lot like drywall. That still holds, so here's the short version. Glass lets the deep low end pass through, and it gets more reflective the higher you go. Thicker glass and more panes push it further towards reflecting everything.

What I didn't get into back then is what that does to the decision everything else in your room depends on: where you sit.

Why does the 38% rule work at all?

You've probably come across it. Put your listening position at 38% of the room length and you'll get a balanced low end.

Here's the idea behind it. Between your front and back wall, a standing wave builds up that fits the length of the room perfectly. At the 38% mark you get the right amount of low-end energy from that wave. Not too much, not too little.

And it's surprisingly accurate. When people go through my listening test for the low-end sweet spot in a plain rectangular room, they really do end up at around 38%.

But only in a room that behaves the way the models expect. The further your room drifts from that box, the less a rule of thumb can tell you about it.

What does a window do to it?

Think of a skipping rope held between two people. If one of them takes half a step back, the whole shape of the rope shifts along with them.

Your front to back standing wave works a bit like that. Some of the low end goes through the glass instead of bouncing straight back. So the wave doesn't turn around exactly at the window. It behaves as if the wall were somewhere behind it, and the mode gets stretched.

When the mode stretches, the spot where you get the right amount of low end moves with it. It might be much closer to the window now. It might be further into the room. It all depends on the exact geometry and materials of the perimeter of your room.

Isn't a leaky window good for your bass, then?

That was Mike's question, and it's a fair guess. If bass escapes through the glass, there should be less of it ringing around your room.

Long story short: I don't think you should expect windows to help. What they do is make your room less predictable. The models and the rules of thumb were built for that sealed concrete box, and every window takes you a step further away from it. (So does drywall, doors or any other irregularities, by the way. Same effect.)

On top of that there is one peculiarity with some windows that you should be aware of. Some single pane windows can really mess with the reflected energy in the transitional frequency range where part of the sound passes through and part of it gets reflected. It can strongly affect the timing of the reflection, and it can really change how that reflection sounds at your listening position. 

I've come across it once or twice in all these years, and when it happens it's usually very obvious. You've got two ways out. Change the window (add glass, or put in a different type of window), or move your setup so that reflection doesn't end up back where you sit.

What does that change about your setup?

Nothing about the process of finding the low end sweet spot. What it does affect is the result.

You still find your listening position by listening for the spot where the low end balances out. You still set your speakers up from there. And the geometry of your reflections stays exactly the same, glass or no glass.

What changes is where you end up. Maybe not where the 38% “rule” says. Maybe not where you'd like to sit either, unfortunately.

But that's all you need to worry about, because now the window is already priced in. You're listening in your actual room, with that window in the wall, letting that bit of bass through. Whatever spot your ears settle on already accounts for it. 

The 38% rule only works in the room it was written for. Your ears work in the room you've actually got.

Mike also asked whether a reflection off smooth glass sounds worse than one off a wall. A little different, sure, the same way wood sounds a little different to tiles. But in a home studio that difference is so small it isn't worth any of your attention. What matters is whether there's a reflection, whether it's causing a problem, and whether you need to deal with it.

Can you keep the daylight?

If your window does sit on a reflection that's causing trouble, treat it like a reflection on any other wall. Depending on how loud it is and which frequencies it covers, that might mean a thick absorber or something lighter.

Which leaves the real question. Is that improvement in sound worth losing some of your light?

A few weeks ago I wrote that sound versus daylight is the actual choice you are making. A window is where that choice stops being abstract. Some people give up the view without a second thought. Others would rather live with the remaining reflection than mix in a dark box.

Neither is wrong. It's a pick-your-poison situation. Just pick with purpose. Everything else, your room has already priced in.