Does Glass Absorb Sound (Soundproofing Perspective)?

When soundproofing a room, you might have wondered, whether glass absorbs sound. Here’s a quick answer:

Glass does absorb sound, but only at its resonant frequency, which is around 400Hz. Any sound waves outside of this frequency are either reflected off the glass or pass through as vibrational waves.

Does Glass Absorb Sound_

In this article, I’ll go into a bit more detail about how glass absorbs sound and how you can work around this in your soundproofing project. Hopefully, by the end, you’ll know how to make the most of glass.

Also read: Soundproof Glass; What Is It And How Does It Work?

The Resonant Frequency of Glass

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As I mentioned, glass only absorbs sound at its resonant frequency. Generally speaking, glass is quite a poor absorber of sound.

But this is mainly because, compared to the surrounding walls, glass windows are relatively thin.

If you were to have windows the same thickness as the walls, their ability to absorb sound would be greatly improved.

This isn’t really an option though. Instead, let’s look at what we mean by resonant frequency and how we can use that information.

Resonant frequency is defined as an object being caused to vibrate at its natural frequency. In the case of sound, this is when sound waves (vibrations) stimulate the particles in an object to move freely.

An easy example to understand is singing wine glasses. Running a wet finger around the rim of a wine glass produces noise. This is a result of its resonant frequency.

For glass, depending on the thickness, the resonant frequency can range anywhere from 200-500Hz, but 400Hz is most common.

But this only covers a small range of viable sound waves that can be detected by the human ear. As a result, glass can be considered a poor absorber of sound waves.

So what happens to the rest?

  • Like all materials, glass reflects a portion of sound waves back into the space.
  • The rest transfer through the glass into (or out of) a space.

Again, these properties largely depend on the thickness of the glass, among other factors.

You should also consider things like dead space, such as the area between 2 panes of double-glazing. This can greatly improve the sound transmission of a window.

In short, the answer to the question is yes, glass does absorb sound. However, its ability to absorb sound is negligible when you think of the vast range of frequencies the human ear can hear.

This of course isn’t very helpful when you’re attempting to soundproof a room. Windows are necessary for light, but can massively impact a space’s ability to contain or block out sound.

For this reason, it’s worth considering ways you can get around this while also keeping the windows.

How to Make Glass Absorb Sound More

I imagine you’re here because you’re planning to soundproof a space and have come across the issue of what to do with the windows.

The best option is to remove them completely and replace them with a different material. But this usually isn’t possible for obvious reasons.

Below I’ll cover the best ways to make glass absorb sound more. This will allow you to effectively soundproof your space while keeping the windows.

But bear in mind that windows will never be completely soundproof. However, these options will at least mean they’re not too much of a weak spot.

1. Make sure you have double-glazing

The first place to start is ensuring you have double-glazed windows. The small gap between the 2 panes is quite effective at reducing noise transmission.

The gap is either filled with a gas (usually argon) or is a vacuum. It’s designed for thermal insulation, but heat and sound share many of the same transmission properties.

You might think it’s worth going another step and getting triple-glazing. However, this doesn’t offer a significant improvement over double-glazing and is quite expensive.

2. Add a layer of plexiglass

plexiglass soundproofing
Adding a sheet of plexiglass (Amazon link) in front of the window will improve its sound transmission class.

This isn’t about adding greater mass to the window, as plexiglass is again not very thick.

Instead, you’re trying to create another layer of void between the room and the window, much the same as the gap between panes of double-glazing.

For best results, fit the sheet of plexiglass into a rubber frame. You can make one with something like weather stripping (Amazon link). This will dampen any vibrations either passing into or out of the plexiglass.

Also, be sure to seal any small gaps with Green Glue (related article). Using this will further dampen any vibrations while also sealing up small gaps in the construction.

I’d recommend fitting the plexiglass with a gap of around 2-4” between it and the window. This will be enough to effectively trap any escaping sound waves.

3. Build a window plug

window plug

Another useful DIY solution is to build a window plug. In short, a window plug will essentially add more mass and emulate the properties of the surrounding wall.

They’re fairly easy to build with a few standard materials. You’ll need:

The method is as follows:

  1. Measure the cavity around the window, including the depth.
  2. Cut out a piece of MDF the height and width of the window cavity.
  3. Cut out 4 more pieces of the right depth, which will become the sides.
  4. Line one side of the largest piece with MLV, with a layer of Green Glue underneath.
  5. Glue a layer of acoustic foam over the top.
  6. Repeat the process with the 4 side pieces.
  7. Assemble the box with the soundproofing materials on the inside.
  8. On the back, add 2 handles so you can move it.
  9. Finally, fit it snugly into the window cavity.

The soundproofing materials will help to block out and absorb sound passing into the room from outside. But it’ll also do a good job of blocking sound escaping from the room.

If you’re trying to stop sound getting out, you can always reverse the design so the materials face into the room.

Of course, the major downside of this design is that it’ll block light coming into the room. This isn’t too much of a problem if you’re only using it on occasions, such as for a home theater or band room.

4. Add Soundproof curtains

Soundproof curtains

I generally don’t recommend soundproof curtains (my top recommendations) on their own. This is because they’re not completely soundproof and so won’t make a massive difference to noise absorption levels.

But if you use them with one of the other options mentioned above, they will be helpful.

Soundproof curtains contain extra layers of material to help reduce sound transfer. High quality ones contain things like felt or wool, which are fairly mass rich (for fabrics).

These curtains are particularly effective against high-frequency sounds, but less effective against low-frequency ones.

Some Final Thoughts

Hopefully, this article has given you some information on whether glass can absorb sound.

If you want the best results, you definitely need to use some dedicated soundproofing materials for reducing noise transfer through glass.

I recommend spending some time researching which option would be best based on your needs and available space.

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