Does Soundproofing Really Work on Both Sides?

A question I’m frequently asked is whether soundproofing really works on both sides of soundproofing material. For example, a customer called recently looking for the best way to soundproof a room in his house that he was using to play instruments and record. He wanted a solution that blocked the loud sounds of his drum kit and guitars from leaking into adjacent rooms of the house, but also was concerned about sounds from outside the room getting in and adding noise to his recordings. His main question was if he was going to have to soundproof the walls in his room as well as the adjacent rooms, or whether just treating his music room would work for both. Another way of asking whether he was going to have to spend twice as much or not!

The answer to whether soundproofing really works on both sides of a wall is, yes it does. In this article, we will explore the science of sound and describe why soundproofing works the way it does so you can plan correctly for your unique acoustic project.

The Physics of Sound and Sound Transmission

To understand soundproofing appropriately, it helps to understand what sound actually is and how it moves. What we call “sound” is a vibration that moves as a mechanical wave through a transmission medium. It’s best to understand this with a simple example of another common type of mechanical wave: waves in water.

If you were on the edge of a completely still pond and wanted to create a wave, you’d need to create a vibration. Let’s say you picked up a rock and threw it into the center of the pond. From where the rock entered the water, a mechanical wave would propagate outwards. The water is the medium, the rock hitting the water is the vibration, and the mechanical wave moves outward from where the rock hit. Depending on how big the rock is, the wave may only be a small ripple before it goes away. If a boulder was dropped in the same spot, the waves would be larger and probably have so much energy they carry past the edge of the water and onto your feet! This is how a tsunami works, except the vibration energy comes from an earthquake at the bottom of the ocean, not an object entering at the top.

Now let’s use this concept to understand sound. While sound can absolutely travel in the medium of water, we are most familiar with it traveling in the medium we live in: air. Air seems empty, but it is full of gas molecules that we cannot see and therefore is a medium, just like water. If we were to pick up a rock and drop it on concrete, the sound we hear is the vibration of the rock hitting the concrete moving through the medium of air as a mechanical wave to our ears. Let’s discuss how this relates to how loud sounds are, or rather how much energy the sound waves carry.

Understanding the Energy of Sound Waves and How it Relates to Loudness

Now that we know what a sound wave is and how it moves, the final step is understanding the difference between loud and quiet sounds. How loud a sound is depends on the energy of the sound wave over a specific area per second, which is known as intensity. While there are important technical differences between intensity, energy, and loudness, for our purposes we can say that the more energy the sound wave has, the louder it will be.

Let’s go back to our pond example and inspect the difference between the rock and the boulder. When we threw the rock, a wave formed but it died out before it made it to the shore. When the boulder dropped, a huge wage formed that not only made it to the shore, it pushed the water over our feet. The difference is the energy of the initial vibration. Since a boulder has way more mass than a rock, that mass accelerated by gravity imparts significantly more energy, which means a larger wave.

Now let’s examine this considering sound. If you were to stand on one end of a football field and your friend on the other, if you whispered something to your friend, they would not hear it. If you instead shouted, they would hear it. The difference was how much energy you imparted to the sound wave when it left your vibrating vocal cords. With the whisper, the energy wasn’t enough for the soundwave to make it across the field through the air. With the shout, the energy was enough for the soundwave to easily travel that distance.

When it comes to soundproofing, or sound blocking, what we’re trying to do is eliminate the energy of the soundwave so that by the time it passes through the soundproofing material, it doesn’t have enough energy for us to hear it anymore.

How Soundproofing Works

To tie the physics of sound to soundproofing, I have one more analogy that I think makes it very clear. Let’s think about soundwaves as arrows and soundproofing material as a shield. If you were to have a shield made of a t-shirt and someone shot an arrow at you, you’re in big trouble! If you have a shield made of wood, the arrowhead possibly might poke through before the wood stops it. If the shield were made of concrete (and you could lift it!), the arrow would bounce harmlessly off of it.

What this analogy shows is how effective different shields are at removing the energy of an incoming arrow. When we soundproof, we are selecting materials to shield, or block, sounds from penetrating. It is removing the sound waves energy until it doesn’t have enough to be heard.

This is why soundproofing really works on both sides. Let’s think about the shield and arrow one more time. If you flipped your t-shirt shield, wood shield, or concrete shield around and the arrow hit the other side, we would get the same exact result. As long as that material sits between us and the arrow, there is no difference between which side the arrow hits. Likewise, if you have a soundproofing blanket or an acoustic barrier panel, it doesn’t matter which side the sound wave passes through, it still has to pass through the material, and therefore will be affected by it.

Understanding the Sound Transmission Class (STC) Rating System

Before we look at a few soundproofing examples and materials to show why it’s important to know that soundproofing really does work on both sides, we need to understand how materials are rated. The most commonly used acoustic rating system for soundproofing is the Sound Transmission Class (STC) rating system. Essentially, the rating determines how much sound can be reduced as it passes through a material to the other side.

All materials technically have an STC rating – think about our t-shirt, wood, and concrete shield analogy. The t-shirt would have a low rating, the wood would have a decent rating, and the concrete would have a great rating. This is how we analyze our project needs in order to calculate how much soundproofing material is needed to achieve our goal.

Effective Soundproofing Products and Techniques

With this knowledge in hand, we can now look at the situations where we soundproof to understand the materials used and why.

Windows

Windows are a good place to start because they can be tricky. Glass is a different material than the components of a wall, so it will have different soundproofing abilities – a different STC rating. Many would like their windows to be more soundproofed, perhaps to reduce or block sounds from the outdoors, but they also want the window to still serve its purpose: to allow light in. To soundproof a window, we need to introduce more material, and also transparent material, to boost the capabilities while maintaining the function of a window.

Our PrivacyShield^® Window Seal Kit is a great example of a product that can be installed over existing windows to increase the windows soundproofing capabilities. If we look at the chart below, you can see an example of STC ratings for common windows, and how the rating is boosted by the addition of the PrivacyShield^® Window Seal Kit.

PrivacyShield® Window Seal Kit
Sound Transmission Loss
Window Type		STC
Double Hung	No seal	24
	With PrivacyShield® Window Seal	37
Triple Casement	No seal	27
	With PrivacyShield® Window Seal	44
Fixed	No seal	33
	With PrivacyShield® Window Seal	51

There is something very important to notice here: you cannot directly add the STC rating of a product to the STC rating of the material it’s being added to. Notice also that the stronger the original STC rating of the assembly you are looking to further soundproof, the more soundproofing material will help.

Bringing this back to the question of whether or not soundproofing works on both sides, the window seal kit will only be applied to one side, but the soundproofing will work no matter which way the sound passes through the window. If for some reason you want additional soundproofing, then you could theoretically soundproof the other side, but that would simply be adding material that would make it even more soundproofed.

Drop Ceilings

I like talking about drop ceilings because they are a curious construction method that soundproofing material can be easily added to. The issue with soundproofing walls, ceilings, and floors, is that you essentially need to renovate them in order to achieve real soundproofing. The benefit of drop ceilings is that they can be renovated without destruction – all you have to do is move tiles.

Let’s say you had a room with a drop ceiling, and you want the ceiling soundproofed because the HVAC system above the tiles is loud and distracting. I would suggest adding PrivacyShield^® Ceiling Tile Barrier to the ceiling tiles, in addition to the PrivacyShield^® Light Hood to cover the light fixtures. What we are doing with these two products is creating a new layer of soundproofing material that the sound has to further pass through. And again, it doesn’t matter if the ceiling tile – the sound still has to pass through, and it will thus be reduced.

For more information about soundproofing projects, see these articles:

• Creating A Noise-Free Retreat: How To Soundproof A Bathroom
• Acoustic Strategies For Effective Warehouse Noise Reduction
• How To Soundproof A Condo: Your Ticket To Serenity
• The Best Products And Strategies For Radio Studio Soundproofing
  How To Soundproof Stairs Treads In 3 Easy Steps

Find the Best in Soundproofing Materials and Techniques with Acoustical Solutions

As I mentioned before, I just love soundproofing because the effects are dramatic and can significantly improve your space no matter its current acoustics. To learn more about soundproofing or discuss your unique project, give us a call today and we can help you understand exactly what it is you need and direct you to those specific products.