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Blocking sound allows you to get the clearest audio possible, whether you’re trying to record a podcast or listen to music while running. However, sound waves are tough to block entirely. Most methods can only reduce sound waves.
Sound waves are difficult to block entirely due to their size and the manner in which they move. They can reflect off of certain objects and also go through or around others. However, sound waves do not travel very far.
Let’s explore the reasons sound waves are notoriously difficult to block. I’ll also illustrate a few different ways to reduce sound waves significantly.
Why Is It So Difficult To Block Sound Waves?
Blocking sound waves is difficult because the waves travel at a fixed speed and do not catch up or cancel out each other. Sound waves can also be absorbed and reflected, given that sound is the propagation or movement of the molecules of a medium in one direction at a certain speed.
Sound Travels Through Air
Even the tiniest leak in your soundproofing material carries sound through. The only way to completely block the propagation of sound waves is to remove the medium—that is, to put it in a vacuum. However, creating and maintaining a vacuum at atmospheric pressure is difficult and expensive.
You might have noticed this effect in rooms where the insulation on windows has worn out. Because small amounts of air can pass through the window, it can carry the sound with it.
Propagation: How Sound Moves Through a Medium
Sound cannot travel without a medium (without a medium, the environment would be a vacuum).
This is because sound is not an object; rather, it is a force or transfer of energy through the vibration of molecules within a medium. Propagation is a term that refers to the way vibrating molecules transmit sound, creating a wave.
A great way to illustrate the movement of sound waves is through liquids like water. Take a look at this fascinating experiment done by YouTuber brusspup, in which sound waves are used to alter the shape of a stream of water:
You’ve seen sound move if you remember the iconic scene in Steven Spielberg’s Jurassic Park (see the video below) when water ripples as the T-Rex approaches.
Let me simplify things further. The sound wave in the scene I’ve mentioned travels through water, which is the medium.
The water molecules at the edge of the glass vibrate with the wave’s energy, leading to vibrations in the neighboring molecules. The molecules push inwards from the glass towards the center with a domino effect.
Sound Waves Are Directional
One unique feature of sound is that it travels with direction. In the example above, the waves traveled inwards from the glass to the water.
Once a wave passes by a row of molecules, it moves on to the next. If a new wave passes that is vibrating in the opposite direction, the molecules vibrate accordingly.
Trying to block a sound wave by using an opposite wave is akin to trying to stop the chain reaction of dominos falling by allowing a portion of the chain to fall in the opposite direction.
How Sound Waves Are Absorbed
Different mediums can absorb sound waves depending on the material and its shape. Absorption is blocking a sound wave by reflection and converting its energy into vibration and heat.
If you’ve ever walked into an unfurnished room, you might’ve noticed your voice become slightly louder. You may have heard the hint of an echo as well. That’s because soft and absorptive materials used in furniture (wood, cotton, etc.) can absorb some sound waves, dampening your voice a little.
On the other hand, hard surfaces such as walls and concrete can reflect the sound and create a slight echo.
How To Block Sound Waves
Some mediums can block sound waves. Incorporating such items into a room is commonly known as soundproofing.
A material that can be used in soundproofing should meet the following criteria:
- Dense enough to make transmission difficult.
- Not too dense, as it would reflect the waves.
- Resistant to the flow of energy.
- Made of cells smaller than the sound wave, with a closed arrangement.
Several insulating materials can be used in soundproofing on a commercial or industrial level. Modern homes are built with insulating foam in the walls.
This is because the foam is ideal for absorbing and slowing down the transmission of sound and heat. Some other soundproofing materials include drywall, acoustic foam, and mass-loaded vinyl.
Mass-loaded vinyl padding is used to prevent the transmission of sound (blocking). It basically acts as a barrier. The material is highly effective at reducing sound waves and preventing their transmission. In fact, the introduction of MLV in the 1960s changed the way we build modern houses.
Sound Blocking vs. Sound Absorption
You might notice I’m using the term sound blocking, not absorption. Blocking sound is a more complex process, and it’s not always feasible. Sound absorption is one of five elements of sound blocking, also known as soundproofing.
For example, a recording studio may use porous materials to absorb sound and allow for clearer audio recordings. On the other hand, a movie theater will need to block out the sound from within the theater to avoid overlapping noise from different movies.
The most popular (and readily accessible) sound absorption method is to use acoustic foam. These are thick polyurethane foam panels that resemble egg crates.
The panels can reduce sound transmission by reducing the amplitude of sound waves. Vloggers and podcasters that do not have access to a studio commonly use these panels to improve audio quality.
Check out this detailed comparison of soundproofing and sound absorption.
You’ve probably heard of noise-canceling headphones, which use little to no padding and still transmit clear audio while blocking out background noise. While they add sound, don’t assume this means sound waves are canceling each other out. It is your perception of the sound that is neutralized and not the sound wave itself.
Sound waves are a force or motion traveling through a medium, and they are difficult to block. However, various methods have been developed to isolate sound or reduce sound waves.
Noise cancellation has proven more effective than sound absorption. For most soundproofing applications, a combination of the two works best.