Simply put, sound travels through walls by transferring the vibrational energy of a sound from the air on one side of the wall, to the solid wall itself, and then back to the air on the other side of the wall. Some of the sound energy will get lost (reflected by the wall or turned into heat), which is why the sound isn’t as loud on the other side of the wall.
Sound is nothing more than a vibration. Well, everything that exists in fact is actually vibration, but we won’t go into that at the present moment. Even the most cleverly designed space will fail to live up to what you expect if you haven’t taken into account how sound will travel inside of it.
You can design the peaceful indoor oasis of your dreams but if you fail to account for indoor and outdoor noise entering your space, or indeed noise from inside your home, you’ll fail to get what you’re likely after; silence.
Too much noise can lead to a whole host of problems. Aside from the potential hearing damage that over-exposure to loud noises can cause, too much noise pollution can affect your sleep, nervous system, and ultimately your mental health. Consequently keeping noise out when you want to is vital for your overall health and wellbeing.
This leads us to the question; how does sound travel through walls? In order to answer this, we need to look first at what exactly sound is.
What Is Sound?
A sound is a form of energy, produced by vibrating waves that travel through a medium (often air, sometimes water) that reaches our ears. This vibrational energy eventually reaches our eardrums, causing them to vibrate as well. Our brain then perceives this as noise. What we understand as noise is produced by the vibrating body (usually air, but can be any other medium) causing the particles around itself to vibrate.
As sound needs a medium through which to transmit itself, it cannot exist in the vacuum of space. (At least not in human terms, that is to say in a time frame or frequency that we would be able to hear.)
Sound is what’s known as a compressional, or longitudinal wave. The particles of a longitudinal wave move parallel to the direction in which the wave is traveling. Whilst sound travels as a wave, the individual particles of a wave do not travel with the wave. Rather, they vibrate back and forth centered on a pivot spot, known as the equilibrium position.
Compressional/ longitudinal waves cause local regions of compression and rarefaction, compression’s opposite.
Now that we have a basic understanding of what sound is, let’s look at how it travels through different mediums.
How Sound Travels Through Solids.
As has already been said, sound needs to travel through a medium such as solids, liquids, or gases. Sounds moves through these different kinds of matter via the vibration of the particles in the matter itself.
What makes each of these mediums what they are? What makes a solid a solid, a liquid a liquid, or a gas a gas? In the simplest terms, in a solid, the particles are packed very tightly together. The particles in a liquid are packed much looser together, whilst those of gas are packed even looser still.
The fact that the particles are packed much tighter together in a solid means that sound can travel much faster in this medium compared to the others. Sound travels about 4 times faster in water than it does in air, which is why whales are able to communicate through such vast distances in the oceans of the world.
In a solid such as wood, sound travels up to thirteen times faster than it does in the air alone. Sound waves also travel faster on a hot day than on a cold one, as the higher temperature causes the particles to bump into each other more.
Sound And Walls
Let us consider an analogy. Think of a drum head producing sound after being stricken. What happens is that the drum head vibrates up and down, pushing the air particles around it away from it, and back again. The vibrations on the drum head material are periodic stretches and contractions of the drum head material.
Air, on the other hand, conducts sound from slight differences in air pressure. These fluctuations in air pressure travel away from the drum head, in the same way that throwing a stone into a still pond will cause ripples to move away from where the stone landed.
The vibrating drum head will pass some of its momentum and energy to the surrounding air, with that energy and momentum traveling as a pressure wave through the room. As sound waves hit a wall, some of the momentum and energy transported by the pressure waves will be reflected back into the room. The rest of these energy waves, however, will be taken in by the wall, causing it to sync with the vibrating air around it.
See For Yourself
To see this in action for yourself, turn your speakers up loud and place your hand on a window or any pane of glass nearby. You should be able to feel the glass shake in time with the music as the acoustic sound waves reach the glass. This is because the acoustic pressure waves from the speaker are causing the glass to vibrate.
As the sound vibrates from the sound on one side, the other side will act as a drum head, giving off its energy and momentum to the air on the other side of the wall. This is a simplified explanation of how sound can travel through walls.
Sound And Walls Made Of Different Materials
As should be clear now, how sound travels through walls will depend on the type of material that the wall is made from, as well as the type of sound being transmitted. Sound can travel through a whole array of different mediums, with the speed of sound differing with each variation of material.
For example, if you’re walls happen to be made of wood, you’ll experience sound traveling through them differently to if your wall was made of another material, such as stone or cement for example. This is due to the composition of the material; to how spaced out the particles are from one another. As wood is often soft and porous, there can be a lot of attenuation (depending on the wood type and its condition/environment, etc) causing scattering of the pressure wave.
Density
As we’ve already been through, sound needs a medium to travel through, such as a gas, liquid, or solid. Sound moves through these mediums by vibrating the molecules in the matter through which it’s traveling.
You may remember from your time in school that sound travels at a constant rate. This doesn’t mean, however, that it’ll reach your ears at a constant rate. This is where the material through which the sound waves are traveling must be taken into consideration.
How does density affect the speed of sound? As sound waves involve the transfer of kinetic energy between adjacent molecules, the closer the molecules are to one another, the faster the sound waves will travel.
With this in mind, we can then understand why sound travels faster in solids than in liquids or in gases; because the molecules are more tightly packed together.
Density Of Different Materials
In the USA, most houses are made of wood, even if the walls appear to be made of stone or brick on the exterior. The brick and stone are only a veneer and don’t hold up the roof. Wood in North America is readily available and quick to assemble. To form the walls, you then nail up what is known as plasterboard, sheetrock, wallboard, or drywall to the wood.
Essentially this is plaster in between two layers of heavy cardboard. Because it’s made of plaster on the inside, it does not expand or shrink with changes in humidity or temperature as solid wood does. This makes it useful at suppressing sound, and also makes it easy to drill into.
Drywall is easy and quick to put up and can be done by most people in a matter of hours. On the whole, your average drywall is not very good at preventing sound from traveling through them. This is because they are quite thin and not very dense, which are characters you’d want to avoid if you were to want to soundproof your walls.
The air cavity between the walls is also an excellent place for sound waves to propagate. They pass through one side of the plasterboard and end up in the cavity. In this space, which consists of a lot of flat surfaces, the sound waves are able to bounce around and reflect off plenty of the other plasterboard surfaces.
This bouncing around inside the cavity then causes the sound waves to amplify when they escape into the other room. So the air cavity inside the drywall can act as an echo chamber, thereby amplifying the noise of others sharing the wall. However, it is possible to have soundproof drywall which we won’t go into here.
Many homes in Europe, depending on the region, are built entirely of masonry (concrete, bricks, mortar, and plaster.) This makes them stronger, and more expensive to build. It also means that sound waves behave differently inside these walls due to the different materials used in the construction.
As has already been mentioned, sound travels faster in solids than liquids or gases, as the particles are more tightly packed together. Remember sound is the transference of vibrational energy from molecule to molecule. The closer the molecules are to one another, the faster sound can travel between them, which is why sound travels faster in solids than liquids or gases.
But not all solids are created equally. The velocity of a sound wave is affected by two different properties of sound waves; density and elasticity.
Elasticity
The speed of sound is also different in different solids, liquids, and gases, depending on the type of elastic properties of the particular material. Elastic properties refer to the tendency of a material to keep its shape and not deform once a force is applied to it. Steel, for example, will experience a smaller deformation than rubber when a force is applied to it.
On a particle level, rigid materials are characterized by atoms and or molecules with strong forces of attraction between them. These forces of attraction act like springs dictating how quickly particles spring back into their original position. Particles that return to their original position quickly are ready to move again sooner than those that take a longer time to do so. As such, they can vibrate at higher speeds.
Therefore, sound can travel faster through mediums with higher elastic properties, such as steel, than those will lower ones such as rubber.
To Conclude
It should be apparent by now that how sound travels through walls is dependent on a range of factors pertaining to the construction of the particular wall in question. A sound is a form of vibrational energy that travels through a medium to reach our eardrum which in turn vibrates and is perceived by our brain as sound.
To understand how sound travels through walls, we have to first understand how sound waves travel in different mediums, and what different materials our walls are made of. Sound waves behave differently depending on the density of the medium through which it is to travel.
The density of a material refers to how closely the molecules are packed together. As sound is the transfer of kinetic energy between, molecules, the denser a material, the more readily kinetic energy can pass through it.
We must also take the elasticity of the medium into consideration, which is how readily the particles bounce back into position after vibrating. The more elastic a medium, the quicker it’ll return to equilibrium and be ready to pass on further vibrations.
So how sound travels through walls depends on the material from which the wall is made; more specifically still, the density and elasticity of the material. The typical drywalls that are common in the USA are not particularly effective at keeping sound out unless you soundproof them. The stone, cement, bricks and mortar typically used in many European nations cause sound waves to behave differently still.
