The Science of Soundproofing


Noise is transferred to us in the form of sound waves. Each sound wave will have a particular frequency or pitch. A typical noise might have lots of sound waves of differing frequencies, ranging from low frequency such deep bass music or a man’s voice, all the way up to much higher frequencies such as a crying baby, and all the other frequencies in between. Each of these frequencies has its own set of sound waves that is passing through your wall (or ceiling, or floor).

Believe it or not, the existing wall you have is acting as a soundproofing material in some capacity. But for various reasons it might not be doing a great job. So the goal when trying to soundproof a wall is to add acoustic materials which will combat as best we can the whole range of frequencies of sound that is coming through the wall, and complement the existing wall.

Why is some sound easier to hear than others?

We need to distinguish between the loudness of the noise and the frequency/pitch of the noise. In general, as sounds of the same volume pass though a wall the low bass frequencies will penetrate the wall easier than the high frequencies. Low frequency sounds lose less energy as they pass through a solid object. This is for two reasons; their longer wavelength, and the fact that low frequency sounds create resonance/vibration in walls and this helps propagate the passage of sound.

We have found that the problem with a standard brick wall is that it vibrates at a certain frequency allowing much easier penetration of certain frequencies (normally bass/low frequency). As such it is important to choose acoustic wall treatment materials that aim to tackle this resonance problem head on.

Airbourne sound versus impact sound

There is is a difference between the types of noise that are measured and produced. They falls into two categories and its very important to understand the difference between the two. The first is “airborne sound” and the second “impact sound”:

Airborne sound
This is the sound that you hear in a house which is generated by things like a television, talking, music, and even computer gaming.

Impact sound
This is the sound generated by impact energy causing a surface, typically a ceiling, but also walls and floors, to vibrate. This kind of sound tends to be much lower frequency and hence tougher to eliminate.

Frequencies and wavelengths and why some sound is harder to combat than others?

Each of those frequencies have a different wavelength. The deeper the sound you are hearing the longer the wavelength.  The noise from neighbours people hear is invariably lower frequencies, and they transmit though walls, floors and ceilings with less loss. 

For the best effect when we disrupt the passage of sound we want to disrupt right across the whole frequency spectrum especially those low frequency sounds. When tackling low-frequency sound there are two best practices:

Firstly, increase the thickness of the wall. However, bare in mind that very low frequency sound can have a wavelength of 5 metres or more! So whilst just making walls thicker will improve things more from a low frequency perspective, it isn't the most effective solution for most people;
Alternatively, apply a soundproofing solution that will focus on the introduction of many different specialist materials that will act to break up the resonance in walls and effectively absorb the sound as it passes through the wall.

So the takeaways here are yes make your wall thicker but also soundproof smarter:

"Thicker"

The highest performing soundproofing solutions will tend to make the wall thicker. Often increasing the soundproofing barrier by 50 to 100%. Whilst they provide the best performance over all, these independent soundproof stud walls are not always the most practical solution from a space loss perspective, especially in smaller urban accommodation where every inch of living space is highly valued.

"Smarter"

What do we mean by smarter? The materials we want to use have to try and fulfil a few key roles in a very limited space. The materials must combine: Mass, Resilience, and Absorption:

Mass
The most well-known law of soundproofing is the Mass Law. This states that by doubling the weight of the acoustic barrier you will gain roughly a 6dB improvement in sound reduction. In other words, if you doubled the size of your brick wall separation you would get around a 30-40% improvement in soundproofing. This is why just gluing a couple of layers soundproofing plasterboard direct to a wall will just not work! In addition, this will not provide any way of addressing the problem of the resonance in a brick wall

Resilience
To enhance the effect further, we have to add the mass discussed above in a much smarter way. The solution is to make it springy so that when the sound waves hit it, the product vibrates and absorbs significantly more passing energy. The traditional method of creating resilience was using "resilient bars". These are corrugated steel bars onto which heavy plasterboard is mounted. Whilst this solution provides sound protection, and is still used today, it was quite quickly found that rubber mounted bars do a better job as rubber dissipates vibration much better than steel. The result is a number of "clip" systems - the foremost of which is the GenieClip system. 

It should be noted that good acoustic membranes, such as Tecsound, can act in this way as well, providing a level of resilience to the solution.

Absorption
Even more efficient that applying a high-mass product, using separate resilient system is combining resilient layers with materials that have their own internal resilience. Adding products which are intrinsically heavy and resilient makes sense. For example, the SBx board product is a sand filled board that vibrates easily throughout its structure. Equally, rubber layers such as the SM20 mat provide in built resilience. These products are non-uniform throughout their structure and so will absorb this sound energy across a much wider range of frequencies. 

So back to our sound waves... Before applying soundproofing, all the sound waves generated by noise hit the wall in question: the high-frequency waves are more easily absorbed, and fall off, but the low-frequency sound pass through the wall easily and in fact cause the wall to resonate helping ease the passage of sound.

When the same sounds hit the smart soundproofed wall, firstly the wall is a bit thicker and this blocks more of the waves. The sound wave is also encountering multiple different types of mass and air gaps. The sound is absorbed in the wall and the wall cannot resonate as one any more due to the resonant frequency of the applied materials being very different to the original heavy brick.

This science of soundproofing is often more of an art than a science! We bring together multiple products that work differently, on different parts of the sound wave mix, to maximize the possibility of successfully soundproofing your wall. All you need to do is think how much space you can afford to lose and how much budget that you wish to put to work. We are then happy to offer the best solutions for your particular circumstance, and guide you through installing it yourself, or finding an experienced fitter who can do it for you.