As acoustic engineers the number one thing we are asked about in the pub as well as the office is;
“how do you go about soundproofing a room?” You may have heard a lot of things including using “eggboxes” on the walls, acoustic foam or bass traps.
I want you to forget about these things. Delete them from your mind.
Absorptive foam and bass traps do have their place in specialist recording rooms, where excess reverb (echo) in a room can make a recording sound unprofessional and noisy. They have almost no impact whatsoever on soundproofing a room. Don’t even get me started on eggboxes! There may be a slight effect from eggbox shaped material scattering sound, which might also help to lower the echo in a space. It would do absolutely nothing to stop noise from your neighbors coming through the wall or ceiling, or else your wall would be made from egg boxes instead of bricks or plasterboard.
The weakest link in soundproofing
When it comes to acoustic consultancy and sound proofing, there is one rule of thumb that should always be considered before anything else:
“Sound will pass through the weakest element of a partition and will ignore the strongest parts”
Imagine sound as if it was heat escaping from your home. You could make the walls of the house really thick and heavy, put insulation in them and more, but this will not stop heat from escaping your home if you have old single-glazed windows, a cheap ventilator, or if your door has a gap around the edge of it. These aren’t just metaphors, the exact same rules apply to soundproofing and acoustics!
If there is a hole around the gap in your door, fill that gap with a threshold seal before deciding to insulate your wall. If you have poor glazing or cheap ventilators they will let sound into your home from the outside and increasing the acoustic performance of your wall will have absolutely no impact.
Once you have sealed up the weakest elements of your leaky partition wall or floor you can then look to improve on that performance. To do that there are three items that acoustic consultants look at; mass isolation and insulation. These three items when balanced perfectly create a soundproof partition that doesn’t have to be expensive or unreasonably heavy.
The easiest and arguably the most important aspect of soundproofing in a partition wall or floor is the mass of the element. That is simply put, how heavy the element is for it’s size, or in the technical term we would describe it as the mass per unit area, where we measure it in kilograms per square meter.
We can predict the performance of a partition wall or floor based on the mass of the element using the Mass Law equation.
Sound Reduction Index = 20*log(mass*frequency)-47
Although this is highly simplistic, it does give a rough idea of the elements performance based only on its mass. This Mass Law formula effectively tells us that mass reduces sound better at higher frequencies and worse at lower frequencies, and that for each doubling of mass we add to the wall, we gain another 6dB in sound reduction. As you can imagine, the partition would quickly become too heavy to be practical if we keep increasing the mass.
The simpler the partition construction, the more accurate this equation becomes, as such it is more effective in masonry than lightweight systems. Lightweight partitions are effected by the stiffness of the panels and studs, along with resonance frequencies that we won’t go into here. These can also be calculated, but the more complex the partition, the more complex the equations become.
Next we move onto isolation, the second most important aspect of soundproofing a partition wall or floor. Isolation is simply isolating one part of a separating partition from another part. A standard, lightweight, plasterboard wall would usually consist of two layers of plasterboard separated by air and held together with a metal or timber stud. The reason this provides sound insulation that is higher than the mass law suggests is because the two elements are separated from each other by air. Air is a great insulator. Going back to our heat analogy, air that is trapped between two solid elements is often used to keep heat in and out of elements.
The more separation we have between elements, the better. That means that a bigger air gap might take up more space, but will usually give a higher level of sound reduction. If we can disconnect each side of the partition from the other completely, we get even more performance! This is because sound cannot then travel through the studwork into the other board (remember the weakest element).
Where space cannot permit the inclusion of a larger airgap or a twin studwork partition, we can reduce the stiffness of the studwork between each side. This is often provided by a slightly springy element on one side of a partition which can include resilient bars or acoustic studwork (usually walls) or resilient hangers (ceilings).
When you started reading this I imagine that insulation was the first place you went to regarding soundproofing. It is in fact usually the last item to tick off when improving soundproofing of a partition.
The air gap that is provided between twin wall elements is good for isolation of the partition, but it does introduce a new problem once the partition gets to a certain level of performance. Because we have introduced an air gap, sound has the opportunity to reverberate inside the gap, much like it does in an echoey room. To fix this, we go back to the very first step and introduce an absorber into the air gap (no not eggboxes!). This absorber usually consists of a dense mineral wool or fiberglass product, which absorbs the sound echoing throughout the cavity and converts it into heat.
And there you have it. The fundamentals of soundproofing made easy and no eggboxes used.