Today I received a very interesting comment/question about sound absorption materials to be used in keyboards when you want to build or modify one to achieve the quietest typing experience possible in a mechanical keyboard. The comment was a response to this part of my “How to build a whisper quiet mechanical keyboard” video on YouTube:
J. Pročka, asked the following question:
“Wait why would a more dense foam result in less sound?
Generally, a higher density means more sound transfer—think solid vs liquid. So a lower density foam would result in more absorption wouldn’t it??”
That is indeed a very good point and raises some very good questions about this topic! So, I decided to elaborate the subject a little bit over here at I/O Sam’s blog.
As a required disclaimer, I should clarify that I am not a physicist nor a sound engineer, but the topics I will try to explain here don’t necessarily require one to be a physics PHD to explain or to be able to understand. So, anyone with basic physics knowledge should be able to grasp the inner workings of how to tune a keyboard for low noise if the right analogies are used.
So, back to J. Procka question. First, yes, he makes a good point here! If material density is taken to the limit (if you make a material dense enough), it will become so solid and hard that it will either vibrate with the sound waves and just transfer them to the other side or, if it is thick enough, it will make the sound bounce back to its source, your fingers in this case, instead of absorbing it. Which, obviously, is not what you want in the context of a keyboard, since that could ruin not only the sound, but also the typing feel of a keyboard. But my point in the video was to explain that high density foam works better to do a mix of blocking and absorbing sound than low density foam.
In other words, I was still comparing two types of foam. So even the denser foam is still soft and full of little holes filled with air, which are needed to absorb sound. Without the soft and spongy air bubbles, a super dense and hard material would either just transfer the sound to the other side or reflect the sound back (depending on the material thickness).
If you want to understand the difference between sound proofing (completely block sound waves from traveling, which will cause them to be reflected back to some extent) and sound absorption (where the sound waves are just damped to avoid reflection) I suggest you check out this awesome article by ACOUSTICAL SURFACES, INC. that explains this topic very well, using all the right analogies.
And here is another interesting aspect of the high-density foam I suggested on my video, sorbothane* (a material that is widely used in sound insulation applications), which is to block the vibrations (specially the low frequency ones, which are usually the most difficult ones to block), to some extent, from reaching the material on the other side of the foam. Foam that is too soft and with air bubbles that are too big will not do a good job in preventing the vibrations from reaching the plastic or metal on the side of the keyboard, which can cause rattling, ping and all sorts of funky noises that you usually want to avoid if you want to silence a keyboard.
So, the bottom line is that the best material to fill the empty spaces of a keyboard (if silence is your goal) is in the middle of the bell curve in terms of softness, hardness and/or density, and which can work almost as a composite material (that can do a little bit of sound proofing and a little bit of sound absorption):
If it is too soft it will not prevent vibrations from crossing to the other side, if it is too hard it will not absorb these vibrations at all and will just transfer or reflect the vibrations back. So, what you want is something in the middle: a foam (still soft) that is a bit more dense than regular packing foam, but not too solid and hard (think of a glass or a concrete wall for example) that will just transfer or reflect most of the vibrations. That material seems to be sorbothane.
Sorbothane is not the only material with that desirable trait of course, but it is one you can easily find online (although for a steep price if compared to regular neoprene foam for example, that is also widely used to quiet noisy keyboards down).
As a rule of thumb, I only suggest sorbothane for people who want to go all out in quieting a mechanical keyboard. If silence is your number one priority in a keyboard build, then sorbothane is hard to beat. But if all you want is to remove the loudest switch pings (specially in keyboards that have metal surfaces in its construction) and calm down the rattles to an acceptable level, you can totally use neoprene foam, which will be way cheaper and more than enough for most cases.
I’d like to take this opportunity to thank J. Procka for this great comment and invite you all to check out my entire tutorial on how to build an extremely silent mechanical keyboard on YouTube if you haven’t done already.
* Disclaimer: products linked in this article are affiliated links and will result in a small commission (paid by Amazon) to this website's owner.