Acoustic Room Treat­ment and Build­ing Sound Pan­els, Part 1: Planning

Whether you are running online workshops, recording audio or video, or making music, it’s worth spending some time on acoustic treatment for your room. Shit in, shit out, as they say… In my case, I wanted to improve the sound of voice recordings and live audio in my little office in the attic, which has a quite small footprint. So, a while ago, I made a real job of it and did my (amateurish) best to make the space sound better. Part of that involved building a few sound panels myself. This is a write-up about which considerations went into improving the room and how I built the panels.

I’m not an audio expert, so while I’ve tried to be as accurate and thorough as possible, please take this post with a grain of salt and feel free to correct me if I’ve got something wrong. I’m mainly documenting it for myself. But if you are thinking about improving the sound of your room as well, then hopefully this is interesting for you and gives you a first rough idea of the things you could think about.

Proof or Treat? #

Now, if you want to improve the sound of a room, there is first an important distinction to make: do you want to keep sound out? Or do you want to improve the sound inside the room? Both things can be important if you want to improve the quality of your recordings. But they are fundamentally different things.

Keeping sound out of a room – or also preventing it from leaving the room – is commonly referred to as soundproofing. Whether you are living right next to a street with a bit of noisy traffic during peak hours, like I am, or you don’t want the sound that other people in your house are making to be heard, or, you want to save your neighbours from hearing the sound of your drums – you basically want to block sound waves and stop their transmission through vibrations and so on.

What you need in this case, is first and foremost: mass. I always like to think of it like the water in the ocean: there are smaller mini waves in the ocean and also gigantic monster waves. If you now put a wall into the water with the intention of breaking all the waves, the small waves will easily be stopped by the wall, because they are low on energy. The monster waves, however, are so full of energy that they will “roll” right through your laughable attempt to disturb the forces of nature. So, especially if you want to block sound in the lower frequencies, there is no better way to stop a wide spectrum of sound waves than thick, dense materials like concrete walls – the thicker the better. However, changing the walls themselves is rarely an option. You can still try to add mass by adding an extra layer of drywall, OSB, MDF, of MLV (Mass Loaded Vinyl). And it can also be quite effective to create airtight seals around doors or other gaps through which sound could leak in or out. But soundproofing is not the primary topic of this post. So let’s move on for now.

If you want to improve the sound inside a room, that’s called acoustic treatment. And that’s what we’re doing here. We want to shape how sound behaves in our room. We want to reduce echo, reverberation, reflections, unpleasant frequencies, and bass buildup, to create a more controlled, even, and pleasant frequency response. For that, different materials can be used. And it is crucially important which material you pick.

Picking the Right Tools #

A common myth, for example, is that if you want to treat your room, you buy cheap egg-crate foam and slap it onto the walls. This won’t work, however, because all this very lightweight material does is reduce reflections in the very high frequencies. Think about it: this foam consists basically of a lot of bubbles of air trapped inside thin plastic – most sound waves can pass right through without a hitch. Only the very high frequencies with very little energy will be reduced a bit. Ultimately, all egg-crate foam really does is create an unbalanced sound that can make a room sound lifeless and “dull”, because it is now missing the high frequency information that audio engineers often refer to as “air”. Besides that, many cheap foams marketed as “acoustic” on Jeff’s online shop are not fire-retardant and can emit toxic fumes when burned. So we need something else.

What actually works better for effective acoustic treatment, are acoustic panels made of materials that are thicker and more dense than egg-crate foam. Very often, you will find people suggesting rock wool or mineral wool, for example, which is primarily used as an insulation material. This works much better, because it absorbs a higher frequency range while still being quite lightweight. And, the thicker the material, the lower the frequencies it can absorb. The only problem I had with using such a material: although it is marketed as being safe, and it is also not allowed in the EU to sell rock or mineral wool that has particles that are so small they will stay in your lungs and cause cancer, it still didn’t feel right to put it onto the walls of a room in which I’m sitting every day – for the next 40 years. Also, people recommend to always wear a pair of gloves when working with rock wool to avoid skin rash and itching. What?!? That surely sounds “safe” as heck to me…

Luckily, there are now better options, like recycled denim, hemp or wood fibre, or also Basotect, a dense melamine resin foam which is flame resistant and has good sound absorption properties. Basotect has been tested for harmful substances and is certified to the STANDARD 100 by OEKO-TEX®. Yes, it is more expensive than rock wool, but the idea is that it also lasts much longer – and doesn’t emit fumes or microparticles. Lastly, Basotect is also really good in keeping its shape which makes it easier to work with for a noob like me. So I went with that. I ordered it from a German store called schaustofflager.de – I actually don’t know where to get it outside of Germany, but you might be able to find a reseller.

A Little Room Acoustics Primer #

But before we actually can start to build our panels, we need to take a step back and answer an important question: what acoustic problem are we actually trying to solve?

Every room has unique challenges. Are you recording in a huge office with church-like acoustics? Then you will have completely different (reverb) issues than in a small 3 × 3 meter attic space like mine. Every room is different and requires other treatment. Very often, what is needed is a combination of absorption, to reduce unwanted frequencies, and diffusion, to balance the whole frequency response.

Standing Waves #

In a tiny room, however, you will most likely have a problem with low frequencies: because the walls are so close together, long sound waves (those with lower frequencies) don’t have enough time to travel before they hit a wall and, still full of energy, are reflected back. When a wave is reflected back and overlaps with the next incoming wave, the two waves will interfere. And that’s called – you guessed it – interference. Actually, waves do interfere in rooms of any size, the problem is just more serious and apparent in smaller rooms. The two overlapping waves now might either reinforce each other, because the amplitudes of the waves add up, or also cancel each other out, when they are “out of phase” – so one of them has a “dip” where the other one has a “peak”.

At certain frequencies that depend on your room’s size, this creates so-called standing waves, first described scientifically by Michael Faraday in 1831, by the way. Yes, that’s the guy with the cage ⚡️.

Standing waves are called standing, because they’re waves that oscillate but don’t move in space. Like this red wave here:

These standing waves create louder and quieter spots in your room at specific resonant frequencies. Where the waves cancel out (called nodes), there’s barely any sound. Where they reinforce each other (called antinodes), the sound is amplified.

Room Modes #

All these resonances together are called room modes. If you want to dive deeper, watch this excellent video by Kyle Mathias. Room modes make sound at certain frequencies appear louder or even completely quiet, depending on your listening position. Put on A Milli by Lil Wayne or Legend Has It by Run the Jewels and walk into one of the corners of your room … see, I mean, hear?

This can become a real problem, though, when you’re mixing music and you are listening at a position where certain frequencies are boosted or cancelled out. “But Matthias,” I hear you say, "we’re trying to improve our room for recording stuff, not listening.” Now, what if the speaker emitting the sound is your mouth, and the ear that’s listening happens to be a microphone? 🤓

Room modes depend on your room dimensions because sound waves have specific wavelengths. Waves that will create standing waves (= modes) between two walls are those with their highest points of pressure, the antinodes, at the reflection points. The lowest mode occurs when half a wavelength fits between two walls. For example, if your walls are 3 meters apart, a wave with a 6-meter wavelength will produce a standing wave. That’s a wave with a frequency of about 57 hertz. The same happens at multiples of this frequency, like 114 Hz, 228 Hz, and so on.

This room mode calculator is fantastic for finding out where the room modes of your room (approximately) are: amroc Room Mode Calculator This is how the room modes look for a room approximately the size of my office, for example:

Just as an aside: knowing those room modes will also be really useful to correct recordings with a parametric EQ later, because you know where your room might still sound a bit “boxy”. And then, you gently attenuate those frequencies a little bit.

Reducing Resonances #

Now that we know our room has modes and roughly where those resonant frequencies sit, the next step is actually treating them – but only if they're causing real problems. In my case, I could clearly hear an issue around 124 Hz, right at one of my room modes, for example.

So how do we fix this? The answer lies in understanding how porous absorbers like Basotect actually work. They absorb sound by converting the energy of moving air molecules into heat – through friction. And here’s the key: they work best where particle velocity is highest, not where pressure is highest. In a standing wave against a wall, the air molecules barely move at the wall surface itself (because pressure is high there). But at a quarter-wavelength distance from the wall, particle velocity reaches its maximum. And that’s why a rough rule of thumb is that the thickness of your material ideally is at least a quarter of the wavelength you want to absorb. For that 57 Hz wave with its 6-meter wavelength, a quarter wavelength would be … 1.5 meters. Which is … err … kind of impractical if your room is 3 meters wide. As you can see: there is just no way to effectively reduce the lowest frequencies in such a tiny room. And that’s why we focus on the problematic low-mid frequencies in the 100 – 300 Hz range.

But it should now be really obvious, why effective acoustic panels need to be substantial — at least 10 to 15 cm thick when using materials like Basotect foam. A 10 cm panel, for example, might absorb down to around 300 Hz before it gets less effective, as you can see on this “absorption coefficient” graph For Basotect:

And that’s where the air gap trick comes in: When you mount a 10 cm Basotect panel with a 10 cm air gap behind it, the panel sits closer to where air molecules are moving more for lower frequencies. This effectively extends the panel’s absorption range downward – it’s like as if you were actually using a thicker absorber! A simple way to get better low-frequency absorption without using twice the material. So using the thickest material strength available in my case (10 cm), should actually be an effective way to at least attenuate some of the lower-mid frequencies.

That’s about it for this post, which turned out much longer than expected. That’s why I’ll cover the actual building of the panels in part 2 of this mini series. And I’ll probably write a third part as well, which is about how and where to mount the panels and the actual results of the treatment – as far as I could measure the outcome.

Let me know if this was interesting or helpful and also if you have treated your room as well. It would be really interesting to hear what worked for you.

❦

This is post 17 of Blogtober 2025.

~