We needed some chairs, here at Umbel, and we found some that we really like on Craigslist for $7 each. They’re Haworth chairs from the 1990’s and quite good for our use. They're well designed; we like the color blue. They are durable and pretty comfortable. 

But if you had to sum them up in one word, you'd probably choose “stackable.” 

“Stackable” implies impermanence. These chairs are not really designed to be in a space for any length of time, other than for that immediate need. When you're done with them, you literally remove them from the room. 

Furniture like this invites questions about permanence. How long do you want somebody to linger in a space? They do have a certain type of permanence, a durability. These chairs were around for decades before we found them, and they work just as well as the day they were made. But they don't have a permanence as far as the room they're in. They don't have a belonging in the space. 

You can try to plot this lingering-versus-permanence of design on a graph. The stackable chair occupies a very particular place on this plot.

It's not only impermanent. It also does not invite you to linger. A more comfortable chair, maybe a larger chair, occupies a different location. It invites you to stay simply because it's comfortable and it's larger and more difficult to move. So it has a greater sense of permanence. There are other pieces of furniture you can think about in this way.

Things like hostile architecture, like benches that are immovable, literally bolted to the ground, but are designed so that you use them the minimal amount and then move on.

And then there are other pieces, like a 19th-century armoire, where even though it's not bolted to the ground, it has a weight, it has a presence that makes it feel very permanent, that makes it feel very much like home.

But this isn't really a story about furniture. This is a story about acoustics.

What do the spaces that we inhabit typically look like?

You can tell a great deal about a space acoustically based on its visual appearance. Our spaces are usually rectangular boxes with very flat surfaces.

Sound energy in a rectilinear box tends to propagate for a very long time. It has a long reverberation time, and that can be problematic. Spaces that have more complicated boundaries, more furniture, more ornate details, more variation in their boundaries tend to dissipate sound more quickly.

That means they have a lower reverberation time. Communication tends to be clearer, and these spaces tend to be nicer to be in. As soon as these flat surfaces, the rectangular forms, the stripped away ornamentation become the dominant features in these rooms, that style starts to move into other elements like furniture, and the rectangular, minimal box starts to become a benchmark.

It's another kind of hostile architecture. It's loud.

It promotes the propagation of sound. It promotes long reverberation. It creates spaces that are uncomfortable to be in. 

This is what happened when we reduced the permanence of our design. We started to remove the ornamentation from our rooms. We started to remove it from our furniture. We made things lighter, more temporary, more stackable, and our environment got louder, got more reverberant. 

And it's not that no one had noticed this happening. People have been writing about the rise of noise for at least a century. In 1913, Luigi Russolo wrote The Art of Noise, where he actually discusses the increasingly loud environment in which he found himself. 

But most of that conversation about rising noise was coming from the standpoint of the machines that were generating that noise.

Our thesis is that our built spaces also contributed to the noisiness.

It's not a coincidence that the removal of ornamentation and the simplification of our designs coincided with the rise of acoustic products. These changes in the way that we design gave rise to reverberation. Reverberation was originally pretty well controlled just by the style of our rooms and the style of our furniture.

Now we needed a way to bring that acoustic functionality back, but within the constraints of modernism: minimizing the footprint, minimizing the visual effect. But then you end up in a situation that's very similar to the stackable chair situation. 

If you're designing acoustic products that can go anywhere, how do you make them look like they belong in any given space?

There's this really interesting experiment that was done at Riverbank Acoustical Laboratories. They're the original testing laboratory for acoustics. They decided one day that they were going to test empty beer bottles to see if they absorbed any sound. 

The dominant way of thinking in architectural acoustics is generally in terms of “hard” and “soft” materials. Something made out of fabric, foam, or fiberglass is considered “soft” and generally would absorb sound. Something made out of metal, concrete, or glass, like a beer bottle, would be considered a “hard” material and would not absorb sound.

They ran the test: They laid a bunch of empty beer bottles out on the floor and they found that they absorbed sound.

This isn't really a surprise. There's a mechanism for this that we know about. It's called Helmholtz resonance. Any time you have a cavity and an opening of that cavity, it can resonate at certain frequencies, which causes sound to absorb.

Riverbank does this on occasion with anything weird that wouldn't be paid for by a manufacturer. Most of what they do is test acoustic products sponsored by manufacturers, and they realized that if that's all you test, then you never test anything that a manufacturer wouldn't sponsor. And so they launch this program, and they just occasionally test weird things to see how they behave acoustically. 

So we were sitting around on our blue chairs talking about the beer bottle experiment, and we thought, maybe we should test the chairs.

Would a minimal stackable hard chair, from the 1990’s laid out in an array, absorb any sound? 

And of course, they do. 

Although hard versus soft materials are a good way to think about acoustics, it turns out to be not strictly true. Acoustic performance is more about shape than it is hard vs soft.

When you look closely, you can see this everywhere. The fact that a plate glass window tends to reflect sound, and a beer bottle tends to absorb it in a very particular way, and fiberglass, which is also made out of glass, tends to absorb it, indicates that this isn't really about hard versus soft material, it's about the configuration of that material.

So the blue chairs absorb sound simply because they have a significant shape. They are very different from having a flat plane.

There's this long-standing mystery in acoustics called the “Absorption Coefficient Problem.” Essentially, when you send a surface or an object to a laboratory, you're trying to find out how absorptive it is, how much sound it absorbs. If you take a sample and you send it to one lab, which is highly controlled and standardized, you get an answer for how absorptive it is. If you take that exact same sample and you send it to another lab that's also standardized and highly controlled, but just a different location, you tend to get a different answer.

These answers are close enough to each other that we can still do our job in acoustics, but they're different enough that they can't be explained by experimental uncertainty alone.

They're different enough that it's cause for concern in the acoustics community, and it has been for more than 100 years now. 

Maybe these stackable chairs can help us illuminate that. 

“Why are these chairs blue?” we can ask. They're blue because when white light falls on them, some colors are absorbed and some colors are reflected. In this case, blue is reflected. But if you were to take one of these chairs and put it into a pitch-black room, no light at all, you could argue that it doesn't really have a color. You could argue that it only has potential for color when you put it in a certain context.

If you're an artist and you're trying to do a realistic painting of a blue chair, you probably know that it's not just about choosing a blue color. There are many other colors mixed in that give the chair its particular look.

In other words, the color of something, like the color blue, depends on where it is and the light that's falling on it.

Maybe absorption coefficients aren't really “real” in the sense that they aren't innate in the surface or object that we're talking about. Maybe absorption is just context-dependent. This is why we get a different result in one lab and another.

But the thing that remains the same between those is the form and the material.

Maybe you can see where this is going. If form and material determine both how something looks and how it sounds, and those two things are both highly context dependent, then maybe we shouldn't be thinking about acoustic materials as hard or soft. Maybe we shouldn't be overly concerned about absorption coefficients. Maybe we should lead with form and material, and we should always be thinking about the entirety of the room that we're designing. 

That includes the elements that fit within it. A piece of furniture, because it has form and material, will influence the sound of a room, and the more complex, large, and present that furniture is, the more it will influence the sound of the room. The more permanent it is, the more it will consistently influence the sound of that room.

For architects, this means you can design and shape hard materials to influence the sound of a space in a beneficial way. 

For product designers, it means you can think about the design of a chair or whatever it is you're designing, in the context of acoustic performance as well. If it's larger and more complex, it will have more of an influence on the space. 

And maybe we should also think about what we do with our stackable chairs when we're not using them. Maybe there's a way to make them more present, less temporary, more permanent.