The immensely satisfying sound of snap-fit design
by Paul Lukas
I’m mildly asthmatic. It’s not a major factor in my life, but I always carry an inhaler with me, just to be safe. There are lots of generic inhaler brands on the market, and they’re all pretty much the same — or at least that’s what I thought until my pharmacy recently started giving me a brand called ProAir, which has completely reconfigured my relationship with asthma medication.
The difference is the ProAir’s cap, which snaps onto the inhaler’s mouthpiece with an extremely satisfying “Click!,” as seen in this short video clip:
Unfortunately, the video doesn’t quite capture the full-bodied sound of the click. Although the mouthpiece and cap are both plastic, the click sounds naturalistic, almost woody. It has depth, and even a hint of an echo.
This aural experience is matched by the cap’s tactile pleasures. It takes just the right amount of force, which is met by just the right amount of resistance, to push the cap onto the inhaler’s mouthpiece. When the force overcomes the resistance and the cap snaps into place, it feels like a perfect marriage of subject and object, action and reaction, and you can feel the impact of the snap resonating up through your arm and into the core of your being. It all feels just so, to the point of being borderline addictive. When the inhaler is in my jacket pocket, I instinctively remove the cap, snap it back on, savor the click, and then do it again. I’m not quite taking it to OCD levels, mind you, but it’s easy to see how the ProAir could entice someone to go down that road. It gives a whole new dimension to the term “clickbait.”
Clicks, of course, can be produced by all sorts of items. They’re most common in products with caps (pens, beverage containers, ChapStick) but are also lurking within other types of mechanisms (Ethernet cable jacks, kiss-lock purse closures, even the computer mouse or trackpad that you may be using to navigate this article). In each case, the user must apply gentle pressure to overcome mild resistance. If the elements aren’t quite in harmony, the experience can feel either unpleasantly harsh or unsatisfyingly empty. But when it’s all engineered just right, as in the case of my inhaler, the resulting click feels like a little moment of perfection, as if the entire universe were briefly coming into alignment.
This type of closure is called a snap-fit. “It’s typically used when you don’t want to use a mechanical fastener, like a screw or a latch,” says Ross Bergman, a mechanical engineer who currently works at the office furniture company Humanscale. “It’s a low-cost solution, because you design it into the part, so a lot of it comes down to mold-making. I agree that there’s something assuring and satisfying about one that goes on just right.”
Most snap-fit designs are exceedingly simple — a raised ridge here, a rounded lip there. My inhaler is an instructive example: The underside of the mouthpiece has two small ridges, and the lower inner surface of the cap has two rails, each with a small notch:
When the cap is pushed onto the mouthpiece, the cap flexes ever so slightly, and then the ridges and notches mate and everything snaps into place. The click’s unusually resonant sound is caused by the cap’s wide-mouthed shape, which creates a miniature echo chamber. It’s a mini-masterpiece of industrial design.
Plastic has a natural elasticity, which makes it well suited to snap-fits. Still, all that flexing and friction can take their toll. “You want it to deform just enough when that pressure is applied,” says Bergman. “But you don’t want it to, as we say, ‘yield,’ which is when it deforms permanently and doesn’t come back. That’s called the plastic limit. Basically, you want it to be repeatable for however many times you need during the product’s anticipated life cycle. If you exceed that life cycle, the material may no longer react the same way.”
But how do you get just the right dynamics for that perfect click? “If button or part has to travel too far before it clicks, that doesn’t feel good, so you can graph out the force required versus the distance traveled, even if that distance is only a millimeter,” says David Lean, a mechanical designer who’s worked on a wide range of consumer electronics products. “But it’s still very much a trial-and-error thing. No matter how many of them you’ve done in the past, each design has its own challenges.”
Lean says the snap-fit’s sound component — that audible “Click!” — is important. “I don’t have a target level for the decibels or anything like that, but you want to give the user the right amount of click to confirm that they’ve completed the action correctly,” he says.
“It’s tricky, because you’re basically trying to quantify something that’s more of an emotional response.”
Steve Wisler, a mechanical engineer who’s worked on pens for Paper Mate, agrees. “For some of our products, there was actually a requirement to have an audible snap,” he says. “I tested a lot of pens while I was there, and some had designs where the experience was more tactile than audible — you could feel the snap but not necessarily hear it. And that’s fine, but I think the audible snap really adds that extra level of security and reinforcement.”
So are hearing-impaired people, in addition to all the other challenges they face, missing out on a key aspect of the snap-fit? In an admittedly unscientific attempt to find out, I tried a few of my favorite snap-fit items — a pen, a juice container, and of course my inhaler — while wearing noise-canceling headphones. The satisfaction level was definitely diminished.
But even without the audible click, I still enjoyed the feeling of repeatedly playing with these snap-fits. Lean and Wisler both refer to this as the “fidget factor.” Lean says he picked up this term from Apple industrial designer Jony Ive’s biography, which is funny, because the term used in that book is actually “fiddle factor.” I like “fidget” better, but whatever — by either name, it accurately captures the compulsion to engage with a snap-fit’s simple but enticing duality of pressure and release. It’s probably more than a coincidence that many people, myself included, are similarly drawn to knuckle-cracking and bubble wrap popping, both of which involve pressure/release dynamics and audible snaps.
Are snap-fit designers aware that their work can have this type of behavioral effect on people? “Absolutely,” says Wisler. “At Paper Mate, I was working on this one project for a mechanical pencil with an eraser cartridge that goes in and out of the back end with a very strong snap-fit. When we were developing it, we definitely thought about the fidget factor, and that someone would be clicking this thing on and off, over and over. I know I do it constantly myself.”
(As an aside: We can’t talk about Paper Mate and clicks without mentioning retractable pens — the ones with the clickable button at the top, which some people find so irresistible that the term “Habitual pen-clicking” has its own Wikipedia subheading. But these pens operate via a ratchet mechanism and, strictly speaking, are not snap-fits. Consider them a topic for further study at a later date.)
But if a well-designed snap-fit is addictive, a bad one is a turn-off. My ChapStick, my toothpaste tube, my shampoo bottle, the cover to the battery compartment on my TV’s remote control — all provide muffled, cheap-sounding clicks and profoundly unsatisfying tactile experiences. And the caps on a few of my pens require so much pressure that the whole process feels jarring and unpleasant. If some snap-fits are so successful, why do others fail so miserably? “Because dialing it in to get that exact sweet spot is very difficult for a mass-produced item,” says Wisler.
Fair enough. All the more reason, then, to appreciate that rarefied strata of products, like the ProAir, that get it just right.
Each month this column will examine small, easily overlooked aspects of the designed world — some obscure, others commonplace, but all worthy of close inspection. Suggestions for future topics are welcome here.