I’ve been using Ultimate Ears for about four years now, and one thing I can say for sure is they are dedicated to producing great products and serving their customers. Oh, and the ears sound pretty darn good, too. A few weeks ago, they invited me up to the office to take some photos to show you all of you. 

Like me, you probably never considered how custom IEMs are made. I was frankly quite surprised how much hand work was involved; I expected much more of the process to be automated. 

The process starts with a set of impressions. Below is a really bad picture of me having my impressions made. We have become really spoiled with our iPhone cameras the last few models… But I digress.

A two part gooey substance is squirted into your ears (after they put a cotton ball in to keep it from hitting your ear drums). You’ll note that there is a bite block in my mouth. This opens your jaw and changes the shape of your ear canal slightly which is especially important for singers. If they weren’t molded “open mouth,” every time the singer would open his or her mouth to sing, the seal would break and sound would be comprised.

It’s a weird feeling to have this stuff squirted into your ear. It’s a bit like being underwater. Thankfully, it cures in about 3 minutes. This substance forms an exact mold of the inside of your ears. In fact, when they pull them out, they look like this. 

The strings are attached to the cotton ball "plugs." 

The strings are attached to the cotton ball “plugs.” 

Once the molds are made, they head back to the shop where a dozen skilled technicians craft and shape the shells that will become the custom IEMs. One of the things about custom molded IEMs is that they rely on being exactly the same shape as your ear canal to maintain a seal. If the shape is off, the seal will be off and sound is compromised. Unlike universal fits, which have a foam seal that conform to anyone’s ear, the shape in customs has to be spot on. UE takes great pains to make sure the shape is right. 

The first step in the process is to refine the actual mold. As you can see from the photo above, there are strings and a cotton ball that needs to be removed, and they need some slight clean up. That happens at this “grinding” station. 

Click to enlarge

Click to enlarge

At this station, the mold is cleaned up and prepared for the next step. This phase is especially critical because any mistake here will translate to the final shell, and a new mold will be required. It takes a lot of skill to know how to buff this up so it’s useable, but not too small.

The next step is to make an inverse mold from silicone. I don’t have any shots of that as they weren’t doing any when I was there. But you’ll see what the mold looks like in the next few steps. 

Once the inverse mold is created, the shell beings to take shape. I hadn’t seen this step before, but it’s pretty cool. 

You can see the polished impressions on the table. Click to enlarge.

You can see the polished impressions on the table. Click to enlarge.

A UV-cured resin is poured into the mold. Remember, the mold is the exact shape of the inside of your ear, so the resin will be the exact shape. But the shell needs to be hollow, and the exact thickness all around. So how do you do that? This part is cool.


Once the molds are filled, they are placed in a special UV oven. Lids are placed on top of the molds, the door is closed and the timer started. Each color plastic has a very specific time that will cure the resin to the exact thickness. As it cures from the outside in, it only cures the outside. 

The molds are removed from the oven and the excess resin is poured off. Because it’s a UV cured resin, there’s no hurry. The molds then go back in the oven to be cured again. This time, the lid is left off as they want the whole thickness cured. 


After curing, the molds head to the next station. In the photo above, you see all the steps in the process so far. The original impression, the mold, and the custom shell. If everything has gone according to plan, the shells should be the exact same shape as the impression. It seems easy, but there’s a lot that goes into it up to this point.


Enough things can and do go wrong that UE employs over 20 different QC check points to make sure everything fits the way it should. Here, a technician checks the shell against the mold. Any gaps will effect the sound, and that is unacceptable. It has to be perfect. 

After the shells are approved at this point, it moves on to the electronics station. 


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