September 23, 2019
It all comes down to occlusion and isolation.
An in-ear monitor — often called by it's initials as an IEM and sometimes mislabeled as a canal-phone — blocks and seals the ear canal from external noise. It achieves this feat through 2 very different methods.
The easiest way to block the ear canal is by use of a large foam or silicone tip that is inserted directly into the ear. In order for this to work, the tip has to be slightly larger than the canal itself so the pressure of the expanding tip keeps everything fitting nice and snug. Many IEM's also are built to rest comfortably in the bowl of the ear and many models incorporate a strain-relief cable system that loops up and over the ear to help keep everything comfortably in place.
The second way to block the canal is by inserting a custom made in-ear monitor that fits perfectly snug into your ear canal and ear bowl. But this can only be made if you visit an audiologist or hearing aid dispensary to take an impression of your inner and outer ear. They'll fill your ear canal up with a liquid silicone that will quickly harden into the shape of your ear and from there, the in-ear monitor manufacturer will use that mold to make a cast of your exact ear dimensions. Each manufacturer has their own unique way of going about this — some will hand pour, some will use 3D printers, some will offer silicone in-ears while others will offer acrylic while others offer hybrid bodies — the common thread being that you seal your entire ear completely and in theory, there is no pressure holding it into place. When it is built right, it just fits. This is a true custom solution.
Once you have isolation, a few things happen sonically that we need to discuss. The first is that by sealing the canal, you block roughly -26dB of ambient noise. This is true for universal IEM's using foam or silicon tips or for custom in-ear monitors. Blocking the canal with anything should give you roughly -26dB of isolation if you have a complete seal.
This matters because it means that the sound coming out of the in-ear monitors does not need to compete with external noise. Not only are the drivers literally right next to the ear drum, but all exterior noise is significantly reduced. This gives the feeling of immersion, of fullness, of full presence, of a wide soundstage — of all the sonic characteristics that in-ears are known for.
The second thing that happens is that when you completely seal the ear canal, the low-end frequencies are heard / perceived via bone conduction. This makes a HUGE difference in how in-ears sound compared to earbuds. With a properly sealed in-ear monitor, the bass is surprisingly full and powerful. Low-end frequencies can be present at +/- 3dB all the way down to 20Hz.
Earphones — or earbuds as they're often called — do not block or seal the canal. They just sit lightly in the concha bowl. The best example of this is the classic iPod earphone or the AirPods. Earphones are typically small and have a single driver voice coil speaker. Since they don't seal the canal and since they are competing with ambient noise, the bass response is often lackluster. As a result, the user often runs these fairly hot. They might not sound like they are very loud because of all the other sounds that are going on, but they definitely are pushing some SPLs.
When an in-ear monitor isn't fitting well or when the seal is compromised, you can think of it as a very expensive earphone. The low-end completely disappears.
As a sidenote — we've all seen photos of young bands using earbuds on stage as in-ear monitors. This set-up will never deliver the desired results simply because earphones lack the ability to provide an adequate seal. Not to mention that in order to compete with typical stage noise, the earbuds would need to be excessively and dangerously loud.
Anything that sits on top of your ears that has a headband or a strap that goes over or around your head is a headphone. Headphones can sit on your ears (Supra-aural) like the old Walkman headphones or they can completely enclose and wrap around your ears (Circumaural headphones.) Both types of headphones can be further subdivided as open or closed backs — depending on how much ambient noise can bleed out and in.
When it comes to studio headphones, closed-back headphones are used for recording (because they don't bleed) and open-backed phones are used for mixing.
On stage, DJ's will be using closed-back headphones and every once-in-awhile, you'll see a Keith Moon disciple using closed-back cans for stage monitors.
Like anything else, it comes down to personal preference whether you prefer using a headphone, an earphone, or an in-ear monitor for casually listening to music. But when it comes to using the right tool for the right job, I don't want to see earbuds on stage anymore!