"My ears weren't working!" has been an artist's rejoinder for everything from cueing the wrong playback to singing off-key. But sometimes things really do go wrong.

Tiny though they may be, in-ear monitors can certainly have a huge impact on a live performance, delivering a custom audio feed directly into an artist's head. As integral as an amp or microphone, in-ear monitors also isolate noise and protect hearing. Not least of the benefits, the monitors are a handy excuse when things don't go well. "My ears weren't working!" has been an artist's rejoinder for everything from cueing the wrong playback to singing off-key.

But sometimes things really do go wrong. In the case of Adele stopping and restarting her performance of George Michael's "Fastlove" during her Grammy tribute to the late singer on Sunday, she had issues with a high-frequency sound in her in-ear monitor during rehearsals, Grammy producer Ken Ehrlich tells Billboard. As for what happened during the show, however, the Grammy producer "can only speculate" that it was an in-ear issue, though the matter is being investigated, according to a Grammy rep.

Tour manager Harry Sandler, who has worked with Katy Perry and John Mellencamp, tells Billboard that frequency issues can often be an issue during a live show. "There are so many frequencies in a building, they can overlap and create problems, whether with a guitar and a wireless monitor or an in-ear that goes to a belt pack."

Interference from a neighboring channel is not uncommon. Working a busy stage, monitor engineers can easily find themselves negotiating among 40 different audio frequencies, allocated among the band, backup singers and dancers, as well as key engineers and sometimes security personnel. Separate channels that are used for things like police and TV crew communications can also interfere.

"You work to make the wireless 'clean,' which has to do with antenna placement and assigning the principal performers the most robust frequencies -- things that usually get worked out during rehearsal," says monitor engineer Kevin Glendinning, who got his start in radio frequencies with The Rolling Stones in 2000 and is about to go on tour with John Mayer.

As the staging setups and communications demands have gotten more complicated, the in-ear technology has also improved since being introduced sometime around 1995 by Jerry Harvey, a monitor engineer working for Van Halen who has since launched his own firm, which dominates the category. An estimated 11 of the 17 acts performing at the Grammys used JH in-ears (Adele not among them).

"What Van Halen wanted he couldn't get off the shelf, so he began innovating, making stuff in the tour bus," says JH Audio creative director Zac Penrod. "The idea was to get away from the floor monitors, which were deafening and didn't offer any subtlety."

Today, in-ear monitors are quite sophisticated. Twenty years ago, Harvey made the leap from single-driver systems (roughly the equivalent of mono) to three drivers: one for low range, one for mid-range and one for high range.

Today, JH's high-performance unit offers a quad of drivers per range, or 12 per ear. Penrod stressed more isn't necessarily better. "You do get more sound separation and detail, breaking up those frequencies, but it's a very personal thing. Some top artists only want 8 or 10 per ear."

"It's like a vocal microphone: different ones work better for different uses," Glendinning says. Independent of the driver capacity, the in-ears are mixed to specification by the monitor engineer to meet the audio needs of each artist.

"Some want just their voice, others want an album mixed in their head -- like a freshly pressed CD," Glendinning notes. "I have one client that has tremendous hearing loss in her left ear only, and the engineers at JH were able to augment the high frequency on the one side to achieve a 50/50 frequency balance in her ears."

For the professional market, in-ear units are all custom-molded (for which clients must visit an audiologist). At JH Audio, the entry-level three-driver custom model starts at $599 and goes up to $2,725 per pair, exclusive of design work.

Bruno Mars requested gold-plated in-ears with his Mars logo in red. Lady Gaga has rhinestones on hers. "She is getting ready to launch a new tour, and came to us for all new in-ears in different colors, to go with all her outfits," Penrod says.

Last year, JH introduced its Performance Series of off-the-shelf ears that retail for $999 to $1,599 and are used as high-end earbuds and also go out to professional clients who need a quick replacement. "They have a generic ear shape and we provide foam and silicone tips to improve the fit," Penrod said.

These days, virtually all in-ear monitors are wireless, since performers want the ultimate mobility, "although sometimes I hard-wire the drummers and others in fixed positions," Glendinning notes, adding, "Less is more in your final count on the radio frequency plot."

Although the in-ears are shielded from ambient noise, some artists want a bit of house sounds mixed in, so they can "feel the room," because "that's part of live concerts, feeling the audience," Sandler said. "It's not supposed to be a studio experience."

You can really get creative. Justin Timberlake spent a long time working "to get something that sounded like a piano in his head," Glendinning said. "You can try different reverbs, different EQ [equalization], adjust the low and high balance. When you can sit down with the actual guy that's singing down the mic, it's invaluable. A lot of people just won't take the time; they'd rather be doing something else and let the technical crew figure it out."

That type of care can help ensure things go perfectly during a live performance, but even when things derail, a canny artist can sometimes plow through. "I lost [connectivity to] my mixing desk at Madison Square Garden with Alicia Keys and Jay Z," Glendinning said. "They were looking at me; they could tell we were having issues, but the audience never knew."