Beyonce performs at the 2013 inauguration of President Obama.Corbis

Beyonce is scheduled to perform live at the Super Bowl halftime show this Sunday. But after she admitted lip-syncing the National Anthem for President Obama’s inauguration, fans may be wondering whether she’s actually singing.

Lip syncing, by either mouthing the words or singing along to a prerecorded track, is part of many professional pop singers' repertoires these days.

Several factors make lip syncing more common than many fans might like to think. While some argue over the ethics of it, others might look on the practice as an art form in and of itself.

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"It's accepted way more in the industry than it is in the public," said Christopher Blood, head of the music production department at McNally Smith College of Music in St. Paul, Minn. "It's a quality control issue, the ability to have the best representation of the artist out there."

Artists often lip sync when they find themselves on a stage without the proper sound system, Blood said, or if the monitoring system fails -- or if they get a cold. Certain sounds are also difficult to recreate in a live setting. Some artists sing their own harmonies, for example, layering in their own background vocals in the studio. In many cases, the artist will do a mixture of singing and lip syncing, especially during performances that require a lot of aerobic dancing.

While voice teachers generally don't teach lip syncing, vocal coach Justin Stoney of New York Vocal Coaching has some advice for artists working on their lip syncing skills:

* Nail the timing of sustained notes: It's very hard to time when a sustained note should end, because a lot of recordings don’t cut it off right on the beat -- but it's an easy giveaway if you get it wrong.

* Show some vibrato: Take fuller breaths. "You can't make yourself look like you're doing a natural vibrato, but breathing should look physical," Stoney said.

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* Position the mike: "You have to make sure you're moving the mike in a way that makes sense for the recording. When it's loud, move it away; when it's soft, come close." If you hear the volume change but the mike didn't move, it's pretty obvious the artist is lip syncing.

* Don't smile: "It's tough to keep a really wide facial posture when singing," Stoney said. Spreading the lip corners wide will brighten the tone and a mouth position toward the center will darken it. Most people in the audience wouldn't be able to explain what it was, but they would sense something seemed a bit strange if the singer's mouth shapes didn't match the sounds.

* Don't use a perfect recording: "If the intention is to dupe the audience, the original recording shouldn’t be too perfect," Stoney said. Mix in some sounds of the microphone moving around, for example.

* Sing along: "That's the No. 1. tip," Stoney said, because the motions of singing will come naturally.

"It's more like an acting exercise than a singing exercise," Stoney said. Still, there are occasions when it's acceptable, he said -- such as the National Anthem at the inauguration.

"In that case, one's presence itself benefits the event," he said, "and it's historic so you don't want to screw it up."