Apart from the music that’s performed on the House of Blues music hall stage, concerts there can be loud affairs. A boisterous, drinking crowd is just naturally noisy.

The House of Blues never pretended to be a listening room where earnest songwriters sing their hearts out as they gently pluck and strum acoustic guitars. Which makes Thursday’s concert at House of Blues by First Aid Kit — a duo featuring Swedish sisters Klara Söderberg, 21, and Johanna, 24 — an exceptional event for the venue.

A large crowd filled the music hall for the all-ages, no-alcohol show. The show’s timing — early Thursday evening — had something to do with the quiet in the hall but, even more so, the audience overwhelmingly showed up to actually listen to the mesmerizing Söderberg sisters.

Before their performance of “A Long Time Ago,” Klara warned the crowd about the regret-filled ballad.

“This is a sad one,” she said. “Most of our songs are sad, but this one is especially sad.”

A murmur of amusement moved through the audience. The Söderbergs’ exquisite melancholy and sibling harmony was exactly what brought the concertgoers there in the first place.

Younger sister Klara does most of the lead singing. She has a beautiful voice, on the sweet side of brassy but also bell-like. Typically, Klara starts the duo’s songs and then Johanna falls seamlessly in for choruses. Together, the Söderbergs produce a lovely sibling vocal blend, singing and playing together with unconscious ease.

Older sister Johanna’s lower voice contrasts Klara’s higher range. Johanna sings lead as well, usually not until the middle of a song, making a dramatic transition from one sister to the other.

First Aid Kit strode confidently on stage a few minutes after 7 p.m. The sisters opened with the dark, folkie title song of their 2012 album, “The Lion’s Roar.”

Visually, the blonde, willowy Johanna is the more dramatic of the sisters. She moved rhythmically to the music, sometimes to the point of dancing. During “The Lion’s Road,” her right hand floated lyrically above her keyboard. As the song ended, she fell to her knees for the final notes.

Klara, dark-haired and compact, is much less demonstrative than Johanna. She prefers to simply stand at her microphone stand, singing and playing her acoustic guitar. Drummer Niclas Lindstrom and pedal steel, mandolin and electric guitar player Melvin Duffy provided evocative backup for the sisters.

As sad as First Aid Kit songs can be, a theme of strength runs through much of the material, often accompanied by militaristic drums. “Master Pretender,” for one, despite its contradictory lyrics, soldiered forth triumphantly, lifting the audience with it.

First Aid Kit fans, no doubt, knew which of the Söderbergs’ many wonderful songs would end the show. “Emmylou” arrived last during Thursday’s three-song encore. Before its opening notes sounded, the attention the audience had given First Aid Kit throughout the show magnified into suspenseful, absolutely silent anticipation.

Mid-song in “Emmylou,” Klara invited the crowd to sing. “I’ll be your Emmylou and I’ll be your June, if you’ll be my Gram and my Johnny, too,” the concertgoers sang on their own. Of course, they knew the words to “Emmylou” as well as the words to many other songs the band performed that night.

Early in the show, Klara told the audience how happy the group was to be back in magical New Orleans. But Thursday night on Decatur Street, the Söderbergs brought the magic.