The fall of the house of “Idol” continues unabated this season, despite the introduction of improved production values on the show and an appealing new lineup of judges.

A onetime ratings juggernaut, “American Idol” on the Fox network hit a new ratings low Thursday night, one that would have seemed incomprehensible even two years ago.

“American Idol” sank below the 2.0 ratings barrier, scoring a 1.9 in the group that Fox sells to advertisers, viewers between the ages of 18 and 49. That 1.9 figure – which will improve somewhat when delayed viewing is counted – is surely a psychological blow to the series, which once routinely hit a double-figure rating in the 18-49 category.

Thursday’s show also had the smallest overall audience – 8.4 million viewers – since a night in July in its first year, when “Idol” played in the summer. A year ago on the same night, “American Idol” averaged a 2.9 rating, which means the show sank about 34 percent from year to year.

Fox executives went into this season hoping to stanch, not to reverse, the ratings plunge of recent seasons, looking for relief, in part, from a new roster of judges. The country artist Keith Urban is the only holdover from last year; he is joined by the pop superstar Jennifer Lopez (a fan favorite back after a year away) and the pianist-singer Harry Connick Jr.

“Idol” had been down about 30 percent each of the last two seasons. As recently as April 2012, the show made news when it hit what was then a ratings low – of 3.9. That 30 percent drop-off figure may again be the average this season, surely putting the show’s long-term survival into more question.

The diminished audience for “Idol” needs to be put into perspective. Even with a 1.9 rating, “Idol” remained a well-above-average performer for Fox, which had a scary prime-time average last week of 1.4. Still, with a downward trend showing no signs of bottoming out, the future for “Idol” looks more gloomy than hopeful.

The precipitous drop has been driven by several factors, one being a literal factor: the show “The X-Factor,” the short-lived similar singing competition Fox tried three seasons ago, which succeeded only in diluting the appeal of “Idol.” (“X-Factor” has now been canceled.)

Another crucial factor was NBC’s introduction of “The Voice” three years ago, which shook up the singing format and became an instant hit. “Voice” is still a hit, while “Idol” continues to erode alarmingly.