Apple's AirPods weren't exactly hailed a revolutionary, must-have product when they were announced last year, but that hasn't stopped the wireless headphones from dominating the market. CEO Tim Cook recently admitted that, despite increasing production capacity, Apple is struggling to meet "strong level of demand for AirPods."

As is the case with the Apple Watch, the iPhone maker has never revealed how many AirPods have been sold since they arrived in December. But according to research firm NPD, over 900,000 totally wireless headphones were sold in the US since the start of the year, and AirPods made up 85 percent of the revenue on those sales.

While NPD doesn't say exactly how much revenue Apple generated from its headphones (or the precise number of units it has shifted), the AirPods' $160 price tag means they're another big earner for Apple.

'Totally wireless' is a new segment of the Bluetooth headphone market that refers to products consisting of two wireless earbuds with no connecting cable. Samsung's Gear IconX, which comes with an optical heart rate tracker and 4GB of onboard memory, falls into this category.

The AirPods' success comes despite concerns that wearing them can make users look a bit strange. They failed to impress many critics, some of whom complained about the mediocre sound quality, high price, and the fact they're easily misplaced.

Apple classifies AirPods under its "Other Products" category, which includes the Apple Watch and iPods. The segment has generated $5.61 billion in 2017 so far, an increase of around 27 percent from the same period last year.