Audio chat app Clubhouse has doubled its download numbers in just over two weeks, according to download estimates by one analytics firm.

App Annie reports that the app saw global downloads grow from 3.5 million to 8.1 million between 1 and 16 February.

The invite-only app has surged in popularity amid appearances by some high-profile celebrities.

But that popularity has also seen the firm attract significant criticism.

Experts highlighted that data from the app is sometimes routed through Chinese servers - even if users are in the United States or other countries. And privacy concerns have also been raised over how the app checks users' contact books.

But those concerns appear to have had little impact on user interest.

The growth has happened despite the restrictions of Clubhouse's model: it is only available for iPhones, and each user is only able to invite a few others.

About 2.6 million of the recent downloads were in the United States, App Annie says in its report, but there was also a wide spread around the world.

Part of the demand may have been fuelled by the appearance of technology figures such as Elon Musk and Mark Zuckerberg - and Facebook is now rumoured to be working on a similar product to compete.

However, download numbers may not be an accurate indicator of the number of actual users.

The app can be downloaded from the Apple App Store without actually having the invite needed to use it.

Clubhouse is encouraging users to download the app anyway - because doing so allows new users to reserve their username for the future.

Usernames can be a hot commodity on leading social networks, where competition for real names - or short, snappy ones - can be intense.

Clubhouse does not release user numbers itself, but some estimates place the number of users at more than six million.