The surge of voice assistant usage in the US raises a natural—and as yet unanswered—question.

How exactly will ads be served to consumers using such services?

New data from Invoca might provide some insight into the answer.

The call tracking and analytics firm surveyed US voice-enabled speaker owners, asking them what factors would make them willing to listen to ads delivered through their devices.

Three in 10 said they would entertain ads via voice assistants if they were simply asked if they wanted to hear one before it played. In addition, 28% were open to ads if they got to choose the brands doing the advertising.

Personalization was also a strong selling point: One-quarter of respondents were willing to listen to voice assistant ads customized for them.

The traditional search/display landscape certainly stands to be disrupted by voice. eMarketer predicts that the number of voice-enabled digital assistant users in the US will grow from 60.5 million this year to 75.5 million by 2019.

And users of such services are expected to skew young. This year, for example, nearly half of voice-enabled digital assistant users will be millennials, eMarketer estimates.

Invoca's survey also underscored how much voice assistants have become a part of users' daily routines. It found that nearly nine in 10 people who had a voice assistant talked to it every day, while one-third said they used it more than five times per day.

One reason for the sudden rise of voice assistants might lie in the fact that they are now often available on smartphones. In fact, a poll of US internet users conducted by Pew Research Center earlier this year found that 42% of respondents used a virtual assistant on their smartphone—more than any other device, including smart speakers like Google Home and Amazon Echo.