# Audio Simulations: Which TTS Model Powers the Simulated User, and How Is the Simulated User Billed? **URL:** \ **Category:** Agents\ **Tags:** testing\ **Created:** [September 23, 2026, 2:42pm UTC](https://community.livekit.io/t/audio-simulations-which-tts-model-powers-the-simulated-user-and-how-is-the-simulated-user-billed/2110 "2026-09-23T14:42:42Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![Benjamin\_Lowe](https://sea1.discourse-cdn.com/flex001/user_avatar/community.livekit.io/benjamin_lowe/32/314_2.png) [@Benjamin\_Lowe](https://community.livekit.io/u/Benjamin_Lowe)\ **Post date:** [September 23, 2026, 2:42pm UTC](https://community.livekit.io/t/audio-simulations-which-tts-model-powers-the-simulated-user-and-how-is-the-simulated-user-billed/2110/1 "2026-09-23T14:42:42Z")
Hi LiveKit team, I’m evaluating the new audio simulation feature for Agents and had a couple of questions about the simulated user voice pipeline. In audio mode, the docs mention that the simulated user “speaks your scenario aloud” and joins the room as a participant, but I could not find details about the implementation of that synthesized voice. It user STT generated on the agent, at livekit servers or at the site of the simulation CLI command? I assume livekit servers from what I can see. Specifically: 1. What TTS model/provider is used for the simulated user’s speech in audio simulations? 2. Is the simulated user’s synthesized audio billed separately from the agent’s own STT/TTS usage? 3. If yes, how is that billing represented? Is it: - included in “audio turns” - billed as provider TTS usage - bundled into simulation pricing - or handled another way? 4. Is there any way to configure or override the simulated user voice/model? Relevant docs page: [Agent simulations | LiveKit Documentation](https://docs.livekit.io/agents/start/testing/simulations/) Thanks! ---
**Author:** ![CWilson](https://sea1.discourse-cdn.com/flex001/user_avatar/community.livekit.io/cwilson/32/135_2.png) [@CWilson](https://community.livekit.io/u/CWilson)\ **Post date:** [September 24, 2026, 3:26pm UTC](https://community.livekit.io/t/audio-simulations-which-tts-model-powers-the-simulated-user-and-how-is-the-simulated-user-billed/2110/2 "2026-09-24T15:26:01Z")
> What TTS model/provider is used for the simulated user’s speech in audio simulations? I am not sure exactly how the simulation works and I don’t believe the team has disclosed that so I probably should not either. > Is the simulated user’s synthesized audio billed separately from the agent’s own STT/TTS usage? We bill the entire simulation per-turn rather than just paying the sum(inference costs). (currently free) > Is there any way to configure or override the simulated user voice/model? Can you tell me more about the usecase for why you need this or why it would be useful? ---
**Author:** ![Benjamin\_Lowe](https://sea1.discourse-cdn.com/flex001/user_avatar/community.livekit.io/benjamin_lowe/32/314_2.png) [@Benjamin\_Lowe](https://community.livekit.io/u/Benjamin_Lowe)\ **Post date:** [September 24, 2026, 7:39pm UTC](https://community.livekit.io/t/audio-simulations-which-tts-model-powers-the-simulated-user-and-how-is-the-simulated-user-billed/2110/3 "2026-09-24T19:39:31Z")
> We bill the entire simulation per-turn rather than just paying the sum(inference costs). (currently free) I see, interesting. I guess free whilst in beta > Can you tell me more about the usecase for why you need this or why it would be useful? Regarding the voice model - it is partly just curiosity, but also because we have had challenges in past trying to get voice models to work well in Japanese, I haven’t done enough testing with the LiveKit simulated TTS to know if those problems apply, but if I knew what model it was that might save me some testing there. Regarding the simulated LLM and LLM-as-a-judge, aside from curiosity, it would be useful to know as it could impact how complex a prompt we can issue to describe the simulated user behaviour and judge criteria (and also how much support for Japanese we can expect). Again I should note I haven’t put the Livekit Simulations through in depth scenarios yet, so I do not yet know if it would meet our requirements, but I noticed that it seems some of the metrics in the evaluation might not be supported in Japanese like Word Error Rate etc. (I should also note after writing this post I sent a small support email asking a few more technical questions specific to a simulation (SR\_YmqoN4WyEb52), to better understand the output I was seeing when I did some test runs, and to confirm how much of the “not measured” might be due to the language choice.) ---
**Author:** ![system](https://sea1.discourse-cdn.com/flex001/user_avatar/community.livekit.io/system/32/26_2.png) [@system](https://community.livekit.io/u/system)\ **Post date:** [October 9, 2026, 7:40pm UTC](https://community.livekit.io/t/audio-simulations-which-tts-model-powers-the-simulated-user-and-how-is-the-simulated-user-billed/2110/4 "2026-10-09T19:40:13Z")
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