Time to first audio
Time to first audio is the interval between a defined input event and the moment a voice system begins playing an audible response. In a conversational test, the starting event is often the detected end of the caller's turn; in a synthesis test, it may be the moment a text response is submitted for audio generation.
What the metric includes
The metric can include response preparation, the start of speech generation, buffering, network transport, decoding, and playback. Its exact scope depends on where the timer starts and stops. A result measured inside a speech service is not directly comparable with one measured from a live phone call, because the latter also includes conversation and network stages.
Streaming can improve time to first audio by allowing playback to begin before the entire spoken response has been generated. That does not necessarily shorten the total time required to finish the response. It also does not reveal whether the system chose the correct moment to begin speaking.
Time to first audio is related to latency, but the terms are not interchangeable. End-to-end voice latency can include endpointing: the delay while the system decides that the caller has finished. A narrow time-to-first-audio measurement may begin only after that decision, leaving out a pause the caller still experiences. Conversely, measuring from the last packet of caller audio can make the metric useful for understanding the full turn transition.
Why it matters for AI phone calls
The first audible signal tells the caller that the system heard the turn and is still present. If it takes too long, callers may repeat themselves or speak into the response. But optimizing only for a quick first sound can reward empty filler, premature interruptions, or a fast opening followed by stalled playback.
A useful evaluation defines the measurement boundary, separates simple replies from tool-assisted ones, and checks the audio itself. Teams should pair this metric with interruption rate, task accuracy, response completion, and slow-outlier review. The aim is not merely to start sound quickly; it is to begin the right response at a conversationally appropriate time.