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Speed controls use different parameters and value directions by model. Use this table before changing a request:

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Adjusting the overall speed

How you adjust overall speaking speed depends on the model.

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Coda, Arcana, and Mist v3

Use the timeScaleFactor parameter. A value above 1.0 slows the audio; a value below 1.0 speeds it up.
Coda, Arcana, and Mist v3 also accept speedAlpha, but it works in the opposite direction from timeScaleFactor (higher than 1.0 is faster, lower is slower). Going forward we recommend timeScaleFactor for these models; it’s the parameter we’ll continue to support across future flagship models.
timeScaleFactor works over both HTTP and WebSocket endpoints. For /ws, /ws2, and /ws3, pass it as a query parameter when opening the connection.

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Mist v2

Use the speedAlpha parameter. Lower than 1.0 is faster; higher than 1.0 is slower.

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Adjusting the speed of individual words

Per-word speed adjustment via inlineSpeedAlpha is a Mist-family feature. Coda and Arcana do not support it.
To adjust the speed of individual words or phrases on the Mist family, use the inlineSpeedAlpha parameter. It takes a comma-separated list of speed values applied to words in square brackets. On mistv2 and mistv3, values < 1.0 speed up speech and values > 1.0 slow it down.
On Mist v3, inlineSpeedAlpha and speedAlpha go in opposite directions; speedAlpha follows the modern flagship direction (higher = faster), but inlineSpeedAlpha follows the legacy Mist direction (lower = faster). Don’t infer one from the other.
For example: “This sentence is [really] [fast]” with inlineSpeedAlpha “0.5, 3” will make “really” fast and “fast” slow on the Mist family.

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Evaluate the result

Start near 1.0 and evaluate representative audio before increasing the adjustment. Large changes can reduce naturalness and clarity.