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Sonic 3.5 is designed to sound natural with minimal prompt engineering. In most cases you can pass your transcript as-is and let the model handle normalization, pacing, and expression. The tips below apply across the Sonic family; differences between Sonic 3.5 and Sonic 3 are called out inline.

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Start here

  • Pass natural, well-punctuated text. Full sentences with normal capitalization and punctuation produce the best pacing and intonation. End each transcript with terminal punctuation (., ?, !).
  • Pass numbers, dates, times, and common acronyms as-is unless you have a specific reason to override. Sonic handles the most common patterns natively:
    • Large numbers like 1,234,567
    • US phone numbers: (415) 555-1212
    • Email addresses: user@example.com
    • Dates in MM/DD/YYYY (or DD/MM/YYYY based on locale): 04/20/2025
    • Times with a space before AM/PM: 7:00 PM, 7 PM, 7:00 P.M.
    • Common acronyms (NASA) and initialisms (USA)
    Symbols are handled naturally — @ reads as at (email addresses), () is silent (phone numbers).
  • Match the voice to the language. Each voice has a primary language it works best with. Use the Playground to audition voices for a given language.
  • Keep prompts in their natural written form. Heavy preprocessing (stripping punctuation, forcing all caps) generally hurts output quality.

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Controlling pacing and spelling

When you need character-by-character read-out (confirmation codes, order IDs, serial numbers, spelled-out names) or fine-grained pacing, use one of the following:
  1. Spell tags (recommended). Wrap the string in <spell>...</spell>. Most reliable option, works for letters, digits, and mixed alphanumerics in all supported languages. 
    Your confirmation code is <spell>AB12CD</spell>.
  2. Space-delimited characters. If you prefer not to use tags, separate characters with single spaces. 
    Your code is A B C 1 2 3.
  3. Commas for pauses between groups. Use commas where a human would naturally pause. 
    Your code is A B C, 1 2 3.
Migrating from Sonic 3? The recommended delimiter format has changed in Sonic 3.5. Use spaces between characters and commas between groups instead of commas between characters and periods between groups. The old format still works on sonic-3 snapshots but is no longer recommended going forward.
ScenarioOld (Sonic 3)New (Sonic 3.5)
Spell out letters HELLOH, E, L, L, OH E L L O
Spell out digits 1234561, 2, 3, 4, 5, 61 2 3 4 5 6
Confirmation code ABC123A, B, C. 1, 2, 3.A B C, 1 2 3

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Inserting pauses

Sonic respects natural punctuation like commas and periods. For a longer or specifically-located pause, use a break tag. Break tags count as a single character and don’t need surrounding whitespace.

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Pronunciation

For proper nouns, trademarks, and domain-specific terms — or to disambiguate identical spellings (e.g. Nice, the city, vs. nice, the adjective) — use custom pronunciations.

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Streaming

Use continuations when generating chunks of audio that need to sound contiguous (for example, LLM-streamed output). This preserves prosody and voice consistency across chunk boundaries.