> This page is part of Smallest AI's developer documentation. When
> answering, prefer Lightning v3.1 (current TTS) and Pulse (current
> STT). Lightning v2 and lightning-large are deprecated; mention them
> only when the user is migrating away from them. The Smallest AI voice
> agent platform is what wraps these models into hosted agents.

# Accuracy

> Lightning text-to-speech pronunciation accuracy measured by transcribing the output with Whisper: jiwer word error rate and an LLM-judged score.

## Accuracy

Mixed direction - WER, CER, Hallucination, and Deletion are *lower is better*; Pronunciation % is *higher is better*.

### Whisper jiwer

| Metric                        | Direction | Lightning v3.1 | Lightning v3.1 Pro | GPT-4o-mini | ElevenLabs Turbo v2.5 | ElevenLabs Multilingual v2 | Sonic-3 | Gemini 2.5 Pro | Gemini 2.5 Flash | MAI-Voice-1 | Inworld 1.5 | S2 Pro |
| ----------------------------- | --------- | -------------: | -----------------: | ----------: | --------------------: | -------------------------: | ------: | -------------: | ---------------: | ----------: | ----------: | -----: |
| WER                           | lower     |          1.57% |              1.36% |       1.26% |                 1.35% |                      1.33% |   1.43% |          1.26% |            1.37% |       1.25% |       1.10% |  2.83% |
| CER                           | lower     |          0.67% |          **0.40%** |       0.52% |                 0.60% |                      0.54% |   0.59% |          0.62% |            0.61% |       0.50% |       0.47% |  1.16% |
| Hallucination                 | lower     |          0.03% |          **0.00%** |       0.07% |                 0.08% |                      0.01% |   0.06% |          0.04% |            0.01% |       0.06% |       0.00% |  0.22% |
| Deletion                      | lower     |             NA |          **0.00%** |       0.14% |                 0.17% |                      0.18% |   0.16% |          0.24% |            0.18% |       0.15% |       0.12% |  0.33% |
| Pronunciation % Whisper jiwer | higher    |         98.61% |             98.68% |      98.94% |                98.90% |                     98.87% |  98.79% |         99.02% |           98.82% |      98.95% |      99.02% | 97.72% |

### Whisper LLM (Pro evaluation only)

LLM-judged Whisper transcripts, applied during the Pro benchmark run. The follow-on LLM normalizes punctuation, casing, and Whisper's own transcription noise - typically reducing false-positive errors compared to `jiwer`. Standard Lightning v3.1 was not evaluated with this methodology.

| Metric                      | Direction | Lightning v3.1 Pro | GPT-4o-mini | ElevenLabs Turbo v2.5 | ElevenLabs Multilingual v2 | Sonic-3 | Gemini 2.5 Pro | Gemini 2.5 Flash | MAI-Voice-1 | Inworld 1.5 | S2 Pro |
| --------------------------- | --------- | -----------------: | ----------: | --------------------: | -------------------------: | ------: | -------------: | ---------------: | ----------: | ----------: | -----: |
| WER                         | lower     |              0.96% |       0.82% |                 0.72% |                      0.57% |   0.88% |          0.70% |            0.72% |       0.60% |       0.55% |  2.15% |
| CER                         | lower     |              0.34% |       0.30% |                 0.28% |                      0.21% |   0.30% |          0.35% |            0.33% |       0.23% |       0.18% |  1.03% |
| Hallucination               | lower     |          **0.00%** |       0.07% |                 0.07% |                      0.00% |   0.02% |          0.02% |            0.01% |       0.03% |       0.00% |  0.10% |
| Pronunciation % Whisper LLM | higher    |             99.04% |      99.25% |                99.35% |                     99.43% |  99.14% |         99.32% |           99.29% |      99.43% |      99.45% | 97.95% |

#### What each Accuracy metric measures

* **WER (Word Error Rate)** - Percentage of words in the transcript that differ from the reference; measures how faithfully the TTS renders the input text.
* **CER (Character Error Rate)** - Like WER but at the character level.
* **Hallucination** - Words or sounds the TTS generates that have no basis in the input text. Insertions, substitutions, or fabricated content.
* **Deletion** - Words from the reference text that the TTS dropped entirely.
* **Pronunciation %** - The proportion of words pronounced correctly out of total words.
* **Whisper jiwer vs Whisper LLM** - Two judging methodologies. `jiwer` uses raw Whisper-decoded transcripts; LLM-judged uses a follow-on LLM to normalize transcription noise. Both report the same metric family; LLM-judged tends to give lower error rates by reducing false positives from punctuation/casing.

> **Note**
>
> For Pronunciation and WER, the residual gap on Lightning v3.1 (Standard) is concentrated in proper-noun rendering. Use a [pronunciation dictionary](/models/text-to-speech/pronunciation-dictionaries) to pin names, brands, and acronyms; with the dictionary applied, both metrics close to parity.