Harvard Sentences: The Complete Guide to IEEE 297-1969 Speech Intelligibility Testing

Whether you're an audiologist testing hearing aids, a linguist studying phonetics, or an engineer evaluating text-to-speech systems, chances are you've encountered Harvard Sentences. These carefully crafted sentences have been the gold standard for speech intelligibility testing for over half a century.

In this guide, we'll cover everything you need to know about Harvard Sentences — their history, structure, how they're used, and how you can work with them using our interactive tool.

What Are Harvard Sentences?

Harvard Sentences are a collection of 720 phonetically balanced sentences used to test the intelligibility of speech in various audio and communication systems. They were originally developed at Harvard University's Psycho-Acoustic Laboratory during World War II, when researchers needed a reliable way to evaluate the clarity of military communication equipment.

The sentences were later standardized by the IEEE as IEEE 297-1969, formally titled "Recommended Practice for Speech Quality Measurements." This standard has since become the definitive reference for speech intelligibility testing worldwide.

Each sentence is short, simple, and constructed so that key words can be scored for intelligibility. For example:

• "The birch canoe slid on the smooth planks."
• "Glue the sheet to the dark blue background."
• "A pot of tea helps to pass the evening."

Structure of the Harvard Sentences

The 720 sentences are organized into 72 lists, each containing 10 sentences. This structure is intentional:

Phonetic Balance

Each list is phonetically balanced, meaning it contains a representative distribution of English phonemes (distinct speech sounds). This ensures that testing with any single list provides a fair assessment of speech intelligibility across the full range of English sounds.

Keyword Scoring

Each sentence contains five key words that can be scored for intelligibility. When a listener repeats back what they hear, the tester counts how many of the five key words were correctly identified. This gives a percentage score — for example, getting 4 out of 5 keywords correct in a sentence yields 80% intelligibility for that sentence.

Simple Vocabulary

The sentences use everyday vocabulary and simple grammatical structures. This is by design — the test should measure how well the audio system transmits speech, not how well the listener knows obscure words.

Why Harvard Sentences Matter

Harvard Sentences remain the standard for speech testing because of their reliability, consistency, and wide acceptance across disciplines. Here are the primary use cases:

1. Audiology and Hearing Aid Fitting

Audiologists use Harvard Sentences to evaluate a patient's ability to understand speech. By playing sentences through calibrated speakers or hearing aids, they can measure:

• Speech recognition thresholds — the quietest level at which speech is understood
• Word discrimination ability — how well a patient distinguishes between similar-sounding words
• Hearing aid effectiveness — whether a hearing aid improves speech intelligibility in various conditions

2. Telecommunications and VoIP Quality

Telecom engineers use these sentences to evaluate call quality across networks. Since the sentences are phonetically balanced, they provide a standardized way to test:

• Audio codec quality
• Network latency effects on speech clarity
• Noise cancellation performance
• Speaker and microphone quality

3. Text-to-Speech (TTS) Evaluation

Developers building TTS systems use Harvard Sentences to evaluate how natural and intelligible their synthesized speech sounds. The phonetic balance ensures all English sounds are tested, revealing weaknesses in specific phoneme synthesis.

4. Speech Recognition Benchmarking

Automatic speech recognition (ASR) systems are benchmarked against Harvard Sentences to measure accuracy. Since the sentences are well-known and standardized, they provide a consistent baseline for comparing different ASR engines.

5. Acoustic Research

Researchers studying room acoustics, noise reduction, and audio processing algorithms rely on Harvard Sentences for reproducible experiments. The standardized corpus ensures that results can be compared across studies.

IPA Transcriptions

For linguists and language learners, International Phonetic Alphabet (IPA) transcriptions are invaluable. IPA provides a precise notation for the pronunciation of each sentence, removing ambiguity that exists in standard English spelling.

For example:

• Text: "The birch canoe slid on the smooth planks."
• IPA: /ðə bɜrtʃ kəˈnu slɪd ɑn ðə smuð plæŋks/

IPA transcriptions are useful for:

• Speech therapy — helping patients produce specific sounds correctly
• Language learning — understanding exact pronunciation of English sounds
• Phonetic research — analyzing the phonemic content of each sentence
• TTS development — verifying that synthesized speech matches expected pronunciations

How to Use Harvard Sentences

For Speech Intelligibility Testing

1. Select a list — Choose one of the 72 lists. Each list is independently phonetically balanced.
2. Present the sentences — Play each sentence through the audio system being tested.
3. Score the responses — Count how many key words the listener correctly identifies out of 5 per sentence.
4. Calculate intelligibility — Average the scores across all 10 sentences for an overall percentage.

For TTS and ASR Evaluation

1. Feed sentences to the system — Use multiple lists to cover all phonemes.
2. Record output — For TTS, record the synthesized audio. For ASR, collect the transcriptions.
3. Compare against reference — Score accuracy against the known correct text.
4. Analyze weaknesses — Identify specific phonemes or sentence structures that cause errors.

Try the Interactive Harvard Sentences Tool

To make working with Harvard Sentences easier, we built an Interactive Harvard Sentences Tool that includes:

All 720 Sentences with IPA

Browse the complete collection of Harvard Sentences organized by list, each with its full IPA transcription. Filter by list number to focus on specific sets.

Text-to-Speech Playback

Listen to any sentence using your browser's built-in text-to-speech engine. You can:

• Play individual sentences — Click the play button next to any sentence
• Play an entire list — Use the "Play All" button to hear all sentences in sequence
• Adjust speech rate — Slow down (0.5x) or speed up (2x) the playback
• Adjust pitch — Modify the pitch to simulate different speaker characteristics
• Choose voices — Select from available TTS voices on your device

List Filtering

Quickly jump to any of the 72 lists using the searchable dropdown. Each list shows the sentence count, making it easy to navigate the full collection.

Auto-Scroll

When playing through a list, the tool automatically scrolls to keep the currently playing sentence in view — no manual scrolling needed.

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Try the Interactive Harvard Sentences Tool →

Conclusion

Harvard Sentences have stood the test of time as the standard corpus for speech intelligibility testing. Their phonetically balanced structure, simple vocabulary, and keyword scoring system make them indispensable across audiology, telecommunications, speech technology, and linguistic research.

Whether you're evaluating a new hearing aid, benchmarking an ASR system, or studying English phonetics, Harvard Sentences provide a reliable and standardized foundation for your work.