SSML (Speech Synthesis Markup Language)

What is SSML?

Speech Synthesis Markup Language (SSML) is an XML-based markup standard developed and maintained by the W3C that enables developers, designers, and linguists to precisely control how text is rendered in synthetic (machine-generated) speech. SSML is essential for applications requiring natural, expressive, and contextually appropriate spoken output—from AI chatbots and voice assistants to accessibility tools and interactive voice response systems.

Without SSML, text-to-speech (TTS) outputs are often robotic, monotonous, and prone to mispronunciations or awkward intonation. SSML solves these issues by providing fine-grained control over pronunciation using phonetic alphabets (IPA, X-SAMPA), prosody specification (pitch, rate, volume, emphasis), natural pause insertion and sentence structuring, explicit handling of special content (dates, times, acronyms, currency), voice and language switching mid-stream, and audio file embedding.

Industry Adoption: SSML is the de facto standard for all major cloud TTS providers including Amazon Alexa Skills Kit, Google Cloud Text-to-Speech, Microsoft Azure Speech Service, IBM Watson Text-to-Speech, and Speechify.

Core Capabilities

Prosody Control

Adjust how text is spoken by setting attributes for pitch, rate (speed), and volume, making speech sound more human and expressive. Subtle prosody changes dramatically improve naturalness and engagement.

Pronunciation Management

Override default pronunciations for technical terms, brand names, or foreign words using phonetic alphabets or text substitutions. This prevents embarrassing mispronunciations in professional applications.

Special Content Handling

Direct explicit reading of dates, times, abbreviations, numbers, emails, and currency. SSML ensures “12/25” is read as “December twenty-fifth” and “$19.99” as “nineteen dollars and ninety-nine cents.”

Expressiveness and Style

Inject emphasis, emotional nuance, and stylistic variation using attributes or vendor-specific extensions. Amazon Alexa supports excited and disappointed emotions, while Azure offers neural voice styles for news reading, customer service, and cheerful delivery.

Accessibility Enhancement

Make speech output clearer and more understandable for users relying on assistive technology, improving comprehension for visually impaired users.

Audio Integration

Insert pauses matching natural conversational flow or embed sounds and music to enhance user experience. Background music, sound effects, and audio cues enrich voice interactions.

Basic Structure

The Root Element: <speak>

All valid SSML documents begin with the <speak> root element, which defines the boundaries of speech synthesis content. This is required by all TTS engines supporting SSML.

<speak>
  Welcome to your AI assistant.
</speak>

Omitting the <speak> tag results in errors or the TTS engine falling back to plain text rendering.

Essential SSML Tags

<break>: Insert Pauses

Adds a pause or controls the boundary between words or phrases.

Attributes:

• time: Exact pause duration (e.g., “500ms”, “2s”)
• strength: Relative pause (“none”, “x-weak”, “weak”, “medium”, “strong”, “x-strong”)

<speak>
  Please wait.<break time="1s"/>Processing your request.
</speak>

<prosody>: Control Pitch, Rate, Volume

Changes expressiveness of speech.

Attributes:

• pitch: “x-low”, “low”, “medium”, “high”, “x-high”, or percentage ("+20%")
• rate: “x-slow”, “slow”, “medium”, “fast”, “x-fast”, or percentage ("-20%")
• volume: “silent”, “x-soft”, “soft”, “medium”, “loud”, “x-loud”, decibel ("-6dB"), or percentage

<speak>
  <prosody pitch="high" rate="fast" volume="+20%">
    This is spoken with higher pitch, faster, and louder.
  </prosody>
</speak>

Best Practice: Avoid extreme values; subtle changes produce more natural speech.

<emphasis>: Highlight Words

Increases or reduces emphasis on specific words or phrases.

<speak>
  You must <emphasis level="strong">complete</emphasis> the task.
</speak>

Levels: “strong”, “moderate”, “reduced”

<say-as>: Interpret Content Type

Directs TTS to read text as specific types.

Common interpret-as values:

• “cardinal”: Numbers (123 → “one hundred twenty-three”)
• “ordinal”: Ordinal numbers (1st → “first”)
• “characters”: Spell out (“SSML” → “S S M L”)
• “date”: Dates with format specification
• “time”: Time values
• “telephone”: Phone numbers
• “currency”: Currency amounts
• “fraction”: Fractions (“3/4” → “three quarters”)
• “unit”: Measurements

Examples:

<speak>
  <say-as interpret-as="characters">SSML</say-as>
</speak>

<speak>
  <say-as interpret-as="date" format="yyyymmdd">20230610</say-as>
</speak>

<speak>
  <say-as interpret-as="currency" language="en-US">$19.99</say-as>
</speak>

<phoneme>: Custom Pronunciation

Specifies exact pronunciation using phonetic alphabets.

Attributes:

• alphabet: “ipa”, “x-sampa”
• ph: Phonetic string

<speak>
  <phoneme alphabet="ipa" ph="ˈniːʃ">niche</phoneme>
</speak>

<sub>: Substitute Text

Reads the alias value instead of the enclosed text.

<speak>
  Welcome to the <sub alias="World Wide Web Consortium">W3C</sub>.
</speak>

Use Cases: Brand names, acronyms, foreign words

<audio>: Insert Audio Clips

Embeds recorded audio in speech output (sound effects, music).

<speak>
  Please listen to this sound.
  <audio src="https://www.example.com/sound.mp3">
    Unable to play audio.
  </audio>
</speak>

Provider Limitations:

• Google Cloud: Format and time restrictions apply
• Amazon Alexa: 240-second max, HTTPS required, size limits
• Azure: Supported with restrictions

<voice>: Change Voice or Persona

Switches to a different voice, language, or persona.

<speak>
  <voice name="en-US-Wavenet-D">Hello, I am the default voice.</voice>
  <voice name="en-GB-Wavenet-B">And I am a British voice.</voice>
</speak>

<p> and <s>: Structure Text

Defines paragraphs (<p>) and sentences (<s>) for better pacing and grouping.

<speak>
  <p>
    <s>This is the first sentence.</s>
    <s>This is the second sentence.</s>
  </p>
</speak>

<lang>: Specify Language

Specifies language for a text segment, enabling proper pronunciation and accent.

<speak>
  Here is a word in French: <lang xml:lang="fr-FR">bonjour</lang>.
</speak>

Provider-Specific Extensions

Amazon Alexa

<amazon:emotion>: Adds “excited” or “disappointed” emotion

<speak>
  <amazon:emotion name="excited" intensity="medium">
    Congratulations on your achievement!
  </amazon:emotion>
</speak>

<amazon:domain>: Changes delivery style (news, music, conversational)

Microsoft Azure

<mstts:express-as>: Neural voice styles and roles

<speak>
  <mstts:express-as style="cheerful">
    Welcome to our service!
  </mstts:express-as>
</speak>

Styles include: cheerful, sad, angry, fearful, friendly, hopeful, newscast, customer-service

Speechify

<speechify:style>: Proprietary style control for enhanced reading experiences

Practical Example

<speak>
  Welcome to the demo.
  <break time="500ms"/>
  Your appointment is on
  <say-as interpret-as="date" format="yyyymmdd">20230610</say-as>.
  The amount due is
  <say-as interpret-as="currency" language="en-US">$19.99</say-as>.
  For assistance, call
  <say-as interpret-as="telephone">18001234567</say-as>.
  <prosody rate="slow">Thank you for using our service.</prosody>
</speak>

Expected Output: “Welcome to the demo. [pause] Your appointment is on June tenth, twenty twenty-three. The amount due is nineteen dollars and ninety-nine cents. For assistance, call one eight hundred one two three four five six seven. [slower] Thank you for using our service.”

Common Use Cases

AI Chatbots and Virtual Assistants

Enhance conversational AI with natural-sounding responses, appropriate pauses, and emotional expression

Customer Service IVR Systems

Guide callers through menus with clear pronunciation and professional tone

Accessibility Applications

Provide high-quality speech output for screen readers and assistive technologies

E-Learning Platforms

Create engaging educational content with varied voices and appropriate pacing

Audiobook Production

Generate natural-sounding narration with character voices and emotional expression

Smart Home Devices

Deliver notifications and responses with appropriate context and emotion

Best Practices

Subtle Adjustments: Small prosody changes are more effective than extreme modifications

Test Across Platforms: Verify SSML rendering across target TTS providers

Use Phonetic Sparingly: Only override pronunciation when necessary

Structure Logically: Use <p> and <s> tags for natural pacing

Balance Speed: Maintain natural rate; avoid overly fast or slow speech

Audio Quality: Ensure embedded audio files are properly encoded and hosted

Accessibility Focus: Consider all users, including those with assistive technology

Implementation Considerations

Platform Compatibility: Different providers support different tag subsets and extensions

Fallback Content: Provide fallback text for unsupported tags

Character Limits: Be aware of platform-specific text length restrictions

Processing Overhead: Complex SSML may increase response time

Cost Management: Some providers charge based on character count including markup

Testing Protocol: Establish comprehensive testing across devices and platforms

References

• W3C Speech Synthesis Markup Language (SSML) Version 1.1
• Amazon Alexa: SSML Reference
• Google Cloud: SSML Documentation
• Microsoft Azure: Speech Synthesis Markup
• IBM Watson: SSML Elements
• Speechify: SSML Features
• International Phonetic Alphabet (IPA)
• Azure Speech Voice Gallery
• Google Cloud Supported SSML Elements
• Microsoft Azure Supported SSML Elements
• Adaptive Cards Designer