Designing for Voice User Interfaces (VUI)

Voice user interfaces (VUI) have transformed our interactions with technology, enabling us to engage in natural conversations and control devices with voice commands. Popular VUIs include virtual assistants like Siri, Google Assistant, and Amazon Alexa, as well as voice-activated smart speakers and other voice-controlled devices.

Designing VUIs requires a different approach compared to standard graphic interfaces. While graphic interfaces rely on visual elements and interactions, VUIs solely depend on voice-based interactions.

Designers must also prioritize accessibility by incorporating features that support users with disabilities, such as alternative input methods for those with speech impairments or visual captions for users with hearing difficulties. Bear in mind that designing VUIs goes beyond assisting users — crafting an experience that feels natural, prioritizes accessibility, and addresses privacy and security concerns.

What’s a voice user interface (VUI)?

Voice user interface (VUI) is an incredible technology that allows users to engage in natural language conversations with machines. Here are some examples of voice user interface (VUI) in action:

• Voice assistants: Well-known voice assistants like Amazon Alexa, Google Assistant, and Apple's Siri are excellent illustrations of VUI. Users can interact with these assistants by simply speaking commands or asking questions to obtain information, perform tasks, or control connected devices.
• Voice-activated smart devices: Devices such as smart speakers or smart home systems make great use of VUI. Users can effortlessly give voice commands to play music, control smart home devices, set timers, receive weather updates, or carry out other tasks without the need for physical buttons or interfaces.
• Voice-enabled applications: Many applications integrate voice commands to offer hands-free control and enhance the user experience. For instance, a mobile application with voice search capability allows users to verbally search for information or navigate through the app using voice commands.
• Automotive voice control systems: Modern vehicles often incorporate VUI for convenient, hands-free control of various functions. Drivers can use voice commands to make phone calls, adjust temperature settings, play music, get directions, or send text messages, all while keeping their hands on the steering wheel.
• Interactive voice response (IVR) systems: IVR systems, commonly employed in customer service or call centers, rely on VUI. Users can easily navigate menus, provide spoken responses to prompts, or access information by interacting with an automated voice system.

Understand user needs

Understanding user needs is a crucial aspect of designing voice user interfaces (VUI). To begin with, it's essential to conduct comprehensive user research and gain insights into the characteristics, behaviors, and expectations of your target audience. Consider the following aspects:

• Context: Take into account the specific context in which users interact with the voice interface, such as the environment, device, or scenario. For instance, users in a noisy environment may have different needs compared to those in a quiet setting.
• Goals and tasks: Understand the primary objectives that users aim to accomplish and the specific actions they are likely to perform using voice commands. For example, if designing a voice assistant for a grocery store, a common goal could be to add items to a shopping list or inquire about product availability. However, the checkout and payment process may not be the most suitable use case, as it typically involves providing sensitive information that requires additional security and confirmation.
• User expectations and prior experiences: Familiarize yourself with the conventions, patterns, and any challenges or frustrations that users may have encountered when using voice interfaces in the past. This understanding enables you to design a voice interface that aligns with users' mental models and addresses their pain points.

Define your voice and tone

The voice and tone of the VUI should align with the brand identity and values of the product or service. Consistency between the VUI and the brand helps establish trust and familiarity with users. For example, if the brand is known for being friendly and approachable, the VUI's voice and tone should reflect that by using warm and welcoming language.

To define your brand's voice, consider the following:

• Understand your brand's personality traits, values, and target audience. Think about how you want users to perceive your brand and the emotions you want to evoke. Determine whether you prefer a friendly and casual tone or a more professional and formal one.
• Explore how other brands in your industry use VUI and analyze their voice and tone. Look for patterns and identify what resonates with users in your market. Consider how you can differentiate your brand by developing a unique voice and tone that sets you apart while still being appropriate and appealing to your target audience.
• Develop clear VUI voice and tone guidelines. Document specific language styles, preferred vocabulary, and guidelines for different scenarios. This helps ensure consistency and provides guidance for the VUI design and content creation process.

Define voice commands

By studying user personas, conducting interviews, or analyzing existing data, designers gain valuable insights into how users communicate, including their vocabulary, intonation, and speech patterns.

When designing voice commands, it is important to prioritize the following:

• Simplicity and clarity: Voice commands should be concise, easily understandable, and aligned with users' natural language patterns. Using familiar terms and phrases that users commonly employ in everyday conversations helps bridge the gap between user expectations and the VUI system.
• Users' mental models and tasks: Consider users' mental models of interacting with the system and the specific tasks they want to accomplish. For example, when designing a smart home control VUI, commands like "Turn on the lights," "Set the thermostat to 72 degrees," or "Lock the front door" align with users' mental models and facilitate seamless interaction with their home environment.
• Distinct and unambiguous voice commands: To avoid confusion or misinterpretation, voice commands should be distinct and free from ambiguity. Careful consideration should be given to avoid command collisions or similar-sounding commands that may lead to unintended actions. For example, using distinct commands like "Play music" and "Stop playback" reduces the risk of confusion and ensures accurate execution of user intentions.

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Optimize language and prompts

Like voice commands, prompts in voice user interfaces (VUI) should effectively communicate and guide users. The goal is to create a natural and intuitive dialogue, making users feel as if they're interacting with a helpful assistant or consultant rather than a machine.

To optimize prompts in VUI design, consider the following practices:

• Use plain language. Avoid technical jargon and complex sentence structures to ensure that prompts are easily understood by users. Opt for simpler language that is clear and straightforward.
• Adopt a conversational tone. Make the prompts feel like a natural conversation by using a conversational tone. Use contractions, appropriate phrasing, and a tone that aligns with the intended user experience.
• Provide explicit prompts. Ensure that prompts clearly communicate the available options and guide users on how to interact with the system. Avoid vague or generic prompts. Instead, offer specific and actionable instructions that leave no room for ambiguity.
• Consider context and flow. Design prompts that take into account the context and flow of the conversation. Anticipate users' needs and provide proactive guidance when necessary. Offer suggestions or examples to help users understand available commands or options.

Design for error handling

Anticipating user errors and implementing effective error-handling mechanisms is crucial for good usability in VUI design, following Nielsen's heuristics. By proactively addressing errors, designers can show empathy towards users and demonstrate an understanding that mistakes happen.

To ensure a smooth user experience, consider the following practices for designing error handling in VUI:

• Anticipate common errors and provide proactive solutions. Identify potential user errors and design the VUI to anticipate and address them. For example, if a voice-controlled virtual assistant frequently misinterprets a command, the system can provide clarification or prompt users to confirm their intention.
• Provide informative and actionable error messages. Error messages should clearly communicate what went wrong and offer guidance on how to correct the error. Instead of generic error messages, provide specific and helpful instructions. For instance, if a user's speech is not recognized, a message like "I'm sorry, I couldn't understand your request. Please try again, speaking clearly and slowly" provides specific guidance for correction.
• Offer recovery options for users to navigate back to a valid state. Design recovery options that help users navigate back to a valid state or correct their input. For example, in a voice-controlled menu system, provide options like "Go back to the previous menu" or "Start over."

Feedback and confirmation

Providing immediate and clear feedback in voice user interfaces (VUI) is essential to help users understand that their requests have been received and processed. This instills confidence and assures users that the system is responsive.

Here are some ways to deliver feedback and confirmation in VUI:

• Audio cues and tones: Utilize auditory feedback such as chimes or beeps to indicate the successful completion of a command or error tones for invalid input.
• Visual indicators: Whenever possible, incorporate visual cues like icons, animations, or status messages to complement audio feedback. They provide an additional layer of confirmation, especially for users who rely on visual cues for comprehension.
• Speech confirmation: The VUI can provide spoken confirmation or feedback to reinforce user commands or actions. Using phrases like "Okay, I've set a reminder for your meeting at 2 p.m." or "I'm sorry, I didn't understand. Could you please repeat that?" reassures users that their input has been recognized and understood.
• Haptic feedback (optional): In certain VUI scenarios, haptic feedback can be used to provide tactile confirmation. Subtle vibrations or touch responses on a device can indicate successful command execution. Note that not all VUI systems have haptic capabilities, so this technique may not always be applicable.

🧠 Pro Tip: Discover more usability heuristics in our engaging Usability Heuristics lesson.

Manage long dialogues

Managing long dialogues in VUI involves ensuring that the system can effectively remember and refer back to the dialogue history as users engage in extended conversations.

To design for long dialogues, consider the following key practices:

• Remember previous interactions. The VUI should be capable of recalling user inputs, responses, and context from earlier parts of the conversation. This allows for continuity and context-aware responses. For example, if users ask about the weather in New York City and then follow up with "What about tomorrow?" the VUI can recognize the context and respond with the forecast for the next day in New York City.
• Provide navigation and recap options. To assist users in navigating through long dialogues, offer mechanisms such as "Repeat," "Go back," or "Recap." These options allow users to access or summarize previous parts of the conversation, helping them review information or clarify their understanding without restarting the dialogue.
• Offer contextual help and suggestions. In complex or lengthy dialogues, provide contextual help and suggestions to guide users. If users are unsure of available actions, the VUI can say, "You can ask for the weather forecast, set a reminder, or get news updates. How can I assist you today?" These prompts offer guidance and remind users of available options.
• Provide visualizations or transcripts (when applicable). When a visual interface is available, consider providing visualizations or transcripts of the dialogue history. This can include displaying a conversation timeline, showing user inputs and system responses, or providing a written summary of the dialogue. Visualizations or transcripts can enhance understanding and serve as a reference for users during and after the conversation.

Contextual awareness

By leveraging contextual information such as location, time, and user preferences, designers can create personalized and relevant interactions that cater to the specific needs and preferences of users.

Follow these key considerations when leveraging contextual awareness in VUI design:

• Location-based information: Utilize location data to provide users with contextually relevant information. For example, if users ask for nearby restaurants, the VUI can leverage their location to provide a list of nearby dining options.
• Time-based context: Consider the time of day or day of the week to provide more relevant and timely information. For instance, if users ask for the weather forecast, the VUI can not only provide the current weather conditions but also offer insights into how the weather might change throughout the day or week.
• User preferences and history: Leverage user preferences and historical data to create personalized experiences. For example, if a user frequently orders vegetarian meals, the VUI can prioritize vegetarian options when providing restaurant recommendations or meal suggestions.
• Integration with other services: Integrate the VUI with other services and platforms to access additional contextual information. For example, by integrating with a calendar app, the VUI can provide personalized reminders, schedule updates, and event notifications.

Accessibility considerations

Accessibility considerations play a crucial role in voice user interface (VUI) design. It is important to ensure that VUIs are accessible to all users, including those with disabilities such as speech impairments or hearing difficulties.

Here are the key considerations to follow when it comes to accessibility in VUI design:

• Alternative input methods: Provide options for alternative input methods to accommodate users with speech impairments or those who prefer not to use voice commands. This can include text-based input options, such as typing or selecting choices through a visual interface.
• Visual captions: Include visual captions for audio responses to accommodate users with hearing difficulties. Displaying text captions for spoken responses allows users to read the information instead of relying solely on audio.
• Clear and concise language: Use clear and concise language in prompts and responses to enhance comprehension for users with cognitive disabilities or language barriers. Avoid using complex jargon or ambiguous terms that might confuse or alienate users.
• User testing with diverse participants: Conduct user testing with individuals who have a range of disabilities to gain insights into their specific needs and challenges. This helps identify any usability issues and provides valuable feedback for optimizing accessibility.

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Privacy and security

Building trust with users is essential in voice interfaces. To address privacy concerns and ensure security, follow these key practices:

• Clear communication: Explain to users how their data is collected, used, and protected in simple terms.
• Consent and control: Ask for explicit permission before collecting personal or sensitive data. Give users the ability to manage their data preferences and choose whether to share it.
• User control and data management: Let users access, modify, or delete their personal data according to their preferences.
• Monitoring and compliance: Regularly check for privacy and security risks in the voice interface. Stay up-to-date with local data protection laws and guidelines to ensure compliance, such as GDPR or CCPA.

Ultimately, successful VUI design hinges on understanding user needs, leveraging technological capabilities, and creating seamless, natural interactions that enhance people's daily lives. As we move forward, VUIs will likely play an increasingly important role in how we interact with technology, making it essential for designers to master these principles and stay abreast of emerging trends and best practices in the field.