n8n vs Postman: Which platform for Voice AI workflows?

This tutorial builds a complete Voice AI workflow system that automatically processes audio files from upload to final result delivery. You'll create an automated pipeline that receives audio through webhooks, sends files for transcription, processes the results, and routes them based on content—all without manual intervention.

We'll use n8n for workflow automation and Postman for API testing during development. The system integrates AssemblyAI's speech-to-text API for transcription, n8n's visual workflow builder for orchestration, and Postman's request testing for validation. You'll learn to test individual API endpoints first, then connect them into a production-ready automated workflow that handles real-world audio processing at scale.

What are n8n and Postman?

n8n is a workflow automation platform that connects different services through a visual interface. This means you can build multi-step processes that trigger automatically—like receiving audio files, sending them for transcription, and routing results based on content. When you need to process audio files automatically without manual intervention, n8n handles the entire workflow.

Postman is an API testing tool where you validate that individual API calls work correctly. This means you test authentication, request formats, and response structures before integrating APIs into your production system. You'll use Postman to ensure your speech-to-text API calls return the expected results.

Different tool categories

These tools belong to completely different software categories, which explains why comparing them directly doesn't make sense.

n8n handles workflow automation: You connect multiple services into automated processes using visual nodes. n8n processes webhook events, transforms data between services, and manages multi-step operations. For Voice AI workflows, n8n coordinates your entire pipeline from audio upload to final result delivery.

Postman handles API development: You test individual API endpoints to verify they work correctly before integration. Postman validates request parameters, checks response formats, and documents API behavior for your team. For Voice AI projects, Postman ensures your transcription API calls succeed before you build workflows around them.

Key features compared

For Voice AI specifically:

• n8n capabilities: Webhook triggers, HTTP nodes for API calls, data transformation, conditional routing, scheduled processing
• Postman capabilities: Endpoint testing, authentication validation, parameter documentation, response monitoring

Which platform for Voice AI workflows?

Use n8n for building Voice AI workflows because it creates automated multi-step processes that Voice AI requires. Postman can't build workflows—it only tests individual API endpoints during development.

Voice AI applications need multiple connected steps: receiving audio, calling transcription APIs, processing results, and triggering actions. n8n orchestrates these connected processes through its visual workflow builder. Postman helps during development by testing each API call, but it doesn't connect those calls into an automated system.

Voice AI automation with n8n

n8n excels at Voice AI workflow orchestration through triggers, processing nodes, and integrations.

Webhook triggers: n8n receives audio files from your application automatically. When users upload audio, your app sends it to n8n's webhook URL, starting the workflow instantly.

HTTP nodes for API calls: n8n makes API calls to transcription services without requiring code. You configure headers, authentication, and request parameters through the visual interface.

Data transformation: After receiving transcription results, n8n processes the response data. You can perform entity detection to extract specific information, filter by confidence scores, or format data for your database.

Here's how n8n processes audio automatically:

// n8n Function node - process transcription results
const transcription = $input.first().json;

// Extract key information
const result = {
  text: transcription.text,
  confidence: transcription.confidence,
  duration: transcription.audio_duration,
  speaker_count: transcription.utterances ? 
    new Set(transcription.utterances.map(u => u.speaker)).size : 1
};

// Route based on content
if (transcription.text.toLowerCase().includes('urgent')) {
  result.priority = 'high';
  result.department = 'escalation';
} else {
  result.priority = 'normal';  
  result.department = 'standard';
}

return result;

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API testing with Postman

Postman validates your Voice AI APIs before you integrate them into n8n workflows. This prevents configuration errors and failed automations.

Authentication testing: You verify your API keys work by sending test requests. Postman shows exactly what authentication headers you need and confirms your credentials are valid.

Parameter validation: You test different audio formats, quality settings, and feature options. Postman reveals which parameters are required and which are optional for your transcription service.

Response inspection: You examine the complete response structure from API calls. Understanding the response format helps you build proper data extraction in your n8n workflows.

Example Postman request for testing AssemblyAI:

// POST https://api.assemblyai.com/v2/transcript
// Headers:
Authorization: YOUR_API_KEY
Content-Type: application/json

// Body:
{
  "audio_url": "https://example.com/audio.mp3",
  "speaker_labels": true,
  "auto_chapters": true,
  "punctuate": true,
  "format_text": true
}

// Test script:
pm.test("Transcription submitted successfully", function() {
  pm.response.to.have.status(200);
  const response = pm.response.json();
  pm.expect(response).to.have.property('id');
  pm.environment.set("transcript_id", response.id);
});

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Building a complete Voice AI workflow

Let's build a Voice AI system that processes audio files automatically using both tools. You'll use Postman to test the APIs first, then build the automation in n8n.

Set up your development environment

Install n8n on your local machine:

npm install n8n -g

‍

Start n8n with basic authentication:

export N8N_BASIC_AUTH_ACTIVE=true
export N8N_BASIC_AUTH_USER=admin
export N8N_BASIC_AUTH_PASSWORD=your_secure_password

n8n start

‍


Access n8n at http://localhost:5678 using your credentials.

Set up Postman with your API credentials:

• Environment variable: assemblyai_api_key with your API key
• Base URL: https://api.assemblyai.com/v2
• Webhook URL: Your n8n webhook endpoint

Test APIs with Postman first

Before building your n8n workflow, validate that your API calls work correctly in Postman.

Create a request to submit audio for transcription:

POST {{base_url}}/transcript
Authorization: {{assemblyai_api_key}}
Content-Type: application/json

{
  "audio_url": "https://storage.googleapis.com/aai-docs-samples/nbc.mp3",
  "speaker_labels": true,
  "auto_highlights": true  // Enables the Key Phrases feature
}

Create another request to check transcription status:

GET {{base_url}}/transcript/{{transcript_id}}
Authorization: {{assemblyai_api_key}}

Test both requests and save the working configuration for n8n implementation.

Build the n8n workflow

Create a new workflow in n8n with these connected nodes:

1. Webhook Node (Trigger):

• Webhook URL: audio-processor
• HTTP Method: POST
• Response Mode: Immediately

2. HTTP Request Node (Submit Audio):

• Method: POST
• URL: https://api.assemblyai.com/v2/transcript
• Headers: Authorization: YOUR_API_KEY
• Body: JSON with audio_url from webhook

3. Wait Node (for tutorial simplicity - use webhooks in production):

• Duration: 30 seconds
• Resume on webhook: Disabled
• Note: For production workflows, use AssemblyAI's webhook_url parameter to receive notifications when transcription completes instead of polling

4. HTTP Request Node (Check Status):

• Method: GET
• URL: https://api.assemblyai.com/v2/transcript/{{$node["HTTP Request"].json["id"]}}
• Headers: Authorization: YOUR_API_KEY

5. IF Node (Check Completion):

• Condition: {{$json.status}} equals "completed"

6. Function Node (Process Results): ‍


// Process completed transcription
const transcript = $input.first().json;

const processed = {
  id: transcript.id,
  text: transcript.text,
  confidence: transcript.confidence,
  word_count: transcript.words ? transcript.words.length : 0,
  speakers: transcript.utterances ? 
    [...new Set(transcript.utterances.map(u => u.speaker))] : ['A'],
  created_at: new Date().toISOString()
};

// Determine routing
const text = transcript.text.toLowerCase();
if (text.includes('support') || text.includes('help')) {
  processed.department = 'support';
} else if (text.includes('sales') || text.includes('purchase')) {
  processed.department = 'sales';
} else {
  processed.department = 'general';
}

return processed;

Test your complete workflow

Test your workflow by sending a webhook request:

curl -X POST http://localhost:5678/webhook/audio-processor \
  -H "Content-Type: application/json" \
  -d '{
    "audio_url": "https://storage.googleapis.com/aai-docs-samples/nbc.mp3",
    "webhook_url": "https://your-app.com/transcription-complete"
  }'

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Monitor the execution in n8n's interface to verify each step works correctly.

// Postman WebSocket request using v3 Streaming API
const socket = new WebSocket('wss://streaming.assemblyai.com');

socket.onopen = function() {
  // Authenticate connection with v3 format
  socket.send(JSON.stringify({
    type: 'authenticate',
    api_key: pm.environment.get('assemblyai_api_key')
  }));
  
  // Configure streaming session
  socket.send(JSON.stringify({
    type: 'configure',
    sample_rate: 16000,
    encoding: 'pcm_s16le'
  }));
};

socket.onmessage = function(event) {
  const response = JSON.parse(event.data);
  console.log('Real-time transcript:', response);
  
  if (response.type === 'transcript' && response.transcript) {
    pm.environment.set('final_transcript', response.transcript);
  }
};

// n8n Function node for WebSocket handling with v3 Streaming API
const WebSocket = require('ws');

const ws = new WebSocket('wss://streaming.assemblyai.com');

ws.on('open', function() {
  // Send authentication for v3 API
  ws.send(JSON.stringify({
    type: 'authenticate',
    api_key: $env.ASSEMBLYAI_API_KEY
  }));
  
  // Configure streaming session
  ws.send(JSON.stringify({
    type: 'configure',
    sample_rate: 16000,
    encoding: 'pcm_s16le'
  }));
  
  console.log('WebSocket connected to v3 Streaming API');
});

ws.on('message', function(data) {
  const message = JSON.parse(data);
  
  if (message.type === 'transcript' && message.transcript) {
    // Process transcript from v3 API
    const result = {
      text: message.transcript,
      confidence: message.confidence,
      timestamp: new Date().toISOString(),
      session_id: $json.session_id
    };
    
    // Send to next node
    $node.sendOutput(result);
  }
});

// Send audio data from webhook
const audioData = $binary.data;
ws.send(JSON.stringify({
  type: 'audio',
  data: audioData.toString('base64')
}));