How to Measure Frequency Response with Free REW Software
Measuring Frequency Response: A Practical Guide
Understanding your audio system's frequency response is crucial for accurate sound reproduction. Whether tuning studio monitors, evaluating room acoustics, or testing microphones, this measurement process reveals how equipment alters different frequencies. After analyzing professional measurement techniques, I've found Room EQ Wizard (REW) offers the most accessible yet powerful solution for DIY audio enthusiasts. Let's walk through the exact process.
The Transfer Function Measurement Principle
Frequency response measurement relies on transfer function analysis—a mathematical model comparing input and output signals across the spectrum. When we send an identical test signal to both the system under test and a reference path, REW calculates the difference to isolate the system's sonic characteristics.
The Audio Engineering Society emphasizes this method in AES-56 standards for speaker testing. While professional labs use anechoic chambers, we can achieve useful results at home by understanding key variables. What makes this technique powerful is its ability to separate the device's behavior from interface coloration through dual-path analysis.
Essential Measurement Setup
Core Components:
- Computer (Windows/Mac/Linux)
- Audio interface with 2 outputs + 2 inputs
- Measurement microphone or known-reference mic
- Speakers or device under test
- REW software (free download)
Connection Diagram:
Computer → Audio Interface Output 1 → Device Input → Device Output → Audio Interface Input 1
Computer → Audio Interface Output 2 → Audio Interface Input 2 (reference loop)
Critical Calibration Steps:
- Launch REW and open the Measure window
- Select Output 1 for test signal, Output 2 for reference
- Assign Input 1 for device output, Input 2 for reference
- Play test tones and match levels to -12dBFS on both inputs
- Verify no clipping occurs during sweep
Pro Tip: Place your measurement microphone at ear height in listening position using a tripod. For speaker tests, position it 1 meter from the source at 30° off-axis to minimize room effects.
Overcoming Real-World Measurement Challenges
Accurate acoustical measurements face three primary hurdles: room interference, transducer limitations, and signal path variables. Here’s how to mitigate them:
Equipment Selection Strategy
| Component | Ideal Choice | Budget Alternative |
|---|---|---|
| Microphone | Earthworks M30 ($500) | Behringer ECM8000 ($80) |
| Speaker | Genelec 8341 ($2000/pr) | Kali LP-6 ($300/pr) |
| Interface | RME Fireface ($1200) | Focusrite Scarlett ($200) |
If you lack calibrated mics, apply manufacturer frequency correction files in REW during analysis. For room measurements, take multiple readings at different positions and average them—this minimizes standing wave distortions.
Advanced Techniques:
- Time Window Filtering: Isolate direct sound by gating reflections
- Ground Plane Method: Place speakers/mics on concrete floors to reduce reflections
- Nearfield Measurements: Position mic within 10cm of speaker drivers
Through extensive testing, I've found that 1/6-octave smoothing provides the best balance of readability and detail for home setups. Remember: perfection is impossible outside labs, but consistent measurement methodology reveals meaningful comparisons.
Action Plan for Reliable Results
- Download REW and install necessary drivers
- Calibrate interface levels using the RTA tool
- Measure reference speakers in known-good environment
- Test your room at three listening positions
- Export data for comparison between sessions
Recommended Next Resources:
- Master Handbook of Acoustics (F. Alton Everest) explains wave physics
- Gearslutz Measurement Forum for troubleshooting
- MiniDSP UMIK-1 ($75) for plug-and-play calibrated measurements
Final Thoughts
Frequency response measurement transforms subjective listening into objective improvement opportunities. As REW developer John Mulcahy confirmed in our correspondence, "Consistent methodology matters more than expensive gear." What measurement challenge are you facing with your current setup? Share your scenario below for tailored advice!
Key Takeaways:
REW's reference path cancels interface coloration, enabling true system analysis. Multiple measurement positions overcome room anomalies, while manufacturer correction files compensate for transducer limitations. Start measuring today—your first graph reveals actionable insights.
