What Is Room EQ Wizard?

Room EQ Wizard (REW) is a free, Java-based acoustic measurement and analysis tool widely used by audio enthusiasts, home theater installers, and professional sound engineers. Originally developed by John Mulcahy, REW provides a comprehensive suite of features for measuring and optimizing the frequency response of audio systems in real rooms. It supports a wide range of microphones, sound cards, and DSP hardware, making it a versatile choice for both beginner and advanced users. The software runs on Windows, macOS, and Linux, and can generate correction filters for parametric EQ, FIR filters, and even convolution engines. For the latest version and downloads, visit the official REW website.

Essential Preparation and Equipment

Accurate measurements begin with proper hardware setup. You will need a measurement microphone – ideally a USB microphone like the miniDSP UMIK-1 or the Dayton Audio UMM-6, which come factory-calibrated with individual calibration files. If using an analog microphone (e.g., Behringer ECM8000) with an audio interface, you must obtain a calibration file for that specific mic. REW uses calibration files to correct for microphone frequency response deviations.

Your audio interface or sound card should be capable of low-latency playback and recording. For best results, use an external USB audio interface with at least two channels (one output, one input). Set your system’s volume at a comfortable level – typically aiming for a measurement level around 75–85 dB SPL at the listening position. Avoid clipping by checking the input level in REW’s soundcard settings before measuring.

Before starting, ensure your microphone is placed at the primary listening position (or multiple positions for averaging) and pointed toward the speakers. Use a microphone stand to avoid hand-held inconsistency. Also, close windows and doors, and turn off any noisy appliances (HVAC, refrigerators) to reduce background noise.

Performing Your First Measurement

Once your equipment is connected, launch REW and configure it:

  • Go to Preferences > Soundcard and select your input and output devices. Choose a sample rate of 48 kHz (common for home theater) – higher sample rates require more memory but offer no benefit for typical room measurements.
  • Click Preferences > Mic/Meter and load the correct calibration file for your microphone (if you have one). Set the SPL offset if your calibration file provides it, otherwise leave it at 0.
  • Click the Measure button on the toolbar. This opens the measurement dialog. Set the sweep parameters:
    • Sweep type: Use “Log sweep” for most room measurements. The sweep duration should be long enough to excite low frequencies – at least 2–3 seconds. For a typical living room, use 1/25th octave resolution with 2–4 seconds sweep length.
    • Level: Start with a middle level (e.g., -12 dBFS test tone) and adjust so the REW input meter reads around -6 dBFS peak during the sweep. You can use the “Check Levels” button to run a short chirp.
  • Click Start Measuring. REW will play the sweep tone through your system. The microphone picks up the sound, and REW records it. Ensure no other sounds interfere during the sweep.
  • After the sweep finishes, REW will process the recorded signal and display the measurement results automatically. If the measurement fails (e.g., input underrange), increase the output level or check connections.

Understanding the Measurement Results

The primary graph REW displays is the Frequency Response – SPL (dB) vs. Frequency (Hz). A flat line would represent perfect reproduction, but real rooms introduce peaks and dips. Look at the overall trend: typically, a slight downward tilt from low to high frequencies is natural. Avoid drawing conclusions from a single measurement – take multiple measurements at slightly different positions (within a few inches) and compare.

Use the Phase graph (available in the top controls) to see time delays across frequencies. Phase shifts are often caused by room reflections or subwoofer distance misalignment. The Impulse Response view shows reflections; the first large peak is the direct sound, and subsequent peaks are reflections from walls, floor, ceiling. A good measurement will have a clean, sharp impulse with minimal noise before the peak.

Key graph controls:

  • Window: Apply a window to see “gated” measurements. A 10 ms window removes reflections from surfaces farther than about 3.4 meters, helping you see the speaker’s direct response. But in small rooms, very short windows may lose low-frequency data.
  • Frequency limits: Set the x-axis from 10 Hz to 20 kHz. Most room issues are below 500 Hz, but full-range is useful.
  • Y-axis: Use 5 dB per division for clarity.

Common Room Acoustic Issues

Room Modes (Standing Waves)

At low frequencies, axial modes between parallel walls create deep nulls (cancellation) and high peaks (reinforcement). You’ll see a series of peaks and dips that change with microphone position. Moving the microphone even a few inches can alter the null depth. REW’s Room Sim tool can help predict mode locations based on room dimensions.

SBIR (Speaker Boundary Interference Response)

If a speaker is placed close to a wall (e.g., <1 meter), the reflection from that wall arrives early enough to cause dips in the response – often a notch at the frequency whose quarter-wavelength equals the distance to the wall. You can identify SBIR by comparing measurements with the speaker moved away from boundaries.

Reflection-Induced Comb Filtering

Multiple reflections from nearby surfaces cause ripples in the frequency response. This appears as fine comb filtering above about 500 Hz. Gating can reveal the direct sound, but in a real listening room, these reflections affect perception. Use Waterfall and Spectrogram views to analyze time-frequency behavior.

Using the Waterfall and Spectrogram Views

The Spectrogram shows frequency vs. time in a colored 2D plot. It highlights decaying energy – a lingering coloration (e.g., a long decay at 80 Hz) indicates a room mode that rings. The Waterfall view (3D) shows the same data but easier to see decay slopes. Look for “persistent ridges” – frequencies that take a long time to decay. These are problem areas that make sound boomy or muddy.

To open these views, click on the graph title and choose Spectrogram or Waterfall. Set the time window to show at least 500 ms. A good room will have low-frequency decay times (RT60) that are smoothly varying; sudden drops or waterfalls that hang for >200 ms need treatment – either EQ or physical absorption/diffusion.

Applying Corrections with REW

Once you have identified problem frequencies, you can generate filters in REW and export them for use with DSP gear. Common methods:

  • Parametric EQ (PEQ): REW can automatically calculate PEQ filters to flatten the response. Go to Filters tab, set the target curve (or use “Flat” with a house curve slope), and choose the number of filters (typically 5–10). REW will create a set of filters that correct peaks and dips within a defined range (e.g., 20–500 Hz). Use “Match SPL to Target” to generate the filter.
  • FIR Filters: For more precise phase correction, REW can create finite impulse response (FIR) filters. This requires advanced DSP hardware that supports convolution (e.g., miniDSP with FIR capability, or software like Equalizer APO). Go to Filters > FIR and set parameters. FIR filters allow independent magnitude and phase control, but they add latency – not ideal for live sound.

After generating filters, you can export them to formats like *.txt (for miniDSP), *.fir (for convolution), or *.xml (for some AVRs). For miniDSP products, you can directly import the filter files into the device’s plugin software. Always re-measure after applying corrections to verify.

Integration with DSP and External Hardware

REW works seamlessly with popular DSP platforms. For example:

  • miniDSP 2x4 HD or DDRC-24: These support both PEQ and FIR filters. REW can export filters directly for these devices using the miniDSP plugin.
  • Behringer DCX2496: Use REW to generate PEQ biquad filters in a text format that can be copied into the DCX software.
  • Equalizer APO (Windows): REW can export filters as configuration files for this free system-wide equalizer.
  • Audyssey, Dirac Live, etc.: These commercial room correction systems often have their own measurement process, but you can use REW to verify their results and make manual tweaks.

If your AV receiver or processor has a built-in parametric EQ for each channel, you can manually enter frequencies, gains, and Q values derived from REW’s filter wizard. This approach is common in high-end home theater setups.

Advanced Techniques

Target Curves and House Curves

Instead of targeting a perfectly flat frequency response, many audiophiles prefer a downward tilt (e.g., -3 dB from 20 Hz to 20 kHz) to compensate for Fletcher-Munson hearing sensitivity and to avoid an overly analytical sound. REW’s target curve editor lets you define any slope or arbitrary contour. For home theater, a slight gradual rise below 100 Hz (+3 dB) is common for a more impactful bass.

Multi-Subwoofer Alignment

Using REW’s Alignment Tool, you can measure multiple subwoofers and find optimal delay and level settings to smooth the combined response. The tool lets you import measurements from each sub alone, then computes delays and gains that minimizes variation. This is essential for integrating two or more subs in a room.

RTA (Real-Time Analyzer)

REW includes an RTA mode for live spectral analysis. Use it to identify resonances while playing music, or to measure ambient noise floor. The RTA also shows a moving average useful for real-time adjustments.

Tips for Accurate and Repeatable Measurements

  • Microphone calibration: Re-calibrate your microphone annually if possible – calibration drift over time affects accuracy. Use a reliable source like Cross-Spectrum Labs for professional calibration.
  • Consistent test positions: For a seating area, take measurements at 3–5 positions (left ear, right ear, center, plus a few inches apart). REW can average them using the Average function under the Graph menu – this gives a spatially averaged response representing the listening zone.
  • Gating vs. No Gating: Use gating to isolate the direct sound for speaker evaluation, but use full-range (no gate) for room correction. The best approach: measure both and overlay them.
  • Check system volume: Keep playback within 75–85 dB SPL to avoid distortion. If you increase volume to overcome noise, the harmonic distortion may pollute the measurement. Verify with the Distortion view (available after measurement).
  • Calibrate soundcard loopback: For timing measurements (e.g., subwoofer distance), REW needs a loopback connection to compensate for driver latency. Connect a cable from an output to an input on your audio interface. Go to Preferences > Soundcard and set Loopback Input. Then when measuring, REW uses the loopback reference for absolute timing.

Conclusion

Room EQ Wizard is one of the most powerful and accessible tools for analyzing and improving the acoustics of any listening environment. By setting up proper equipment, performing careful measurements, and interpreting the graphs, you can identify problems like room modes, reflections, and frequency imbalances. REW’s filter generation features then let you apply targeted corrections through DSP or EQ, transforming the sound quality of your system. With practice, you’ll learn to hear the differences that small adjustments make and develop a systematic approach to achieving a flat, neutral, and immersive audio experience. For deeper tutorials and community support, visit the AV NIRVANA REW forum or the official REW documentation.