What Is BFD in REW and How Does It Help Control Room Resonances?

Updated

In Room EQ Wizard (REW), BFD refers to a parametric EQ filter approach — specifically the kind of narrow "notch" filter popularized by the Behringer Feedback Destroyer, a classic budget parametric equalizer that REW users have long employed to cut room modes. In practice, "BFD" in REW discussions means using REW's measurement and EQ tools to identify resonant peaks in the low-frequency response and then configuring parametric filters (frequency, bandwidth/Q, gain) to reduce them — whether in a BFD unit or in REW's own EQ engine. This applies to anyone treating low-frequency room resonances in a listening room, studio, or home theater; it is not a substitute for speaker placement or acoustic treatment, and it targets peaks rather than broad tonal shaping.

What BFD means in the REW workflow

REW is free software for room acoustic measurement, loudspeaker measurement, and audio device measurement, and it helps you "configure EQ to help tackle room problems" (roomeqwizard.com). The BFD is one of the hardware equalizers that REW's EQ features were designed to work with: a multi-band parametric EQ with adjustable frequency, bandwidth, and gain per filter.

The term shows up in REW's own keyword set alongside "resonances," "modes," "parametric," and "equalisers," which reflects the intended pairing: measure the room, find the modal peaks, and generate parametric filter settings.

BFD vs. REW's built-in EQ

Aspect BFD (hardware parametric EQ) REW's EQ features
Purpose Apply filters to the signal chain Calculate and model filter settings from measurements
Parameters Frequency, bandwidth/Q, gain per band Same parameters, entered or exported
Where filters act In your audio path (e.g., subwoofer chain) In software, or exported to hardware/other EQ
Typical use Cutting low-frequency room modes Identifying peaks and generating filter values

How to set up BFD-style filters in REW

  1. Measure first. Take a frequency-response measurement of your speakers or subwoofer at the listening position. The measurement is the input that tells you which resonances exist.
  2. Identify resonant peaks. Look at the low-frequency region (typically below a few hundred Hz) for narrow, pronounced peaks. These are the room modes a BFD-style notch filter is meant to cut.
  3. Set the filter frequency. Center each parametric filter on the peak you want to reduce.
  4. Set the bandwidth (Q). Use a narrow enough bandwidth to affect the peak without pulling down neighboring frequencies. Narrow peaks call for higher Q; broader humps need lower Q.
  5. Set the gain. Apply negative gain (a cut) rather than boosting dips. Cutting peaks is the safer, more effective use of parametric EQ for room modes.
  6. Verify. Re-measure with the filters active and compare against the original measurement to confirm the peak is reduced and no new problems were introduced.

The expected result is a smoother low-frequency response with the modal peaks attenuated. If a filter changes the response in ways you didn't intend, adjust frequency, Q, or gain and re-measure.

Common sticking points

  • Boosting instead of cutting. Deep nulls are usually caused by cancellation and cannot be filled by EQ boost without side effects; cut peaks instead.
  • Too-wide filters. Overly broad cuts remove useful output along with the resonance.
  • Skipping re-measurement. Filter settings that look right on paper may behave differently in the room; always verify with a new measurement.
  • Chasing every ripple. Focus on the audible, significant peaks rather than flattening every small variation.

Limitations and when to use something else

BFD-style parametric EQ is a targeted tool for resonances, not a cure-all:

  • Placement comes first. REW is explicitly positioned to help "find the best locations for your speakers, subwoofers and listening position" (roomeqwizard.com). Moving speakers or the listening seat often reduces a mode more effectively than any filter.
  • Acoustic treatment addresses resonances physically and can reduce the workload on EQ, especially above the low-frequency range where EQ corrections become less reliable.
  • Broad tonal balance is better handled by other EQ approaches or system setup, not by narrow notch filters.
  • Multiple seats. A filter tuned for one listening position may not help others; decide which position matters most.

If your goal is precise mode control at one primary seat, BFD-style parametric cuts in REW are a practical fit. If you need to fix the whole room or correct broad response trends, prioritize placement and treatment, and use EQ only for what remains.