⚙️ ENGINEER LEVEL: Minimum-Phase vs Non-Minimum-Phase Responses
Classification of Acoustic Measurements
Minimum-phase response: The phase response is uniquely determined by the amplitude response. Every dB of amplitude change has a corresponding predictable phase shift. Minimum-phase anomalies CAN be corrected by EQ — correcting the amplitude also corrects the phase.
Non-minimum-phase response: Phase is not determined by amplitude. Created by: - Acoustic cancellation between two sources arriving at different times - Room reflections (multipath effects) - FIR filtering with symmetric impulse response
Non-minimum-phase anomalies in the acoustic measurement CANNOT be corrected by IIR EQ — correcting the magnitude makes the phase worse, or vice versa.
Distinguishing minimum-phase in REW:
- Measure impulse response
- Use REW's "Minimum Phase Response" feature to compute the minimum-phase version of your measurement
- Compare to actual measurement
- Regions where actual phase ≠ minimum-phase phase: non-minimum-phase anomalies
- In those regions, EQ amplitude correction will not fix the phase — accept the limitation or address with time alignment
Practical implication: In a car, below approximately 300 Hz, the response is largely minimum-phase (dominated by single direct path and room modes). Above 300 Hz, multipath becomes significant — expect non-minimum-phase regions. Aggressive EQ above 1 kHz risks making the phase response worse.