Understanding Acoustics
Phase and Time Alignment
Phase describes the timing relationship between frequencies in an audio signal. Proper phase alignment is essential for accurate transient reproduction and natural-sounding bass.
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What is Phase?
Phase describes where a wave is in its cycle at any given moment. For a sine wave, phase is measured in degrees (0° to 360°) or radians. When we talk about "phase" in audio, we usually mean the relative timing between different frequency components.
In a perfect system, all frequencies would arrive at your ears at exactly the same time. In reality, several factors cause different frequencies to arrive at different times:
- Speaker crossovers: Passive crossovers in multi-driver speakers introduce phase shifts
- Room reflections: Reflected sound arrives later than direct sound
- Room modes: Bass frequencies get "stored" in the room and released later
- Minimum-phase EQ: Traditional EQ changes phase when it changes magnitude
Two sine waves showing in-phase and out-of-phase relationships
Group Delay Explained
Group delay is a measurement of how much different frequencies are delayed relative to each other. It's measured in milliseconds and tells you how much a particular frequency "lags behind" others.
Constant group delay across all frequencies means the signal is time-aligned—all frequencies arrive together. Variable group delay means some frequencies are "smeared" in time.
Typical Group Delay Sources
| Source | Typical Delay | Frequencies Affected |
|---|---|---|
| Speaker crossover | 1-5 ms | Near crossover frequency |
| Ported speaker (bass) | 20-50 ms | Below port tuning |
| Room modes | 50-200 ms | Modal frequencies |
| Minimum-phase EQ | Varies | Where correction is applied |
Audibility Threshold
How Phase Problems Sound
Smeared Transients
When different frequencies in a transient (like a drum hit) are delayed differently, the attack becomes less defined. A tight, punchy kick drum sounds "soft" or "bloated."
Bass "Overhang"
Room modes store energy and release it slowly. This manifests as bass that continues after it should have stopped—a "one-note" boomy quality where all bass seems to ring at the same pitch.
Reduced Clarity
When frequencies are time-smeared, fine detail becomes harder to perceive. Vocals can sound "thick" or lacking articulation.
Imaging Problems
Phase inconsistencies between left and right channels (from asymmetric room reflections) can cause instruments to be poorly localized or shift position with frequency.
Waterfall plot showing bass ringing from room modes
Phase Correction Approaches
IIR (Minimum-Phase) Correction
Traditional parametric EQ uses IIR (Infinite Impulse Response) filters. These correct magnitude (how loud each frequency is) but inherently add their own phase shift—they trade one phase problem for another.
IIR filters are efficient and work well for correcting broad tonal issues, but they cannot correct the time-domain problems caused by room modes.
FIR (Linear-Phase) Correction
FIR (Finite Impulse Response) filters can correct both magnitude AND phase. They can:
- Flatten frequency response without adding phase shift
- Reduce modal ringing by correcting excess group delay
- Improve transient response and clarity
The tradeoff is that FIR filters require more processing power and add latency (typically 20-50 ms for full bass correction).
Sounn's Hybrid Approach
What Sounn Corrects
Sounn's phase correction targets:
- Modal ringing: Reduces the decay time of room modes
- Group delay: Aligns frequency arrival times
- Excess phase: Removes phase shifts from room acoustics
The result is tighter, more controlled bass and improved transient clarity throughout the frequency range.
Viewing Phase Data
After taking a measurement in Sounn, you can view:
- Phase response: Shows phase angle vs frequency
- Group delay: Shows frequency arrival time differences
- Waterfall/Spectrogram: Shows energy decay over time at each frequency
These views help you understand your room's time-domain behavior and see how much improvement correction provides.