Cancellation Null
Canonical Definition
A cancellation null is a reduction in measured magnitude caused by vector addition of two or more contributions that arrive with opposing phase and comparable level at a particular frequency, position, and condition.
Measure the contributors separately, move the microphone, and inspect magnitude and phase before deciding what the graph means.
Why Cancellation Happens
When two equal signals arrive in phase, their pressure adds. When they arrive with opposite phase, their pressure subtracts.
Combined pressure = contribution A + contribution BIf A and B have similar magnitude and opposite phase at one frequency, the combined result can approach zero. The phase opposition may come from:
- reversed polarity
- path-length difference
- filter phase
- channel delay
- reflections and boundaries
- driver or enclosure phase behavior
- multiple sources covering the same band
Not Every Dip Is The Same
| Type of dip | Typical evidence | Likely action |
|---|---|---|
| Level or tonal deficiency | Broad and similar across positions; individual source also low | Level, shelf, broad EQ, or driver / enclosure review |
| Crossover cancellation | Individual drivers are healthy; combined response dips in overlap | Polarity, delay, level, filter, all-pass, or crossover-region change |
| Boundary or reflection null | Frequency and depth change with microphone or source position | Placement, mounting, treatment, seating compromise, or limited correction |
| Directivity / lobing hole | On-axis or off-axis response changes strongly with angle | Geometry, spacing, crossover frequency, or driver choice |
| Driver rolloff or protection filter | Dip appears in the individual driver response and follows the filter | Respect the limit, reshape target, or redesign the band |
Why Equal Boost Often Fails
Suppose two sources are equal in magnitude and opposite in phase:
1 + (-1) = 0Boosting both equally gives:
2 + (-2) = 0The amplifiers and drivers work harder, but the acoustic subtraction remains at that point. In real systems the cancellation may not be perfect, but deep narrow boost still tends to consume headroom and excursion for little stable benefit.
Diagnosis Workflow
- Repeat the measurement to rule out noise or setup error.
- Move the microphone a small known distance and observe whether the dip moves or changes depth.
- Measure each suspected source individually.
- Measure them combined with the same timing reference.
- Compare magnitude and phase through the affected region.
- Test deliberate polarity, delay, level, and crossover changes one at a time.
- Repeat at nearby positions before choosing a correction.
Correction Categories
- Polarity: correct wiring or intentionally invert one path when the acoustic target requires it.
- Delay or phase shaping: improve relative phase through the overlap region.
- Level: reduce one contributor when equal magnitudes create the deepest subtraction.
- Crossover: reduce overlap, change the transition region, or reshape the acoustic slopes.
- Placement or geometry: move the driver, listener, enclosure, or reflecting boundary relationship.
- Source management: disable or band-limit a redundant source.
- EQ: use only when the feature is sufficiently stable, correctable, and within system limits.
Minimum-Phase Versus Spatial Cancellation
Some response features are tied closely enough to the system's minimum-phase behavior that magnitude EQ can improve both the response and associated time behavior. Other dips are caused by path interference, multiple arrivals, or spatial geometry and cannot be made universally flat with one channel EQ.
Use phase, excess group delay, impulse response, individual sources, and positional measurements to classify the feature instead of treating every dip as uncorrectable—or every dip as EQ-able.