Why the crossover transition region is so sensitive
In the transition region of a crossover, both drivers – for example woofer and tweeter – are active at the same time. Exactly here the fine interplay of level and phase decides whether the summed result is calm, stable in imaging and natural – or whether soundstage, localisation and sibilants become “nervous”. Both Macro-microphony (movement of windings/foils) and Micro-microphony (very small, materialinternal effects) can create timevarying minichanges in component values. In the transition region such Micro-modulations have a disproportionate effect, because two sources are summing simultaneously, phase is rotating steeply around the crossover frequency, and directivity (lobing) reacts very sensitively.
What really happens during the transition
Beyond Ohm's Law (R), reactive components dominate around the crossover frequency:
- Inductor: reactance X_L = 2π f · L – the larger L, the stronger the "resistance" to rapid changes in current.
- Capacitor: reactance X_C = 1/(2π f · C) – the larger C, the "easier" current flows at high frequencies.
The crossover frequency (simplified for a second-order LC filter) is
[ f_c \approx \frac{1}{2π\sqrt{L·C}} ]
Small temporal fluctuations in L(t) or C(t) – whether due to macro or micro-microphonics – shift this f_c slightly.
In other words: If L or C "breathe," the crossover frequency breathes with them.
These tiny shifts hit three amplifiers:
- Dual source: Near f_c are both drivers active. Tenth-of-a-dB changes per branch alter the sum significantly.
- Steep phase shift: filters rotate by f_c the phase rapidly. A small Δf_c generates noticeable phase shift → the addition pattern changes audibly.
Directivity (lobing): As soon as the phase relationship fluctuates, the main radiation lobe shifts. This leads to an unstable soundstage, wandering imaging, and glassy sibilance.
A clear numerical example
At f_c = 2 kHz, a change of just ΔC/C = +0.5% results in approximately Δf_c/f_c ≈ −0.25%, or about 5 Hz. This sounds small, but due to the steep phase shift around f_c, it can be enough to shift the sum by tenths of a dB, noticeably alter imaging and sibilance, and cause resonance combs to form.
How macro- and micro-microphony interact
- Macro-microphony: movement of conductors/foils generated by field forces induced voltages and time-variable L(t)/C(t) – the filter effect modulates itself.
- Micro-microphony: material effects (e.g., piezo/electrostrictive micro-movements, dielectric relaxation) generate very small but continuous ΔL/L and ΔC/C as well as subtle interference components.
In the transition, both add up: macro sets the dynamic framework, while micro compresses it across a broad band. Together, they shift the level, phase, and Q factor of the branches – exactly where the ear is most sensitive.
Conclusion – and how the effects interact
The transition zone is the acoustic focal point of the crossover. This is where time-variable component values – whether from macro-microphony or micro-microphony – have a disproportionate impact, because summation, phase response, and directivity are all sensitive at the same time. This is why the chapters on macro-microphony and micro-microphony refer to this section: It explains why small modulations have such a large effect here – and why structural stability and material-stable components allow for audibly more music in the transition.
Key points:
- When L or C breathe, the crossover frequency breathes with them.
- Where two drivers sum, every phase becomes a top priority.
- Calm structure = calm result.
- The truth is decided in the transition.
- No modulation, no nervousness – just music.
- Macro moves components, micro moves materials – both shifts Level, phase, quality in focus.
