Seamless Crossover Transition
48 dB/octave linear-phase FIR filters. Controlled driver overlap, acoustic phase alignment and a smooth sum through all four ways.
One acoustic sum
Crossovers split the audio signal so each driver handles only its intended frequency range. In the Baffless system, crossover frequencies are set at 170/800/3000 Hz using 48 dB/octave FIR filters.
Each driver is first linearised across the range surrounding its crossover. The FIR branches are given a common delay and adjusted from acoustic measurements so adjacent drivers arrive phase-aligned on the calibrated listening axis. The target is a flat magnitude sum with constant group delay through each transition — not zero phase or zero latency.
Why 48 dB/octave?
A shallower 24 dB/octave slope creates a broad overlap region in which two drivers reproduce the same frequencies. That pushes each driver farther outside the range around its first dipole peak and weakens the aim of making every band behave as one compact acoustic source.
A 96 dB/octave slope reduces that overlap further, but the sharper FIR transition extends the filter response in time. When the low- and high-pass branches arrive together, much of their complementary pre- and post-ringing cancels in the acoustic sum. Vertically off-axis, their path lengths differ and that cancellation is no longer exact, making a very steep crossover more sensitive to listening height.
During development, 24, 48 and 96 dB/octave crossovers were compared in the complete loudspeaker. Moving from 24 to 48 dB/octave produced a clear improvement. The change from 48 to 96 dB/octave was less conclusive because filter overlap, off-axis behaviour and time response all changed together. 48 dB/octave was therefore retained as a conservative Baffless-specific balance: narrow enough to control driver overlap, without pursuing unnecessary filter steepness. It is a design choice for this four-way system, not a universal optimum for every loudspeaker.
Further reading
Bodzio Software, “Pre- and Post Ringing of Impulse Response” is an accessible visual introduction to temporal masking and to the way complementary low- and high-pass responses can cancel ringing in their combined output.